Applications of ZEKE spectroscopy

JOURNAL OF ELECTRON SPECTROSCOPY

and Related Phenomena

ELSEVIER Journal of Electron Spectroscopy and Related Phenomena 75 (1995) 35-46

Applications of ZEKE spectroscopy

Klaus Miiller-Dethlefs’

Irzstitut,fir Physikrrlische und Theoretische Chemie, Technische Universittit Miirzchen, LichrcnherRsrrussc 4, D-85747 Gorching, Germutlj~

Received 27 June 1995: accepted 2 August 1995

Abstract

Typical applications of high resolution Zero kinetic energy (ZEKE) photoelectron spectroscopy to molecules and clusters are reviewed. The high resolution of ZEKE spectroscopy compared to conventional photoelectron spectroscopy allows the rotational structure of larger molecular cations, for instance the benzene cation, to be obtained. The dynamic Jahn-Teller effect and the shape of the benzene cation is apparent from these rotationally resolved ZEKE spectra. For molecular clusters, the phenol-water and phenolLmethano1 complexes show dense, clearly resolved intermolecular vibrations.

Keywrds: Application; Synchrotron radiation zero kinetic energy photoelectron spectroscopy

1. Introduction

Molecular photoelectron spectroscopy (PES), developed in the early 1960s [l-3], provides information about electronic and vibranic states of molecular ions [4,5] through the “back-titration” of the kinetic energy of the ejected photoelectron. Though there is no principal restriction for the resolution limit of PES (from energy conservation the energy of the ejected electron is quantized assuming sharp photon energy and sharp states for the cation and the neutral molecule) the practi- cal resolution limit of common geometrical or time-of-flight photoelectron analysers is found around IOmeV, but recently in favorable cases 3- 5 meV has been demonstrated for a combination of He I source and spherical analyser [6]. When

’ Present address: Chair of Physical Chemistry, University of York, Heslington. York YOI 5DD, UK.

combined with resonance enhanced multi-photon ionization (REMPI) a resolution of around 3- 5meV [7,8] has also been achieved in some cases. Hence, with the notable exceptions for hydrogen [9,10] where the rotational structure was resolved by the Shirley group [l l] and for high lying rotational states of NO’ [7,8], conventional PES does not generally allow preparation of rotational levels of molecular ions. Similarly, the typical soft (low frequency) vibrations present in molecular clusters (e.g. the intermolecular vibrations) are difficult to resolve by PES. This limitation in resolution can be overcome by the use of what is now termed zero kinetic energy (ZEKE) photoelectron spectroscopy, a method developed in the author’s laboratory over the last decade [ 12- 191. Many other groups have taken up the method and it is impossible here to review a representative cross section of the present work on ZEKE spectroscopy. which is given in a recent book [20]. Hence

036%2048/95/$09.50 5‘) 1995 Elsevier Science B.V. All rights reserved

SSDI 0368-2048(95)02387-9

36 K. Miiller-DethlejslJournal of Electron Spectroscopy and Related Phenomena 75 (1995) 35-46

this review focuses on two typical applications of ZEKE spectroscopy: the rotationally resolved threshold ionization of benzene to understand the dynamic Jahn-Teller effect in the benzene cation and the resolution of intermolecular vibrations in hydrogen bonded phenol-water and phenol- methanol clusters.

In contrast to conventional PES, ZEKE spectroscopy essentially offers a resolution limited by the typical bandwidth of tunable pulsed dye lasers. With such resolution of around 0.1 cm-’ it is possible to resolve rotational states of larger molecular cations and even prepare specific rovibronic states by ZEKE spectroscopy; this opens the way to new experiments in chemical reaction dynamics. The ZEKE method enables the observation of the rotational structure of molecular ions, the study of excited-state and Rydberg-state dynamics [14,21], photofragmentation and the investigation of the dynamics of photoionization apparent in the coupling between the molecular ion core (in a certain eigenstate) and the ejected photoelectron [20].

When used with REMPI, ZEKE spectroscopy combines the sensitivity and selectivity of REMPI-PES with the high energy resolution of optical spectroscopy. Of particular significance was the discovery of the very long life-time of very high-n Rydberg states within the “magic region” of about 1 meV converging to a certain ion threshold [22]. This discovery led to the conclusion that in addition to the original concept of pulsed field extraction a dominant portion of the ZEKE signal can be generated from pulsed field ionization of highly excited, long-lived Rydberg states with a principal quantum number n > 150. This is also called ZEKE-PFI [22] and has become the most widespread experimental realization of ZEKE spectroscopy. With this method a laser limited resolution of 0.2cm-’ is achieved in practice which allows the study of even larger molecules with rotational resolution [14]. Recent rotationally resolved ZEKE spectra of benzene show that the benzene cation is planar and adequately described in the DSh molecular point group, although it is subject to Jahn-Teller distortion [50]. ZEKE spectroscopy has also been successfully applied for studies of the vibrational structure of large organic molecules [23], molecular [24] and

metal clusters [25,26] and hydrogen-bonded systems [27]. Using ZEKE spectroscopy, ionization energies (IEs) can be determined with an accuracy comparable to that of Rydberg extrapolations but with less experimental effort.

2. Experimental ZEKE technique

The idea and principles of ZEKE spectroscopy have already been discussed in a number of papers [12,13,22,28,29], a Nature article [21], review arti- cles [14,16-191 and a recent book [20]. The term ZEKE spectroscopy is now commonly used to describe any experimental technique that detects electrons from a small, experimentally adjustable energy range around (i.e. above and below) a selected ionization threshold using delayed pulsed electric field extraction [12,13] (for a discussion of the older threshold techniques and their relation to ZEKE see Refs. [14,18,20]). In contrast to PES where a fixed photon energy is used for ionization, the ZEKE method involves the detection of electrons that are formed with zero kinetic energy when scanning the light source across the ionic threshold region. ZEKE electrons (within a certain bandwidth) are produced when the photon energy of the incident light matches the energy of a rovibronic transition between the initial state of the neutral molecule and the final state of the ion [14]. In the original ZEKE concept, as seen in Fig. 1, the free ZEKE electrons produced under (nearly) field-free conditions are extracted using a delayed pulsed electric field. Zero-field ionization and delayed pulsed-field extraction allows ZEKE electrons to be discriminated from kinetic electrons by their respective times of flight as indicated in Fig. 1. It should be noted that there are two different ways to produce electrons contributing to the measured ZEKE signal. Firstly, there are very low kinetic energy electrons, which are generated by photoionization at threshold (a threshold implies, for molecules, a rovibronic eigenstate of the cation). These so called “free ZEKE electrons” are discriminated from kinetic electrons using the steradiancy principle [30] and the time-of-flight separation. Secondly, as discovered by Reiser et al. [22]. electrons from long-lived Rydberg levels

K. Miiller-Dethl~fslJournal of Electron Spectroscopy and Related Phenomena 75 (3995) 35-46 31

* ‘I detector

0

t-0

t

electron signal

2

LLL 1 3

+ time-of-flight

Fig. 1. Free ZEKE photoelectron detection scheme. Top: when ionization occurs (r = 0) the electrons with non-zero kinetic energy start to fly away from the ZEKE electrons. Bottom: after the delay time (t = td) kinetic electrons flying away (1) or towards (3) the detector are spatially separated from the ZEKE electrons (2); the spatial separation is then transformed into a time-of-flight separation as observed in the electron signal vs. time-of-flight curve.

of high principal quantum number n very close to threshold can also contribute to the ZEKE signal. Such electrons are generated by pulsed-field ionization of these high-lying Rydberg states by the applied extraction pulse (see below). The free ZEKE electrons, for instance, can contribute to most of the signal when the bandwidth of the ionizing radiation is comparable to or bigger than the magic region of long-lived ZEKE Rydberg states (see below). This is the case for typical VUV synchrotron radiation ZEKE experiments employing a normal incidence monochromator. The resulting detection window is then described by a transmission function, which gives a (theoretical) detection probability as a function of the energy displacement from a certain threshold. For delayed pulsed extraction this function extends to energies below threshold. The shape of the transmission function can be influenced by four factors: the geometry of the analyser, the shape and delay time of the extraction pulse and width of the time-of-flight detection gate for the electrons. By

varying these parameters one can adapt the transmission function to the photon bandwidth of the light source. The ability to adapt the energy resolution to the bandwidth of the light source is a particular advantage of ZEKE spectroscopy. The example in Fig. 2 was measured at the BESSY 3 m normal incidence monochromator and shows the ZEKE spectrum of argon in the region of the

+/? and ‘P,,z ionization thresholds [31]. An extraction pulse of 0.8 V cm-’ synchronized with the BESSY multi-bunch emission characteristic of 208 ns (with a dark region of 60 ns) was employed in these experiments. It can be seen that all auto- ionization resonances between the two thresholds are completely suppressed. The ‘Pli2 peak is pre- sented on an expanded scale in the insert showing an experimental FWHM of 50 cm-‘. With a photon resolution of 36cm-’ at this energy the FWHM of the analyser transmission function can be estimated to be 35 cm-’ (4.3 meV). The only slightly asymmetric shape of this peak also shows that the transmission function of the electron analyser goes very rapidly to zero in contrast to a typical TPES experiment.

The experimental observation of long-lived (i.e. up to tens of microseconds) Rydberg states converging to (ro)vibronic levels of the ion has led to a second variation of the ZEKE technique: the detection of pulsed-field-ionized Rydberg electrons [22]. This method is generally applicable: not only

t2py2 800 soan” IL 0 : 1285ao 12woa 128700 ' . .

. . *'

. . . .

. . -.

/,

.>

I I I 4 127ooO 127500 128cm 128500 12Qow

Photon Energy (cm-‘)

Fig. 2. ZEKE spectrum of argon in the region of the first ionization thresholds obtained with multi-bunch synchrotron radiation and pulsed-field extraction [31]. The insert shows the ‘Pl,z peak on an expanded scale. Reproduced from Ref. [31] with permission.

38 K. Miiller-DerhlefslJournal of Electron Spectroscopy and Related Phenomena 75 (1995) 35-46

for small molecules such as NO have long-lived high-n Rydberg states been found, but also ZEKE Rydberg states within the magic region with a lifetime of up to tens of microseconds are clearly evident for rather large molecules such as benzene and for molecular clusters [ 17,321. Besides the advantage of having the full accuracy of optical spectroscopy, the ZEKE technique offers a spectral resolution of up to 0.2 cm-‘, which is sufficient for the rotational resolution of molecules such as NH3 [28], (NO)z [24], and C6H6 [36,37,50]. However, the PFI technique is not applicable to photodetachment studies, because no Rydberg states exist for anions. In this case the original ZEKE method involving the detection of free electrons with negligible kinetic energy has to be applied. Appli- cation to the photodetachment of mass selected anions yields the vibronic structure of the corresponding neutral species [25,33,35]. A very interesting result came from the application of ZEKE photodetachment to anions using the original concept [12,13] of delayed pulsed field extraction. Particular interest, as an example of transition state spectroscopy, was generated by the ZEKE photodetachment spectrum of IHII. This spectrum, reproduced in Fig. 3 shows resolved vibrational structure of the (metastable) activated complex IHI of the iodine hydrogen reaction [34]. This structure is resolved by ZEKE but cannot be resolved by PES.

E, IeVl 0.60 0.40 0.20 0.00

3) 1

0.20 0.40 0.60 0.80 I .oo

Eki”leVl

Fig. 3. Comparison of ZEKE photodetachment spectrum (solid line) [34] and TOF photoelectron spectrum (dashed line) of IHI-. Reproduced from Ref. [76] with permission.

With the discovery of very long life-time high-n molecular Rydberg states it became possible to “park” electrons in ZEKE Rydberg states and generate the ZEKE signal from pulsed field ionization (ZEKE-PFI) [22]. In combination with a slowly rising electric field extraction pulse (i.e. with a linear or multi-step slope) the very high resolution (up to 0.2cm-‘) which is essential to get rotational resolution for larger molecules such as benzene [36,37,50] has been obtained. By a variation of the slope of the pulse, the spectral resolution of the ZEKE-PFI technique can be adjusted according to the laser bandwidth and to the needs of the system under study, i.e. whether vibrational or rotational resolution shall be achieved. This can be seen in Fig. 4 where the effect of pulse slope risetime on time-of-flight (TOF) of the corresponding electrons produced by PFI is envisaged. A fast pulse generates all the signal within a narrow TOF distribution whereas a slow pulse spreads the different “slices” of Rydberg states into a broader TOF distribution. By setting a TOF gate and by scanning the photon energy of the light source a smaller spectral “Rydberg slice” is thus collected for a slow pulse compared to a fast

Rvdban molecule

(a> E Iii. :: ’ 0.2 I I

0.4 t C]

i Gates: 1 2 3 4

II 1,

Chtes: 1234

Fig. 4. Pulsed-field ionization of long-lived Rydberg states using a fast-rising (a) and a slowly-rising (b) extraction pulse and collection with a certain time gate.

K. Miiller-DethlefsiJournal of Electron Spectroscopy and Related Phenomena 75 11995) 35-46 39

pulse. In fact, using a multi-step staircase-like extraction pulse provides a technique for the exact determination of the ionization energy under field free conditions [38,39].

A schematic experimental setup for a ZEKE experiment is shown in Fig. 5. It consists of a laser system and a vacuum apparatus including the molecular beam source, the extraction plates and a p-metal shielded flight tube with electron detectors (i.e. dual multichannel plates) at its end. In a typical two-colour experiment both dye lasers (typically frequency doubled) are pumped simul- taneously by an excimer laser or a Nd:YAG laser. The first dye laser excites a specific vibronic or rovibronic level of the intermediate state and the second laser ionizes the molecules or promotes them into long-lived Rydberg states (n > 150) converging to (ro)vibronic levels of the electronic ground state or an electronically excited state of the cation. After a delay time of several microseconds, an extraction pulse is applied by either a simple electric pulsing device or by using an arbitrary function generator. The electrons are detected with multichannel plates and their TOF signal is recorded with boxcar integrators or a transient digitizer by setting narrow time gates (lo-30 ns).

The field ionization of Rydberg states allows the adjustment of the spectral resolution by a variation of slope and magnitude of the delayed extraction pulse from the magic region. The advantage of spe- cially tailored, slowly rising extraction pulses is exploited in the present work (note that throughout this article “rising” is meant in the sense of rising in

Jet-Valve

R

Fig. 5. Experimental setup for a two-colour ZEKE experiment with two dye lasers. MCP, multichannel plates of frequency W, and LL12.

field strength; the voltage of the pulse itself is negative). In particular, pulses with a multi- step “staircase slope” are of interest here. Such “staircase slope” pulses can be used for an exact determination of the IE.

3. Benzene

The neutral benzene molecule has a hexagonal, planar structure with Dhh symmetry [40-431. In the electronic ground state the electron configuration is (a2,)2(els)4. Upon ionization, one elg electron is removed from the highest molecular orbital, thus leaving one elg electron unpaired. The results in a double degenerate 2Els electronic ground state of the cation. Due to the Jahn-Teller theorem [44], for any nonlinear polyatomic molecule in a degenerate electronic state, there exists a distortion of the nuclei along at least one non-totally symmetric normal coordinate that results in a splitting of the potential energy function so that the potential minimum is no longer at the symmetrical position [44-46,521. Several authors have discussed struc- tural distortions of the benzene cation [47,48]. According to quantum chemical ab initio calculations, in-plane distorted geometries with an elon- gated or compressed DZh structure are more stable than the highly symmetrical Dhh configuration [47]. The experimentally determined value of the stabil- ization energy of 266cm-’ [49] is about the half of the zero-point energy of the lowest JahnTeller active normal vibration. The question arises whether the symmetry reduction deduced from the static Jahn-Teller distortion and predicted by theoretical calculations has an influence on the rotational selective ionization dynamics and on the rotational structure of the benzene cation. If the molecule was statically distorted to lower symmetry, transitions should appear in the rotationally resolved ZEKE spectra, which are forbidden in the undistorted, highly symmetrical Deb configuration. Thus, the observed rotational transitions are a sen- sitive and clear indication of the symmetry of the cation.50

If one quantum of a JahnTeller active normal vibration (for benzene normal modes with e2s symmetry [44], z+_ 9 in Wilson’s notation [5 11) is excited,

40 K. Miiller-DethlefslJournal of Electron Spectroscopy and Related Phenomena 75 (1995) 35-46

the linear dynamic Jahn-Teller coupling leads to a splitting into two vibronic states withj = f l/2 and j = f3/2 vibronic angular momentum [52,53]. The quadratic dynamic Jahn-Teller coupling further splits the j = f3/2 band into two substates, but leaves the j = &l/2 state doubly degenerate [52]. The excitation of an e2g normal vibration in an El, electronic state leads to vibronic states with B,, $ BZg @ El, vibronic symmetry species. First order interaction lifts the four-fold degeneracy and results in two doubly degenerate vibronic states with B1, @ B2s and El, symmetry, respect- ively. Higher order interactions can only split the B1, $ BZg state, but not the inherently doubly degenerate El, vibronic state.

This splitting of the j = -3/2 and j = +3/2 bands, which is due to quadratic Jahn-Teller coupling [36,37,54] is related to the coupling strength. Without full rotational resolution, however, a symmetry assignment of the two substates is impossible, because depending on the (as yet unknown) sign of the quadratic Jahn-Teller coupling parameter g either the higher or the lower lying vibronic state is of B1, symmetry (the other having Bzg symmetry). With full rotational resolution the spectrum shown in Fig. 6 is recorded. For this spectrum a slow rising extraction pulse was used which resulted in a spectral resolution of 0.2 cm-‘. The ZEKE spectrum in Fig. 6 was obtained via the S,6l(J’ = 2, K' = 2, -,) rovibronic state, i.e. the lower Coriolis level of the intermediate resonance. The rotational structures of the two bands can be assigned to K+ = 6 and K' = 0. For KS = 6 the states with total angular momentum N+ = 6 and 7 are observed for both vibronic bands with similar intensity. However, the K+ = 0 behave in a completely different way. As can be seen from the assignments given in Fig. 6, for transitions to rotational levels with projection quantum number K+ = 0 only either progressions with even (left) or odd (right) total angular momentum quantum number N’ occur. This leads to an unambiguous assignment of the vibronic symmetry of these two bands.

In the molecular symmetry group Dhh the rovibronic levels with projection K = 0 of vibronic states with A (Al,, AL Azg or AZ”) or B (Bl,, B1,, B2g or B2u) symmetry alternate with J (or N), i.e. have alternating symmetry species depend-

Do 6’ (+3/2) <- S1 6l (J=2, K’=2, - l)

Kc=0 Kf=O

m m 02 4 N+ 13 5 N+

J

K+= 6

l-l 6 7 N+

K+= 6

l-l 6 7 N+

36296 36306 36316 36326

Ionising Laser Energy [cm?] Fig. 6. ZEKE spectrum of the 6’ (j = f3/2) band of the u6 vibrationally excited electronic ground state of the benzene cation via the rovibronic state S, 6’ (J’ = 2, K’ = 2, -I), i.e. the lower Coriolis level of the intermediate resonance (r,,, = B1, @ Bzu). The centre part shows the term diagram for the K+ = 0 states in the B,, (left, N+ = 0,2,4. .) and Bzu (right, N+ = 1.3.5.. .) vibronic JT states.

ing on whether the total angular momentum quantum number is even or odd. For a vibronic state with B,, symmetry, the rovibronic symmetry species are B,, and B,, for even and odd total angular momentum Nf (see Fig. 6), and vice versa for a vibronic state with B2s symmetry. Because only transitions to either even or odd N’ occur in the spectrum shown in Fig. 6, one of the two final rovibronic symmetry species must be favoured. The symmetry species of the intermediate state is B1, @ Bzu, so both states (i.e. both symmetry species) are populated. However, the nuclear spin wave function of benzene with DSh symmetry has irreducible representations rns = 13A1 @ A2 @ 7B1

K. Miiller-DethlefslJournal qf Electron Spectroscopy and Related Phenomena 75 (199.5) 35-46 41

$3B2 CE 9E1 $ 1 l&. As already mentioned, the overall allowed symmetry, which is the combination of the rovibronic symmetry and the symmetry of the nuclear spin wave function, is Bi for D6h benzene. Rovibronic states with symmetry species B,, or B,, have Ai nuclear spin symmetry, those with BZu or BZg have A2 nuclear spin symmetry. Thus the population (i.e. the statistical weight) of rovibronic levels with Bi symmetry (Bi, or B,,) is a factor of 13 higher than those of B2 symmetry (B2s or BzU). Transitions to rovibronic levels of the cation with Bi, symmetry species therefore must be much stronger, exactly as seen in the spectrum. Because these states have even total angular momentum in B,, vibronic states and odd total angular momentum in BZg vibronic states, the lower state (left in Fig. 6) must be of Bi, and the higher state (right in Fig. 6) of BZg vibronic symmetry.

Only rotationally resolved ZEKE spectroscopy thus enables a symmetry assignment to be made for the two substates of the 6’ (j = f3/2) band, which are split due to quadratic dynamic Jahn-Teller coupling. In addition, another conclusion can be drawn from this result. If either the intermediate state or the final state or both states belonged to a less symmetric molecular point group such as &, or Dz~. the corresponding rovibronic symmetry species would not have such different nuclear spin statistical weights. The rotational structures of the ZEKE spectrum via the S,6’(J’ = 2, K’ = 2, -I) rovibronic intermediate resonance therefore would not be that different, i.e. for projections K- = 0 only either the even or odd total angular momentum would have intensity. The ZEKE spectrum of the 6’ (j = *3/2) band shown in Fig. 6 hence gives strong evidence that benzene has Dbh symmetry in both the neutral intermediate state and the electronic ground state of the cation with the &, vibration excited. This agrees with and further supports the above-stated De,, symmetry of the benzene cation, which was concluded from the transitions observed in the ZEKE spectra of its vibronic ground state [50].

4. Hydrogen bonded clusters

A subject that has received enormous attention

over the years is hydrogen bonding, owing to its ubiquity in biological systems [55]. The detailed study of hydrogen bonding in such systems is not straightforward, however, and attention has been focused on rather smaller systems in environments that are more amenable to interpretation by the chemists. The biggest successes have come from the study of 1 : 1 bonded complexes in molecular beams [56,57]. Although numerous studies have been performed on neutral complexes, there are very few studies that have concentrated on ionic species. These species are of obvious importance because fundamental processes, such as solvation, depend on ionic species interacting with neutral molecules. The reason why there is a paucity of such studies is due to the difficulty of producing significant quantities of ionic complexes in the gas phase (although infrared spectroscopy of ions is an extremely active field [58,59]). Conventional PES does not offer sufficient resolution to determine the soft intermolecular vibrations.

The study of hydrogen-bonded species using ZEKE spectroscopy has mainly taken place in my laboratory, over the last three years. To date, five different hydrogen-bonded species have been stu- died, all involving phenol as the proton-donating moiety. Specifically, these complexes are phenol- water (Ph-H20) [27,60,61], phenol-methanol (Ph-MeOH) [62], phenol-ethanol (Ph-EtOH) [63], phenol dimer (Phz) [64], and phenol- dimethylether (Ph-DME) [65]. The hydrogen- bonded clusters discussed here have recently been reviewed in a broader context (see Ref. [ 171). The experimental modifications used to study phenol- water are discussed in detail in the previous ZEKE studies. Briefly, phenol (heated up to 100°C) and water were expanded through a 300 pm nozzle into a vacuum chamber with up to 5 bar of argon or neon as carrier gas to produce the phenol- water complex. To record a ZEKE spectrum, the first dye laser was fixed resonant with the S, state of the complex under study, while the second dye laser was scanned through the ionization thresholds to populate ZEKE Rydberg states; these were subsequently field-ionized by a delayed extraction pulse of 0.7 Vcm-’ (delayed 2 W).

42 K. Miiller-Dethlefs~Journal of Electron Spectroscopy and Related Phenomena 75 (1995) 35-46

4.1. Phenol-water

This complex, and the series of complex phenol- (water), (n = l-4) have received much attention over the last lo- 15 years. Several experimental techniques have been employed (see Ref. [17] for a comprehensive review). A number of ab initio studies on the neutral ground electronic state have also been published [66-691, all agreeing that the water is bonded symmetrically with respect to reflection in the plane of the phenol ring: the symmetry is thus C,. The experimental studies on the 1: 1 complex have concentrated on the first excited singlet state Si (formed by a X* +- 7r excitation on the phenol ring) in the main, although some information is available on the ground state (S,). The 1 + 1’ REMPI spectrum of the Ph-Hz0 complex shows only a meagre amount of structure. The transition into the vibrationless state is the most intense transition, suggesting that the geometries of the Si and S,, states are fairly similar, at least along the hydrogen-bonding coordinates; two other intermolecular vibrations appear, the reason- ably strong stretch vibration (u) at 156cm-’ and the weak in-plane wag vibration (7’) at 121 cm-‘. The assignments of these bands have been refined over the years and can now be considered as certain, as shown by comparison of a high quality 1 + 1’ REMPI spectrum of the phenol-water complex with ab initio calculations [66], confirming the previous spectral assignment [70].

Information about the low-frequency intermolecular vibrations of the ionic complex (in contrast to the neutral states of the 1 : 1 phenol-water complex) was rather sparse before the ZEKE studies. A conventional (one-colour, two-photon) time-of- flight photoelectron spectrum (REMPI-PES) [71] via the vibrationless Si state does not show resolved structure due to intermolecular modes, the vibrations of most interest. Two-colour photoionization efficiency (REMPI-PIE) [72] measurements via the vibrationless Si origin and the Si level with one quantum of the intermolecular stretch excited showed the presence of at least one intermolecular vibration which was observed as a progression of steps in the ion yield spectra. This vibration of 242cm-’ was assigned to the intermolecular stretching mode of the ionic

complex. However, no further (out of a possible six) intermolecular modes have been obtained.

Very recently, the 1 : 1 phenol-water complex has been investigated in a more complete ZEKE spectroscopic study [61] including the threefold deuterated complex (where the two water hydro- gens and the phenolic hydrogen have been substi- tuted by a deuterium atom). For the protonated (h3) complex, ZEKE spectra were obtained by exciting through three intermediate vibronic states: the S, O”, the Sia’ and the Sly” states: the resulting spectra are shown in Fig. 7. From the ZEKE experiment, the (field-free) ionization energy (including the correction for the extraction field) was determined as IE = 64027 f 4cm-‘. This same value was, within experimental error, obtained from all three intermediate states. The IE of the h3 complex is lower by 4601 f 8 cm-’ compared to the ionization energy of the isolated phenol molecule (68626 f 4 cm-‘) [73].

From the recent work, five out of a possible six intermolecular vibrations were extracted. As noted above, the most obvious feature is the progression of the intermolecular stretch (see Fig. 7) also seen in combination with other intermolecular modes. This long progression (consisting of five quanta) of the intermolecular stretch is indicative of the expected large change in the hydrogen bond length upon ionization. It is also interesting that this progression is so strong; it is also seen in the photoionization efficiency measurements [72], and in fact a bond decrease of 0.018 A was estimated in that work from a one-dimensional Franck-Condon factor calculation. The progression itself was found to be quite anharmonic, a general finding from the ZEKE spectra of the different hydrogen- bonded complexes. The increase of frequency of the intermolecular stretch of the cation (240 cm-‘) over that in the Si state (157 cm-‘) [61] and the So state (155 cm-‘) [66] illustrates the large increase in binding energy upon ionization. The other intermolecular vibrations observed were the out-of-plane bend (0”) at 67cm-‘, the in-plane bend (p’) at 84cm-‘, the first overtone of torsional mode (27) at 257cm-’ and the in-plane wag (7’) at 328 cm-‘. Hence, five intermolecular modes are believed to be identified. Various argu- ments were used for the specific assignment of these

K. Miiiler-Dethlefi/Journal qf Electron Spectroscop! and Related Phenomena 75 (19951 35-46 43

u 2a

lBb+a

IE o]

0 500 1000 Ion Internal Energy [cm?]

Fig. 7. ZEKE spectra of the 1 : 1 phenolLwater complex, [Ph- HzO]-h,, via different intermediate S, levels: S, 0’ (top), S, u’, intermolecular stretch (middle) and S, y”, in-plane wagging mode (bottom). Reproduced from Ref. 1611 with permission.

modes, including the magnitude of vibrational frequency shifts on deuteration, comparison with the ab initio calculations [69] that were carried out con- currently and observation of the changes in intensity as the intermediate Si vibronic level was changed. One good example for the latter point may be clearly seen in Fig. 7: in the ZEKE spectrum via the intermolecular Si y” level (bottom) the ionic intermolecular in-plane wag (7’) is strongly enhanced (and also the intermolecular stretch progression starting on that particular vibrational origin) compared to the other two ZEKE spectra via the Sia’ (middle) and Si origin level (top).

One point of interest was that the intramolecular Vi8b vibration of the complex at 450Cm~’ was found to couple strongly with the intermolecular stretch: this point was used to explain why this vibration appeared so prominently in the ZEKE spectrum of this complex, while it was seen only weakly in the ZEKE spectrum of the isolated phenol molecule. The latter coupling was identified from normal mode pictures obtained via the ab initio calculations [69] of the frequencies.

4.2. Phenol-methanol

The study of the Ph-MeOH complex was carried out in a similar way to that of the Ph-Hz0 complex: a mixture of phenol and methanol was heated and the resulting vapour was expanded with Ar. The 1 + 1’ REMPI spectrum was difficult to interpret owing to the proximity of a number of vibrations (see Ref. [17] for details). Vibrational levels in the S, state were used as intermediate states on the way to ionization. The ZEKE spectrum obtained by excitation via the Si vibrationless level is particularly striking (see Fig. 8) with progressions of about ten quanta of a low-frequency vibrational mode of 34cm-‘, denoted ql [74], appearing in combination with components of an anharmonic progression of the intermolecular stretch of 278cm-‘. This pattern obviously sug- gests a rather substantial change of geometry upon ionization. The adiabatic ionization energy was derived as 63207 f 4cm-’ (field-free value) which represented an increase of bonding over that in the So state of 5421 * 8 cm-- ‘, again showing a large increase in bond strength. The latter point is also exemplified by the large increase in the intermolecular stretch over the values of 176 cm-’ in the Si state [62] and the value of 162cm-’ in the So state obtained by dispersed fluorescence spectroscopy [75]. Additionally, between the latter components, another set of progressions of the intermolecular mode of 34cm- ’ were seen, this time in combination with a third intermolecular mode of 52cm-‘. The ZEKE spectrum obtained via the Si state with one quantum of the lowest frequency intermolecular mode excited showed the same vibrations but with a substantially changed Franck-Condon envelope which allowed

K. MiiNer-DethlefslJournal of Electron Spectroscop.v and Related Phenomena 75 (1995) 35-46

1

2&o 28Ao Ionizing Laser Energy [cm-l]

Fig. 8. ZEKE spectrum of phenol-methanol via the vibrationless Si origin. Reproduced from Ref. [62] with permission

the identification of a fourth intermolecular mode, denoted q4, at 153 cm-i. A slightly different envelope of the 34cm-’ vibration was also obtained when exciting through the St state with one quantum of the intermolecular stretch (a) excited. The other two intermolecular modes of the Ph-MeOH cationic complex were identified from the ZEKE spectrum obtained via a combination band; their values were 76 cm-’ and 158 cm-‘. Therefore, similarly to Ph-Hz0 where five intermolecular vibrations were assigned, for phenol-methanol all six intermolecular modes of the cationic complex were identified by using different intermolecular vibrational Si levels as intermediate resonances.

Acknowledgements

The author is grateful to E.W. Schlag for his support and encouragement while this work was performed. Financial support for this work from the Deutsche Forschungsgemeinschaft (Grant No.

Mu 547/7-l) the Bundesministerium fur Bildung und Wissenschaft (Grant No 055WOFAI) and the European Community under the Science Program (Grant No. SC1 *-CT90-0642-MD) is gratefully acknowledged.

References and notes

[l] F.I. Vilesov, B.I. Kurbatov and A.N. Terenin, Sov. Phys. Dokl. Akad. Nauk SSSR, 138 (1961) 1329.

[2] D.W. Turner and MI. Al Jaboury, J. Chem. Phys., 37 (1962) 3007.

[3] D.W. Turner, C. Baker, A.D. Baker and CR. Brundle, Molecular Photoelectron Spectroscopy, Wiley, London, 1970.

[4] J. Berkowitz, Photoabsorption, Photoionization and Photoelectron Spectrocopy, Academic Press, New York, 1979.

[5] K. Kimura, S. Katsamuta, Y. Achiba, T. Yamazaki and S. Iwata, Handbook of He I Photoelectron Spectra of Fundamental Organic Molecules, Japan Sci. Sot. Press, Tokyo, 1981.

[6] P. Baltzer, L. Karlsson, M. Lundqvist and B. Wannberg, Rev. Sci. Instrum., 64 (1993) 2179.

K. Miiller-DethlefslJournal qf Electron Spectroscopy and Related Phenomena 75 (1995) 35-46 45

[7] K.S. Viswanatan, E. Sekreta, E.R. Davidson and J.P. Reilly, J. Phys. Chem., 90 (1986) 5078.

[8] D.J. Leahy, K.L. Reid, H. Park and R. Zare, J. Chem. Phys., 97 (1992) 4948.

[9] L. ksbrink, Chem. Phys. Lett., 7 (1970) 549. [IO] A. Niehaus and M.W. Ruf, Chem. Phys. Lett., ll(l971) 55. [ 1 I] S. Southworth, C.M. Truesdale, P.H. Kobrin, D.W. Lindle,

W.D. Brewer and D.A. Shirley, J. Chem. Phys., 76 (1982) 143.

[14] K. Mtlller-Dethlefs and E.W. Schlag, Annu. Rev. Phys. Chem., 42 (1991) 109.

[12] K. Miiller-Dethlefs, M. Sander and E.W. Schlag. Z. Naturforsch. Teil A, 39 (1984) 1089.

[13] K. Miiller-Dethlefs, M. Sander and E.W. Schlag, Chem. Phys. Lett., 112 (1984) 291.

[15] T.G. Wright, G. Reiser and K. Mtiller-Dethlefs, Chem. Br., Feb. 1994, p. 128.

[16] I. Fischer, R. Lindner and K. Milller-Dethlefs, J. Chem. Sot. Faraday Trans., 90 (1994) 2425.

[17] K. Miiller-Dethlefs, 0. Dopfer and T.G. Wright, Chem. Rev., 94 (1994) 1845.

[ 181 K. Mtiller-Dethlefs. High resolution spectroscopy with photoelectrons, in I. Powis, T. Baer and C.-K. Ng (Eds.), High Resolution Laser Photoionization and Photoelectron Studies, Wiley, Chichester, UK, 1995, Chapter II, p. 21.

[19] K. Mtlller-Dethlefs, E.W. Schlag, E. Grant, K. Wang and B.V. McKay, Adv. Chem. Phys., 90 (1995) 1.

[20] I. Powis, T. Baer and C.-K. Ng (Eds.), High resolution Laser Photoionization and Photoelectron Studies, Wiley, Chichester, UK, 1995.

[21] E.R. Grant and M.G. White, Nature, 354 (1991) 249. [22] G. Reiser, W. Habenicht, K. Miiller-Dethlefs and E.W.

Schlag, Chem. Phys. Lett., 152 (1988) 119. [23] G. Reiser, D. Rieger, T.G. Wright, K. Miiller-Dethlefs and

E.W. Schlag, J. Phys. Chem., 97 (1993) 4335. (241 I. Fischer, A. Strobel, J. Staeker, G. Niedner-Schatteburg,

K. Miiller-Dethlefs and V.E. Bondybey, J. Chem. Phys., 96 (1992) 7171.

[25] T. Kitsopoulos, C.J. Chick, Y. Zhao and D.M. Neumark, J. Chem. Phys. 95 (1991) 1441.

[26] G. Gantefor, D.M. Cox and A. Kaldor, J. Chem. Phys., 93 (1990) 8395.

[27] G. Reiser, 0. Dopfer, R. Lindner, G. Henri, K. Miiller- Dcthlefs, E.W. Schlag and S.D. Colson, Chem. Phys. Lett., 181 (1991) 1.

[28] W. Habenicht, G. Reiser and K. Mtlller-Dethlefs, J. Chem. Phys.. 95 (1991) 4809.

[29] K. Mtiller-Dethlefs, J. Chem. Phys., 95 (1991) 4821. [30] W.B. Peatman, T.B. Borne and E.W. Schlag, Chem. Phys.

Lett., 3 (1969) 492. [31] E. Waterstradt, R. Jung, H.-J. Dietrich and K. Miiller-

Dethlefs, Rev. Sci. Instrum., 64 (1993) 3104. [32] T.G. Wright, E. Cordes, 0. Dopfer and K. Mtiller-

Dethlefs. J. Chem. Sot. Faraday Trans., 89 (1993) 1609.

[33] T.N. Kitsopoulos, I.M. Waller, J.G. Loeser and D.M. Neumark. Chem. Phys. Lett., 159 (1989) 300.

[34] I.M. Wailer, T.N. Kitsopulos and D.M. Neumark, J. Phys. Chem., 94 (1990) 2240.

[35] D.M. Neumark, Annu. Rev. Phys. Chem., 43 (1992) 153. R.B. Metz, S.E. Bradforth and D.M. Neumark. Adv. Chem. Phys., 82 (1992) 1.

[36] H. Sekiya, R. Lindner and K. Miiller-Dethlefs. Chem. Lett.. (1993) 458.

[37] R. Lindner, H. Sekiya and K. Miiller-Dethlefs, Angew. Chem.. 105 (1993) 1384; Angew. Chem. Int. Ed. Engl.. 32 (1993) 1364.

[38] R. Lindner, H.-H. Dietrich and K. Miiller-Dethlefs, Chem. Phys. Lett., 228 (1994) 417.

[39] R. Lindner, H. Sekiya and K. Mfiller-Dethlefs. J. Chem. Phys., to be submitted.

[40] J.H. Callomon, T.M. Dunn and I.M. Mills, Phil. Trans. R. Sot. London Ser. A, 259 (1966) 499.

[41] M. Oldani, R. Widmer, G. Grassi and A. Bauder, J. Mol. Struct., 190 (1988) 31.

[42] E. Riedle, T. Knittel, T. Weber and H.J. Neusser. J. Chem. Phys.. 91 (1989) 4555.

[43] E. Riedle and J. Pliva, Chem. Phys.. 152 (1991) 375. [44] H.A. Jahn and E. Teller, Proc. R. Sot. London Ser. A. 161

(1937) 220. [45] U. dpik and M.H.L. Pryce, Proc. R. Sot. London Ser. A.

238 (1956) 425. [46] I.B. Bersuker. The Jahn-Teller Effect and Vibronic Inter-

actions in Modern Chemistry, Plenum Press. New York, 1984.

[47] K. Raghavachari, R.C. Haddon. T.A. Miller and V.E. Bondybey, J. Chem. Phys., 79 (1983) 1387.

[48] T.A. Miller and V.E. Bondybey, Molecular Ions: Spectro- scopy, Structure and Chemistry. (North-Holland, Amster- dam. 1983.

[49] S.R. Long, J.T. Meek and J.P. Reilly, J. Chem. Phys., 79 (1983) 3206.

[50] R. Lindner. B. Beyl, H. Sekiya and K. Mtiller-Dethlefs. Angew. Chem., 105 (1993) 631; Angew. Chem. Int. Ed. Engl.. 32 (1993) 603.

[Sl] E.B. Wilson. Jr., Phys. Rev., 45 (1934) 706. [52] G. Herzberg, Electronic Spectra of Polyatomic Molecules,

Van Nostrand, New York, 1966. [53] H.C. Longuet-Higgins, U. epik, M.H.L. Pryce, F.R.S.

Sack and R.A. Sack, Proc. R. Sot. London Ser. A. 244 (1958) 1.

[54] H. Krause and H.J. Neusser, J. Chem. Phys., 97 (1992) 5923. [55] P. Hobza and R. Zahradnik, Intermolecular Complexes:

The Role of van der Waals Systems in Physical Chemistry and in the Biodisciplines, Elsevier, Amsterdam, 1988.

[56] M. Ito, J. Mol. Struct., 177 (1988) 173. [57] See, for example, the following and references cited therein:

M. Ito, in J.R. Durig (Ed.), Vibrational Spectra and Structure, Vol. 15, Elsevier, Amsterdam, 1986. M. Ito, T. Ebata and N. Mikami, Annu. Rev. Phys. Chem.. 39 (1988) 123 M. Ito, S. Yamamoto, T. Aoto and T. Ebata, J. Mol. Struct., 237 (1990) 105.

46 K. Miiller-DethlefsjJournal of Electron Spectroscopy and Related Phenomena 75 (1995) 35-46

M. Gerhards, B. Kimpfel, M. Pohl, M. Schmitt and K. Kleinermanns, J. Mol. Struct., 270 (1992) 301.

[58] T. Oka, Rev. Mod. Phys., 64 (1992) 1141. 1591 E. Hirota, Chem. Rev., 92 (1992) 141. [60] 0. Dopfer, G. Reiser, R. Lindner and K. Miiller-Dethlefs.

Ber. Bunsen-ges. Phys. Chem., 96 (1992) 1259. [61] 0. Dopfer, G. Reiser, K. Mtiller-Dethlefs, E.W. Schlag and

SD. Colson, J. Chem. Phys., 101 (1994) 974. [62] T.G. Wright, E. Cordes, 0. Dopfer and K. Miiller-

Dethlefs, J. Chem. Sot. Faraday Trans., 89 (1993) 1609.

[63] E. Cordes, 0. Dopfer, T.G. Wright and K. Miiller- Dethlefs, J. Phys. Chem., 97 (1993) 7471.

[64] 0. Dopfer, G. Lembach, T.G. Wright and K. Mtiller- Dethlefs, J. Chem. Phys., 98 (1993) 1933.

[65] T.G. Wright, 0. Dopfer, E. Cordes and K. Miiller- Dethlefs, J. Am. Chem. Sot., 116 (1994) 5880.

[66] M. Schtitz, T. Biirgi, S. Leutwyler, and T. Fischer, J. Chem. Phys., 98 (1993) 3763.

[67] M. Schutz, T. Btirgi and S. Leutwyler, J. Mol. Struct. (Theochem), 276 (1992) 117.

[68] D. Feller and M.W. Feyereisen, J. Comput. Chem., 14 (1993) 1027.

[69] P. Hobza, R. Burcl, V. Spirko, 0. Dopfer, K. Milller- Dethlefs and E.W. Schlag, J. Chem. Phys., 101 (1994) 990.

[70] R.J. Lipert and S.D. Colson, J. Phys. Chem., 93 (1989) 135.

[71] K. Fuke, H. Yoshiuchi, K. Kaya, Y. Achiba, K. Sato and K. Kimura, Chem. Phys. Lett., 108 (1984) 179.

[72] R.J. Lipert and S.D. Colson, J. Chem. Phys., 89 (1988) 4579.

[73] G. Lembach, Diplomarbeit, Technische Universitlt. Mtinchen, 1992. G. Lembach et al., to be published.

[74] For the hydrogen-bonded complexes Ph-MeOH, Ph- EtOH, PhhDME and Pha the following nomenclature is used for the six intermolcular vibrations: Ei for bends and the torsion (i = l-5) and a for the stretch in the Si state; ni for bends and the torsion (i = l-5) and g+ for the stretch in the ionic ground state.

[75] H. Abe, N. Mikami, M. Ito and Y. Udagawa, J. Phys. Chem., 86 (1982) 2567.

[76] D.M. Neumark, Act. Chem. Res., 26 (1993) 33.

Documents

Applications of ZEKE spectroscopy