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Chemical Physics Letters 428 (2006) 268–276

Characterization of violet emission from Rb optical pumping cellsused in laser-polarized xenon NMR experiments

Indrajit Saha, Panayiotis Nikolaou, Nicholas Whiting, Boyd M. Goodson *

113 Neckers Building, Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, IL 62901, USA

Received 1 July 2006; in final form 22 July 2006Available online 1 August 2006

Abstract

Visible emission from Rb optical pumping cells was characterized under a range of conditions relevant to the production of laser-polarized xenon (including temperature, partial pressures, and D1-resonant 795 nm laser power). Bright 421 nm (6P! 5S) emissionwas consistent with energy-pooling processes of the type: Rb*(5P1/2) + Rb*(5P1/2,3/2)! Rb*(6P1/2,3/2) + Rb(5S1/2), with processes trans-iting through 5D states likely contributing at higher temperatures/lower N2 partial pressures. Under such conditions a number of Rblines may be observed, indicating population of Rb states to P9D (�31822 cm�1). Such energies exceed those required for efficient pro-duction of laser-induced plasma.� 2006 Elsevier B.V. All rights reserved.

1. Introduction

The sensitivity of conventional NMR is often limited bythe ordinarily low nuclear spin polarization achievable ineven the strongest magnets. Thus, methods for achievinghighly non-equilibrium spin polarization are often soughtto improve the NMR detection sensitivity while maintain-ing spectral information content. For example, opticalpumping (OP) may be employed to dramatically increasenoble gas polarization by transferring angular momentumfrom laser light to electronic and nuclear spins [1]. The cor-responding sensitivity enhancement, combined with theadvantageous properties of noble gases, has permitted avariety of applications across NMR and MRI [2,3]. In sup-port of such applications, a number of experimental alkali-metal spin-exchange (AMSE) OP setups have beendeveloped to produce quantities of a given noble gas iso-tope with some optimized combination of polarization,purity, efficiency, amount, and storage duration (e.g., Refs.[1,2,4–17]). Naturally, studies of fundamental processes

0009-2614/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2006.07.074

* Corresponding author. Fax: +1 618 453 6408.E-mail address: bgoodson@chem.siu.edu (B.M. Goodson).

underlying OP under such conditions have played key rolesin the progression of many applications.

Here we report the details of a modular xenon OP appa-ratus used for enhanced liquids-NMR experiments in ourlaboratory [18]. In the context of these experiments, the vis-ible-light emission from Rb cells was investigated under arange of conditions relevant to AMSE OP [19]. The appa-ratus’ open design permitted the observation of brightviolet emission (�421 nm, 6P! 5S) consistent withenergy-pooling processes [20] of the type: Rb*(5P1/2) +Rb*(5P1/2,3/2)! Rb*(6P1/2,3/2) + Rb(5S1/2), similar toeffects explored by Walker and co-workers [21,22] in thestudy of two-photon collision phenomena in ultracold Rbvapors. Processes transiting through 5D states likely con-tribute at high temperatures/low N2 partial pressures.Under such conditions a number of spectral lines wereobserved – all assignable to neutral, monotomic Rb [23] –indicating Rb* population up to P9D (�31220 cm�1).Such energies exceed those required for production oflaser-induced plasma [24] via a number of possible ioniza-tion pathways [20,24,25]; the observation of an apparentwhite-light baseline is consistent with plasma formation[24]. To our knowledge, this is the first investigation of suchprocesses under a range of conditions relevant to the

I. Saha et al. / Chemical Physics Letters 428 (2006) 268–276 269

production of laser-polarized noble gases, including: reso-nant Rb D1 excitation; significant buffer gas pressures(�0–3 bar Xe, N2, and/or He), moderate cell temperatures(�300–430 K), and high laser power (�0–33 W). Finally,potential relevance of these results for different OP/NMRapproaches is discussed.

1 Indeed, one of us (B.M.G.) was first introduced to the effect nearly 10years ago by Y.-Q. Song and R.E. Taylor during Ph.D. work in the lab ofAlex Pines at U.C. Berkeley (although poorly understood at the time, theobservation of violet emission from the cell was anecdotally correlatedwith poor Xe polarization).

2. Background: Rb energy pooling and ionization processes

While various absorption and emission processes ofalkali metals have been characterized almost since thedawn of spectroscopy [23], so-called energy pooling pro-cesses have been studied only relatively recently (e.g., Refs.[20–22,26]). Briefly, energy pooling involves the through-space, non-radiative transfer of energy between two excitedatoms (or molecules) on a collisional trajectory resulting inone species residing in a higher excited electronic state, andleaving the other in the ground state. Not to be confusedwith true two-photon excitation, such energy-pooling phe-nomena in alkali metal vapors are akin to annihilation pro-cesses well-known in the photochemical community, suchas the recent observation of efficient upconversion duringbimolecular triplet–triplet annihilation in anthracene solu-tions [27].

Collisional energy-pooling processes between two alkaliatoms may be written as

X�ðnLJ Þ þX�ðn0L0J 0 Þ ! X�ðn00L00J 00 Þ þXðn0S1=2Þ � jDEj;ð1Þ

where X denotes the alkali metal, the asterisk indicates anexcited state, DE is the energy defect, and n, L, and J havetheir usual meanings. Naturally, such processes are mostefficient when jDEj is small (e.g., less than the thermal en-ergy) and/or exoergic, and the X�ðnLJ Þ= X�ðn0L0J 0 Þ statesare well-populated. Thus, for X = Rb and resonant D1

(�794.7 nm) laser excitation, processes likely to be centralto the production of �421 nm (6P! 5S) violet light are

Rb�ð5P1=2Þ þRb�ð5P1=2Þ ! Rb�ð6P1=2;3=2ÞþRbð5S1=2Þ þ 1400 cm�1; ð2aÞ

Rb�ð5P1=2Þ þRb�ð5P3=2Þ ! Rb�ð6P1=2;3=2ÞþRbð5S1=2Þ þ 1640 cm�1; ð2bÞ

where the 5P1/2 and 5P3/2 states are respectively populatedby D1 excitation and thermal collisions [DE(5P3/2; 5P1/2) =238 cm�1, �340 K], and the DE values neglect fine-structure energies for the RHS terms [23]. Rb* atoms in6P1/2,3/2 states may then radiatively decay at 421.6 and420.3 nm, respectively. Endoergic processes that may berelevant at higher temperatures include:

Rb�ð5P1=2Þ þRb�ð5P3=2Þ ! Rb�ð5DÞ þRbð5S1=2Þ � 307 cm�1;

ð3aÞRb�ð5P3=2Þ þRb�ð5P3=2Þ ! Rb�ð5DÞ þRbð5S1=2Þ � 69 cm�1;

ð3bÞ

Rb�ð5P3=2Þ þRb�ð5P3=2Þ ! Rb�ð7SÞ þRbð5S1=2Þ� 678 cm�1; ð4Þ

where 5D and 7S populations may radiatively decay to the5P1/2,3/2 states at �762/776 nm and 728/741 nm, respec-tively; alternatively, population may transit through the5D and 7S states to reach the 6P manifold by releasing�1950 and �2550 cm�1 of energy, respectively.

Facile participation of alkali vapors in energy-poolingprocesses, combined with high D-line absorption cross-sec-tions and low electronic binding energies, enables the pro-duction of laser-induced plasma at relatively low photonenergies and laser powers [24]. Potentially relevant ioniza-tion processes for D1 excitation include: ion-pair forma-tion; multi-body association; Penning ionization; andphoto-ionization (Eqs. (5)–(10), respectively) [20,23–25]:

Rb�ð8P1=2Þ þRb! Rbþ þRb�; ð5ÞRb�ð6D3=2Þ þRb! Rbþ2 þ e�; ð6ÞRb�ð6P1=2Þ þRb2 ! Rbþ3 þ e�; ð7ÞRb�ð6S1=2Þ þRbþRb! Rbþ3 þ e�; ð8ÞRb�ð6P1=2Þ þRb�ð5P1=2Þ ! Rbþ þRbð5S1=2Þ þ e�; ð9ÞRb�ð6P1=2Þ þ hmð795 nmÞ ! Rbþ þ e�; ð10Þ

where the lowest-energy (threshold) states for each processare given (note that direct ionization of ground-state Rb,Rb2, and Rb3 requires 33691, 31917, and 26260 cm�1,respectively [24]).

Some recent studies of Rb electronic, dimerization, andclustering phenomena have involved energy-pooling and/or ionization processes described above. For example,Walker and co-workers [21,22] monitored violet light pro-duction following energy pooling (Eq. (2a)) in the study ofcollision processes in ultracold Rb vapors simultaneouslyexcited by two (quasi-D1-resonant) low-power lasers. Moc-hizuki et al. performed time-resolved studies of dimeriza-tion, clustering, and microcrystal formation in high-density Rb vapors [28]. Most recently, Ban et al. measuredspectral emission associated with Rb energy-pooling pro-cesses and Rb2 dimers when pumping dense Rb vaporsusing low-power, narrow-band diode lasers tuned to theD2 line (�780 nm, 40 mW) or the 5S! 6P transitions(�420 nm, 16 mW) to probe collisional processes at hightemperatures (�500–700 K) and low buffer gas pressures(�10–100 Torr) [26]. While D1 excitation of Rb vaporswith high-power lasers is central to the production oflaser-polarized noble gases, to our knowledge only anec-dotal reports exist for the observation of visible emissionfrom Rb OP cells in this context1. Indeed, from informaldiscussions with others in the noble gas OP/NMR commu-nity, we have found that many are either unaware of such

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phenomena (likely due either to the closed design of theirapparatus or the corresponding experimental conditions –see below), or have observed visible emission during noblegas polarization without knowledge of its origin. We setout to characterize the visible emission from Rb OP cellsunder conditions relevant for AMSE OP in order to obtainbetter understanding of the phenomena, and to evaluateany potential implications (and applications) in the contextof preparing laser-polarized gases.

3. Experimental methods

Our OP apparatus ((Fig. 1) includes aspects of differentliterature designs [4,8,10,17,29]; modular and relativelyinexpensive, the apparatus can run in either batch [7] or‘stopped-flow’ [8,10] modes. AMSE OP is performedwithin a Pyrex cell (Rosen design [10]); the 75 cc inner cyl-inder contains the gas mixture, whereas the outer cylinderis connected to a heated air source and serves as an ovenfor the inner cell. After cleaning [10], coating (Surfasil,Pierce [30]), Rb-loading, and evacuation (P8 · 10�6 Torr),the cell is heated to distribute a thin Rb film and placedwithin a Helmholtz coil pair (Walker Magnetics, typicalfield: 30 G; 2200 i.d.). OP is performed using a 40 W fiber-coupled laser diode array (LDA, Coherent) tuned to theRb D1 line (FWHM � 2.2 nm). All gases pass throughO2 filters mounted in a high-vacuum manifold, and pres-sures are measured with gauges upon loading. For simplic-ity, binary (Xe/N2) gas mixtures are used when polarizingXe (�300 Torr/run); �1750 Torr N2 is added to quenchRb fluorescence and to increase the laser absorption effi-ciency via collisional broadening of the Rb D1 line.

Following OP (10–20 min) the cell is cooled and the gasmixture is pumped through a liquid-N2-cooled condensercoil (Fig. 1b), removing the N2 gas [4,8,10,17]. The highpolarization of the frozen Xe is maintained [29,31] by a pairof 400 magnets (Indigo Inst.) producing P1500 G through-out the storage region. Virtually all (±5%) of the Xe is cap-tured by the condenser; similarly, the strong field [31],combined with avoidance of ‘warm solid xenon’ (129XeT1 is 610 s near the triple point [29]), minimizes polariza-tion losses during freezing/sublimation to within the typicalrun-to-run OP variability (ca. 20%). The efficient designallows for cost-effective use of enriched 129Xe, and multipleOP cycles can be used to prepare larger Xe quantities [10];the condenser geometry also permits multiple samples to bestudied using the same Xe batch. Typical 129Xe polariza-tion values are �5–10% (Fig. 2), but this value is expectedto improve when the current LDA is replaced by a 40 Wvolume holographic grating (VBG)-narrowed LDA(FWHM: 60.3 nm [32]). UV/visible and near-IR emissionis monitored with USB2000 and HR2000 Ocean Opticsspectrometers with fiberoptic probes; unless stated other-wise, optical integration times were 100 ms, and spectrawere baseline- and dark-current corrected. 129Xe NMRspectra were recorded at 110.6 MHz using a Varian Inovaspectrometer.

4. Results and discussion

When illuminated by a high-power �795 nm laser andloaded only with Xe gas, a Rb OP cell can emit bright vio-let light (Fig. 3a). This striking phenomenon results fromradiative decay of Rb*6P levels, and is consistent with‘quasi-two-photon’ energy-pooling processes betweenRb*5P atoms (see below). It is well known that the presenceof high Rb* densities can be detrimental to noble gas polar-ization: Decay of Rb* via radiative emission, with concom-itant re-absorption of the resonant, unpolarized (andomnidirectional) photons by optically dense Rb vapors(i.e., ‘radiation trapping’), can result in dramaticallyreduced Rb electronic spin polarization – and hence, corre-spondingly reduced nuclear spin polarization of the noblegas [1,4]. As such intense violet emission reflects the pres-ence of high [Rb*] and extensive radiation-trapping,reduced Xe polarization is expected under such conditions(Fig. 3b). Thus, it has become common practice to add N2

gas to the cell [1,4] (Fig. 3c). The high density of statesoffered by the internal (rovibrational) degrees of freedomof N2 enable it to non-radiatively de-excite Rb* species dur-ing collisions, thereby efficiently quenching radiation trap-ping. Indeed, the addition of 100 Torr of N2 results in adramatic reduction (but not complete elimination) of visi-ble emission from the cell, along with a �10-fold improve-ment in 129Xe polarization (Fig. 3c).

While the 6P3/2,1/2! 5S1/2 transitions give rise to thebright violet emission, under our conditions a simple spec-trometer may be used to record a number of different lines– all of which can be assigned to neutral, monotomic Rb.Fig. 4a shows the temperature dependence of the opticalemission from a Rb-loaded cell containing 300 Torr Xe.In addition to the principal 5P1/2! 5S1/2 emission at795 nm (obscured by laser scatter), at lower temperatures(<100 �C) the spectra are dominated by the neighboring5P3/2! 5S1/2 line [14] at �780 nm and the violet lines(�421 nm). At higher temperatures these lines grow signif-icantly in correspondence with the increase in [Rb*], and arejoined by a number of other transitions resulting from thepopulation of Rb* electronic states possessing increasinglyhigher energies; the most prominent of these lines is at�762 nm resulting from 5D! 5P1/2 transitions (the�776 nm 5D! 5P3/2 line is obscured by the 780 nm signal)[23]. Assignment of the more significant transitions is shownin Fig. 4b, with lines at �741/728 nm, �631/621 nm, �573/566 nm, and �359 nm indicating population of Rb*7S, 6D,7D, and 7P manifolds, respectively [23]. At high tempera-tures and/or with longer integration times (e.g., Fig. 4(a,inset)), additional lines at �617/608, �559, �544/537,�540/533, and �527/520 nm may be observed, respectivelyassigned to 8S! 5P3/2,1/2, 9S! 5P1/2, 8D! 5P3/2,1/2,10S! 5P3/2,1/2, and 9D! 5P3/2,1/2 transitions; under suchconditions, somewhat broader (and weaker) peaks may alsobe discerned at �524/517, �510, and �502 nm, likely orig-inating from overlapping 11S! 5P3/2,1/2, 10D! 5P3/2,1/2,and 11D! 5P3/2,1/2 transitions [23].

Fig. 1. (a) OP apparatus schematic. The k/4 box (Coherent) contains optics used to circularly-polarize the laser light before entering the Rosen cell [10] forOP. The cell temperature is measured using a digital thermometer calibrated with a thermocouple mounted to the exterior wall of the inner cylinder (notshown). Key aspects of the Xe cryostorage region are shown in (b): Following OP, the bottom portion of the Pyrex condenser coil is immersed in liquid N2.The gas contents of the OP cell are expanded into the condenser region, allowing solid xenon to be collected while the non-condensable buffer gas ispumped away. The liquid N2 level is raised for each subsequent run [17]; this procedure, combined with rapid Xe sublimation prior to transfer, helpsminimize polarization loss during freezing/thawing cycles.

I. Saha et al. / Chemical Physics Letters 428 (2006) 268–276 271

Fig. 2. Examples of 129Xe NMR signals from thermally-polarized (a) and laser-polarized xenon gas (b) (�100 Torr Xe). The signal in (b) was acquiredwith a single pulse (tipping angle: a = 4.3�); the emissive phase is the result of the laser light’s helicity. The thermal signal in (a) was obtained with 260 scansfollowing (paramagnetic) O2 addition (a = 90�, recycle delay = 500 s). The enhancement is 8600 ± 1200, corresponding to a 129Xe polarization of7.7 ± 1.1%. The inset shows the relative signal strength from laser-polarized Xe samples with varying cell temperature, showing the useful range for theapparatus.

272 I. Saha et al. / Chemical Physics Letters 428 (2006) 268–276

As with the 6P levels, the population of a given high-energy Rb* state is expected to be governed by a complexinterplay (‘cascade’) of various collisional, energy-pooling,and decay processes. Thus, increased emission from thesehigher-lying states likely reflects the greater Rb densities,laser absorption, and collision rates expected with elevatedtemperatures. Yet wide-line Rb2 dimer emission (e.g., at�600–605 and �680–720 nm from 23Pg ! 1ðaÞ3Rþu and1ðBÞ1Pu ! 1ðXÞ1Rþg transitions [26,28]) was not observedunder any conditions studied. The absence of dimer signa-tures likely reflects the relatively mild temperaturesemployed compared to previous studies [26,28], as well asthe simplicity and low photon energies of our experimentalsetup.

Despite the lack of direct knowledge of local Rb densi-ties or internal gas temperatures (which may vary widelyfrom expectation and from externally-measured values,e.g., Ref. [33]), it is still possible to glean considerable infor-mation regarding the present phenomena using the rathersimple experimental setup reported here. For example,Fig. 5a shows the dependence of violet emission on thetotal incident laser power. The raw dependence is highlynon-linear; however, this behavior is an artifact of the

oscillating LDA linewidth with increasing driving current(and hence variable fractional power at the D1 line). Whenthe same data is plotted as a function of the relative outputintensity at 794.7 nm, good agreement with a quadratic fitis obtained (Fig. 5b) – consistent with excitation processesinvolving two near-IR photons.

While the intense laser scatter prevented effective mea-surement of �795 nm emission, Fig. 5c shows the temper-ature dependence of the normalized 421 nm emissionintensity compared to that observed at 762 nm and780 nm – respectively reflecting Rb* 6P3/2,1/2, 5D, and5P3/2 population trends. As expected the 780 nm line risesfirst (I780), reflecting population of the 5P3/2level fromRb* 5P1/2 atoms via thermal collisions [14]. The 421 nmemission (I421), however, shows good agreement with the(I780)2 curve. Indeed, except at the highest temperatures(where the high Rb density causes most of the laser lightto be absorbed at the front of the cell – resulting in reducedemission at the positions of the fiber optic probes), a plot ofthe 421 nm intensity vs. the 780 nm signal is well-repro-duced by a quadratic fit (Fig. 5(c inset)). A true resonanttwo-photon process (e.g. 5S! 5D at �778 nm [34]) shouldnot have a quadratic [Rb*] dependence. Assuming the

Fig. 3. (a) Unfiltered photograph of the OP cell exhibiting bright violetemission consistent with energy-pooling processes (300 Torr Xe; 140 �C;�33 W incident laser power entering from the left). (b,c) Effect of buffergas change on visible emission and Xe polarization. (b) OP is performedunder the same conditions as (a); (c), as in (b), but with 100 Torr of N2 gasadded, showing significantly reduced (but non-zero) visible emission.Corresponding 129Xe NMR spectra in (b) and (c) were obtained witha = 70� and a = 7� observation pulses, respectively; the signals correspondto 129Xe polarization enhancements of �200 and >2000, respectively (therather low polarization for the latter – relative to Fig. 2 – is primarily theresult of inefficient absorption of the broad-band laser due to low cell gaspressure and correspondingly reduced collisional broadening of the Rbline compared to normal OP conditions).

Fig. 4. (a) Temperature dependence of emission lines from a Rb-loadedOP cell with 300 Torr Xe gas and �33 W incident laser power (10 msintegration time). At 95 �C, only the 421, 780, and �795 nm lines areclearly visible (the 795 line is convoluted with the laser scatter); at highertemperatures, a number of other lines become apparent. The inset showsthe signal obtained from 590 to 640 nm with a longer integration time (1 s)under similar conditions (141 �C, 300 Torr Xe, �10 Torr N2). (b) GrotianRb energy-level diagram, showing relevant atomic levels, transitions, andassignments for the more prominent lines in (a). Dotted horizontal linescorrespond to energies required for various potential ionization processes;the dashed line indicates the average energy possessed by two 5P Rb*

atoms (after [26]).

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bimolecular processes in Eqs. (2a) and (2b), it would beexpected that

½Rb�6P3=2;1=2� / ½Rb�5P3=2;1=2�2; ð11Þ

since [Rb* 5P3/2] should be proportional to [Rb* 5P1/2]. Thus,since I421 � [Rb* 6P3/2,1/2] and I780 � [Rb* 5P3/2] (and [Rb*

5P1/2]), the data in Fig. 5c are consistent with energy-poolingphenomena described by Eqs. (2a) and (2b) – althoughEq. (2a) would be expected to dominate under our condi-tions. A quadratic dependence would also be consistent withEqs. (3–4). Indeed, the 5P3/2 population becomes sufficient

Fig. 5. (a) Dependence of �421 nm emission from a Rb OP cell (300 Torr Xe, 126 �C) on total laser power. (b) Same data as (a), but plotted as a functionof relative laser intensity at 794.7 nm (measured separately from laser emission spectra taken at each power); the corrected data are well-reproduced by aquadratic fit (curve). (c) As in (a), but with fixed laser power (33 W) and variable temperature, showing the dependences of the relative (normalized)intensities of the 421, 780, and 762 nm lines. Points obtained by squaring the 780 nm data are shown for comparison. The inset shows the dependence ofthe 421 nm data upon the 780 nm emission intensity; the curve is a quadratic fit (open squares, not fit, are for data taken at temperatures at which the cell’semission had reached the saturation regime; similar analysis of the 762 line is hampered by the presence of a large shoulder contribution from the �780line, which had to be manually subtracted). (d) Dependence of the normalized emission signals at 421 and 762 nm on the Xe partial pressure. Datareported for 550 nm represent the baseline behavior; linear fits exclude the first point. (e,f) Effects of He and N2 addition: (e) As in (a), but with fixed(33 W) laser power and with �1750 Torr He added, showing much steeper growth with temperature of the 421 nm and baseline signals with the addition ofHe. The inset shows a comparison of the absolute baseline signals obtained with and without (as in (d)) the He added. (f) as in (c), but with fixedtemperature (140 �C) and varying N2 pressure in addition to 300 Torr Xe. Unless stated otherwise, the lines in (a–f) are meant only to guide the eye.

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at high temperatures to permit energy-pooling processes thattransit through the 5D manifold (Eqs. (3a) and (3b)), withthe dramatically increased 5D population manifested bythe late temperature-onset of the 762 nm line; both Eq. (3)processes are probably significant, as the larger endoergicityof Eq. (3a) should be mitigated by the increased concentra-tion of Rb* 5P1/2 vs. 5P3/2. On the other hand, the relativelylow emission from the 741/728 nm lines suggests thatprocesses transiting through 7S (Eq. (4)) are not significantunder our conditions, reflecting the large endoergicity com-pared to the available thermal energy.

Fig. 5d–f show the effects of increased buffer gas pres-sures. Increasing Xe pressure (Fig. 5d) resulted in a corre-sponding rise in the intensities of the 6P and 5D lines.Qualitatively, this observation at least partially resultsfrom increased pressure-broadening of the narrow Rb D1

line [6], which would give more efficient absorption of thebroad-band LDA output (and hence, greater [Rb*]). How-ever, after an initial steep rise the dependence becomesnearly linear for both lines up to 300 Torr Xe. Pressure-broadening of the Rb D1 absorption line is linearly depen-dent upon the buffer gas pressure (�19 GHz/amagat for Xe

I. Saha et al. / Chemical Physics Letters 428 (2006) 268–276 275

[6]), and thus a quadratic dependence would be expectedfor a linear increase in resonant laser absorption. Addition-ally, while the�421 nm doublet is poorly resolved, the 6P3/2/6P1/2 ratio appears to grow steadily with Xe pressure but isunaffected by temperature (data not shown). These obser-vations are not yet understood.

The addition of He gas gives qualitatively similar resultsas observed for Xe. The increased pressure yields corre-spondingly brighter 421 nm emission; the temperaturedependence of the normalized signal from a cell containing300 Torr Xe and 1750 Torr He (Fig. 5e) follows a similarprofile as that observed with Xe alone (albeit with a muchsteeper rise). However, N2 gas has the opposite effect on allemission lines (Fig. 5f). The addition of 50 Torr N2 cuts the421 nm emission in half, and 200 Torr N2 cuts all emissionsnearly completely. This result is expected since N2 effi-ciently de-excites Rb* species [1,4], drastically reducing[Rb*] and effectively quenching the collisional/energy pool-ing processes that populate higher-lying electronic states.Thus, violet emission is a sensitive indicator of the presenceof significant [Rb*] densities and radiation-trapping phe-nomena that can be detrimental to the production oflaser-polarized gases.

As shown in Fig. 4b, the energies of many Rb* statespopulated under examined conditions exceed thoserequired for various ionization processes (e.g., Eqs. (5)–(10)). In addition to the Rb* spectral lines, a variable(‘white-light’) baseline was observed in the spectra; thisbaseline was not the result of the spectrometer’s dark cur-rent (or residual LDA output). Instead, the baseline inten-sity qualitatively tracked that of the 421 nm emission overvarious experiments, as measured from the middle of thevisible region (e.g. �550 nm, away from specific Rb* lines;see Fig. 5d–f). The observation of a white-light baselinewould be consistent with the formation of laser-inducedplasma [24]; indeed, plasma formation would also explainthe whitish glow observed from OP cells under some condi-tions (e.g., Fig. 3c). However, it would be difficult to dis-cern the dominant ionization mechanisms based on thepresent data and indirect observations. For example, thefact that the baseline signal qualitatively follows the 6P/5D lines (e.g., Fig. 5d) would be consistent with polyatomicprocesses described by Eqs. (7) and (8) (based on availableenergies). The possible occurrence of such processes mayalso be supported by the steep temperature profile of thebaseline signal observed with 1750 Torr He added com-pared with 300 Torr Xe alone (Fig. 5(e inset)); Ban et al.observed that increasing buffer gas pressures (Ar, from 15to 60 Torr) can dramatically affect the formation of Rbmolecular species [26].

5. Concluding remarks

We have described a modular OP apparatus for generat-ing laser-polarized Xe for enhanced NMR experiments[18], and characterized visible Rb emission under a rangeof conditions relevant to AMSE OP. While the negative

effects of radiation trapping for generating high spin polar-ization are well established [1,4] we expect that the 421 nmemission is not, in itself, particularly detrimental to OP; theenergy-pooling processes ultimately produce half as manyre-absorbable photons as Rb*5P emission, and [Rb*6P]�[Rb*5P]. Instead, violet emission provides a sensitive, back-ground-free indicator of radiation-trapping processes, andcould prove useful when optimizing the OP gas mixture(e.g., the N2 fraction). Additionally, we have found thebright violet emission (easily observed through near-IRlaser goggles) to be helpful for optimizing the cell excita-tion geometry. Furthermore, because of the sensitive powerdependence, measurement of 421 nm emission could aidoptimization of the laser’s D1 resonance condition; thisapplication could be particularly helpful when using next-generation LDA sources narrowed either by external-cavity [35] or VBG [32] methods – where linewidths couldapproach that of the collision-broadened (and shifted [6])Rb D1 absorption line. On the other hand, the presenceof laser-induced plasma is unlikely to be helpful for noblegas polarization. If formed, such plasmas could damageorganic cell coatings or seals over time, or lead to otherunwanted consequences (indeed, on occasion we haveobserved that cells used for extended periods under condi-tions favoring visible emission would often yield poorer Xepolarization in subsequent runs, although this effect wasnot investigated in detail). In any case, the results presentedhere could be especially relevant when performing OPunder demanding conditions provided by high laser powers[12,17] (particularly with narrow linewidths), reduced N2

gas mixtures (to ease Xe cryotransfer [7] or improve Xe sig-nal-to-concentration ratio [14]), or when performing OP athigh internal cell temperatures [33] and gas pressures.These possibilities await further study.

Acknowledgements

We thank Matthew Rosen (HSCfA) for providing de-sign assistance regarding the optical pumping cell, andKen Owens (UMSL) for his expert glassblowing and ad-vice. We also thank Nick Kuzma (Princeton), Mike Barlow(Nottingham), Mark Conradi and Jason Leawoods(WUSTL), and the Bill Hersman group (New Hampshire)for helpful discussions, and Xiaoxia Li, Matt McCarroll,Kaushik Balakrishnan, and Zeyad Al-Talla (SIUC) forinstrument assistance. B.M.G. is a Cottrell Scholar of Re-search Corporation. Work at SIUC is supported by NSF(CHE-03492550), the ACS PRF, Research Corp., andSIU MTC.

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