Soft X-ray Absorption Spectroscopy of Liquids and Solutions · Soft X‑ray Absorption Spectroscopy of Liquids and Solutions Jacob W. Smith and Richard J. Saykally* Department of

Soft X‑ray Absorption Spectroscopy of Liquids and SolutionsJacob W. Smith and Richard J. Saykally*

Department of Chemistry, University of California, Berkeley, California 94720, United States

Chemical Sciences Division, Lawrence Berkeley National Lab, Berkeley, California 94720, United States

ABSTRACT: X-ray absorption spectroscopy (XAS) is an electronic absorptiontechnique for which the initial state is a deeply buried core level. The photon energiescorresponding to such transitions are governed primarily by the binding energies of theinitial state. Because the binding energies of core electrons vary significantly amongatomic species, this makes XAS an element-selective spectroscopy. Proper interpretationof XA spectra can provide detailed information on the local chemical and geometricenvironment of the target atom. The introduction of liquid microjet and flow celltechnologies into XAS experiments has enabled the general study of liquid samples.Liquids studied to date include water, alcohols, and solutions with relevance to biology and energy technology. This Reviewsummarizes the experimental techniques employed in XAS studies of liquid samples and computational methods used forinterpretation of the resulting spectra and summarizes salient experiments and results obtained in the XAS investigations ofliquids.

CONTENTS

1. Introduction 139092. Experimental Techniques 13909

2.1. NEXAFS/XANES 139102.2. EXAFS 139122.3. Scattering Techniques 13913

3. Theoretical Interpretation of NEXAFS Spectra 139134. XAS of Water 13915

4.1. Liquid Microjet Experiments 139154.2. Liquid Flow Cell Techniques 139174.3. Interpreting the Water NEXAFS Spectrum 139174.4. Perturbing the Hydrogen Bond Structure of

Liquid Water 139195. XAS of Short-Chain Alcohols 139206. XAS of Solutes as a Probe of Solution Structure 13921

6.1. Probing Solute−Solute Interactions 139216.2. Probing Solute−Solvent Interactions 139246.3. XAS Studies of Solvated Biomolecules 13925

7. Concluding Remarks and Outlook 13927Author Information 13928

Corresponding Author 13928ORCID 13929Notes 13929Biographies 13929

Acknowledgments 13929References 13929

1. INTRODUCTIONModern second- and third-generation synchrotron lightsources, utilizing wiggler and undulator radiation sources, arecapable of producing X-ray beams with high flux (ca. 1010 to1016 photons/s), excellent resolving power (typical E/ΔE =1 000−10 000), small spot sizes (ca. a few hundred microns),and broad tunability (e.g., a single beamline may be able to

produce photons with energies spanning 2 orders of magnitude,and a single facility may simultaneously produce beams inenergy regimes ranging from the far-infrared to hard X-ray).Access to such bright, tunable, and monochromatic sources hasenabled a powerful new suite of X-ray spectroscopies. Perhapsthe most straightforward and widespread approach is soft X-rayabsorption spectroscopy (XAS).Soft X-rays are loosely defined as those having short

attenuation lengths in most media, including air. While avariety of figures have been suggested for the upper energybound of this region, 5 keV (∼0.25 nm, 4 × 107 cm−1, 1.2 kHz)can be considered a reasonable cutoff between the soft and hardX-ray regimes. Studies involving X-rays below this energy havetypically been carried out in high-vacuum (HV) or ultrahigh-vacuum (UHV) chambers as a result of the short attenuationlength of such photons in air. This requirement has historicallylimited the applicability of soft X-ray spectroscopies for thestudy of liquidswhile XAS experiments became widespreadbeginning in the 1970s,1,2 they did not become commonplacefor liquid samples until the introduction of liquid microjettechnology into the field by Wilson et al. in 2001.3 Hence, thisReview covers primarily ∼15 years of experiments in this area.A summary review of XAS and other X-ray techniques for thestudy of liquid samples has recently been published by Langeand Aziz.4 In addition, Nilsson and co-workers have publishedseveral multitechnique reviews of X-ray spectroscopy ofwater.5−7

2. EXPERIMENTAL TECHNIQUESThe soft X-ray energy regime corresponds to the typical rangeof binding energies of core-level electrons. Consequently, thedominant interaction between soft X-rays and matter is photon

Received: April 19, 2017Published: November 10, 2017

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© 2017 American Chemical Society 13909 DOI: 10.1021/acs.chemrev.7b00213Chem. Rev. 2017, 117, 13909−13934

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absorption and excitation of core electrons into unoccupiedenergy levels or unbound continuum states as photoelectrons.In the soft X-ray regime, this process is orders of magnitudestronger than scattering processes.8 As core-level bindingenergies vary substantially from atom to atom, XAS techniquesare highly atom-specific. The sensitivity of the unoccupiedvalence electronic structure to the local environment make XASa powerful probe of the local intra- and intermolecular chemicalenvironment of the target atoms. The majority of studiesdiscussed in this Review involve experiments on the absorptionK-edges of low-z atoms, viz. absorptions with an initial state ofn = 1. Absorptions from higher energy levels advance throughthe alphabet (e.g., excitations from n = 2 are termed L-edge,etc.). XAS experiments can be broadly categorized into twoclasses. Studies probing the region from the absorption onset toseveral tens of eV above the edge are typically referred to asnear-edge X-ray absorption fine structure (NEXAFS) or X-rayabsorption near edge structure (XANES) and probe theunoccupied bound states accessible to the target atom.9 Withinthe dipole approximation, it may be said that K-edge NEXAFSspectra reflect the p-projections of the local unoccupied densityof states. Experiments probing above this energy, sometimes toas much as 1000 eV above the absorption edge, are termedextended X-ray absorption fine structure (EXAFS) and probethe local structural environment of the target atom.

2.1. NEXAFS/XANES

Near-edge XAS experiments probe excitations from core levelsinto the unoccupied energy levels accessible to the target atom,producing both an excited electron and a core hole. Relaxationof the core-excited atom can take place radiatively, through theemission of a fluorescent photon, or nonradiatively via theemission of an Auger electron. The processes involved inexcitation and de-excitation are illustrated in Figure 1.

A variety of detection schemes can be employed formeasurement of NEXAFS spectra, exploiting both emissionpathways. The first fluorescence detection of a NEXAFSspectrum was reported by Fischer et al. in 1986 and utilized agas proportional counter to detect the fluorescent signal.10 Inthe years that followed, fluorescence was often measured by thelarger, liquid nitrogen-cooled solid-state detectors that had beenutilized for some decades for detection of X and γ radiationfrom radioactive isotopes (e.g., Si(Li), Ge(Li), drifted silicon,etc.), and which offered the advantage of excellent energy

resolution when properly operated. All of these detectionapparati typically employ windows of varying chemicalcomposition to filter out low-energy photons that may havereached the detector as a result of scattering processes or fromexternal sources.9 Modern studies measuring fluorescence yield,however, tend to utilize semiconductor photodiodes. Thesesimple detectors are small and do not require cryogeniccooling, making them much easier to incorporate intoexperimental designs. However, they do not energy-resolvethe incident photons and are also sensitive to photons withenergies in the visible and UV ranges. While the quantumefficiency of such devices increases with photon energy, andsubstantially larger currents are observed at X-ray energiesrelative to the visible and UV,11 photodiodes generally produceXA spectra with larger but generally constant backgroundsrelative to energy-resolved detection devices. Because photo-diodes do not discriminate photon energies, data collected withsuch detectors are typically referred to as total fluorescenceyield (TFY) spectra.While energy-resolved fluorescence yield XA spectra tend to

have very low background and therefore offer an excellentsignal/background ratio, they often do not exhibit high signal/noise as a result of low total signal strength. For core-excitedlow-z elements, the Auger relaxation pathway dominates, asshown in Figure 2; for example, the average fluorescence

quantum yield of carbon following K-edge excitation has beenmeasured to be 2.8 × 10−3.12 An additional complication forfluorescence yield spectroscopy arises in part because of thislow quantum yield: state-dependent variation in the relativeyield of fluorescent photons is large relative to the averagequantum yield, potentially distorting fluorescence-yield spectrain favor of spectral features resulting from transitions into stateswith high fluorescence quantum yield.13,14 Furthermore, as thefluorescent X-ray photons have energy only slightly below thatof the incident beam, they have a similar attenuation length.Consequently, fluorescent photons from all absorption eventswithin the sample can reach the detector, and the probe depthis defined by the penetration of the beam. For samples with apath length enabling near-complete absorption of the incomingbeamonly hundreds of nanometers around edge resonan-

Figure 1. NEXAFS-relevant electronic transitions: (a) Absorption ofan X-ray photon results in excitation of a core electron into anunoccupied state. (b) Nonradiative relaxation occurs via the Augerprocess. (c) Radiative relaxation occurs via emission of a fluorescent X-ray photon. As depicted in these energy-level diagrams, relaxation ofthe excited electron is slow relative to relaxation of the core hole.

Figure 2. Quantum yields of Auger electrons and fluorescent X-raysper core-excited atom. For second-row elements, Auger yieldrepresents >99% of relaxation events.

Chemical Reviews Review

DOI: 10.1021/acs.chemrev.7b00213Chem. Rev. 2017, 117, 13909−13934

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ces15this results in substantial saturation effects in the TFYspectrum. Self-absorption errorsspectral broadening causedby reduction of peak heights due to absorption of fluorescentX-rays within the samplehave also been observed whensamples are sufficiently concentrated.16

In recent years, a technique known as inverse partialfluorescence yield (iPFY) has been developed to enablefluorescence detection without the interference of saturationand self-absorption effects and state-dependent spectralintensity fluctuations.17,18 In this technique, partial fluorescenceyield is measured at a nonresonant wavelength, typically severaltens of eV below the resonant edge fluorescence energy. Asabsorption events attenuate the beam and reduce the potentialfor nonresonant scattering, resonant absorption results in areduction in the iPFY signal. It has been shown that the iPFYsignal is inversely related to the absorption coefficient and thatthe inverse iPFY spectrum matches well with electron-detectionexperiments.17 Gotz et al. have demonstrated the use of iPFYdetection for liquid samples and further suggest that, whenpaired with PFY, it provides a probe of the Coster−Kronigtransition probabilities of transition metal species in solution.19

The sensitivity of iPFY detection closely matches that of PFYdetection while minimizing the effects of saturation and state-dependent effects; however, both techniques exhibit reducedsensitivity relative to TFY.Prior to the development of fluorescence yield detection

methods, NEXAFS spectra were collected primarily via electronyield. The simplest form of electron-yield detection is totalelectron yield (TEY), which can be measured by placing anelectrode or channeltron detector near the irradiated region ofthe sample, often utilizing a positive bias to collect electronsfrom a larger solid angle. Whereas fluorescence yield detectionprobes a sample depth of ca. the photon mean-free path, TEYdetection probes to a depth governed by the mean-free path ofelectrons in the sample medium. In general, the photon mean-free paths of soft X-rays in condensed phase media are ca. 100nm.20 However, in the vicinity of an absorption edge, this maydrop to a few tens of nanometers. The effective probe depthmeasured by TEY is more difficult to determine quantitatively.While the mean-free path of Auger electrons with energies of afew hundred eV are generally ∼1 nm in condensed phase, theprobe depth is believed to be greater than this. The extension ofprobe depth beyond the mean-free path results from the natureof the interactions of energetic electrons with their chemicalenvironment, which allow individual Auger electrons toproduce a cascade of inelastically scattered secondary electrons.Several studies summarized in ref 21 predict a probe depth ofseveral tens of Å in solid Si, GaAs, and Al. However, the probedepth is thought to vary with sample composition and Augerelectron energy.22,23 The probe depth in aqueous media hasbeen estimated by Smith as 50 Å;24 however, this value is basedon a simple model with a crudely estimated multiplicative factorfor inelastic scattering and should not be consideredquantitative.While electron escape depths of tens of Angstroms represent

a substantially shallower probe depth than that measured viafluorescence yield, TEY still constitutes primarily a bulkmeasurement. It is possible to increase surface sensitivity byenergy-filtering the detected electrons. With each subsequentscattering event, the Auger and secondary electrons becomegreater in number and lower in energy; the TEY signal is largelydominated by the numerous low-energy electrons generated byAuger emission events initiated deep within the sample.9 By

collecting only the higher-energy electrons, a detectionapproach referred to as partial electron yield (PEY), it ispossible to isolate signal from electrons having undergone asmall number of scattering events; such electrons will generallyhave originated near the sample surface. PEY spectra have mostcommonly been measured by placing a screening grid with anegative bias between a sample and a simple electron multiplier.The negative bias of the grid, often referred to as the retardingbias, blocks electrons with kinetic energies below the gridpotential from reaching the detector; thus, a retarding bias of200 V would allow passage and subsequent detection ofelectrons leaving the sample with KE > 200 eV. Typicalretarding biases are 30−80 eV below the characteristic Augerelectron energy of the atom being probed. A comparison of aPEY spectrum obtained using this type of detector, with aretarding bias ∼40 V below the Auger energy, with afluorescence yield spectrum of ethylene chemisorbed on aCu(100) crystal is exhibited in Figure 3. This comparison

makes it clear why PEY has generally been preferred tofluorescence yield for the study of low-z species adsorbed tosolid substrates: while the signal/background ratio is vastlysuperior for fluorescence yield, the signal/noise is substantiallybetter in the PEY spectrum. PEY spectra can also be measuredusing a standard electron energy analyzer, such as a channeltronor hemispherical energy analyzer, by integrating only thedesired range of electron energies.25,26

An additional class of electron detection, also illustrated inFigure 3, is Auger electron yield (AEY). Unsurprisingly, AEYXA spectra measure electrons with a very narrow energy rangearound an Auger emission energy of the target atom. Typically,

Figure 3. NEXAFS spectrum of ∼2 monolayers of ethylene adsorbedon the (100) surface of a copper substrate as measured by (a)fluorescence yield collected in a gas proportional counter, (b) Augerelectron yield collected in a cylindrical mirror analyzer, and (c) partialelectron yield collected with a 220 V retarding bias. All spectra arenormalized to the spectrum of clean Cu(100) measured via thecorresponding method. The listed JR values correspond to the signal/background ratios (measured at 320 eV). Reproduced with permissionfrom ref 10. Copyright 1986 Elsevier.



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such spectra are collected via a cylindrical mirror analyzer(CMA) with its acceptance window centered on the desiredAuger energy. The poor signal/noise evidenced in Figure 3 forthis detection method is largely a result of a shorter counttimegiven equal counting time, AEY spectra of adsorbedspecies generally exhibit superior signal/noise relative tofluorescence yield detection due to the greater quantum yieldof Auger electrons.10 The probe depth for such experiments isdetermined by the mean-free path of the Auger electron, ∼1nm in condensed phase media, providing greatly enhancedsurface sensitivity relative to other methods. However, due tothe fixed and typically short focal length and high-vacuumrequirements for the operation of CMAs, this detectionmechanism has generally been perceived to be incompatiblewith liquid samples.Two additional classes of NEXAFS detection merit

discussion. The first is ion yield detection. Auger electronemission and the subsequent cascade of secondary electronsproduce local pockets of positive charge. This results in theexpulsion of ionized species, which can be detected as an ionyield. Ion yield NEXAFS spectra have been recorded forgases,27 adsorbed molecules on solids,28 and liquids.29,30 It issuggested in these works that ion yield detection may be themost surface-sensitive NEXAFS detection technique as a resultof the very short mean-free path of ionic species in condensedphase. However, it has been demonstrated that ion yield spectraof adsorbed species do not produce a spectrum linearlyproportional to the absorption coefficients of the target atom inthe spectral range.31,32 Moreover, while ions must be generatednear the vacuum interface to escape and be detected,contributions of ions produced by secondary ionization eventsfollowing initial excitation and Auger emission within the bulkhave been found to dominate the spectrum of solid ice,33 andfurther experimentation has shown the ion yield and electronyield spectra of liquids to be generally indistinguishable.34

The final detection scheme used in NEXAFS spectroscopy,which has become increasingly relevant to the study of liquidsin recent years, is transmission mode detection. This is themost straightforward detection method for any absorptionspectroscopy. However, as a result of the small mean-free pathof soft X-rays in sample media, it is primarily useful in the studyof thin films,35 or more recently in the study of very thin liquidsamples, which will be discussed in greater detail in this work.

2.2. EXAFS

Whereas NEXAFS probes the unoccupied bound states of atarget atom, thereby providing information on the localelectronic environment, EXAFS produces excitations into thecontinuum, producing photoelectrons and probing the localstructural environment. Because production of an X-rayphotoelectron generates a core hole that may relax via thesame Auger or fluorescence pathways accessible to core-excitedatoms following excitation into bound states, EXAFS spectracan be collected using the same suite of measurementtechniques employed for NEXAFS. When displayed on signalversus energy coordinates, as is typical for NEXAFS spectra(e.g., Figure 3), EXAFS spectra appear as low-amplitudeoscillations in the signal intensity in the slowly decayingbaseline typically observed at energies above an absorptionedge (Figure 4). The origin of these oscillations is interferencebetween the wave functions of photoelectrons and backscatterfrom nearby atoms.

Extraction of structural information from EXAFS spectrarequires conversion from energy coordinates to momentumcoordinates, typically expressed in terms of the photoelectronwave vector k,

=ℏ

−km

E E2

( )2 0(1)

where E0 is the ionization threshold. The signal intensity isbackground-subtracted, typically by removal of a constantbackground measured from the pre-edge region. The oscillationintensity χ(k) is then obtained by normalization to an isolatedatomic background μ0:

χμ μ

μ=

−k( ) 0

0 (2)

This atomic background can be obtained from experimentalmeasurements or from a simple long-range fitting of the EXAFSspectrum over many oscillations. Plots in k-space are oftendisplayed with χ(k) multiplied by a power of k (typically k2 ork3) to visually amplify the lower-amplitude oscillations at highervalues of k.The “EXAFS equation” can be derived from Fermi’s Golden

Rule with consideration of perturbations resulting from thebackscatter interference. While exact statements of the EXAFSequation vary slightly with the formalism used (e.g., plane wavevs curved wave, single scattering vs multiple scattering, etc.),the most common form of the equation is

∑χ φ= +σ−kNf k

kRkR k( )

( )e sin{2 ( )}

j

j j

j

kj j2

2 j2 2

(3)

Here Nj is the number of nearest neighbors, f j(k) is adecreasing amplitude function, Rj is the interatomic separation,φj is a phase shift function arising from the atomic potential,and σj is the Debye−Waller factor. This final term is a measureof the structural disorder in the sample, proportional to themean-squared fluctuation of symmetry-equivalent values of Rj.Consequently, the exponential term of the EXAFS equationdescribes a function reducing in amplitude at highermomentum at a rate proportional to the disorder in the localstructure resulting from increasingly destructive interference

Figure 4. Raw EXAFS spectrum (μ) at the copper K edge of a singlecopper crystal showing the smooth atomic-like absorption backgroundμ0 and χ, in this case shown in energy coordinates as χ(E). The zero ofthe energy axis has been set to the ionization onset energy.Reproduced with permission from J. J. Rehr & R. C. Albers,“Theoretical Approaches to X-Ray Absorption Fine Structure.” Rev.Mod. Phys. 2000, 72 (3), 621−654.36 Copyright 2000 by the AmericanPhysical Society.



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patterns arising from neighbors at differing radial separation.Consequently, the rates at which the magnitude of EXAFSoscillations decay indicate the degree of disorder in the localstructure.While reasonable values for some of the unknowns in eq 3

can be extracted from the experimental data, fitting of raw χ(k)data is challenging because of the typical complexity of thespectra. The basis of the modern solution to this problem wasdeveloped in 1971 by Sayers et al.: by computing the Fouriertransform of χ(k), they demonstrated that it is possible toextract a plot in terms of radial distance to scattering atoms.37

Back-transforming a single peak from this spectrum produces asimpler EXAFS spectrum in k-space, for which eq 3 can besolved to find the number of atoms and Debye−Waller factorfor neighbors at the distance from the target atom of the back-transformed peak.More detailed discussion of the derivation and solution of the

EXAFS equation for various formalisms can be found in theliterature.21,36,38−42 At present, most EXAFS data processingand interpretation is performed in a generally straightforwardalbeit not trivial in many casesmanner, utilizing one ofseveral preprogrammed packages that are readily available withvarying user inputs. A list of common programs for EXAFSprocessing, including short descriptions and links, can be foundat http://www.ixasportal.net/wiki/Software.

2.3. Scattering Techniques

While they do not truly fit into the category of XAS techniques,it is necessary to briefly mention X-ray and electron scatteringtechniques because of their modern applications to similarproblems. Experiments utilizing inelastic X-ray scattering, oftenreferred to as X-ray Raman spectroscopy (XRS), typicallymeasure the inelastic scattering energy loss at energies aroundan X-ray absorption edgeproducing data analogous to thosemeasured in NEXAFS experimentsor above itproducingdata analogous to those measured in EXAFS experiments. It hasbeen shown mathematically that, for cases in which themomentum transfer is small relative to the inverse radialdistribution of the ground-state electrons (e.g., 1s electrons forK-shell scattering)that is, cases in which the dipoleapproximation can be considered validXRS provides thesame chemical and structural information available from XAS.43

Studies of crystalline substances by XRS have shown that itreproduces spectra measured by traditional NEXAFS44 (Figure5) and EXAFS45 techniques for low-z atom types. Suchexperiments have the substantial benefit of offering anessentially free choice of input photon energies. As scatteringcross sections increase at higher photon energy, XRS studies aretypically performed with hard X-rays and consequently are notsubject to the substantial difficulties associated with vaporbackground from liquid samples and limited penetration depththat make soft X-ray spectroscopy of liquids particularlychallenging.46 However, XRS experiments in the hard X-rayrequire collection and quantitative energy resolution of thescattered hard X-rays, often at E > 10 keV, and consequentlytend to require substantially longer collection times and exhibitinferior spectral resolution compared to equivalent soft X-raytechniques. A thorough overview of the development and utilityof the XRS technique through its early years has been publishedby Krisch and Sette.47

Inelastic electron scattering experiments at soft X-rayenergies, typically referred to as inner-shell electron energyloss spectroscopy (ISEELS) or simply EELS (which may also

refer to the equivalent technique in the vibrational energyregime), can also probe information equivalent to that obtainedfrom XAS and XRS experiments.48 Experimental designs forISEELS typically combine an “electron gun” with a high-resolution electron energy analyzer to map electron energy loss.However, while ISEELS has been utilized to study, for example,the water content of mineral species,49 the requirement for HVor UHV pressures at the detector, typically short detector focallengths, and poor electron penetration through windowmaterials generally render bulk liquid samples incompatiblewith this technique.

3. THEORETICAL INTERPRETATION OF NEXAFSSPECTRA

XA spectra in the NEXAFS/EXAFS energy regimes contain agreat deal of information on the electronic and structuralproperties of chemical species. However, this information canonly be extracted through comparison with appropriatetheoretical calculations. While interpretation of EXAFS canbe achieved by the straightforward solution of the EXAFSequation for a given set of spectral data, interpretation ofNEXAFS spectra is less straightforward and typically requiresindividual first-principles calculations on a system-by-systembasis. For example, a study in our group of the TEY nitrogen K-edge NEXAFS spectrum of guanidinium ions in aqueoussolution found that the lowest unoccupied molecular orbital(LUMO) and LUMO+1 states exchange ordering upon core-excitation of a target nitrogen atom.50 Such effects are notcaptured by a purely analytical computational approach.Instead, the standard approach to analysis of NEXAFS spectracombines molecular dynamics (MD) simulations of appropri-ately chosen model systems with electronic structurecalculationsmost commonly ab initio density functionaltheory (DFT)-based calculations51,52to compute core-excitation energies and intensities. A thorough and detailedreview of MD and electronic structure techniques utilized in

Figure 5. Comparison of XRS (empty points at several momentumtransfer values) and NEXAFS (dashed line) spectra in the area of thelithium K-edge of solid lithium metal. In this figure, qa represents theproduct of the scattering momentum transfer q and the Li 1s inverseorbital radius a. It is clear that for low qathat is, cases in which thedipole approximation is validthe XRS spectrum reproduces theNEXAFS spectrum quite well, while for larger qa the breakdown of thedipole approximation results in greater discrepancies. Reproducedfrom H. Nagasawa et al., “X-Ray Raman Spectrum of Li, Be andGraphite in a High-Resolution Inelastic Synchrotron X-Ray ScatteringExperiment.” J. Phys. Soc. Jpn. 1989, 58 (2), 710−717,44 withpermission from The Physical Society of Japan.



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http://www.ixasportal.net/wiki/Software



such calculations has been recently published.53 Here we willbriefly discuss the specific challenges associated with simulationof NEXAFS spectra of liquid systems and provide relevantdetails of the most commonly utilized computational methodsfor such systems.The first challenge in simulating liquid-phase NEXAFS

spectra is the selection of appropriate molecular coordinates inwhich to compute the theoretical core-excitation spectrum.Because of the inherent disorder in liquid systems, it isnecessary to representatively sample a large region ofconfiguration space while modeling generally transientintermolecular interactions as accurately as is reasonablypossible. Many calculations utilize molecular coordinatessampled from MD simulations of clusters, typically modelingtransitions arising from ground states of atom(s) near thecenter of the cluster. Alternatively, a bulk liquid environmentcan be simulated in some cases by utilizing periodic boundaryconditions. Many liquid studies have utilized fully classical MDsimulations or quantum mechanics/molecular mechanics(QM/MM) simulations in which the central molecule to beexcited and, in many cases, its interactions with nearby solventmolecules are treated quantum mechanically, while the bulkliquid is treated classically. In addition, a number of simulatedNEXAFS studies have utilized full ab initio molecular dynamics,particularly in the simulation of water,54−57 including Car−Parrinello MD simulations58 and path-integral ab initio MDsimulations.59 Inclusion of quantum mechanical effects hasbeen shown to appreciably improve the fit to experimentalspectra for numerous liquid systems.60−62

Calculation of the simulated core-excitation spectrum fromcoordinates sampled from MD simulations has been performedwithin numerous methodologies. The simplest and leastcomputationally expensive standard DFT method for simulat-ing NEXAFS spectra is the so-called “muffin-tin approach,” asingle-electron multiple-scattering approach in which theexcited states are modeled as dispersed within a potentialfield consisting of a spherical potential around nuclei and aconstant interstitial potential. This method is analogous to theanalysis of EXAFS spectra; indeed, muffin-tin calculations forthe interpretation of NEXAFS spectra can be performedsemiquantitatively with the FEFF software package for EXAFSanalysis.36 This simple approach neglects the local electrondensity and core-hole interactions; however, it has been foundto be sufficient to reproduce the K-edge NEXAFS spectra ofsystems with highly delocalized excited states for whichindividual electronic interactions are less significant.63,64 Notethat newer versions of FEFF have been updated to includepackages for NEXAFS analysis that combine multiple scatteringwith, e.g., core-hole interaction estimates as describedbelow.65,66 A methodology utilizing a similar multiple-scatteringapproach but employing an unconstrained potential field (i.e.,free from the muffin-tin approximation) has been published byJoly.67

The exponential growth in computational resources hasenabled DFT-based methods of NEXAFS spectral analysis forrealistic systems employing detailed approximations of the core-hole interaction and the local electronic structure of the finalstates. One such method that has been utilized substantially forliquid systems is transition potential DFT (TP-DFT).68 Thismethod is a philosophical descendant of the Slater transitionpotential method of calculating electronic transition energies69

in that it leverages an electronic configuration averagedbetween the ground and excited states. However, unlike the

Slater method, which computes transitions on a state-by-statebasis, TP-DFT typically removes half of a core electron fromthe atom of interest (for which reason it is often referred to asthe half-core-hole {HCH} method), relaxes the generatedmolecular ion, freezes the resulting density, and computes thefull spectrum of excited states in a single calculation from theresulting potential. Consequently, this methodology is able tomodel directly the interactions of the core-excited structure andthe excited electron.Analogous methodology to the HCH TP-DFT method

utilizing a full-core hole (FCH) has also been explored.54,70

Most commonly, the FCH is modeled using the z+1approximation, in which, rather than explicitly generating acore hole, the nucleus of the excited atom is replaced by the z+1 nucleus (i.e., a carbon target atom would be modeled as anitrogen with 4 valence electrons, effectively generating thesame core charge as a carbon nucleus with a core hole). It hasbeen suggested that this method is problematic for atom pairswherein a substantial electronegativity gap between the twospecies exists, in which case enhanced local valence occupationdue to the electronegative attraction for bonding electron pairsmay substantially impact the relative weights of peaks in theobserved spectrum.71 Furthermore, it has been suggested thatTP-DFT calculations may in general not be quantitativelyaccurate in reproducing the experimental NEXAFS spectra ofliquids based on a random selection of coordinates,72,73

although this claim has been contested.5,6,74,75 The fact thatthe excited half-electron is ignored may also be problematic forsystems in which the excited electron is fairly localized and/orinteracts strongly with the unoccupied orbitals of the excitedatom.74

The other analytical method that has been used effectively inthe analysis of liquid-phase NEXAFS spectra is thePrendergast−Galli excited-electron and core-hole (XCH)methodology.76 In this increasingly popular approach, theexcited atom is modeled with a modified pseudopotential torepresent the presence of the core hole, and the excitedelectron is included explicitly in the lowest available valenceorbital. The electron density of the first excited state is thencalculated self-consistently. The remaining states are generatednonself-consistently from the unoccupied Kohn−Sham orbitalsof the self-consistently generated field. Transition matrixelements to the computed states are calculated within Fermi’sGolden Rule within the dipole approximation. Whereas TP-DFT calculations are typically performed with an orbital basisset utilizing cluster-based sample systems, the XCH calculationutilizes a plane-wave basis. Consequently, calculations onclusters produce substantial nonphysical edge effects; instead,coordinates must be generated under periodic boundaryconditions. For small box sizes, this may result in nonphysicalboundary interactions.7 Critics of the methodology have alsopointed out that it may break down in cases for which the firstexcited state and higher-energy excited states differ substan-tially, and they have also suggested that the nonself-consistentportion of the calculationthe orbitals for which are computedin the presence of the core-excited electron in the LUMO andthus with a net-neutral charge rather than a positive chargemay be a poor model for insulating materials in which metalliccharge screening mimicking this effect would not be expected.74

NEXAFS spectra can also be computed via time-dependentDFT (TDDFT).77 The accuracy of such methods has beenfound to vary substantially with system and choice offunctional.77,78 Only two thorough TDDFT studies of the



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NEXAFS spectra of liquids have been performed, both studyingpure liquid water.79,80 Brancato et al. calculated a spectrum withrelative peak heights in poor agreement with experiment;however, Fransson et al. found that utilizing larger clusters andmultimolecule averaging TDDFT produced a more quantita-tively accurate reproduction of the experimental data than TP-DFT calculations on the same molecular coordinates, asexhibited in Figure 6.

All of the above computational methods simplify the two-body electron−core-hole system into a single-body problem,typically by modeling the core hole via an appropriatepseudopotential for the core-excited atomic center. However,in recent years it has become possible to solve the two-bodyBethe−Saltpeter equation (BSE) in conjunction with a plane-wave DFT treatment of the excited state and GW quasiparticlecorrections to the self-energy (generally speaking, a dynamicscreening correction) to computationally address spectralbroadening.66,81−83 This methodology has been applied towater;84 however, the system size was constrained to only 17water molecules by the computational costs of the theoreticalmethodology, and the calculated spectrum was not consistentwith experiment at high energies.7 The computational methodhas recently been further streamlined for application to larger

systems, but this more-efficient adaptation of the calculationhas not yet been applied to liquid systems.85

4. XAS OF WATER

The hydrogen-bond structure of water has been the subject ofextensive study, as it underlies the remarkable properties ofwater in its condensed phases. The unique liquid properties ofwater are critical to its role in biological solvation and reactiondynamics, e.g., in the stabilization of native protein structure.86

It has long been believed that liquid water at ambienttemperature has a locally ordered tetrahedral structure with asmall number of broken hydrogen bonds, allowing for tighterpacking than that found in ice Ih.

87,88 Indeed, this picture hasbecome “textbook knowledge” and has been supported byseveral experimental techniques. For example, Fourier trans-form infrared spectroscopy (FTIR) studies have indicated asmall number of dangling hydrogen bonds at room temper-ature,89,90 supporting a picture of liquid water in which mostwater molecules are donating and accepting two hydrogenbonds. Neutron91 and X-ray scattering92,93 results have beeninterpreted such that the extracted O−O radial distributionfunctions (RDFs) have a reasonably sharp second peak,indicating an ordered and periodic structure relative to thatof most liquids and with a separation between peak positionsconsistent with tetrahedral bond angles. First-principlessimulations of liquid water have also almost universallysupported the locally tetrahedral structure of water, withexceptions generally being ascribed to an incomplete basis set.94

This tetrahedral picture of liquid water was nearly universallyaccepted until the development of techniques for NEXAFSspectroscopy of liquid water introduced new controversy intothis old discussion, inciting passionate disagreements regardingthe nature of liquid water at ambient conditions.

4.1. Liquid Microjet Experiments

The first XA spectrum of a liquid sample was an EXAFSspectrum of water published by Yang and Kirz in 1987.95 Theauthors utilized a static water cell comprising two 150 nmsilicon nitride windows surrounding a sample of thickness 1 umand a photomultiplier tube (PMT) detector in a transmissiongeometry. Transmission through the empty cell was ∼20%. Theresulting EXAFS spectrum matched qualitatively with theauthors’ calculated spectrum; however, the lack of an accurateamplitude function and phase shift for liquid water, combinedwith the poor spectral resolution available at the time, did notpermit quantitative determination of the O−O pair correlationfunction and Debye−Waller factor.The breakthrough in XAS of liquids occurred with the

publication by Wilson et al. of the EXAFS and NEXAFS spectraof liquid water microjets in 2001.29 Liquid microjet technologyhad been pioneered for ultraviolet photoelectron spectroscopy(UPS) by Faubel et al. in the late 1980s96 and was adapted foruse in XAS at Berkeley. Liquid microjets are formed by forcingpressurized liquids through a micron-scale orifice with a linearflow velocity in the range 20−100 m/s. The streaming jetprovides a continuously renewing sample and avoids thesubstantial absorption losses from the windows observed in thestatic cell experiment of Yang and Kirz. For XAS experiments,the resulting liquid jet is intersected with the X-ray beamdownstream from the jet tip. Any of the various detectionmethods described above compatible with observed chamberpressures can be placed near the region of intersection.Differential pumping allows the X-ray beamline to be

Figure 6. Comparison of simulated liquid water NEXAFS spectraobtained from TP-DFT and TDDFT to the experimental spectrum.Each theoretical spectrum is averaged over 100 sets of molecularcoordinates; each set of coordinates consists of a cluster of 32 watermolecules computed using path integral molecular dynamics (PIMD).Here, the TDDFT calculated spectrum shows a better agreement withexperiment, particularly in terms of the main-edge (∼537 eV) to post-edge (∼541.5 eV) peak ratio. This concept will be discussed in greaterdetail in the discussion of the water NEXAFS spectrum. Reproducedwith permission from ref 80. Copyright 2016 PCCP Owner Societies.



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maintained at ultrahigh vacuum while the local pressure in thesample chamber may be substantially higher (typical chamberpressures are 10−5 to 10−3 Torr). As X-ray beam diameters aretypically larger than liquid microjets, a background signal fromthe vapor jacket evaporating from the liquid jet is usuallyunavoidable for volatile samples, although the recentintroduction of electrokinetic detection of the XAS spectrumby Lam et al. greatly reduces the vapor contribution to themeasured signal.97 A schematic diagram of the experimentalchamber utilized in the earliest XAS studies of liquid water isshown in Figure 7, and a detailed experimental description canbe found in ref 98.Improvements to experimental and computational resources

since the publication of Yang and Kirz, particularly in theresolution and brightness of synchrotron light sources, allowedWilson et al. to fit the EXAFS spectrum of liquid water,exhibited in Figure 8. Utilizing a simple single-scatteringformalism, the authors found a nearest-neighbor O−O radius of2.85 ± 0.05 Å, consistent with the values found from X-ray93

and neutron91 scattering studies. A follow-up study found asimilar value of 2.80 ± 0.05 Å.100 These studies first illustratedthe power of the liquid microjet technique for studying liquidsamples by soft X-ray absorption spectroscopy.Ekimova et al. have recently reported the development of a

liquid flatjet produced by collision of two liquid jets in air orvacuum.101 As can be seen in the photographs of such a jetapparatus in Figure 9, under appropriate flow conditions this

collision results in the formation of several orthogonal flatliquid sheets of roughly elliptical shape. The authors report thatthese sheets are stable over times as long as hours and havefairly stable thickness of ca. 1 μm. This combination of long-term stability and thin sample path length render such sheetsideal for windowless transmission-mode XAS studies of liquidsamples; several applications of this technique are presented inref 101. As transmission-mode spectral detection avoidspotential spectral distortions from absorption saturation,decay path yields, etc., this technology has the potential to

Figure 7. Schematic diagram of the Berkeley endstation used to perform XAS of liquid microjets. This configuration allows pressures of ca. 10−4 Torrto be achieved in the main chamber and pressures of ca. 10−9 Torr to be simultaneously maintained in the X-ray beamline with windowless coupling.Adapted from ref 99 with permission.

Figure 8. EXAFS spectrum above the liquid water oxygen K-edgemeasured using TEY detection and transformed into K-space. (a)Experimental data smoothed via a 5-point Savitzky−Golay smoothingalgorithm. (b) Fourier-filtered experimental data. (c) Calculatedspectrum of water assuming a fixed single O−O scattering distance,calculated to be 2.85 ± 0.05 Å. Reproduced with permission from ref29. Copyright 2001 American Chemical Society.

Figure 9. Two photographs taken in (left) and perpendicular to(right) the plane of the collision of two liquid jets. This jet wasproduced by flowing the liquid sample at identical flow rates of ∼6mL/min through two identical capillary tips with inner diameter 50μm. The initial elliptical flat liquid sheet is formed perpendicularly tothe plane of the collision. In this case the second liquid sheet can beseen breaking into droplets. Reproduced from ref 101 under a CreativeCommons Unported License.



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open a valuable new class of XAS experiments on liquidsamples. Additionally, the authors incorporated a samplerecycling system similar to that described by Charvat et al.for laser desorption mass spectrometry102 and previouslyadapted for XAS by Lange et al.,103 allowing for real-timesample reuse. While introducing an enhanced risk ofcontamination or radiative sample damage, this feature canprove helpful when working with expensive or difficult-to-synthesize liquid samples.While the introduction of liquid microjets into soft X-ray

absorption spectroscopy has enabled the windowless study ofrapidly renewed liquid samples, thereby minimizing radiationdamage, weakly interacting liquids produce large vaporbackgrounds that strongly perturb the liquid signal. Con-sequently, solvents (e.g., hydrocarbons, ethers, ketones, etc.)and solutions of central importance in chemistry and biologyhave been inaccessible by the microjet technology. Recently,Lam et al. have developed a new detection method,electrokinetic or “upstream” detection, in which a positiveelectrode is utilized to remove electrons from the liquidmicrojet sample and a positive current proportional to thenumber of electrons ejected from the sample is measured froma conductive capillary jet tip.104 This greatly reduces the vapor-phase contribution to the X-ray absorption signal whileretaining important advantages of liquid microjet sampleintroduction (e.g., minimal radiation damage). The effective-ness of the upstream detection method was demonstrated inthe first study of the room-temperature liquid hydrocarbons n-nonane and n-decane.104 More recently, Schon et al. haveadapted a similar two-electrode method to detect positive ioniccurrent from samples in liquid flow cells and found that itreproduced the known spectra of several liquid samples verywell.105

4.2. Liquid Flow Cell Techniques

Shortly after the introduction of liquid microjet technology intosoft XAS by Wilson et al., flow cells emerged as an alternativemethod of sample delivery for liquid XAS experiments. Thefirst flow-cell-based liquid XAS experiment was published byFreiwald et al. in 2004.106 Their apparatus, diagrammed inFigure 10a, consisted of a liquid chamber with input and outputtubing allowing liquid samples to be pumped through. IncidentX-rays enter and fluorescent photons escape to the detector viaa single 150 nm thick silicon nitride window. X-ray trans-mittance through such windows generally increases through the

soft X-ray energy regime, ranging from 0.1 at 200 eV to 0.96 at1600 eV.106

Several additional flow-cell designs for XAS of liquidsutilizing silicon nitride windows of 100−150 μm thicknesshave been introduced since the design of Freiwald et al.107−114

Like liquid microjets, flow cells have the advantage of providinga sample that can be continuously renewed without the need tobreak vacuum and remove the sample mount. However, thesample renewal is much slower than that of microjetexperiments as a result of the requirement that the pressureforcing the liquid through the cell not be great enough to burstthe thin windows. Furthermore, the silicon nitride windowintroduces a solid interface to the sample that does not exist inliquid microjet experiments. On the other hand, flow-cellexperiments remove the risk of freezing that is present withliquid jets and thus are generally more robust than microjetexperiments. Most can be performed without the gas-phasebackground observed in microjet XA spectra.Various cell designs have been employed to introduce

different experimental features. For example, some are designedfor active temperature control,109,112 with two parallel windowsto allow for transmission studies,107,110−112 or even to studyliquids at solid interfaces.108,113,114 Several cell designs allow forXA spectroscopy to be conducted simultaneously withelectrochemical cycling.113−117 Figure 10b provides a simplifiedschematic of the cell design of Nagasaka et al., a variable-thickness flow cell with two parallel silicon nitride windows fortransmission detection.110 In this design, windows are sealedexternally to the flow cell via o-rings. The windows areseparated by spacers outside the o-rings relative to the centralinteraction region. This arrangement allows sufficient flexibilityto modulate sample thickness in situ by changing the pressureof the helium outside the windows and maintain the desiredtotal absorption through the sample without removing the cellfrom vacuum to change out a spacer. On the other hand, thedual-window transmission cell of Meibohm et al. utilizes rigidgold spacers to maintain a well-defined sample thickness of 500nm.112

4.3. Interpreting the Water NEXAFS Spectrum

The oxygen K-edge NEXAFS spectrum of water, initiallyreported by Wilson et al.,3 has since been replicated byn u m e r o u s m e t h o d s a n d i s w e l l - e s t a b -lished.26,30,58,107,109−112,118−120 Spectra measured with variousdetection methods are exhibited in Figure 11. From thiscomparison, it is clear that the general spectral features remain

Figure 10. Schematic diagrams of several liquid flow cell apparati used for XAS experiments: (a) Single-window photon-in/photon-out flow cell forTFY experiments utilizing a recycled sample. (b) Dual-window flow cell for transmission mode experiments.



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quite consistent from method to method. However, theresolution of XRS experiments is substantially inferior to thatof the NEXAFS methods (∼0.5−1 vs ∼0.1 eV), and the relativeintensities of the spectral features appear to differ in AEYrelative to the other techniques. The origin of this anomaly isunclear; while AEY is believed to represent the spectrum ofinterfacial molecules even in a bulk sample, the experimentproviding the data for this spectrum was also performed on theso-called “premelted” water, a thin layer of liquid known toexist at the surface of ice in an atmosphere of water vapor.26

This sample may not exhibit the same structural characteristicsof the bulk water samples used for the other detection methods.The water spectrum exhibits three distinct spectral regions,

highlighted in Figure 12: a small pre-edge feature around 535 eVand larger main-edge (centered between 537 and 538 eV) andpost-edge (centered between 541 and 542 eV) features. It wasproposed by Cavalleri et al. in 2002 that the pre-edge featureintensity was related to the number of broken hydrogen bonds,as DFT calculations indicated that broken hydrogen-bonddonor sites result in asymmetry that enhances the p-characterof the 4a1 state and increases the oscillator strength oftransitions from the oxygen 1s.122 This suggestion is reinforcedby the fact that the pre-edge feature is weaker or nonexistent in

oxygen K-edge NEXAFS spectra of ice and has since becomegenerally accepted.59,76,123 Similarly, the post-edge feature hasbeen attributed to highly coordinated long-range tetrahedralstructure and is stronger in ice than liquid water.58,122

Moreover, time-resolved core-excitation decay studies haveindicated that the final state corresponding to the pre-edgefeature is fairly localized, whereas the final state correspondingto the post-edge feature is highly delocalized.124

Substantial controversy exists as to the hydrogen-bondstructure of liquid water implied by the peak ratios observedin the oxygen K-edge spectrum of liquid water. Wernet et al.proposed in 2004 that the spectrum could only be explained bya liquid structure in which most of the water molecules onlymade two strong hydrogen bonds, one donated and oneaccepted.58 Such a hydrogen bond structure results in the one-dimensional “rings and chains” structural motif, entirely at oddswith the traditional picture of liquid water as a three-dimensional quasi-tetrahedral liquid at room temperature.These conclusions and the quantitative validity of theassociated theory were quickly questioned in the litera-ture,76,125,126 and the structure of liquid water has continuedto be the subject of vigorous debate.5,6,24,55,73,74,127−131

Prendergast and Galli found that within the XCH approx-imation the features of the observed NEXAFS spectrum ofliquid water can be well-reproduced without violating the well-established tetrahedral model of water. Kuhne and Khaliullinhave proposed an intermediary theory in which the averagemolecule in liquid water is in a structurally symmetricenvironment (that is, donating and accepting two stronghydrogen bonds), but high-frequency structural fluctuationsresult in significant instantaneous energetic asymmetry betweensymmetry-equivalent bonding sites.123 Such solutions areattractive in that they do not reject the tetrahedral picture ofliquid water, which has been supported by numerousexperimental and computational studies over the years.132,133

The implications of the NEXAFS spectrum of water on the

Figure 11. Comparison of oxygen K-edge NEXAFS spectra of watermeasured via several detection methods: (a) AEY, (b) TEY, (c)transmission mode, (d) XRS, and (e) TFY. Spectra are area-normalized to the full spectral width available in all 5 spectra, thatis, from 532 to 550 eV. They have also been energy-aligned to placethe pre-edge feature at 535 eV. In addition, the TFY spectrum hasbeen corrected for saturation effects as described in ref 121.Reproduced with permission from ref 107. Copyright 2005 AmericanChemical Society.

Figure 12. Comparison of the oxygen K-edge NEXAFS spectra ofliquid water to gas-phase water vapor and water ice indicating thespectral regions comprising the pre-edge, main edge, and post-edge ofthe liquid water spectrum. The dotted line in the ice spectrumrepresents the spectral contribution of ice Ih, with contributions abovethis line arising from amorphous ice and coinciding with features ofthe liquid spectrum. Reproduced with permission from ref 5.Copyright 2010 Elsevier.



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understanding of the hydrogen-bond structure of liquid waterhave been recently reviewed in greater detail in ref 7.4.4. Perturbing the Hydrogen Bond Structure of LiquidWater

The understanding of the spectral features exhibited in theoxygen K-edge NEXAFS spectrum of liquid water has beenenhanced by studies of water under conditions for which thehydrogen-bonding network has been intentionally altered, e.g.,by varying the temperature of the sample or introducing“defects” into the liquid water matrix. Several studies of thetemperature dependence of the liquid water spectrum havebeen carried out, spanning a temperature range fromsupercooled to supercritical.57,58,112,119,134 While Wernet et al.and Smith et al. disagreed on the rearrangement energyrequired to break a hydrogen bond and adopt an asymmetricalconfiguration,58,119 the spectral signatures observed upontemperature variation are consistent across all studies. Astemperature increases, the post-edge intensity decreases, thepre-edge intensity increases, and the entire spectrum shifts tolower energy; these effects are illustrated in Figure 13 for the

temperature range 1.5−39 °C. On the basis of the standardinterpretation of the respective spectral regions, these changesare consistent with an increase in broken hydrogen-bond donorsites and a decrease in long-range tetrahedral order astemperature increases. Alternatively, increasing the pressureresults first in a decrease in pre-edge intensity, suggesting areduction of dangling hydrogens, followed by an increase inpre-edge intensity above 0.3 GPa, perhaps indicating that thestructure becomes too compressed for tetrahedral ordering athigher pressures.135

Lange et al. have demonstrated a spectral effect similar tothat of increasing the temperature by diluting water withacetonitrile, a polar organic solvent that accepts but cannotdonate hydrogen bonds.136−138 Indeed, at 5% water (byvolume), the pre-edges and main edges of the water spectrum

closely resemble a simple blue-shift to the 4a1 and 2b2 peaks ofgas-phase water. The pre-edge is still reduced in intensityrelative to the gas-phase 4a1 peak, consistent with previousfindings indicating that even at very low concentration manywater molecules form 2−3 strong hydrogen bonds inacetonitrile;139 however, the existence of a strong pre-edgeenhancement indicates that acetonitrile may not actually beacting as a strong hydrogen-bond acceptor in these mixtures.Additional studies have found this effect to be highly solvent-dependent.136,140 For example, at similar concentrations, 3-methylpyridine acts as a strong hydrogen-bond acceptor, andwhile the spectrum of 10% D2O in acetonitrile closelyresembles that of H2O in acetonitrile, the spectrum of 10%D2O in 3-methylpyridine much more closely resembles that ofneat water with a reduced post-edge intensity.140 Water at verylow concentrations in nonpolar solvents such as benzene andchloroform exhibits the spectral characteristics of highlycoordinated icelike structure, indicating strong aggregationwithin the hydrophobic solvent and, in the case of benzene,possible formation of clathrate-like solvent traps.136 Havingnoted a heavy water experiment here, it is also notable thatBergmann et al. have reported that the NEXAFS spectrum ofD2O resembles that of H2O at a temperature ∼20 K lower; thatis, isotopic substitution to heavy water results in a spectrumthat is blue-shifted and exhibits a slightly enhanced post-edgeand reduced pre-edge relative to H2O.

141

A related clustering effect to that observed for lowconcentrations of water in hydrophobic solvents has beenobserved for thin films of water on solid substrates. Nordlundet al. found that, while isolated water molecules adsorbed onRu(001) exhibited an oxygen K-edge NEXAFS spectrumlargely indistinguishable from that of the gas phase, continuedaddition of water molecules resulted in an enhancement of thepost-edge and reduction in the pre-edge, and at coverageapproaching one monolayer, an icelike spectrum wasobserved.142 A similar interfacial spectral behavior has beenobserved for bulk water at a gold interface.114 Velasco-Velez etal. utilized an electrochemical liquid flow cell with a ∼20 nmlayer of gold deposited on the 100 nm Si3N4 window and actingas the working electrode and current collector for TEYspectroscopy of the interfacial region. The NEXAFS spectrumof water at the gold interface was found to exhibit minimalsignal in the pre-edge region at neutral or positive potential.This phenomenon has been attributed to electronic coupling ofthe core-excited state with the gold substrate, rather than to alack of dangling hydrogen bonds. By applying a negativepotential to the gold, the authors were able to observe a normalwater spectrum with the pre-edge feature intact, presumablyresulting from a structural rearrangement to rotate waterhydrogens toward the gold surface. The same spectral effectcould be achieved by coating the surface with a hydrophobicsubstrate and preventing the electronic coupling with interfacialwater.More subtle changes to the liquid water NEXAFS spectrum

can be induced by the introduction of certain solutes intoaqueous solution. Naslund et al. observed new peaks in theliquid water spectrum upon orbital mixing with the d-orbitals ofdissolved transition metals in salt solutions.143 In the samework, the authors suggest the presence of a reproducible changeto the water NEXAFS spectrum attributable to hydrogen-bondrearrangement upon addition of salts. The first systematic studyof such a phenomenon was published by Cappa et al. in 2005and indicated linear increases in the intensity of the pre-edge

Figure 13. Temperature dependence of the oxygen K-edge NEXAFSspectrum of liquid water. All black spectra are reproductions of the1.5° spectrum to illustrate spectral changes. All spectra were collectedin transmission mode using a temperature-controlled flow cell withtemperature resolution of ±0.25 °C and have been area normalized.Reproduced from J. Meibohm et al., “Temperature Dependent Soft X-ray Absorption Spectroscopy of Liquids.” Rev. Sci. Instrum. 2014, 85(10), 103102,112 with the permission of AIP Publishing.



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and main edge and a linear decrease in the intensity of the post-edge feature with increasing concentration of sodium halidesalts in a study of liquid microjet samples utilizing TEYdetection (results shown in Figure 14).72 In addition, observed

were a concentration-dependent red-shift for NaI solutions, asmaller red-shift for NaCl solutions, and no meaningful shift forNaBr solutions, relative to neat water. These changes areattributed primarily to local electronic perturbations from near-neighbor halide anions rather than to substantial perturbationof the water hydrogen-bonding network outside of the firstsolvation shell.72,121 It has been alternatively suggested thatiodide and sodium ions both contribute to disruption of thewater hydrogen-bond network, while chloride and bromide donot substantially alter hydrogen bonding.75 Interestingly,utilizing a flow cell with TFY detection, Guo et al. observeda small blue-shift, rather than a red-shift, for the sodium,magnesium, and aluminum salts of chloride;144 the origin ofthis discrepancy is unclear.A substantial discrepancy in the observed effect on the water

NEXAFS spectrum induced by introduction of monovalentcations has also been observed. Liquid microjets of aqueoussolutions of akali metal, ammonium, and guanidinium chloridesalts, detected via TEY, revealed minimal differences betweenthe solution oxygen K-edge spectra of the various salts.145

Spectra of solutions of Li+, Na+, K+, and NH4+ acetate measured

in a single-window flow cell by TFY also exhibited nosignificant difference in the water spectrum among the variouscations.146 However, in a transmission-mode study publishedby Waluyo et al., the alkali chlorides and fluorides from Li+

through Rb+ exhibited the spectral signatures of disruptedhydrogen bonding, viz. reduced post-edge and enhanced pre-edge intensity, with cations larger than sodium also inducing asmall red-shift.75 With the exception of Rb+, the observedspectral changes are more pronounced with each larger cation.As with the preceding study of anions, the cause of thisdiscrepancy is currently unclear.Utilizing the same methodology as that which found a red-

shift for NaCl, Cappa et al. observed a substantially smaller andopposite spectral change for solutions of HCl from those ofNaCl.147 This has been attributed to competing effects; viz.while chloride is implicated as a “structure breaker” in aqueoussolution, the hydronium ion acts as a “structure maker”, withthe combination producing a solution NEXAFS spectrumsimilar to that of neat water. Chen et al. have studied theNEXAFS spectra of aqueous HCl and NaOH and suggest thathydronium structures water within the first solvation shell, buthydroxide induces longer-range ordering.148 Some multivalentcations may also be implicated as structure makers as a result ofapparent offsetting spectral effects in the spectra measured fortheir halide salts.144,149 Spectral effects of multivalent cationsalso appear to be ion-specific and may result from acombination of structural impact on the water hydrogen-bonding network and electronic coupling between the ions andnear-neighbor solvent molecules. The question of whether ionssubstantially impact the hydrogen-bond network of wateroutside the first solvation shell remains open.

5. XAS OF SHORT-CHAIN ALCOHOLSWhile studies of water and aqueous solutions have clearlydominated the field of XAS of liquids since its introduction,short-chain alcohols have also been the subject of an increasingnumber of experiments in recent years. The first and most-common nonaqueous liquid to be studied by XAS has beenmethanol. As it contains only one O−H bond, methanol canonly donate one hydrogen bond per molecule and therefore hasan average of ∼2 H-bonded nearest neighbors.150 Conse-quently, the local liquid structure comprises primarily rings andchains, and methanol exhibits a lower density than water atroom temperature, despite a shorter O−O separation. Wilsonet al. first reported the TEY EXAFS spectrum of methanolabove the oxygen K-edge in a liquid microjet in 2002,100

determining an average nearest-neighbor O−O distance of 2.75Å, slightly shorter than the O−O hydrogen-bond pair length ofbulk water and in good agreement with existing neutron and X-ray diffraction measurements.151 Subsequently, the liquidmethanol NEXAFS spectrum has been measured on both theoxygen and carbon K-edges.152−156 The carbon K-edgespectrum of liquid methanol measured by Cappa et al. is onlymoderately distinguishable from that of the vapor and thusprovides little opportunity for extraction of meaningfulinformation on the liquid structure,153 consistent with thevapor-phase finding that carbon K-edge spectrum of CH3 islargely insensitive to the chemical environment.157 The carbonK-edge spectrum measured by Nagasaka et al. does exhibitsome broadening and enhanced post-edge intensity.155 Incontrast, Figure 15 illustrates that the oxygen K-edge spectrumexhibits a similar behavior to that observed upon condensation

Figure 14. Oxygen K-edge NEXAFS spectrum of aqueous solutions of(a) NaI, (b) NaBr, and (c) NaCl. Spectra were collected via TEY ofliquid microjets and have been area-normalized over the spectralregion 532−550 eV. Difference spectra between each solution and theneat water spectrum are shown at the top of each panel. Reproducedwith permission from ref 72. Copyright 2005 American ChemicalSociety.



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of water: a blue-shift, reduction in 4a1 peak intensity, andbroadening of the higher-energy spectral features.152−156

Indeed, the liquid methanol spectrum closely resembles thatof liquid water, with reduced pre-edge intensity and a moreextended but less distinct post-edge feature (see comparison ofmethanol and water spectra in Figure 16). Computationalanalysis of this spectrum indicates that the spectral signature isconsistent with a 2-dimensional structure of rings and chainswith few dangling hydrogen bonds; however, the calculatedspectral signatures of rings versus chains may be too similar todifferentiate their relative populations from the NEXAFSspectrum.153,154 Moreover, while longer-chain and branchedalcohols of up to 4 carbons exhibit small but distinct spectraldifferences on the oxygen K-edge, the spectral signature ofhydrogen bonding and calculated hydrogen-bonding structureare similar for all such species.154

While the various spectra measured for liquid methanol areall generally in good agreement, one small but potentiallymeaningful difference again occurs between the spectracollected from liquid microjets156 and those collected in flowcells.152,155 As illustrated in Figure 16, when compared directlywith a water spectrum collected under the same conditions, theliquid microjet experiment produces a methanol spectrum forwhich the pre-edge is at a slightly lower energy than that of

water. In flow-cell experiments, the pre-edge of methanol is atthe same or slightly higher energy. As is the case for similaranomalies discussed above in the spectra of salt solutions, theorigin of this discrepancy is not clear.Mixtures of water and methanol are of fundamental interest

due to their unusual thermodynamic properties. It has longbeen known that these mixtures exhibit a negative excessentropy of mixing; that is, mixtures are more ordered thanwould be suggested by an ideal/random mixing description.158

Possible explanations for and previous studies of thisphenomenon are discussed in refs 155 and 156 and referencestherein. The sensitivity of NEXAFS spectra to local hydrogen-bonding structure makes the technique an ideal tool toinvestigate the local structure in H2O/MeOH mixtures. Guoet al. interrogated several water/methanol mixtures on theoxygen K-edge utilizing a photon-in, photon-out (TFY) flowcell.152 Their experimental spectra exhibit a new feature 3 eVbelow the pre-edge for liquid mixtures. Analysis of the observedspectra in light of both calculations and complementary X-rayemission experiments attributes the observed negative excessenthalpy of mixing to two factors: (1) incomplete mixing andmicroimmiscibility and (2) conversion of methanol chains intomore organized ring structures containing one or more watermolecules. Later studies of the oxygen K-edge of water/methanol mixtures by Nagasaka et al. and Lam et al., displayedin Figure 16, observed no new low-energy feature upon solventmixing.155,156 Instead, the pre-edge regions of solution spectraappear as linear combinations of the water and methanolspectra. Nagasaka et al. find a linear relationship between therelative concentrations of the components and the intensities oftheir respective spectral components. Lam et al., however,observed a faster-than-linear decrease in the spectral intensity ofthe methanol pre-edge upon addition of water and thereforehave attributed the negative excess entropy of mixing to areduction in the number of broken hydrogen bonds around themethanol molecules in water/methanol mixtures relative to theneat liquid.

6. XAS OF SOLUTES AS A PROBE OF SOLUTIONSTRUCTURE

6.1. Probing Solute−Solute Interactions

The vast majority of chemical reactions are carried out insolution, wherein details of the interactions of solutes with

Figure 15. Comparison of the oxygen K-edge NEXAFS spectra ofmethanol in the vapor phase (A) and liquid phase (B). Liquid spectrawere collected via TEY of a liquid methanol microjet; the vaporspectrum was collected using the same experimental apparatus bytranslating the jet several centimeters from the X-ray beamline,allowing only the evaporated gas to interact with the X-rays.Reproduced with permission from ref 153. Copyright 2005 AmericanChemical Society.

Figure 16. Oxygen K-edge NEXAFS spectra of water/methanol mixtures. (a) Transmission-mode flow-cell spectra of Nagasaka et al.; the pure waterspectrum is in red, and the pure methanol spectrum is in violet. The pre-edge becomes increasingly pronounced as the fraction of water increases andshifts to lower energy. Reproduced with permission from ref 155. Copyright 2014 American Chemical Society. (b) Liquid jet TEY spectra of Lam etal. around the pre-edge spectral region. In these data the pre-edge also becomes more prominent as the fraction of water increases, but it is observedto shift to higher energy. Reproduced from R. K. Lam et al., “Communication: Hydrogen Bonding Interactions in Water-Alcohol Mixtures from X-ray Absorption Spectroscopy.” J. Chem. Phys. 2016, 144 (19), 191103,156 with the permission of AIP Publishing.



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solvent molecules and with one another are critical forcontrolling their rates and products. Consequently, a clearand detailed picture of the chemical environments of moleculesin solution is critical to a proper understanding of thesereactions. XAS has proven to be a powerful probe of thesolvation environment of appropriately chosen solutes. Itshould be noted that it is not an effective tool for all solutes,however. For example, DFT calculations on a variety ofmolecular configurations indicate that the boron K-edge spectraof aqueous boric acid, borate, and polyborate, while clearlydistinct from one another, are largely insensitive to theformation of contact pairs with sodium ions.159 Similarly, thecalculated nitrogen K-edge spectra of the aqueous sodium saltsof nitrate and nitrite are insensitive to ion pairing, hydrationnumber, and solvent geometry.160 Computational results withrespect to ion pairing in these cases are supported byexperimental data indicating no spectral changes as a functionof salt concentration. Conversely, concentration-dependentinvestigation of the sodium K-edge in aqueous NaCl reveals adistinct spectral signature of ionic association.106,161 Computa-tional spectroscopy further reveals that the observed spectralchanges are associated with the formation of solvent-separated(alternatively “solvent-shared”) ion pairs rather than contaction pairs.161 Moreover, the spectral shape can be assigned tosodium ions solvated by either 6 waters or 5 waters and 1chloride, providing a qualitative understanding of the overallsolvation environment. Addition of hydroxide to solutionscontaining Na+ results in the formation of ion clusters, witheach OH− interacting with 2.4 ± 0.6 Na+ ions as either contactor solvent-separated ion pairs.162

It should be noted that NEXAFS spectral analyses are oftennot so quantitative. For example, observe the concentration-dependent spectra of aqueous NiCl2 on the nickel L3-edgepresented in Figure 17.163 The peaks labeled P1 and P2

correspond to transitions into the triplet and singlet finalstates, respectively. It is clear that, at elevated saltconcentrations, excitations into the singlet state becomeincreasingly favorable. An additional feature above P2 ispredicted for Ni2+ ions in contact ion pairs and is visible inthe spectrum of solid NiCl2. As there is at most a slightindication of the appearance of this peak in the 1.5 M solutionspectrum, it appears that, as with NaCl, any ion pairing occurs

via solvent-sharing and not direct ion−ion interaction.Calculations indicate that enhancement of the singlet excitationcan occur upon reweighting of the transition dipole matrixelements induced by breaking of the dipole equivalence of thesymmetry axes. However, no quantitative picture of thesolvation sphere corresponding to such a shift in the matrixelements can be deduced, merely the existence of a changeresulting from asymmetry generated by formation of solvent-separated ion pairs.In addition to concentration-dependent pairing effects, ion-

specific pairing effects have been observed for carbonylspecies.146,164 On the carbon K-edge, blue-shifts in theNEXAFS spectral features corresponding to the C1s →π*CO transitions of acetate (shown in Figure 18a) andformate correspond to stronger cation interactions with thecarbonyl oxygen.164 Using this formalism, it is established thatLi+ associates more strongly with the carbonyl than does Na+,which in turn associates more strongly than does K+, consistentwith the respective ionic sizes and charge densities of thesespecies. This trend is reproduced in HCH electronic structurecalculations of sodium and potassium acetate, which compute ablue-shift for sodium acetate relative to potassium acetate.Lithium could not be calculated due to the lack of anappropriate core potential basis set. The magnitude of theobserved shift between the sodium and potassium acetatespectral features in the experimental spectra is similar to thedifference in interaction energies previously reported for thesespecies.165 On the oxygen K-edge, enhanced interactionstrength with the counterion is manifested as an enhancementof the spectral feature corresponding to the O1s → π*COtransition in solutions of acetate, formate, and glycine.146

Donation of electron density from the π* orbital to the cationresults in an increased unoccupied density of states in the finalstate, resulting in the stronger spectral signature for thistransition shown in Figure 18b. Interestingly, by this measure,the interaction strength of the carbonyl with lithium is found tobe weaker than that with sodium, whereas in the carbon K-edgeexperiment, affinity was found to monotonically decrease withion size.NEXAFS spectroscopy is also a valuable tool for the study of

solute aggregation. In addition to the aggregation of waterclusters in organic solvents discussed earlier in this Review,solutions of acetic acid provide an interesting example, as thegas-phase oxygen K-edge NEXAFS spectra of monomeric andsmall-cluster acetic acid are well-known166 and provide areference for spectral behavior upon clustering. Figure 19exhibits these reference spectra along with the spectra of severalconcentrations of acetic acid dissolved in hexane andacetonitrile.167 It is clear that both cluster formation anddissolution in these organic solvents results in a systematicnarrowing of the gap between the π* feature at ∼532 eV,corresponding to the transition originating from the carbonyloxygen 1s, and the π* feature at ∼535 eV, corresponding to thetransition originating from the hydroxyl oxygen 1s. Theseeffects can be attributed to the formation of strong hydrogenbonds at both oxygens. Donation of hydrogen bonds by thehydroxyl oxygen and acceptance of hydrogen bonds by thecarbonyl oxygen make the oxygens increasingly indistinguish-able as the hydrogen bonds grow stronger/shorter, causing thepeak positions to move nearer to one another. On the basis ofthe experimental spectra, it is clear that acetic acid in hexanesolution forms dimers or larger clusters at all experimentalconcentrations. In acetonitrile this phenomenon is enhanced

Figure 17. Nickel L3-edge TFY NEXAFS spectrum, collected in a flowcell, of several concentrations of aqueous NiCl2. The solid red linesindicate the projected 50 mM spectrum corrected for saturation effectsat higher concentrations. Thus, the area on each spectrum shaded ingray is the concentration-dependent spectral enhancement at thatconcentration. Reproduced with permission from ref 163. Copyright2007 American Chemical Society.



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with concentration, indicating a cooperative enhancement ofhydrogen-bonding interactions as a function of acetic acidconcentration. Whereas acetonitrile acts as a hydrogen-bondacceptor in MD simulations of this system, hexane cannot act as

a hydrogen-bond donor or acceptor. Thus, interaction of aceticacid molecules with one another must cause the observed shifts.A slightly more pronounced shift has also been observed inglacial acetic acid.168

Perhaps a more unusual example of aggregation of like-typesolutes is the π-stacking cation−cation pair formation observedby Shih et al. in aqueous solutions of guanidinium chloride.169

The TEY nitrogen K-edge NEXAFS spectrum, shown in Figure20, exhibits red-shifts of varying magnitude for its significant

spectral features at concentrations above 1 M. These can onlybe reproduced in the calculated spectra by formation of like-charge ion pairs. In this case the π-stacking energy is found tobe sufficient to overcome the electrostatic repulsion underconcentrated conditions.In addition to these studies of solute aggregation in solution,

Schwartz et al. have utilized NEXAFS to study the auto-

Figure 18. Evidence for enhanced ion−ion affinity in sodium acetate relative to potassium acetate. (a) TEY carbon K-edge NEXAFS spectra of liquidmicrojets of several acetate salts showing a red-shift with increasing cation size that has been attributed to reduced affinity. Reproduced withpermission from ref 164. Copyright 2008 National Academy of Sciences. (b) TFY oxygen K-edge NEXAFS spectra of several salts of acetatecompared to that of neat water (collected in a flow cell). Reproduced with permission from ref 146. Copyright 2008 American Chemical Society.

Figure 19. Oxygen K-edge TFY NEXAFS spectra of acetic acidsolutions in hexane and acetonitrile. Vapor-phase spectra ofmonomeric and clustered acetic acid are provided for reference. Thetwo sharp π* features are shown in greater detail in (b) and (c).Reproduced with permission from ref 167. Copyright 2012 Elsevier.

Figure 20. Comparison of nitrogen K-edge TEY NEXAFS spectra of0.5 and 6.0 M aqueous solutions of guanidinium chloride, collectedusing a liquid microjet, with the calculated spectra of cation−cationstacked pairs and free guanidinium ions indicates that spectral changesobserved at higher concentration are well-reproduced in the theory byformation of cation−cation pairs. Spectral changes observed uponformation of Gdm+−Cl− cation−anion pairs do not agree withexperimental spectral changes at higher concentrations. Reproducedfrom O. Shih et al., “Cation−Cation Contact Pairing in Water:Guanidinium.” J. Chem. Phys. 2013, 139, 35104,169 with the permissionof AIP Publishing.



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oligomerization of pyrrole in aqueous solution.170 Pyrrole iswell-known to polymerize upon exposure to light or air andmore quickly upon exposure to water. The spectra of gas-phasepyrrole on the carbon and nitrogen K-edges were compared tothe spectra collected from a liquid microjet of 0.5 M aqueouspyrrole. Somewhat surprisingly, the spectra on both edges werefound to differ only slightly between the vapor and solutionphases. XCH calculations indicate that the nitrogen K-edgespectrum is only marginally affected by solvation andoligomerization. The small change observed between thevapor and solution on the carbon K-edge has been attributedto “damage” of the liquid pyrrole sample, resulting inoligomerization of the neat liquid. Figure 21 reveals that theexperimental vapor-phase spectrum on the carbon K-edge canonly be reproduced in the calculated spectrum by averaging thespectra of the pyrrole monomer and dimer. It is of note that,while changes in the carbon and nitrogen K-edge spectra uponsolvation were subtle, consisting of slight shifts and, in the caseof the carbon K-edge, disappearance of a low-intensity feature,they were accurately reproduced by the XCH calculations.

6.2. Probing Solute−Solvent Interactions

As described in the previous section, the solvation environmentof Na+ in aqueous solution comprises 6 components (6 watersor 5 waters and 1 counterion).161 The solvation environment ofNa+ in ethanol has been determined to have the samecomposition.106 More recently, the solvation of sodium halideshas been studied in water, methanol, ethanol, propanol, andmixtures thereof.171 The dipole interaction strengths of thealcohol solvents were found to decrease with increasing chainlength. However, Figure 22 illustrates that the sodium K-edgeNEXAFS spectrum of Na+ in a mixture of water and methanolcontains similar contributions from the pure water and puremethanol solution spectra, while water/ethanol and methanol/ethanol mixtures favor water and methanol over ethanol at∼2:1. However, in spite of the apparently similar interactions ofsodium ions with water and methanol, water is found to have aparticularly stabilizing effect on the solvation shell. Stopping the

liquid flow within the experimental cell and exposing thealcohol solutions of NaI to soft X-rays over time results in theformation of I3

− as a result of radiative sample damage.Triiodide binds tightly with the Na+ ion in alcohol solution,which causes a substantial change in the observed spectrum inalcohol solutions (Figure 22d). However, in water and water−alcohol mixed solvents, no spectral changes are observed withirradiation time, which has been attributed to a stabilization ofthe solvation shell by water, which prevents triiodide fromforming contact ion pairs.The solvations of Fe2+ and Fe3+ have also been investigated

in water and short-chain alcohol solvents.106,172 The chargestates of iron differ meaningfully in their spectral fine structureon the iron L2 and L3 edges. The corresponding solvationstructures in alcohols have been determined from ligandmultiplet electronic structure calculations to be octahedral forFe3+ and primarily tetragonal for Fe2+.172 In addition, thespectral differences between the oxidation states have beenutilized by Nagasaka et al. to demonstrate the capabilities oftheir electrochemical flow cell.115−117 The authors have utilizedcell potentials to reversibly cycle between Fe2+ and Fe3+ inaqueous solutions of iron sulfate without evidence of radiativeor electrochemical sample damage.Relevant to the design of advanced lithium ion batteries, the

solvation of Li+ in solutions of LiBF4 in propylene carbonatehas been measured via NEXAFS by Smith et al.173 In thissystem, a blue-shift in the spectral feature in the oxygen K-edgespectrum of propylene carbonate corresponding to the 1scarbonyl→ π* transition is observed when the carbonyl oxygen is in thefirst solvation shell of a lithium ion (Figure 23). Spectralanalysis and electronic structure calculations within the XCHapproximation permit the net shift in experimental solutionspectra to be interpreted as a proportion of Li+-binding andnon-Li+-binding solvent molecules and so determine a solvationnumber.Our group has established the predictive power of XCH

calculations for reproducing spectra of the aqueous carbonate

Figure 21. Evidence for auto-oligomerization of neat pyrrole. (a) Calculated carbon K-edge NEXAFS spectra of various oligomers of pyrrolecomputed within the XCH approximation. (b) The TEY carbon K-edge NEXAFS spectrum of pyrrole in the vapor phase can be reasonablyreproduced by combining 50% of the calculated monomer spectrum with 50% of the flat dimer spectrum, indicating that oligomerization has alreadyoccurred in the neat pyrrole sample prior to introduction into the sample chamber. Reproduced from C. P. Schwartz et al. “Auto-Oligomerizationand Hydration of Pyrrole Revealed by X-ray Absorption Spectroscopy.” J. Chem. Phys. 2009, 131 (11), 114509,170 with the permission of AIPPublishing.



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system. England et al. published the carbon K-edge NEXAFSspectra of aqueous sodium carbonate and bicarbonate andgaseous CO2 in 2011.174 Calculated spectra of these species, aswell as carbonic acid (H2CO3) and aqueous CO2, computedwithin the XCH approximation, were also published; however,the transient nature of the latter two species rendered theirexperimental detection too difficult in the initial experiment.Lam et al. have subsequently developed a fast-flow liquidmicrojet mixing system utilizing a microfluidic mixing Y-cell toacidify bicarbonate ∼5−10 μs before intersecting the solutionwith the X-ray beamline. Utilizing this apparatus Lam et al. havemeasured the spectra of both aqueous carbonic acid175 andhydrated CO2.

176 In particular, the NEXAFS spectrum ofaqueous CO2 exhibits not only the characteristic shift in theprimary 1s → π* transition observed for all members of theaqueous carbonate system but also the disappearance of a smallvibronic peak that is clearly observed in the spectrum ofgaseous CO2, as shown in Figure 24. It is interesting to notehere the significance of this finding relative to the statement

made at the beginning of this section of our Review: “XAS hasproven to be a valuable probe of the solvation environment ofappropriately chosen solutes.” The small but distinctive changein the CO2 carbon K-edge NEXAFS spectrum upon solvationin waterviz. the disappearance of the vibronic peakoccursin both the experimental and calculated spectra in spite of veryweak hydration energies. MD simulations indicate the existenceof only 0.56 hydrogen bonds per solute molecule. Thishighlights the fact that appropriately chosen is not alwaysintuitively obvious, and in many cases, the suitability of systemsfor NEXAFS analysis can only be determined empirically.

6.3. XAS Studies of Solvated Biomolecules

While the utility of XAS for studying biomolecules is largelyrestricted by the size of most such molecules and the resultingspectral complexity, some of the most important contributionsof the liquid XAS techniques have nevertheless resulted fromthe study of solvation of biologically relevant molecules,including isolated amino acids,177−179 polypeptides,177,180−182

Figure 22. Sodium K-edge TFY NEXAFS spectra of Na+ in solutions of water, methanol, ethanol, and mixtures thereof collected in a flow cell. Panel(d) exhibits the spectral changes in alcohol solvents observed following extended irradiation under stopped-flow conditions. Reproduced from ref171 with permission of the PCCP Owner Societies.



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nucleotide bases,183−185 and adenosine triphosphate (ATP).186

A great deal of effort has been directed toward the study of themetal centers of metalloproteins, particularly those withinporphyrin rings. For example, NEXAFS spectroscopy has beenperformed on the iron L2 and L3 edgescorresponding totransitions from the 2p1/2 and 2p3/2 orbitals, respectivelyofmethemoglobin and beef liver catalase, porphyrin species withlow and high activity, respectively, as catalysts for H2O2

decomposition.181 The data from these experiments, alongwith computed spectra calculated via the ligand multipletmethod with consideration for charge transfer and back-bonding,187,188 are exhibited in Figure 25. These data

Figure 23. Comparison of TEY oxygen K-edge NEXAFS spectra ofpropylene carbonate (red) and 1 M LiBF4 in propylene carbonate(blue), collected from a liquid microjet, with calculated spectra ofpropylene molecules associating with Li+ (green) and not associatingdirectly with Li+ (red) in a simulation box containing ∼1 M LiBF4.The blue calculated curve consists of a linear combination of 60%unassociating and 40% Li+-associating solvent molecules, correspond-ing to an average solvation number of 4.5. Reproduced from ref 173with permission of the PCCP Owner Societies.

Figure 24. Carbon K-edge NEXAFS spectra of the acidic pH species of the aqueous carbonate system. (a) Calculated spectra of gaseous CO2,dissolved CO2, and aqueous carbonic acid. (b) Experimental spectra of 1 M HCl and 1 M NaHCO3 combined in the fast-flow mixing system withsufficient interaction time to allow H2CO3 to decompose into water and CO2 (top panel) and well before interrogating the system, allowing for CO2to leave solution (bottom panel). Reproduced with permission from ref 176. Copyright 2015 Elsevier.

Figure 25. TFY NEXAFS spectra of the Fe L2 and L3 edges of (a) 1mM aqueous solution of methemoglobin and (b) a 1 mM solution ofaqueous beef liver catalase collected in a liquid flow cell at 30 °C andpH = 7 (physiological conditions). Solid lines indicate calculatedspectra from ligand multiplet calculations. Inset (c) shows thestructures of the iron−porphyrin complex of beef liver catalase.Note the proximal tyrosine ligand, which is found to be responsible forreducing electron density at the iron center and conferring some Fe4+

character upon the observed spectrum. Reproduced from ref 181 withthe permission of the PCCP Owner Societies.



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unambiguously demonstrate that, while the iron centers in thecrystalline forms of these species exist in the low-spin state, inthe solution phase they are almost exclusively in the high-spinstate, as shown by the agreement between experimental spectraand calculated spectra for these spin states. In addition, spectralanalysis reveals the extent of σ and π donation and back-bonding from the ligands to the iron center and quantifies thetotal charge transfer between iron and the porphyrin ring. Thecalculations reveal back-donation from the iron center to thephenolic oxygen of the tyrosine proximal ligand in beef livercatalase, imparting a small Fe4+ character to the iron andenabling fast bidirectional electron transfer between the ironcenter and porphyrin ring. This effect facilitates the high activityof catalase in hydrogen peroxide decomposition, which involvesboth oxidative and reductive stages. The same technique hasmore recently been applied to the study of charge transferbetween the Fe-porphyrin active region of myoglobin andvarious transport ligands.189 The spectra of aqueous myoglobinunder ambient conditions in a liquid flow cell exhibited inFigure 26 show a clear reduction in the intensity of the iron L3-

edge upon ligand binding. This reduction is substantially moresignificant for the more tightly bound ligands (CO and CN)and has been attributed to charge transfer between the ligandsand iron. Additional studies of metal active sites in proteins viaX-ray spectroscopies have been previously reviewed by Aziz.190

In addition, a similar L-edge NEXAFS study has investigatedthe interaction between Ni2+ and nucleotide bases in (Ni)·M-DNA, in which the metal ion participates in the base-pairbonding interaction.183

Additional studies of amino acid and protein structure andsolvation in solution have been carried out directly on thestructural edges of the polypeptide backbone (i.e., C, O, and NK-edges). Such studies are inherently limited to small-moleculesamples, because large polypeptides contain numerous non-equivalent atoms of these types and thus would produceNEXAFS spectra with too much complexity for reasonableinterpretation. For example, Schwartz et al. studied the nitrogenK-edge of triglycine in several salt solutions to study specific-ioninteractions of the protein backbone with ions across theHofmeister series.182 A detailed pH-dependent study of allthree structural edges of the peptide backbone focused on theisolated glycine amino acid for spectral simplicity.178 TheNEXAFS C, O, and N K-edge spectra of aqueous glycine as acation, zwitterion, and anion are shown in Figure 27. The

oxygen and nitrogen K-edges reveal distinctive spectral changesupon protonation/deprotonation of the target atom, as mightbe expected. Changes to the carbon K-edge spectrum as afunction of pH are subtle. Interestingly, the most obviouschange is the red-shift in the sharp feature corresponding to the1sCO → π* transition at high pH. The protonation states ofthe carbonyl oxygens, however, are identical at high and neutralpH. Instead, this shift is thought to arise from a decrease in theelectronegativity of nitrogen upon deprotonation resulting in asmall rearrangement of the electron localization along themolecular backbone.A similar pH-dependent study has been performed on the

carbon and nitrogen K-edge NEXAFS spectra of aqueous ATP,a nucleoside triphosphate with a critical role in storing andtransferring energy in biological systems (Figure 28).186 Asobserved for glycine, solvation and delocalization effects spreadthe spectral effects of protonation/deprotonation of the aminonitrogen in the adenine ring to features associated with atomsfar from the amino group. At pH 2.5, the spectral feature in thecarbon K-edge spectrum centered at 287.3 eVthought to beassociated with ring carbons adjacent to the ring nitrogens191is split. Moreover, while only one nitrogen changes protonationstate, all spectral features in the nitrogen K-edge spectrum areobserved to change in intensity and energy.

7. CONCLUDING REMARKS AND OUTLOOKSince the introduction of liquid microjet technology into XASin 2001, X-ray absorption spectroscopy has evolved into ageneral and powerful tool for the study of the structures anddynamics of liquids and solutions. NEXAFS and EXAFStechniques, coupled with electronic structure calculations, havenow been utilized to characterize the local environments ofmany important species in the liquid phase. The atom-specificnature of this spectroscopy permits the chemical environmentsof specific atoms in fairly complex molecules to becharacterized, providing a valuable probe of solute−solute,solute−solvent, and solvent−solvent interactions. However,XAS of liquids is rapidly becoming a mature field, in which themajority of the “low-hanging fruit” has already been explored.This Review has provided an overview of current technologyand capabilities of XAS for the study of liquids and hashighlighted some of the salient results obtained from theseexperiments. Moving forward, we will see experiments ofgreater complexity than most described herein. The fast-flowliquid microjet mixing system described herein for the study ofcarbonic acid175 and hydrated carbon dioxide176 presents atechnological pathway to the study of other short-lived reactionintermediates. The mixing of bulk liquid samples is inherentlynonspontaneous and therefore presents a substantial obstacle tothe determination of quantitative kinetic data (i.e., determi-nation of a rate constant) via this technique. However, theexperimental design offers a valuable probe of mechanisticdetails of liquid- and solution-phase reactions in situ. Thistechnology enables the study of important multistep reactions,e.g., radical reactions in aqueous solution, which are of interestin numerous biological systems.A recent trend in NEXAFS has been the utilization of

multiple spectral-detection methods for the same sample toextract electronic structure information that may not bediscernible from a single technique. Golnak et al. havecompared PFY and PEY measurements for various decaychannels following excitation of the Fe L-edge in aqueoussolutions of FeCl2

192 and FeCl3.193 In addition to ascertaining

Figure 26. TFY NEXAFS spectra of the Fe L2 and L3 edges ofmyoglobin with various transport ligands. The spectra have beennormalized to the intensity of the most intense feature of the L2 edgecentered at ∼721 eV. Reduced intensity of the primary feature of theL3 edge, centered at ∼707.5 eV, has been attributed to charge transferbetween the ligands and the iron center. Reproduced from ref 189with permission of The Royal Society of Chemistry.



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that PFY and PEY spectra collected at the energy of the iron 3s→ 2p relaxation channel best reproduce the true absorptionspectrum measured via transmission, the authors suggest thatlocal electronic structure (e.g., electronic interactions with thesolvent, etc.) can be revealed by comparison of the spectraobtained from various decay channels. While the validity ofrelated strategies for determining the local electronic structurehas been a subject of debate in the past,194−198 the utility ofsuch a method for measuring the local electronic interactions ofa transition metal center is unquestionable. Transition metalsplay important catalytic roles in numerous reactions of

biological and commercial importance, and the ability tomeasure electron density and/or interaction energy at ligand-binding sites enables a substantial new class of XASexperiments with broad application.We have revealed in this Review a trend of subtle but

unmistakable discrepancies between spectra of liquid systemsmeasured in liquid microjets and in flow cells, particularly withregards to changes to the water oxygen K-edge NEXAFSspectrum upon addition of solutes. The origin of thisdiscrepancy has not yet been examined. In addition, we havehighlighted the ongoing debate regarding the interpretation ofthe liquid water NEXAFS spectrum and its implication for ourunderstanding of bulk water structure. Note that all of thefeatures in the liquid water XA spectrum can be reproducedusing standard tetrahedral water models without invoking 1-and 2- dimensional hydrogen-bonding structures (“rings andchains”) as the dominant motif in the liquid, according toPrendergast and Galli.76 Questions such as these can only beadequately answered through continued application of state-of-the-art theoretical methods. Recent progress in the develop-ment of many-body potential energy functions has enabled, forthe first time, simulations of water capable of accuratelyreproducing important properties of this critical material in allof its phases.199 Continued improvement to MD methods forsimulation of water and aqueous solutions must provide thebasis for a universally acceptable interpretation of the waterNEXAFS spectrum and will help to interpret the spectral dataobtained from increasingly complex experiments such as thosedescribed above.

AUTHOR INFORMATIONCorresponding Author

*E-mail: [email protected].

Figure 27. pH dependence of the NEXAFS spectrum of glycine at (a) the nitrogen K-edge, (b) the oxygen K-edge, and (c) the carbon K-edgeshowing spectra of glycine as an anion (A), a zwitterion (B), and a cation (C). Spectral changes upon deprotonation of the amino group andprotonation of the carboxyl group are clearly visible in the high- and low-pH spectra, respectively. Adapted with permission from ref 178. Copyright2005 American Chemical Society.

Figure 28. TEY NEXAFS spectra of 0.2 M aqueous ATP on thecarbon (A) and nitrogen (B) K-edges at pH 2.5 and pH 7.5 collectedfrom a liquid microjet. Reproduced from Kelly, D. N. et al.“Communication: Near Edge X-ray Absorption Fine StructureSpectroscopy of Aqueous Adenosine Triphosphate at the Carbonand Nitrogen K-Edges.” J. Chem. Phys. 2010, 133, 101103,186 with thepermission of AIP Publishing.



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mailto:[email protected]




ORCID

Richard J. Saykally: 0000-0001-8942-3656Notes

The authors declare no competing financial interest.

Biographies

Jacob Smith is a 2016 chemistry Ph.D. graduate of the Saykally Groupat the University of California, Berkeley. His thesis research focused onthe use of XAS to study ion solvation in electrolyte solutions. Dr.Smith received his B.S. in chemistry from James Madison University,where he worked in the research group of Dr. Daniel Downey.

Richard Saykally is the Class of 1932 Endowed Professor of Chemistryat the University of California, Berkeley. He received his B.S. inChemistry from the University of Wisconsin, Eau Claire and his Ph.D.from the University of Wisconsin, Madison. Prof. Saykally has been therecipient of over 70 awards from 10 different countries, including theIrving Langmuir Award in Chemical Physics and Peter DeBye Awardin Physical Chemistry from the American Chemical Society and theFaraday Lectureship Prize from the Royal Society of Chemistry. Hehas been an author of over 400 scientific papers that have been citedover 40 000 times. Prof. Saykally and his students have pioneeredmany important advances in spectroscopy, including velocitymodulation spectroscopy of ions, terahertz laser vibration−rotation−tunneling spectroscopy of clusters, infrared photon counting spec-troscopy, cavity ringdown spectroscopy, and X-ray spectroscopy ofliquid microjets.

ACKNOWLEDGMENTS

The Berkeley X-ray spectroscopy effort is supported by theDirector, Office of Basic Energy Sciences, Office of Science,U.S. Department of Energy (DOE), under Contract no. DE-AC02-05CH11231, through LBNL Chemical Sciences Divi-sion. J.W.S. thanks the Berkeley Center for Solvation Studies(CALSOLV) for support.

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Soft X-ray Absorption Spectroscopy of Liquids and Solutions · Soft X‑ray Absorption Spectroscopy of Liquids and Solutions Jacob W. Smith and Richard J. Saykally* Department of