Resonant multiphoton ionization dynamics of N2 via the a 1Π g (v=10–14) states:Preparation of stateselected N+ 2 X 2Σ+ g (v +=0–4) ionsG. S. Ondrey, C. Rose, D. Proch, and K. L. Kompa Citation: The Journal of Chemical Physics 95, 7823 (1991); doi: 10.1063/1.461311 View online: http://dx.doi.org/10.1063/1.461311 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/95/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Direct observation of collision induced transitions between the a 1Π g (v=0) and a’ 1∑− u (v=0) levels of N2via double resonance enhanced multiphoton ionization spectroscopy J. Chem. Phys. 97, 2820 (1992); 10.1063/1.463023 2+1 resonantly enhanced multiphoton ionization of CO via the E 1Π–X 1Σ+ transition: From measured ionsignals to quantitative population distributions J. Chem. Phys. 93, 8557 (1990); 10.1063/1.459293 The production and characterization by resonance enhanced multiphoton ionization of H2(v=10–14) fromphotodissociation of H2S J. Chem. Phys. 91, 6113 (1989); 10.1063/1.457430 Stateselective ionization of nitrogen in the X 2Σ+ g v +=0 and v +=1 states by twocolor (1+1) photonexcitation near threshold J. Chem. Phys. 91, 6006 (1989); 10.1063/1.457417 Multiphoton ionization of O2 X 3Σ− g , a 1Δ g , and b 1Σ+ g via the twophoton resonant n sσ g , n dσ g ,and n dπ g Rydberg levels J. Chem. Phys. 91, 5185 (1989); 10.1063/1.457589

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Resonant multiphoton ionization dynamics of N2 via thea 1llg (v=10-14) states: Preparation of state-selected Nt X 2~;(V+ = 0-4) ions

G. s. Ondrey, C. Rose, D. Proch, and K. L. Kompa Max Planck Institutfur Quantenoptik, D 8046 Garching bei Munchen, Germany

(Received 16 July 1991; accepted 21 August 1991)

The photoelectron spectra are reported from resonantly enhanced multiphoton ionization of N2 via the a IIIg (v = 10-14) states. Ionization from these levels is found to follow a (2 + 1) pathway resulting in Nt X2~g+ (v+ = 0-4) ions. The observed branching ratios in the photoelectron spectra reveal a strong propensity for forming ground state ions in a single vibrational state. The results are explained qualitatively by favorable Franck-Condon overlap in the ionizing X 2~t -a I I1g transition.

I. INTRODUCTION

Considerable effort has been spent in discovering meth­ods for the production of vibrationally state-selected ions in quantities sufficient for use in reactive scattering experi­ments. Resonance-enhanced multiphoton ionization (REMPI) via a Rydberg intermediate state is a promising scheme because of the Au = 0 propensity rule imposed by the Franck-Condon (FC) factors ofthe nearly superimpo­sable Rydberg and ionic states. Although some success has been achieved for a few molecular systems,I-1O there are many examples 11-15 where the FC factors do not govern the ionization dynamics due to complications arising, e.g., from autoionization or the mixing of the Rydberg with valence states. Because of this, no general rule governing the ioniza­tion dynamics exists and vibrational distributions for each system must be determined experimentally.

In the case of the N2+ X2~g+ (v + ) ion, several REMPI schemes have been studied with moderate to good vibra­tional state selection. Using (3 + 1) REMPI via the C3 Illu Cv) and c';' l~: (v) Rydberg states, Pratt et al. 14

showed that the ground state ions thus produced were in the v + = v vibrational state for v = 0 and 1. Unfortunately, this ionization scheme is complicated by the fact that a consider­able fraction of the ions are electronically excited as well due to mixing ofthe C3 [I1u and c';' [~u states with valence states converging to A 2I1u. The production of electronically excit­ed N2+ can be avoided by using a second laser to ionize from the C3 Illu Cv) and c';' l~u (v) Rydberg states, in which Case, the excess energy can be kept below that of the A 2I1u state. Two separate two-color ionization schemes have been stu­died where this energetic constraint has been imposed. The two experiments differed in their method of preparing the C3 Illu and c';' I~u states. Trickl et a/.9 chose single photon absorption from the ground state, whereas Opitz et al. 8 opt­ed for single photon absorption from the a [I1g state pre­pared previously by two-photon excitation. Despite their successful preparation ofN/ X2~g+ (v+ = 0,1), both ioni­zation schemes suffer from limitations making them incon­venient sources for Nt X2~g+ (v+) ions. The (2 + l' + 1') ionization requires careful control of the laser intensities to prevent undesirable ionization pathways from occurring si·

multaneously. On the other hand, the (1 + 1') ionization scheme, while avoiding this problem because the lasers need not be focused, requires a source of tunable VUV photons. In both cases, the need for a second laser is a disadvantage, if only for reasons of economy.

In this communication, an alternative REMPI scheme is described which is found to be a convenient and reliable source of N2+ X 2~g+ (v + = 0-4) ions having a high degree of vibrational state selection. As shown in Fig. 1, a single laser is tuned to be two-photon resonant with the v = 10,11, ... , or 14 vibrational levels of the a lIlg(v) state. Subsequent absorption of a third photon is sufficient to ion­ize the molecule into the v + = 0,1, ... , or 4 vibrational level of the ground state of N 2+ • For all five cases, 3hv is below the I.P. of the A 2I1u state. The relevant energetics are summar­ized in Table I. The vibrational distributions of the

I. P.

'///. £ A2

Ttu

23 X2r+

15 01 9

1413

10 1tll 0 1 Ttg 9

5

x1rg

FIG. 1. A diagram of the excitation schemes used in this work.

J. Chern. Phys. 95 (11),1 December 1991 0021-9606/91/237823-05$03.00 © 1991 American Institute of Physics 7823

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7824 Ondrey et 81.: Preparation of state-selected Nt X 2,E % ions

TABLE I. Possible electron kinetic energies for the (2 + 1) photoionization ofN2 via the a IIIg(v:= 10-14)

states.

a IIIz(v)

10 11

12

13

14

a From Ref. 25. b From Ref. 26 .

So wavelength (nm)" hv (eV)

237.022 5.232 233.177 5.317

229.603 5.400

226.154 5.483

222.752 5.566

• Calculated using the constants of Ref. 26.

X2~t (v+)

0 0 1 0 1 2 0 1 2 3 0 1 2 3 4

N2+ X2~g+ (v+) ions have been measured by kinetic energy analysis of the ejected electrons using a novel time-of-flight (TOF) apparatus capable of collecting up to 25% of all elec­trons having kinetic energy (KE) in the range of 0.05 to 5 eV with a resolution of 10 meV.

II. EXPERIMENTAL

The apparatus consists of a pulsed nozzle source, an excimer pumped dye laser, and an electron TOF spectrom­eter. The laser, molecular beam, and detector axes were mu­tually orthogonal. The excimer laser (Lambda Physik LPX 300) delivers up to 700 mJ of 308 nm radiation with a pulse full width at half-maximum (FWHM) of 20 ns. For this experiment, less than 300 mJ were required,to pump a dye laser (Lambda Physik FL2oo2 with FL582 computer con­trol) operating with Coumarin 47 or 102. Tunable radiation from 440 to 480 nm was then frequency doubled with a BBO I crystal to produce up to 1 mJ ofUV having a bandwidth of 0.2 cm - 1. The laser was polarized along the detector axis. The UV was focused by a f = 25 cm lens to intersect the molecular beam 105 mm downstream .. The laser energy was sufficiently low to avoid complications due to space charge effects.

The molecular beam was formed by expansion of 5 bar of pure nitrogen through a 0.2 mm diameter nozzle of a pulsed valve (Laser Technics LPV) and subsequently colli­mated with a skimmer (beam dynamics, ¢ = 1 mm). Differ­ential pumping of the nozzle and TOF chambers was per­formed with two 450 I turbomolecular pumps and a 14 K cryopump resulting in a pressure of less than 2 X 10- 7 mbar in the flight tube with the valve operating at 10 Hz.

The TOF spectrometer consisted of an electrostatic lens used for either collimation of the photoelectrons or extrac­tion of the ions, a magnetically shielded 514 mm long flight

KE = 3hv-I.P.b ~~ Eu+ C (eV)

0.112 0.371 0.100 0.620 0.349 0.085 0 .. 867 0.596 0.332 0.069 1.118 0.847 0.583 0.320 0.065

tube, and a multichannel plate (MCP) detector (Galileo LPD 25) wired for either electron or positive ion detection. The signal was fed to a preamplifier (Ortec 9301), discri­minator (Ortee 9302), and -to either a boxcar integrator (Stanford Research SR250) for measuring excitation spec­tra, or a 200 MHz transient digitizer (Tektronix 7612) for measuring TOF distributions. A single PC was interfaced to a computer-aided measurement and control (CAMAC) crate for supplying voltages to the electrostatic lens and to the digitizer for data acquisition.

Characteristics of the electrostatic lens system will be described in greater detail elsewhere, 16 only a brief summary ofits properties being given here. It was designed to increase the electron collection efficiency over that of conventional TOF spectrometers, especially for'slow « 100 meV) elec­trons which are most susceptible to stray fields: Selection of the voltages to be applied, as determined by trajectory calcu­lations, results in an electric field inside the lens such that the photoelectrons are created at a saddle point of the potential energy surface, i.e., a point of zero potential gradient. Elec­trons leaving this point with a velocity component along the detector axis are channeled towards the MCP. Outside the lens, the potential'rapidly (within 30 mm) drops to zero allowing the field-free drift necessary for energy resolution. Urider optimum conditions, an acceptance angle of ± 38° with respect to the detector axis is achieved, compared to ± 1° when the lens is off. The-collection efficiency will de­pend on the electron angular distribution. The lens system thus has an effect similar to the magnetic bottle of Kruit et al. I7 with the advantage that strong magnetic fields, whose influence on the spectroscopy of the molecules being studied is questionable, are avoided. On the other hand, the magne­tic bottle has a collection efficiency independent of the elec­tron angular distribution.

The construction of the lens is such that a - 1000 V

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Ondrey et a/.: Preparation of state-selected Nt X 2 I; ions 7825

extraction potential may also be supplied, making it an effi­cient (100%) collector of positive ions. This is useful for the identification of the resonant intermediate states by record­ing their excitation spectra.

One characteristic of the electrostatic lens is that the collection efficiency depends strongly on the operating vol­tages. Although the complete photoelectron spectrum can be obtained at a given set of operating voltages, the spectrum must be corrected for the transmission function of the spec­trometer. To mipimize this difficulty, we first obtained an overview of the complete photoelectron spectrum and iden­tified the energies of all the observed electron peaks. Then, for each electron peak, averaging was done with the lens voltages optimized for that specific KE. This was done in a staggered fashion to compensate for the gradual decrease in laser energy due to dye degradation. In this way, each TOF peak was recorded under conditions of nearly identical col­lection efficiency and the branching could be determined directly. Even in those cases where electron peaks were not observed in the overview spectrum, but were energetically allowed (cf. Table I), averaging was carried out with the lens optimized for those kinetic energies in order to guarantee the absence of weak signals.

Another characteristic of the electrostatic lens is that there is no longer a simple t - 2 relationship between the electrons' time offtight and their kinetic energy. As a result, extensive trajectory calculations have been performed 16 to correlate arrival times with starting energies for a given set of lens voltages. Using the monoenergetic electrons ejected from the (2 + 1) photoionization via the a Ing(v = lO) state for calibration, we find the agreement between calcu­lated and measured arrival times to be within lO ns [trans­forming the TOF distribution to KE (see below) gave a peak at 0.12 eV, which is only 8 meV away from the expected value]. Because the arrival times for electrons which differ by one vibrational quantum of energy are more than 400 ns apart, there is no problem in the interpretation of the data. TOF spectra are transformed to energy distributions using an nth-order polynomial of the form

n

E= L ait i,

f-O

(1)

where E is the kinetic energy, t the time after the laser pulse, the coefficients ai are determined from trajectory calcula­tions, and the order used is selected to give the best fit (for n = 6, the correlation coefficient is nearly I). With this method, the electron KE peaks agree with the predicted va­lues of Table I to within 20 meV.

III. RESULTS AND DISCUSSION

Figure 2(a) shows the TOF spectrum for positive ions when the laser is two-photon resonant with the So line of the a Ing (10) - X l~g+ (0) transition. The sharp peak at t = 0 is due to scattered UV photons reaching the MCP detector and is a convenient marker for to. Unfortunately, the scat­tered photons, having energies above the work functions of most metals, also give rise to photoelectrons generated at surfaces. These electrons are accelerated inside the lens suffi­ciently to cause electron-impact ionization and dissociation

.?:' 'iii c GI 'E c .2 GI > ti 'ii a::

0 5 6 7 8 Time of Flight (\Js)

.?:' 'iii c GI

E 51 50

c Q1 .2 +N Z GI > ~ Qj a::

237.0 237.1 Laser Wavelength (nm)

FIG. 2. The ion TOF spectrum (upper) and corresponding excitation spec­trum (lower) for the (2 + 1) photoionization ofN2 via thea lIIg(v = 10) state. The broad, nonresonant N + and Nt signals are due to electron im­pact ionization and dissociation of the N2 beam, the electrons being gener­ated from scattered 1igh~ (cf. the t~xt). The excitation spectrum assign­ments are from Vandershce et al. (Ref. 25).

of the N z beam, as is apparent in the broad features from 4.8 to 5.5 f.ls (N+) and from 6.7 to 7.7 f.ls (N2+). The sharp peak at 7.1 f.ls is the resonant N 2+ ion signal. The arrival time was found to be identical to that determined by trajectory calculations. With the gate of the boxcar set to the resonant N2+ ion signal and scanning the laser wavelength, the excita­tion spectrum was recorded, as shown in Fig. 2 (b). The ap­pearance of only three peaks ofthe S branch corresponds to a N z rotational temperature of Trot = 15 K. Similar spectra are obtained for the a IITg Cv = 11-14)-X I~g+ (v = 0) two­photon transitions.

With the laser tuned to the So line of the a IITg (v = lO)-X I~~+ (v = 0) transition, the voltages sup­plied to the TOF lens and the MCP detector were changed so that the photoelectrons could be collected yielding the TOF spectrum shown in Fig. 3. Again, one observes the scattered photon peak at to and a broad signal caused by the scattered photons ejecting electrons from surfaces. Because the nonre­sonant background electrons are present even without the N 2 beam, it was easily eliminated by alternate addition and subtraction of spectra with the beam on or off. Another TOF

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7826 Ondrey et al.: Preparation of state-selected Nt X 2 I; ions

c:

~ u .. 0; o '0 ..c: 0..

L.--' ____ L.. ___ "'--___ -'-___ J __ ._~

o 0.5 1.0 1.5 2.0 2.5 Time of Flight IllS)

FIG. 3. The electron TOF spectrum for the (2 + 1) photoionization ofN2

via the a IIIg (v = 10) state. The scattered light at t = 0 gives rise to nonre­sonant electrons from metal surfaces. This background signal can be elimin­ated by alternate averaging with the nitrogen beam on and off. Another sp.ectrum was recorded in this way and is also shown with a displaced ordin­ate.

spectrum in which the background was removed in this manner is also shown in Fig. 3. The sharp peak at t = 1.29 p,s is the (2 + 1) resonant electron signal. The absence of other peaks shows that the absorption of more than three photons does not occur. This is also consistent with laser-induced fluorescence (LIF) measurements.8

TOF spectra were also recorded for the (2 + 1) ioniza­tion via thea IIIg (v = 11-14) states. In all cases, the So line was used. For these transitions, more than one vibrational level oftheX 2'£g+ state is accessible and averaging was per­formed for each of the possible electron kinetic energies in a staggered fashion, as discussed in Sec. II. The resulting elec­tron KE distributions are presented in Fig. 4. For all five ionization wavelengths, one observes a pronounced prefer­ence for the formation of a single vibrational state of N2+ X2'£g+, thus illustrating the utility of this method for the preparation of vibrationally state-selected N2+ ions.

The results can be explained qualitatively by the favora­ble Franck-Condon overlap which occurs between the left side of the a I I1g and X 2'£g+ potential energy curves. 18

Franck-Condon factors were calculated for the ionizing X 2'£t -a 1 I1g transition. As an approximation, Morse wave functions l9 were used according to Halmann and Lau­licht.20 The relevant FC factors were then used to predict vibrational branching ratios for ionization. As seen in Fig. 5, the measured branching ratios follow a trend similar to that predicted from FC arguments.

It is interesting to compare our results with the LIF measurements ofEbataetal,21 on theNl X2'£g+ (v+) ions formed via the (2 + 2) REMPI via the a I I1g (v = 1) state. They found a v + distribution in good agreement with FC predictions for the observed v + = 0, 1, and 2 states. At their excitation energy, ground state vibrational levels up to v + = 7 were accessible as well as the first three vibrations of

4 3 2 0 v+ ,-:----..:;'------=:'-------::---------',

1

1: o .2

o .2 .4 .6

v=12

-~-~l-~

.4 .6 .8

v=13

Electron Kinetic Energy (eV)

FIG. 4. Photoelectron spectra for the (2 + 1) photoionization ofN2 via the a iIIg(v= 10-14) states. In alI cases, the So line of the a-X transition was

used.

1.00 .2 ~ 0.75

.50.50 V.11 'li e 0.25 In

1.00 2 & 0.75 CI .!: 0.50 'li E 0.25 In

V.12

V'

2 V'

o Measured

E'J Fe

1.00l > '>"~ .2 &0.75

.5'0.50 V.13 Jl .. !o.25 ~ aD - 'j

o 0 1 2 3'

.2 1.00

& 0.75

~0'50 c: ~ 0.25 In

o 0

V'

V.14

2 V'

FIG. 5. Vibrational branching ratios for N,+ X2"£t Cu+) following (2 + 1) photoionizationofN2 via thea IIIg(u = 10-14) states. The hollow boxes are the measured ratios and the hatched boxes are calculated ratios determined from the Fe factors for the ionizing X2"£g+ -a III. transition. Experimental and theoretical results were normalized by dividing each peak by the sum over the integrated peak intensities of energetically allowed v + levels. The error associated with the measured branching ratios are 5%, 10%, 15%, and 20% in going from v = 11-14. Excitation via a III~(Ii= 10) (not shown) must produce 100% N,+ XI"£g+(v+ =0)

from energetic reasons C cf. Table I).

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Ondrey et af.: Preparation of state-selected Nt X 2:s t ions 7827

the A III state. Unfortunately, if ions were formed in these states, they would have eluded detection by LIF, so their results are somewhat incomplete.

At this time, we offer no explanations for the quantita­tive differences between the measured and predicted branch­ing ratios. The errors associated with the measurements be­come larger in going from v = 10-14 of the a lIIg state because of the increasing difficulty in observing an electron signal at the higher vibrations. For single-photon absorp­tion, the FC factors for the a lIIg_X l~g+ transition become increasingly smaller for the (10-0) to (14-0) bands. IS It is expected that the two-photon transition probability for the (14-0) is significantly smaller than that for the (10-0) band as well. Therefore, we cannot rule out the possibility that the failure to observe the the lower vibrations is due to our limits of detection.

Another reason for the differences in the measured and predicted results may be due to the approximate Morse wave functions used in calculating the FC factors. These wave functions become increasingly inaccurate as v increases. Also, the possible interaction of the s~/ state, which is thought to be responsiblel8 for the weak predissociation ob­served in emission from the a 1 IIg state above v = 6, may also playa role in the ionization dynamics. Accurate wave functions for the higher vibrational levels of the a I IIg state which account for any interactions of nearby states would be helpful.

Finally, all measurements were performed with the laser polarized along the detector axis and the reported vibra­tional branching ratios tacitly assumed identical angular dis­tributions. If the angular distributions of the v + 'channels are drastically different, additional uncertainties would arise in our branching ratios. An estimate on the upper limit for this error can be determined by considering the case of v = 11. If one of the channels is isotropic and the other a cosine squared distribution, the reported branching ratio of 0.9 would change to a value of 0.95 or 0.79. 16 On the other hand, although v + -dependent angular distributions have been ob­served for Hz (Ref. 22,23) and NO,24 the differences were much smaller and similar differences would not be observa­ble in our measurements.

IV. CONCLUSIONS We have measured vibrational branching ratios for

(2 + 1) resonant ionizationofNz via thea IIIg (v = 10-14) states. A strong preference for the formation ofN/ Xz~/ ions in single vibrational levels was observed, in qualitative agreement with FC calculations. The extremely high degree of state selection resulting from this ionization scheme makes it an excellent means for the production of N z+

X Z~g+ ions in the v + = 0-4 states. Because the method re­quires only one laser and avoids the formation of electroni­cally excited N2+ ions, we regard it as superior to previous schemes using Rydberg intermediate states.

ACKNOWLEDGMENTS

We would like to thank Hans Bauer for technical assis­tance and Dr. S. Kaesdorf for fruitful discussions. GSO is grateful for the support ofthe Max Planck Gesellschaft and the hospitality of the Max Planck Institut fUr Quantenoptik.

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