A vacuum ultraviolet laser photoionization and pulsed field ionization studyof nascent S(3P2,1,0) and S(1D2) formed in the 193.3nm photodissociationof CS2Jingang Zhou, Brant Jones, Xueliang Yang, W. M. Jackson, and C. Y. Ng Citation: J. Chem. Phys. 128, 014305 (2008); doi: 10.1063/1.2816749 View online: http://dx.doi.org/10.1063/1.2816749 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v128/i1 Published by the AIP Publishing LLC. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

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A vacuum ultraviolet laser photoionization and pulsed field ionizationstudy of nascent S„3P2,1,0… and S„1D2… formed in the 193.3 nmphotodissociation of CS2

Jingang Zhou, Brant Jones, Xueliang Yang, W. M. Jackson,a� and C. Y. Ngb�

Department of Chemistry, University of California, Davis, California 95616, USA

�Received 27 August 2007; accepted 1 November 2007; published online 3 January 2008�

The photoionization efficiency �PIE� and pulsed field ionization–photoion �PFI-PI� spectra for sulfuratoms S�3P2,1,0� and S�1D2� resulting from the 193.3 nm photodissociation of CS2 have beenmeasured using tunable vacuum ultraviolet �vuv� laser radiation in the frequency range of82 750–83 570 cm−1. The PIE spectrum of S�3P2,1,0� near their ionization threshold exhibitssteplike structures. On the basis of the velocity-mapped ion-imaging measurements, four strongautoionizing peaks observed in the PIE measurement in this frequency range have been identified tooriginate from vuv excitation of S�1D2�. The PFI-PI measurement reveals over 120 previouslyunidentified new Rydberg lines. They have been assigned as Rydberg states �3p3�4S��nd3D� �n=17–64�� converging to the ground ionic state S+�4S�� formed by vuv excitations of S�3P2,1,0�. Theconverging limits of these Rydberg series have provided more accurate values, 82 985.43±0.05,83 162.94±0.05, and 83 559.04±0.05 cm−1 for the respective ionization energies of S�3P0�, S�3P1�,and S�3P2� to form S+�4S��. The relative intensities of the PFI-PI bands for S�3P0�, S�3P1�, andS�3P2� have been used to determine the branching ratios for these fine structure states,S�3P0� :S�3P1� :S�3P2�=1.00:1.54:3.55, produced by photodissociation of CS2 at 193.3 nm.© 2008 American Institute of Physics. �DOI: 10.1063/1.2816749�

I. INTRODUCTION

The relatively high abundance of sulfur �S� in the solaratmosphere has made the investigation of the ultraviolet �uv�and vacuum uv �vuv� spectroscopy of S important in themodeling of stellar atmospheres.1–4 The spectroscopic mea-surement of the Voyager-1 mission through Jupiter hasshown that the atomic sulfur ion �S+� is a dominant speciesin the torus plasma that surrounds Jupiter’s Io satellite.1,2

Being an open shell atom with relatively strong spin-orbitcoupling, the vuv photoionization studies of S atoms havealso served as a benchmark system for the understanding offundamental photoionization dynamics.5,6

The lowest electronic configuration for the neutral Satom is 3s23p4, which gives rise to the 3P2, 3P1, 3P0, 1D2,and 1S0 states with the relative energies in ascending order of0, 396.055±0.005, 573.640±0.016, 9238.609±0.005, and22 179.954±0.005 cm−1.7,8 The removal of a 3p electronfrom S atom results in the formation of the ground 4S�

3/2 andexcited 2D�

5/2,3/2 and 2P�3/2,1/2 states for S+. The

photoabsorption,8–10 photoionization efficiency �PIE�,11–13

and photoelectron14,15 spectra of S atoms in the vuv havebeen reported previously. Although the ionization energy�IE� of S atom was determined directly from the He I photo-electron measurement to be 10.36 eV,14,15 the most preciseIE values for S atom have been obtained by Rydberg seriesanalyses.15–17 The IE of S�3P2�, IE�S�3P2��, to formS+�4S�

3/2� has been recommended16 as 83 559.1±1.0 cm−1

based on the convergence limits of the four Rydberg series,�4S��ns 5S� �n=4–10�, �4S��ns 3S� �n=4–11�, �4S��nd 5D�

4�n=4–9�, and �4S��nf 5F �n=4–8�, observed in the previousvuv photoabsorption and photoionization studies.8–11 The re-cent uv laser �2+1� resonance-enhanced multiphoton ioniza-tion study17 has revealed the even-parity Rydberg states withthe S+�2D , 2P� ion cores, which are allowed by two-photonexcitation from the ground state. The fitting of these states tothe Rydberg formula has provided a value of83 560.8±0.1 cm−1 for the IE�S�3P2��.17

We have recently constructed a comprehensive tunablevuv laser system with a time-sliced velocity ion-imaging18,19

apparatus. The vuv laser has made the selective photoioniza-tion detection of nascent photofragments in different internalstates possible. In this report, we present the vuv laser PIEand pulsed field ionization–photoion19 �PFI-PI� or mass ana-lyzed threshold ion20 spectra of S�3P2,1,0� and S�1D2� neartheir IEs produced by the 193.3 nm photodissociation21–24 ofCS2,

CS2 + h��193.3 nm� → S�3P2,1,0� + CS �X,v = 0 – 13� ,

�1�

→S�1D2� + CS �X,v = 0 – 4� . �2�

By performing vuv laser PIE and PFI-PI measurements ofthe nascent photofragments, S�3P2,1,0�, S�1D2�, andCS�X 1��, we have not only gained insight about the photo-ionization spectroscopy for these transient species and theirions but also the energetics and dynamics for the neutralphotodissociation reactions �1� and �2�. An important infor-

a�Electronic mail: jackson@chem.ucdavis.edu.b�Electronic mail: cyng@chem.ucdavis.edu.

THE JOURNAL OF CHEMICAL PHYSICS 128, 014305 �2008�

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mation obtainable from this study is that the branching ratiosfor the fine structure states S�3P0� :S�3P1� :S�3P2� producedfrom the 193 nm photodissociation of CS2 can be unambigu-ously determined from the vuv laser PFI-PI bands for S�3P0�,S�3P1�, and S�3P2�. The high-n Rydberg states, with n up to64, resolved here in the PFI measurements, have allowed thedetermination of more accurate IE values for S�3P0�, S�3P1�,and S�3P2�. Equipped with a comprehensive tunable vuv la-ser system, one of our motivations for this work was to ex-amine the feasibility of performing Rydberg tagging time-of-flight �TOF� detection25 of nascent S�3P2,1,0� atoms. Theobservation and finding the vuv energy positions of the high-n Rydberg states originating from the S�3P2,1,0� atoms in thisstudy represents the first step for such an experiment.

II. EXPERIMENT

The PIE and PFI-PI measurements were performed us-ing the laser photodissociation time-sliced velocity-mappedion-imaging apparatus, which has recently been described indetail.18 The newly implemented tunable vuv laser systemwas similar to that described previously19,26 and was used forphotoexcitation and photoionization sampling of photofrag-ments produced in the uv laser photodissociation processes.

Figure 1 depicts the arrangement of the nonlinear mixingchamber for vuv generation, the vuv alignment aperture as-sembly, and the molecular beam ion-imaging apparatus. Tun-able vuv laser radiation was generated by nonlinear four-wave difference-frequency mixing schemes �2�1−�2� usinga Kr jet as the nonlinear medium.26 Here, �1 and �2 repre-sent the respective uv and visible laser fundamental frequen-

cies. The �1 and �2 frequencies were generated by two dyelasers �Lambda Physik, FL3002�, which were pumped by thesecond harmonic of an identical neodymium-doped yttriumaluminum garnet laser �Spectra Physics, model PRO-290�operated at 30 Hz. The Kr gas jet assembly was housed in anonlinear mixing chamber, which was evacuated by a100 l /s turbomolecular pump �TMP� �13�. A pulsed valve�General valve, 30 Hz, not shown here� was connected to themiddle entrance of a T-shape channel �diameter of 1 mm�,such that Kr from the pulsed valve entered the middle of theT-shape channel and exit symmetrically through the oppositeends of the perpendicular section of the channel. The axis ofthis section, which defines a 30 mm long gas channel �14�,was aligned along the propagating axis of the �1 and �2 laserbeams. The �1 and �2 laser beams were temporally compen-sated and merged by a dichroic mirror �19� before enteringthe nonlinear mixing chamber through a fused silica window�15� to pass through the Kr gas channel, where the vuv ra-diation was generated. The �1 and �2 laser beams were em-pirically focused at the Kr gas channel to yield the optimalvuv intensity. The vuv radiation generated, along with thefundamental �1 and �2 beams, intersected an off-axis bicon-vex MgF2 lens �11�, where the vuv beam of interest wasdispersed from the uv and visible beams due to the differentdeflection angles experienced by these laser beams at thesurfaces of the biconvex lens. The biconvex lens wasmounted on an aluminum flange, which allows the lens toslide off axis under vacuum conditions. By adjusting the po-sition of off axis of the biconvex lens, the deflection angleand thus the direction of the vuv beam can be varied. Thevuv beam emerging from the convex MgF2 lens was directedthrough two skimmer apertures �8� �diameter of �3 mm�prior to intersecting the CS2 molecular beam perpendicularlyat the photodissociation/photoionization �PD/PI� region �4�.These apertures are used to define the vuv laser beam and toeffectively block out the deflected fundamental �1 and �2

beams from entering the PD/PI region. In order for the vuvbeam to follow the path defined by the two skimmer aper-tures, which is perpendicular to the ion TOF axis of the ion-imaging apparatus, it is necessary to slightly bend the axis ofthe nonlinear mixing chamber from perpendicularity with re-spect to the ion TOF axis. This bending is made possible bythe use of the flexible bellow �10�. While the uv �1 fre-quency �obtained by tripling of a dye laser output� was fixedat 202.316 nm to coincide with the two-photon transition�4p�5�2P1/2�5p��1 /2��J=0�← �4p�6 1S0 at 98 855.1 cm−1

�=2�1�, the visible �2 frequency was tuned in the range of620–655 nm to generate the desired vuv laser energy rangebased on the four-wave difference-frequency mixing scheme.The frequency of the visible laser was continuously moni-tored by a wavemeter �Coherent, Wavemaster� during theexperiment. The vuv resolution achieved was shown to be0.45 cm−1 �full width at half maximum �FWHM�� based onexperimental linewidth measurements of autoionizing Ryd-berg levels. We find that this compact vuv generation andseparation arrangement offers a significantly higher vuvtransmission efficiency than that obtained using a vuv mono-

FIG. 1. �Color online� The arrangement of the nonlinear mixing chamber forvuv generation, the vuv alignment aperture assembly, and the molecularbeam ion-imaging apparatus. �1� Pulsed nozzle 1, �2� beam source chamber�to 500 l /s TMP�, �3� entrance port for uv photodissociation laser, �4� PD/PIregion, �5� dual skimmers for molecular beam, �6� 45° MgF2 window, �7�view port, crossed-beam chamber �to 500 l /s TMP�, �8� skimmer aperturesfor defining the vuv laser beam path, �9� Cu photoelectric detector, �10�bellow, �11� slide holder of biconvex focusing lens, �12� ion-imaging lenses,�13� to 100 l /s TMP, �14� gas channel �30 mm�, �15� entrance port forfundamental uv �1 and visible �2 laser beams, �16� ion TOF tube, �17� to500 l /s TMP, �18� imaging MCP detector, and �19� dichroic mirror.

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chromator; however, this setup is limited to vuv radiationwith wavelengths longer than �1050 Å, which is the cutoffwavelength of LiF.

After passing through the PD/PI region, the vuv beamexits the photoionization chamber through a MgF2 windowtilted at 45° �6�, where a small fraction of the vuv beam isreflected perpendicularly by the window surface onto a Cuphotoelectric detector �9�. The vuv signal observed by thisphotoelectric detector is useful for optimizing the variousparameters employed to generate the vuv radiation and fornormalizing the PIE and PFI-PI spectra.

A beam of CS2 seeded in Ar was produced by supersonicexpansion through a pulsed valve �Evan-Lavie pulsed valve,EL-5-2004, nozzle diameter of 0.2 mm� �1� operated at30 Hz at a total stagnation pressure of 50 psi. The partialnozzle stagnation pressure of CS2 was kept constant at�400 Torr, which was determined by the vapor pressure ofCS2 at 298 K. The narrow pulse width �20 �s� and highbeam intensity of the CS2 beam achieved by using the Evan-Lavie valve are essential for attaining the high signal-to-noise ratios for ion detection in the present study.

The photolysis 193 nm ArF excimer laser �LambdaPhysik, Complex 205� beam was linearly polarized along theCS2 beam and the TOF axis using a stack of quartz platesand was defined by two small irises �diameter of 2 mm�without focusing. The 193 nm photodissociation laser beamwas directed to counterpropagate along the direction of thevuv laser beam by entering the PD/PI region through the 45°tilted MgF2 window. Typically, the pulse energy of the193 nm laser beam used in this experiment was �5 �J asmeasured outside of the 45° tilted entrance window.

In the PIE and PFI-PI measurements, ions of differentmasses were resolved by TOF mass spectrometry over an ionflight path of 72.5 cm prior to detection by a microchannelplate �MCP� ion detector �active diameter of 7.5 cm� �18�. Toobtain the optimal S+ ion signal, the photolysis laser pulseentered the PD/PI region 120 ns before the vuv laser pulse. Alow pulse energy was used for the ArF laser so that noisefrom the formation of background ions for S+, CS+, andCS2

+ was negligible. The free of ion background for S+ de-tection was also confirmed by our ion-imaging measurementof S+ at various vuv wavelengths.24 Firing the vuv laser gaverise to a strong CS2

+ signal because the energies of the vuvphotons used were above the IE of CS2 at 10.078 eV. How-ever, no S+ signal due to vuv photoionization was observedwithout turning on the ArF photodissociation laser. At theoptimized experimental conditions, the S+ intensity wasfound to be stronger than that of CS2

+ at the autoionizingRydberg resonance of S observed at 83 512.89 cm−1.

In addition to the vuv-PIE and vuv-PFI spectra forS�3P2,1,0� and S�1D2� atoms measured in the vuv region of82 750–83 570 cm−1 presented in this report, we have alsoperformed extensive velocity-mapped ion-imaging measure-ment of S+ formed by vuv photoionization of these nascent Satomic photofragments in the wavelength region of74 232–84 930 cm−1.24 The results of the latter measure-ments will be published separately.

III. RESULTS AND DISCUSSION

As pointed out earlier, nascent S�3P2,1,0� and S�1D2� pre-pared by the respective photodissociation reactions �1� and�2� are subject to vuv laser PIE and PFI-PI measurements.The results of these measurements will be discussed in thesubsections below.

A. vuv-PIE spectrum of S„3P2,1,0… and S„1D2…

Figure 2 depicts the vuv-PIE spectrum for S�3P2,1,0� andS�1D2� in the frequency range of 82 750–83 570 cm−1 nearthe IE of S�3P2,1,0� obtained by setting a dc ion extractionfield F=100 V /cm in the PD/PI region. The spectrum exhib-its step-function behavior, along with sharp autoionizing fea-tures as expected for a typical vuv-PIE spectrum of atoms.

Four sharp autoionization peaks are observed in the PIEspectrum of S�3P2,1,0� at 83 201.62, 83 216.97, 83 381.39,and 83 512.89 cm−1 �marked as �a�, �b�, �c�, and �d�, respec-tively, in Fig. 2�. The FWHMs of these autoionizing peakswere found to fall in the range of 0.8–2.7 cm−1, which aregreater than the optical bandwidth of the vuv laser. We havetaken time-sliced velocity-mapped ion images of S+ by set-ting the vuv laser frequencies at these autoionizing peaks,and the ion images thus obtained are consistent with theconclusion that these autoionization peaks arise from Ryd-berg transitions originating from S�1D2� formed by reaction�2�. The ion images of S�1D2� formed by reaction �2� aredistinctly different from those of S�3P2,1,0� formed by reac-tion �1� as observed by setting the vuv laser in the frequencyrange of Fig. 2 �Refs. 23 and 24� but away from the fourautoionization peaks. The ion-imaging measurements showthat the four autoionizing Rydberg states are formed by vuvexcitation of S�1D2�. On the basis of the linewidths observed

FIG. 2. vuv-PIE spectra of S�3P2,1,0� and S�1D2� in the frequency range of82 750–83 570 cm−1. The dc field is F=100 V /cm. The Stark shifted IEvalues for S�3P0�, S�3P1�, and S�3P2� are marked by short dropped lines,whereas the IE values for S�3P0�, S�3P1�, and S�3P2� after correcting for theStark shift of �=63 cm−1 are marked by long dropped lines. Four PIE peaksmarked as a, b, c, and d are identified as autoionizing Rydberg states ofS�1D2� based on the ion-imaging measurements.

014305-3 S�3P� and S�1D� from CS2 photodissociation J. Chem. Phys. 128, 014305 �2008�

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for these autoionizing states, the autoionizing lifetimes areestimated to be �2.0–7.1 ps. The detailed analysis of theseautoionizing Rydberg states, along with other autoionizingRydberg lines from S�1D2� observed at vuv energies belowthe IE�S�3P0��, according to the Fano line shape formula27

will be reported in a separate24 publication.The PIE spectrum of Fig. 2 exhibits three sharp steps at

82 922.4, 83 099.9, and 83 496.0 cm−1, which can be as-signed as the respective thresholds for direct vuv photoion-ization of S�3P0�, S�3P1�, and S�3P2�. After taking into ac-count the Stark shift for the applied dc field �F=100 V /cm�, �= �6.3�F1/2 cm−1=63 cm−1, the IE values forS�3P0�, S�3P1�, and S�3P2� based on the vuv-PIE measure-ment are determined as 82 985.4, 83 162.9, and83 559.0 cm−1, respectively. These are in excellent agree-ment with the known IE�S�3P2�� �Refs. 16 and 17� and en-ergy intervals of the S�3P2,1,0� spin-orbit states.7,8

Considering that the degeneracy �2J+1� for S�3P0�,S�3P1�, and S�3P2� are 1, 3, and 5, respectively, the PIE stepratios for these spin-orbit states should be identical to thesedegeneracy ratios, provided that the photoionization crosssections for the spin-orbit states are identical and that thedegenerate mJ sublevels for a given J state are equally popu-lated in the photodissociation process. Without any perturba-tions of near resonance autoionizing states, it is a good as-sumption that the photoionization cross sections for finestructure states are proportional to their degeneracy factors.A well-known example is the relative photoionization crosssections of the rare gas atoms �Rg=Ar, Kr, and Xe� to formthe Rg+�2P3/2� and Rg+�2P1/2� ion states, which are experi-mentally shown to bear the ratio of Rg+�2P3/2� :Rg+�2P1/2�=2:1.28 The relative PIE step heights for the S�3P0�, S�3P1�,and S�3P2� ionization thresholds are measured as 1.0, 1.1,and 3.1, respectively, which are found to deviate from thestatistical predictions. In an ideal case, the ratios of the PIEstep heights are identical to the ratios of the intensities forthe photoelectron bands of S�3P0�, S�3P1�, and S�3P2� andthus can be used to determine the branching ratios forS�3P0�, S�3P1�, and S�3P2� populated by the photodissocia-tion reaction �1�. We note that a more reliable measure of therelative fine-structure-state populations of S atoms can beobtained from the measured PFI-PI bands for S�3P0�, S�3P1�,and S�3P2�, which will be presented in Sec. III C.

B. Combined vuv-PIE and vuv-PFI-PI spectrum ofS„3P2,1,0… and S„1D2…

Figure 3 shows the combined vuv-PIE and vuv-PFI-PIspectrum of S�3P2,1,0� and S�1D2� formed by reactions �1�and �2� measured in the frequency range of82 750–83 570 cm−1. This spectrum was obtained by apply-ing a positive pulse of 450 V /cm �pulse width of 10 �s�superimposed on a 1.5 V /cm dc field �see the inset in thelower panel of Fig. 3�. The vuv laser pulse was applied 80 nspreceding the rising edge of this electric field pulse. Here, the450 V /cm electric field pulse serves to produce PFI-PIs byPFI of high-n Rydberg states and to extract prompt ions andPFI-PIs toward the TOF mass spectrometer. Due to the use ofa small dc field of 1.5 V /cm, prompt ions and PFI-PIs can-

not be resolved by the TOF mass measurement. Thus, thespectrum that is obtained is a combined vuv-PIE and vuv-PFI-PI spectrum of S�3P2,1,0� and S�1D2�. As shown in Fig.3, the observed spectrum exhibits all the features of the vuv-PIE spectrum depicted in Fig. 2. In addition, the spectrumreveals three groups of Rydberg features, which can be as-signed as Rydberg series converging to the IEs of S�3P0�,S�3P1�, and S�3P2�. The Rydberg peaks are found to haveFWHMs of �0.8 cm−1. These three sets of Rydberg featurescan be ascribed as the PFI-PI bands of S�3P0�, S�3P1�, andS�3P2�.

The assignment of the Rydberg levels resolved in thePFI-PI spectrum will be discussed in Sec. II D. On the basisof such an assignment, the principal quantum numbers �n� ofthe observed Rydberg levels are marked on top of Fig. 3. Thespectrum of Fig. 3 reveals a sharp drop in Rydberg peakintensity for n�30, indicating a Stark shift of �128 cm−1.The latter value is estimated as the shift in energy of theRydberg state n=30 measured with respect to the corre-sponding converging limit or IE value. By increasing the PFIfield to F=952 V /cm �spectrum not shown�, the sharp dropin Rydberg peak intensity was observed for n�25, whichcorresponds to a Stark shift of �186 cm−1. Again, theseStark shifts observed are consistent with the predictions of134 and 194 cm−1 for the respective Stark electric fields ofF=450 and 952 V /cm based on the adiabatic Stark shift for-mular �= �6.3� ·F1/2 cm−1.

The highest Rydberg states observable that used the PFIscheme are in the range of n=62–64, which is limited by thevuv resolution and the separation electric field of 1.5 V /cm.This dc “separation” field has the effect of inducing Starkmixing between adjacent Rydberg levels of high n values.The increase of the separation electric field higher than1.5 V /cm was found to wash out the Rydberg peaks of high-

FIG. 3. Combined vuv-PIE and vuv-PFI-PI spectra of S�3P2,1,0� and S�1D2�measured in the frequency range of 82 750–83 570 cm−1. The electric fieldpulse used is shown in the inset. The vuv laser was turned on 80 ns preced-ing the rising edge of the electric field pulse. The assignments of the nvalues for the Rydberg peaks observed are marked on top of the spectrum.

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n Rydberg states. That is, the weak separation field used hereis designed to identify the high Rydberg states n=50–64.

On the basis of the Stark ionization consideration, weonly expected to observe Rydberg levels n�30. However,weak Rydberg peaks corresponding to n=17–29 Rydberglevels converging to the IE�S�3P2��, which are energeticallyinaccessible, are discernible in the magnified spectrumshown in the lower panel of Fig. 3. The observation of theselower Rydberg states can be attributed to collision-assistedionization processes involving excited S atoms in high-n Ry-dberg states and ambient neutral molecules in the PD/PIchamber.29 The collision-induced ionization cross section fora Rydberg state of a given n value is expected to be propor-tional to n4, whereas the lifetime of the Rydberg state isproportional to n3. Hence, the relative intensities for the Ry-dberg states should be proportional to n, which are in fairagreement with those observed in the PFI-PI spectra of Fig.4.

C. vuv-PFI-PI spectra of S„3P0…, S„3P1…, and S„3P2…

The vuv-PFI-PI spectra for S�3P0�, S�3P1�, and S�3P2�,together with the electric field pulse used for acquiring thespectra, are shown in Fig. 4. The PFI process is initiated bythe falling edge of an electric field pulse from342 to 10.5 V /cm applied to the PD/PI region. The electricfield was maintained at 10.5 V /cm for 2.5 �s and was thenraised back to 342 V /cm. The application of the vuv laserpulse is delayed by 150 ns with respect to this falling edge. Itis understood that a small decaying oscillating electric field,resulting from the falling of the electric field in the PD/PIregion, can serve as the scrambling field to induce l mixings,which have the effect of lengthening the lifetimes of excitedhigh-n Rydberg species initially populated by vuvexcitation.30,31 The delay of 150 ns is determined experimen-tally to obtain the highest PFI-PI signal for S atoms. The

electric field pulse scheme employed here provides a separa-tion field of 10.5 V /cm and a PFI field of 342 V /cm andallows a complete separation of prompt and PFI-PI ions asobserved in the TOF measurement. Thus, the PFI-PI spec-trum of Fig. 4 can be recorded by gating the mass peak forthe PFI-PIs of S+. The spectrum of Fig. 4 can be consideredto consist of three independent spectra associated with thePFI-PI measurements of individual S�3P0�, S�3P1�, andS�3P2� states. We note that the four autoionizing peaks ofS�1D� and the step functional feature of the PIE spectra ob-served in Figs. 2 and 3 are not evident in the spectrum of Fig.4. The observed PFI-PI spectra of S atoms are typical ofthose for atoms.11,15 Again, a sharp drop in Rydberg peakintensity is observed for n�32, corresponding to a Starkshift of 112 cm−1. The latter value is also in accordance withthe adiabatic Stark shift prediction of �= �6.3� ·F1/2

=116.5 cm−1.Although the ionization of lower Rydberg states n

=17–31 are energetically inaccessible, the observation of S+

ions associated with these states �see Fig. 4� can be partlyattributed to collision-assisted ionization processes29 due tocollisions between these excited S atoms in these Rydbergstates and ambient neutral species in the PD/PI region. Theintensities of these energetically inaccessible states comparedto those of the inaccessible states appeared in Fig. 3 areconsiderably higher. This can be rationalized by the fact thata scrambling field31 is applied in the measurement of thespectra of Fig. 4, such that the lifetimes for Rydberg levelsassociated with the PFI-PI spectra are generally longer thanthose associated with spectrum of Fig. 3.

Assuming that the vuv-PFI cross sections for S�3P0�,S�3P1�, and S�3P2� are identical, the relative populations ofthese S atomic states can be reliably deduced from the rela-tive intensities of the vuv-PFI-PI bands for S�3P0�, S�3P1�,and S�3P2�, which are estimated to be 1.00, 1.54, and 3.55,respectively. After taking into account the experimental un-certainties, these values are in fair agreement with those de-duced based on the relative step heights of 1.0, 1.1, and 3.7measured by the vuv-PIE spectrum of Fig. 2. The fine-structure-state distribution for S atoms formed by the photo-dissociation reaction �1� was measured previously using thevuv laser induced fluorescence �LIF� method, yielding theratio of S�3P0� :S�3P1� :S�3P2�=0.04:0.25:0.71 �or1.00:6.25:17.75�.22 Comparing the latter ratio with the ratiosfor the PFI-PI band intensities determined in the presentstudy, we conclude that our value for the population of thehighest excited spin-orbit state, S�3P0�, is considerablyhigher than that reported by Waller and Hepburn.22 The sig-nificantly higher populations of the lower spin-orbit statesobserved in the LIF study indicate that some relaxationsfrom the higher to the lower spin-orbit states might haveoccurred due to secondary collisions in the latter experiment.

The theoretical study on photodissociation by Singer etal.32 shows that the fine structure distribution of the atomicfragment of a photodissociation process may proceed in twolimiting cases, depending on the relative kinetic energy ofthe dissociating pair. At low recoil energies, the fine structuredistribution is governed by the adiabatic potential energy sur-faces. In this limit, symmetry correlation determines the fine

FIG. 4. vuv-PFI-PI bands for S�3P0�, S�3P1�, and S�3P2� in the frequencyrange of 82 750–83 570 cm−1. The inset illustrates the electric field pulseused for the PFI-PI measurement. The firing of the vuv laser was delayed by150 ns with respect to the falling edge of the electric field pulse.

014305-5 S�3P� and S�1D� from CS2 photodissociation J. Chem. Phys. 128, 014305 �2008�

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TABLE I. Energies �En’s� of Rydberg transitions for n=17–64 observed in the present vuv-PFI-PI measurement of S�3P0�, S�3P1�, and S�3P2� formed byreaction �1� and the assignment of these transitions as the Rydberg series 3p3 �4S��nd 3D� �n=17–62�←S�3P0� �S�3P1� and S�3P0��. Energies �En’s� ofRydberg transitions determined in the present PFI-PI measurement have error limits within ±0.05 cm−1. The quantum defect ��� values are calculated usingthe best fitted IE values obtained in the least square fits of En �n�30� values �see text�.

n

3p3�4S��nd 3D�← 3P2 3p3�4S��nd 3D�← 3P1 3p3�4S��nd 3D�← 3P0

Ena � En

a � En �

16¯ ¯ �82 698� ¯ ¯ ¯

17 0.64283 148.949 �82 754� ¯ ¯ ¯

18 0.65383 194.381 �82 798� ¯ ¯ ¯

19 83 232.607 0.665�83 231� �82 838� ¯ ¯ ¯

20 83 265.224 0.674 ¯ ¯ ¯

�83 266� �82 868�21 83 293.180 0.684

�83 294� �82 899� ¯ ¯ ¯

22 83 317.425 0.689�83 318� ¯ ¯ ¯ ¯

23 83 338.558 0.691 ¯ ¯ ¯ ¯

24 83 356.965 0.697 82 961.084 0.684 82 783.671 0.67825 83 373.093 0.707 82 977.338 0.684 82 799.7896 0.68626 83 387.456 0.711 82 991.627 0.691 82 814.085 0.69327 83 400.246 0.712 83 004.391 0.691 82 826.802 0.69828 83 411.638 0.715 83 015.726 0.697 82 838.198 0.69929 83 421.810 0.722 83 025.923 0.700 82 848.382 0.70330 83 431.040 0.720 83 035.110 0.700 82 857.550 0.70631 83 439.331 0.724 83 043.419 0.699 82 865.831 0.70932 83 446.864 0.723 83 050.910 0.702 82 873.339 0.71133 83 453.724 0.721 83 057.736 0.703 82 880.173 0.71134 83 459.934 0.725 83 063.984 0.699 82 886.385 0.71435 83 465.620 0.727 83 069.638 0.705 82 892.084 0.71336 83 470.865 0.723 83 074.877 0.699 82 897.296 0.71337 83 475.650 0.725 83 079.645 0.703 82 902.085 0.71438 83 480.071 0.723 83 084.027 0.709 82 906.500 0.71339 83 484.138 0.725 83 088.125 0.701 82 910.565 0.71440 83 487.892 0.728 83 091.855 0.709 82 914.320 0.71641 83 491.392 0.725 83 095.353 0.705 82 917.813 0.71442 83 494.505 0.765 83 098.600 0.700 82 921.065 0.70843 83 497.573 0.748 83 101.589 0.706 82 924.063 0.71244 83 500.345 0.762 83 104.401 0.702 82 926.870 0.71045 83 503.010 0.746 83 107.014 0.702 82 929.490 0.70846 83 505.517 0.721 83 109.462 0.700 82 931.929 0.70947 83 507.745 0.748 83 111.749 0.699 82 834.217 0.70948 83 509.971 0.711 83 113.881 0.703 82 936.360 0.70949 83 511.936 0.735 83 115.905 0.696 82 938.371 0.70950 83 513.820 0.740 83 117.780 0.704 82 940.260 0.70951 83 515.624 0.727 83 119.545 0.711 82 942.043 0.70652 83 517.252 0.757 83 121.246 0.695 82 943.720 0.70553 83 518.850 0.748 83 122.819 0.699 82 945.295 0.70854 83 520.330 0.759 83 124.306 0.702 82 946.790 0.70655 83 521.820 0.703 83 125.720 0.699 82 948.200 0.70656 83 523.168 0.693 83 127.055 0.698 82 949.530 0.70957 83 524.415 0.706 83 128.325 0.693 82 950.796 0.70858 83 525.650 0.674 83 129.512 0.701 82 951.992 0.71059 83 526.782 0.677 83 130.655 0.696 82 953.134 0.70660 83 527.850 0.687 83 131.736 0.694 82 954.211 0.70961 83 528.880 0.683 83 132.758 0.699 82 955.338 0.70862 83 529.860 0.678 83 133.738 0.695 82 956.209 0.71563 83 530.791 0.676 ¯ ¯ ¯ ¯

64 83 531.670 0.683 ¯ ¯ ¯ ¯

014305-6 Zhou et al. J. Chem. Phys. 128, 014305 �2008�

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structure distribution. At the high energy limit, a frame trans-formation would result in a statistical distribution. The TOFmeasurements of the CS�X ,v�+S�3P2,1,0� indicate that a sig-nificant fraction of CS�X� molecules are formed in highlyvibrationally excited states with v up to 13.21 Thus, we ex-pect that a major portion of the dissociating S+CS productpairs have low relative kinetic energies, giving rise to a non-statical distribution of the fine structure states of the S atoms.This expectation is consistent with the S�3P2,1,0� fine struc-ture distribution determined here based on the vuv laserPFI-PI measurement.

D. Assignment of Rydberg series converging to theIEs of S„3P0…, S„3P1…, and S„3P2…

The energy levels for the lowest Rydberg states,S�3p3�4S��nd�n=3–14��, can be found in the 1990 compila-tion of Martin et al.16 A few low-n Rydberg states associatedwith the Rydberg transitions, S�3p3�4S��nd�n=19–22��←S�3P2� and S�3p3�4S��nd�n=16–21��←S�3P1�, whichconverge to the ground S+�4S�� state have also been observedin a previous absorption study by Joshi and co-workers.9,10

These known Rydberg states, along with the energies of theRydberg levels �En, n=17–64� resolved in the vuv-PFI-PIspectra of Figs. 2 and 3, are listed in Table I. The En valuesdetermined here is limited to n�64 values. This is consistentwith the fact that the energy spacings between adjacent Ry-dberg states for n64 are comparable to or smaller than thevuv optical resolution. Taking into account the uncertainty�±1 pm or 0.025 cm−1� of the wavemeter used in the presentexperiment, the overall uncertainty for the energies of Ryd-berg levels determined in this PFI-PI measurement is be-lieved to be within ±0.05 cm−1.

In order to determine more precise IE values for S�3P0�,S�3P1�, and S�3P2�, we have performed least square fits ofthe observed En values according to the Rydberg formula,

En = IE − RS/�n − ��2. �3�

Here, � and RS represent the quantum defect and the Ryd-berg constant for sulfur atom. The RS value,109 735.4328 cm−1, is obtained by the relation RS=R / �1+me /M�, where R=109 737.3157 cm−1, M is the mass of Satom, and me is the rest mass of electron. We found that theleast square fit of all the observed energy levels to the Ryd-

berg equation does not produce good results as indicated bythe gradual increase in � values as n is increased from 17 to29. The observation is not surprising due to the valence-Rydberg interactions for low-n Rydberg states. As a conse-quence of the valence-Rydberg interactions, the IE value de-duced based on a least square fit of low-n En values to theRydberg equation is often known to result in a large error forthe IE value. Due to this observation, we have omitted the En

�n=17–29� values in the least square fitting for deriving theIEs of S�3P0�, S�3P1�, and S�3P2�. On the basis of the leastsquare fits of the experimental En �n=30–64� values forS�3P2�, En �n=30–62� values for S�3P1�, and En �n=30–62� for S�3P2� to Eq. �3�, we have determined the bestfitted �IE, �� values of �83 559.04±0.01, 0.725±0.024� forS�3P2�, �83 162.94±0.01, 0.702±0.004� for S�3P1�, and�82 985.43±0.01, 0.710±0.003� for S�3P0�. Although thestandard deviation for the IE values deduced by the leastsquare fits is ±0.01 cm−1, we have assigned an error limit of±0.05 cm−1 for the IE values for S�3P0�, S�3P1�, and S�3P2�.The recommended IE values for S�3P0��82 985.43±0.05 cm−1�, S�3P1� �83 162.94±0.05 cm−1�, andS�3P2� �83 559.04±0.05 cm−1� based on the present mea-surement indicate that the excited S�3P1� and S�3P0� statesare higher than the ground S�3P2� state by 396.11 and573.62 cm−1, respectively. These energy spacings are foundto be in excellent agreement with the values determined byprevious spectroscopic studies.7,8

Figure 5 shows the plot of the energy residual�En�expt�−En�fit�� as a function of n in the range of n=17–64 for the three Rydberg series, where En�expt� andEn�fit� represent the respective experimental and least squarefit values for En. The rapidly increasing trend of the energyresidual as n is lowered from 29 to 17 is evident in thisfigure. This plot also reveals the relatively large fluctuationsfor the energy residuals for n=42–64 of the S�3P2� series.This observation may be indicative of the perturbation ofthese Rydberg states by the nearby autoionizing state forS�1D� at 83 512.89 cm−1.

Using the best fitted IE values for S�3P0�, S�3P1�, andS�3P2�, we have calculated the quantum defects for indi-vidual Rydberg transitions, which are also given in Table I.Figure 6 depicts the plot of the quantum defect � vs n forn=17–64 of the three Rydberg series. In this range of Ryd-

TABLE I. �Continued.�

n

3p3�4S��nd 3D�← 3P2 3p3�4S��nd 3D�← 3P1 3p3�4S��nd 3D�← 3P0

Ena � En

a � En �

83 559.04±0.01b 0.725±0.024b 83 162.94±0.01b 0.702±0.004b 82 985.43±0.01b 0.710±0.003b

83 559.04±0.05c 83 162.94±0.05c 82 985.43±0.05c

83 559.1±1.0d

83 560.8±0.1e

aEn values in parentheses are obtained by photoabsorption measurements of Ref. 9.bBest fitted IE values or � for S�3P0�, S�3P1�, and S�3P2� determined by the least square fits of En �n=30–64� for S�3P2�, En�n=30–62� for S�3P1�, and En

�n=30–62� for S�3P0�.cRecommended IE values. The assigned uncertainty of ±0.05 cm−1 for these IE values has taken into account the error limit of experimental En values �seetext�.dIE value determined by fitting of the Rydberg series 3p3�4S��nd 3D�← 3P �n=4–11� �Ref. 16�.eIE value determined by fitting of the even-parity Rydberg series �4S0�np 3P �n=9–25� �Ref. 17�.

014305-7 S�3P� and S�1D� from CS2 photodissociation J. Chem. Phys. 128, 014305 �2008�

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berg states, the fluctuation for n=42–64 of the S�3P2� seriesis also found to be relatively high, which is consistent withthe fluctuation observed for the energy residuals of thesestates. The fluctuation observed for the energy residuals andquantum defects for the n=17–64 Rydberg states of S�3P2�may be indicative of the perturbation of these Rydberg statesby the nearby autoionizing state for S�1D2� at83 512.89 cm−1. Plot in Fig. 6 also includes the quantumdefect for the lower n=3–14 Rydberg states of the S�3P2�series.16 This section of the quantum defect curve for n=3–14 seems to connect smoothly with that for n=17–64 ofthe S�3P2� series determined in the present measurement. Asshown in Fig. 6, the quantum defect was found to change

from �=0.138 at n=3 to �=0.722 at n=29 and then staysnearly constant at n�30 for the S�3P2� series. This observa-tion can be taken as support for ignoring the low-n Rydbergstates in the least square fits to obtain more precise IE valuesfor S�3P0�, S�3P1�, and S�3P2�. The functional form for theexperimental ��n� curve shown in Fig. 6 is in good accor-dance with the theoretical prediction33 of the ��n� functionfor the 3p3�4S��nd 3D� Rydberg series of S atom.

IV. CONCLUSIONS

A compact coherent vuv laser source �optical bandwidthof 0.45 cm−1 �FWHM�� based on four-wave frequency mix-ing in a rare gas jet has been constructed and implementedwith a time-sliced velocity-mapped ion-imaging apparatus.Using this vuv laser ion-imaging apparatus, we have mea-sured the vuv-PIE and vuv-PFI-PI spectra for nascentS�3P2,1,0� and S�1D2� formed in the ArF laser �193.3 nm�photodissociation of CS2. The PFI-PI spectra observed forS�3P0�, S�3P1�, and S�3P2� in the region of82 750–83 570 cm−1 reveal well-resolved Rydberg features,which have been assigned as the three Rydberg series3p3�4S��nd 3D� �n=17–62�←S�3P0� �S�3P1� and S�3P2��.The assignment and least square fit of the En values of theseRydberg transitions to the Rydberg formula have allowed thedetermination of more precise IE values for S�3P0�, S�3P1�,and S�3P2�. Four strong autoionizing lines resolved in thevuv-PIE measurement have been shown to originate fromS�1D2� based on ion-imaging measurements. The relativestep heights associated with the ionization thresholds forS�3P0�, S�3P1�, and S�3P2� observed in the PIE spectra andthe relative intensities for PFI-PI bands for S�3P0�, S�3P1�,and S�3P2� have provided information about the relativepopulation for the S�3P0�, S�3P1�, and S�3P2� states preparedby photodissociation of CS2 at 193.3 nm. Assuming that thephotoionization cross sections for S�3P0�, S�3P1�, and S�3P2�are identical, we estimate that relative populations for theS�3P0�, S�3P1�, and S�3P2� fine structure states to be non-statistical at 1.00, 1.54, and 3.55, respectively.

ACKNOWLEDGMENTS

This work was supported by the Director, Office of BasicEnergy Sciences, Chemical Sciences Division of the U.S.Department of Energy. C.Y.N. also acknowledges partialsupports by the NSF Grant No. CHE-0517871 and theAFOSR Grant No. F49620-03-1-0116. William M. Jacksonand Xueliang Yang acknowledge the support of the Chemis-try Division of NSF under Grant No. CHE-0503765.

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FIG. 5. The energy residual �En�expt�−En�fit�� plotted as a function of n inthe range of n=17–64 for the Rydberg series of S�3P0�, S�3P1�, and S�3P2�,where En�expt� and En�fit� represent the respective experimental and leastsquare fit values for En. The energy residuals for the Rydberg series ofS�3P0�, S�3P1�, and S�3P2� are shown as ���, ���, and ���, respectively.Note that the energy residuals for n�28 have been multiplied 30-fold.

FIG. 6. The quantum defect � plotted as a function of n for the Rydbergseries of S�3P0�, S�3P1�, and S�3P2�. The quantum defects for the Rydbergseries of S�3P0�, S�3P1�, and S�3P2� are shown as ���, ���, and ���, re-spectively. The upward solid triangles display the quantum defects for n=3–14 of the Rydberg series of S�3P2�, which are taken from Ref. 16. Notethat the quantum defect increases smoothly from 0.138 to 0.591 for n from3 to 14 and becomes nearly constant for n=30–64 �see text�.

014305-8 Zhou et al. J. Chem. Phys. 128, 014305 �2008�

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014305-9 S�3P� and S�1D� from CS2 photodissociation J. Chem. Phys. 128, 014305 �2008�

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