3588
J. Phys. Chem. 1994,98, 3588-3591
Vibrational Populations of CS(AIII) Produced by Electron-Impact Dissociation of CS2 and OCS Ikuo Tokue,. Masanobu Kusakabe, Hiroshi Ogawa, and Yoshio Ito Department of Chemistry, Faculty of Science, Niigata University, Ikarashi, Niigata 950-21, Japan Received: November 23, 1993; In Final Form: January 25, 1994’ The CS(AIII-XIZ+) emission spectra produced by electron impact on CS2 and OCS have been measured from threshold up to 120 eV. Emission cross sections of this band from CS2 and OCS are 10 f 2 and (0.75 f 0.15) X 10-l8 cm2, respectively, a t 100 eV. At low impact energies (below 20 eV), the vibrational state distributions of CS(AIII, u’ = 0-8) measured from CS2 and OCS can be represented by temperatures of 12 500 f 2000 and 26 000 f 4000 K, respectively, while the rotational temperature of the u’ = 0 level is estimated to be 3850 f 400 K from both parents. Vibrational distribution data are approximately represented by distributions predicted from the impulsive half-collision model.
Introduction Dissociation processes of CS2 and OCS have been extensively studied by photoexcitationl-7 and electron-impact excitation,8-10 and in flowing afterg10w~l-I~ and discharge.20 Lee and Judge2 determined the vibrational distribution for the v’ = 0-4 levels of the CS(AlII, u’) state produced through vacuum ultraviolet (VUV) photodissociation of CS2 and OCS by analyzing the CS(AIlI-XIZ+) emission spectra with the assumption that the electronic transition moment is constant. They found a population inversion between the u’ = 0 and 1 levels. Later, Coxon et al.13 studied the dependence of the electronic transition moment on r-centroid on combining the band intensities of the CS(A-X) system with RKR Franck-Condon factors and determined the value of 0.40 f 0.08 for the coefficient of from the first-order fit of the electronic transition moment. Ashfold et al.5 confirmed the result given by Coxon et al.” and evaluated the vibrational populations of CS(A, u’ = 0-6) produced by VUV photodissociation of CS2. The vibrational distribution of CS(A) thus obtained strongly dependson the wavelengthoftheincident photon and differs from those reported by Lee and Judge.2 Wu19 studied the CS(A-X) fluorescence produced in the energy-transfer reaction Ar(3P) + CS2 and determined thevibrational populations of CS(A, u’ = 0-6). The resultant populations were similar to those obtained from the photodissociation5 of CS2, and no population inversion was observed. H e concluded that the fragmentation dynamics from the photon and from the Ar metastable impact are very similar in nature and that the presence of Ar does not have any significant dynamical effect. Recently, Xu et al.zl reexamined the vibrational populations of CS(A) resulting from the Ar(3P) + CS2 reaction under single-collision conditions and found a population inversion at u‘ = 1,2. Thus, the vibrational population data of CS(A) accumulated from photodissociation and energy-transfer reaction and their interpretations have not always been consistent with each other.2v5J9.21 In electron-impact excitation of CS2, Ajello and Srivastavalo measured the emission cross section of the CS(A-X) band and found a population inversion between u’ = 0 and 1 with the assumption of a constant transition moment. Nevertheless, it is evident that the analysis of vibrational populations of CS(A) must take into account the dependence of th_etranjition moment on FJ,P. Horani et a1.* observed the OCS+(A2II-X2II) emission produced by electron impact on OCS at 120 eV but did not mention formation of excited fragments. Thus, the amount of information on vibrational populations of CS(A) produced by electron-impact excitation of CS2 and OCS is apparently less than those obtained from other methods. Analysis of the internal state distribution of CS(A) provides a simple way of probing the dynamics of dissociation of CS2 and OCS. It is desirable to investigate Abstract published in Advance ACS Absfracts, March 1, 1994.
fragmentation of these molecules by electron impact and to compare the result with those derived from other measurements. In this context, the emission cross sections of the CS(A-X) transition resulting from electron impact on CS2 and OCS have been measured and the internal state distribution of CS(A) has been determined within the framework of the first-order dependence of the electronic transition moment on the r-centroid. The observed vibrational populations have been compared with results calculated from the impulsive half-collisional model.22 The dynamics for formation of CS(A) is discussed.
Experimental Section The apparatus and experimental details were essentially the same as those reported p r e v i o u ~ l y .Briefly, ~~ target gases were introduced into the collision chamber by a multicapillary array under the effusive beam condition. The molecular beam crossed an electron beam perpendicularly about 10 mm downstream from the array. Under the operating conditions, a typical electron beam current was 40 pA a t 12 eV and the ambient pressure of the collision chamber measured by an ionization vacuum gauge was 1.2 mPa. ThesampleofCS2(Wako,purityat least 99%) wasusedwithout further purification, while the sample gas of OCS (Matheson, purity of 97.5%) was vacuum-distilled prior to use in order to remove several impurities in particular C02.
Results and Discussion Emission Spectra. For the population analysis, the CS(A-X) emission spectra in the 245-275-nm region have been measured from CS2 at an impact energy of 12 eV with the 0.1-nm fwhm resolution and from OCS at 17 eV with the 0.3-nm fwhm resolution. Figure 1 shows the CS(A-X) spectrum in the 245275-nm region resulting from electron-impact excitation of CS,; the relative sensitivity of the total photon-detection system was calibrated with a deuterium lamp. Vibrational bands for v’ up to 11 were assigned with the aid of the Deslandres table given by Bergeman and Cossart.24 The CS(A-X) emission bands observed from both CS2 and OCS were found to be overlapped with an unknown continuous band. The spectrum shown in Figure 1 is the resultant after a constant background was subtracted (about 3% of the total intensity) from the observed spectrum with the assumption that the intensity of the unknown band is uniform in this wavelength region. The 1B2(S3)-XlZ+ fluorescence25 of CS2 and several triplet-singlet systems24 of CS are candidates for this continuous band. For electron-impact dissociation of OCS at 17 eV, the constant background reaches 11% of the total intensity in the 245-275-nm region. The CS(A-X) spectra observed from CS2 at impact energies above 25 eV are similar to that observed at 12 eV, while those
0022-3654/94/2098-3588~04.50~0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3589
Vibrational Populations of CS(AIII)
V 18. ENERGY / E V
11 10
0.0
Q-
ocs
8 7 8-
’8
V”
5 4
I-
2
20.5 0
1
a
3-
J
1
n
I
-
I
I
0.21, , , , I , I I I , I I 0 1 2 3 4 5 6 7 8 91011
-I
V’
Figure 2. Relative vibrational populations P#/Poof the CS(AIII, v’) state produced by electron impact on CS2 at 12 eV and on OCS at 17
eV: the observedvaluesareindicated (0,X); thelines representvibrational temperatures of 12 500 K for CS2 and 26 000 K for OCS.
255
265
275
WAVELENGTH/n m Figure 1. Observed CS(A-X) emission spectrum (dots) produced by electron impact on CS2 at 12 eV with the 0.1-nm fwhm resolution; the upper trace is the best-fit synthetic spectrum and the lower is the residual. from OCS above 25 eV come to be overlapped with the CO+(B-X) band. Moreover, the continuous background increases with the impact energy for both molecules. Thus, the internal state distribution of CS(A) was analyzed only for the spectra observed at impact energies below 20 eV. Internal State Distributions. To evaluate the internal state distribution of the nascent CS(A) state, the observed spectrum was compared with band envelopes simulated. Synthetic spectra were calculated within the framework of the r-centroid approximation. Coxon et al.13 have concluded that the transition moment is given by R e ( f d d f )= c [ 1 - (0.40 f 0,08)Fdd?],with Fddf in units of 1O-Io m. Nevertheless, the coefficient of FddT was treated as an adjustable parameter in a least-square analysis, since a fairly large uncertainty is attached to the reported value. The transition frequencies of the CS(A-X) system were calculated from the spectroscopic data” by neglecting the A-type splitting, and the line strengths were given by the HBnl-London formulas.26 Bergeman and C0ssart2~have calculated the FranckCondon factors ( q d d t ) for u’ = 0-1 1 and u” = 0-16 from RKR potentials, while they have not given Fddf values. The fi/df values have been reported only for v’ = 0-5 and d’ = 0-8.13 Therefore, we have calculated the Fddt values for v’ up to 11 and for u’ up to 16 from the RKR potentials on the basis of the spectroscopic parameters determined by Bergeman and C ~ s s a r t . ~ ~ The rotational state distribution for each vibrational level was assumed to be a single Boltzmann distribution because the spectral resolution is insufficient for resolving rotational lines. A further constraint was applied to rotational temperatures: the rotational temperature (Td) for the u’ level is assumed to be represented by the formula T d = TO-A d , where TOis the rotational temperature of the u’ = 0 level and A is a fitting parameter. Band envelopes were computed by means of convolution of the calculated intensity for J’up to 300 of the u’ = 0-1 1 levels with the slit function. This function was estimated by a helium atomic line at 388.8 nm measured under the same optical condition. The vibrational populations P d for u’ = 0-8, the parameters of rotational temperature ( TOand A ) , and the coefficient of Fddl have been determined independently from the least-square analysis.
The P d values for u’ = 9-1 1 are estimated on the assumption that those for v’ = 0-8 are represented by a Boltzmann temperature. The best-fit spectrum and residual thus obtained are displayed in Figure 1. A fairly good agreement with the observed spectrum is obtained as a whole except near 251, 259, and 268 nm. The discrepancies near these wavelengths are probably caused by overlapping of several triplet-singlet emission bands of CS; in fact, the intense 3-1 and 3-2 bands of the e3Z--X1Z+ transition appear at 258.4 and 267.1 nm, respectively (Figure 2 in ref 24). The a3n, a’3Z+, d3A, and e3Z- states of CS can be produced directly by electron impact on CS2 and OCS at the impact energies employed in this measurement (e.g. 12 eV for CS2). The TOvalues of CS(A) are calculated to be 3800 f 400 K from CS2 and 3900 f 400 K from OCS, and the A value is estimated to be 0 f 50 K from both molecules. The latter means that the rotational distributions for u’ = 0-8 can be represented by a Boltzmann temperature TO.Figure 2 shows the relative vibrational populations P d / P o for u’ = 0-8 when compared with the lines, which represent Boltzmann temperatures of 12 500 K for CS2and 26 000 K for OCS; the uncertainties of P d values for u’ = 0-4 are about 5% and those for v’ = 5-8 are 10-20%. The observed distributions are only approximately represented by Boltzmann temperatures, as indicated in the figure. The value of 0.40 f 0.04 for the coefficient of F d f obtained is in good agreement with the value given by Coxon et al.13 The internal state distributions of CS(A) thus obtained represent the nascent distribution produced by electron impact, if cascadings from upper states are insignificant. For CS, no upper state which can cascade to the A state is known except the well-known effects of triplet-singlet perturbations via collision^.^^ Collisional relaxations are obviously insignificant in this experiment. Therefore, the internal state distributions seem to be nascent. The TOvalueobtained by electron impact on CS2 is considerably higher than an average temperature (1500 K) estimated from the double-Boltzmann distribution with 675 K (80%) and 4800 K (20%) obtained by photodissociation of CS2 at 130.4 r ~ mThe .~ rotational energy of the fragments mainly originates in the rotation of parent CS2(at room temperature) and the bending motion of the intermediate CS2* state. In the photodissociation at 130.4 nm,5 the former and latter sources lead to the components with lower and higher temperatures, respectively. The amount of rotational excitation from the second source becomes rather small since the linear equilibrium configuration should be preserved in the photoexcited intermediate state, and then final rotational levels of CS(A) are restricted by angular momentumconservation;
Tokue et al.
3590 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 formation of CS(A) is correlated with the Rydberg transitions due to the excitation of a nonbonding l?r, electron.' On the contrary, TOobtained in the energy-transfer reaction of Ar(3P) CS2 is very high (lo4 K).zl This high temperature is ascribed to some dynamical effects: when Ar and CS2* separate after the energy transfer, CS2* intermediates are highly rotationally excited since CS(A) is produced mainly when the CS2 molecular axis is perpendicular to the relative velocity vectoraZ8 In the present result, the PI/P0ratios are evaluated to be 0.92 f 0.04 for CSz and 0.97 f 0.05 for OCS, and then no population inversion occurs. This is, however, inconsistent with the previous result,I0which shows the population inversion between J = 0 and 1 (PI/P0 = 1.25). The discrepancy probably originates in the procedure for the population analysis since the emission intensity of the 1-1 band relative to that of the 0-0 band in Figure 1 is very similar to the ratio estimated from the spectrum reported (Figure 7 in ref 10). In their analysis, the electronic transition moment is assumed to be constant and vibrational populations only for u' = 0-4 have been analyzed. Whereas, we have found that POand P1are strongly correlated with Piand Ps, respectively, because of overlapping between the 0-0 and the 4-5 bands and between the 1-1 and the 5-6 bands. The vibrational populations of CS(A) obtained from CS2 in this study are enhanced apparently more than those in photodissociation of CS2 in the 121.6-130.5-nm r e g i ~ n .At ~ these wavelengths CS(A) can only be formed via the spin-forbidden excitation CS2- CS(A1II) S(3P). This fact together with the result for To indicates that the formation dynamics of CS(A) by electron-impact dissociation differs from that in photodissociation above 121.6 nm. In order to examine the formation mechanism of CS(A) produced from CSI and OCS, the vibrational populations of CS(A) obtained have been compared with thedistributions predicted from the impulsive half-collision mode1.22 This simple model has the advantage of being easy to apply. When a CSZor OCS molecule is excited to a repulsive state, the CS(A) fragment will occupy a range of initial vibrational levels determined by the Franck-Condon principle. During the separation of CS (and S or 0) fragments, CS(A) is transferred into a set of final levels by the recoil force. On the assumption that the vibrational level of CS(A) is that of a harmonic oscillator and the recoil force corresponds to an exponentially repulsive potential, the final population P,, of the u' level is given byz329.30
V I B. ENERGY 10V
0.5
1.(
+
+
min(i,v')
P,, =
N,i!v'!(Au)'+' ex*(-nu)[ i
(-A~)-/
c o I!(i - l)!(v' - I)!
1
2
where Ni is the population of the initial vibrational level i and Au represents the average number of vibrational quanta transferred. If we assume that the initial levels are only the ground level i = 0, the final populations are reduced to a Poisson distribution.z Nevertheless, it is evident that the observed populations cannot be represented by a Poisson distribution. This suggests that initial levels also form in higher levels. Thus, we have calculated vibrational populations under two types of constraint, assuming that the Au value is the same for all transitions; for type I it is assumed that the i = 0 and 1 levels are initially populated, and for type I1 the i = 0-2 levels are initially populated. The best-fit distributions are displayed in Figure 3 to be compared with the observed values and fitting parameters listed in Table 1. The agreement between the observed data and the populations obtained for type I1 is slightly better than that for type I. For OCS the calculated PI/Poratio is, however, stiIl higher than the observed. The Au values obtained in this study are apparently larger than the value of 1.9 obtained in photodissociationZof CS2 and OCS at 92.3 nm and the value of 1.6 obtained from the Ar(") + CSZ reaction.19 This indicates that theexcess energy in thedissociation by electron impact is larger than those for the others.
=I
n0.50 n
t
0.2t.
0
,
,
,
,
1
2
3
4
,
,
,
:
5
6
7
6
V'
Figure 3. Comparison of the vibrational populations of CS(A) (0) measured by electron impact on CS2 and OCS with the best-fit results calculated from the impulsivehalf-collision model; broken and solid lines indicate the populations obtained under type I and type I1 constraints,
respectively.
.
A f16t 0
0
X-
ocs
10
i 0
20
40
60 80 100 120 ELECTRON ENERGY/oV
140
160
Figure 4. Emission cross sections of the CS(A-X) band obtained from (0)CS2 and (X) OCS: the ECSs for OCS represent
of the symbols.
Emission Cross Sections. The emission cross section (ECS) of the CS(A-X) band has been evaluated on the basis of the ECS of the 0-0 band for the N2+(B-X) transition from N2 at 100 eV, (1 -74 f 0.17) X 10-1' cm2.31 The procedure for evaluation of ECS has been described elsewhere.23 The CS(A-X) emission intensity was measured in the 245275-nm range. Those intensities at the wavelengths shorter than 245 nm and longer than 275 nm were estimated to be (7.5 f 1 3 % of the total emission from the best-fit synthetic spectrum. Thus, the ECSs of the CS(A-X) transition produced from CS2 and OCS are evaluated to be (10 f 2) and (0.75 f 0.15) X 10-18 cmz, respectively, a t an impact energy of 100 eV. The ECS from OCS is less than 1/10 of the ECS from CS2 a t 100 eV since the CO+(A-X) and CO+(B-X) bands produced from OCS are prominent a t 100 eV. The ECS measured from CSz is slightly cm2.IO smaller than the reported values (12 3) X Figure 4 shows the ECSs for the CS(A-X) band plotted against the impact energy. These excitation functions show similar features: first peaks appear in the 14-17-eV range and second peaks in the 30-40-eV range. The excitation function observed from CSzis very similar to those reported by Ajello and Srivastava.lo TheonsetsoftheCS(A-X) bandfromCS2andOCSareevaluated to be 9.7 f 0.3 and 12.8 f 0.6 eV, respectively, on the basis of the onset (1 1.2 eV) of the N2(C-B) band from Nz.32 These first
*
Vibrational Populations of CS(AIII)
The Journal of Physical Chemistry, Vol. 98, NO. 14, 1994 3591
TABLE 1: Parameters Used for the Best-Fit Vibrational Distributions of the Impulsive Half-Collision Model parent type Av No Ni Ni cs2 I 2.61 1 .oo 2.13 I1 I I1
ocs
-
1.oo 1.00 1.oo
2.72 2.93 3.23
1.51 2.26 1.85
0.76 0.84
onsets correspond to the following dissociations:
CS,
+ S(3P) CS(A'II) + O(3P)
CS(A'II)
AH = 9.37 f 0.26 eV
(1)
OCS AH = 11.73 f 0.26 eV (2) The thresholds for these reactions were calculated by using the enthalpies of formation33and the electronic energy of the CS(A) ~ t a t e . 3 Processes ~ 1 and 2 are induced by the spin-forbidden excitations via electron exchange between the incident electron and a target electron. The next channel for formation of CS(A) is the process of CS2 CS(AIII) S(lD) with AH = 10.5 f 0.3 eV or OCS CS(AIII) + O(1D) with AH = 13.7 f 0.3eV via the spin-allowed excitation. Thus, these processes can partly contribute to the internal state distribution of CS(A) measured in this study. Although the second onsets near 20-25 eV are relatively difficult toassign, themost probableare the dissociative ionizations as follows:
--
CS,
-
OCS
CS(A'II)
+
+ S+(4S) + e
AH = 19.63 f 0.26 eV (3)
CS(A'II) + O'(4S)
+e
internal energy distribution of CS(A) suggests that the repulsive potential curve on which the fragments dissociate is very steep and that linear geometry is preserved during dissociation. The dissociationdynamics is approximately described by the impulsive half-collision model. Nevertheless, the fraction of translational energy estimated for dissociation of OCS is inconsistent with the impulsive half-collision model. Other experimental results, especially the translational energy disposal,35 are required for more quantitative understanding of the excited-state fragmentation dynamics of CS2 and OCS.
AH = 25.35 f 0.26 eV
(4) Nevertheless, several dissociations containing an excited atom or ion as a byproduct can occur above the second onsets. The differences between the observed onsets and the thresholds calculated for processes 1 and 2 should be the excess energy, which is available as the translational and internal energies of the fragments; the differences are 0.3 eV for CS2 and 1.1 eV for OCS. On the other hand, the vibrational energy disposal (Ev) of CS(A) estimated from the vibrational temperatures is 1.1 eV for CS2 and 2.2eV for OCS, while the rotational energy disposal (E,) derived from TOis 0.3 eV for both parents. It follows that the available energies for formation of CS(A) from electron impact on CS2 at 12 eV and on OCS at 17 eV are far higher than the differences between the observed onset and the calculated threshold. The maximum available energy (E,,) can be estimated from the difference between the impact energy and the thresholds for process 1 and 2;E, for CS2 and OCS are 2.6and 5.3 eV, respectively. The vibrational energy disposals are much larger than the rotational energy disposals. Thus, the collinearity is very likely maintained in dissociation of CS2 and OCS. Since the dissociations of CS2 and OCS can be approximately described as impulsive models, the fractions of the translational energy released in the dissociations of CS2 and OCS can be estimated to be 0.47 and 0.74,re~pectively.3~The translational energies (E,) estimated from these fractions combined with the E, values are 1.2 eV for CS2 and 3.9 eV for OCS, while the differences Et = E, E, - E, are 1.2 eV for CS2 and 2.8 eV for OCS. This result indicates that dissociation of CS2 at 12 eV is represented by the impulsive half-collision model, whereas that of OCS at 17 eV is not. The available energy from dissociation of OCS seems to be selectively converted into the vibrational energy of CS(A).
Conclusions In this study, the excited-state fragmentation dynamics of CS2 and OCS is extracted from the fluorescence spectra produced by electron-impact dissociation. The experimental evidence for the
Acknowledgment. We are grateful to Dr. H. Ito of Nagaoka University of Technology for helpful comments on the calculation of the RKR potentials and to Dr. M. Tsuji of Kyushu University for fruitful discussions. References and Notes (1) Okabe, H. J . Chem. Phys. 1972,56, 4381. (2) Lee, L. C.; Judge, D. L. J. Chem. Phys. 1975,63, 2782. (3) Kligler, D. J.; Pummer, H.; Bischel, W. K.; Rhodes, C. K. J. Chem. Phys. 1978,69,4652. (4) Yang, S.C.; Freedman, A.; Kawasaki, M.; Bersohn, R. J . Chcm. Phys. 1980, 72, 4058. (5) Ashfold, M. N. R.; Quinton, A. M.; Simons, J. P. J . Chem. Soc. Faraday Trans. 2 1980, 76,905. (6) Butler, J. E.; Drozdoski, W. S.; McDonald, J. R. Chem. Phys. 1980, 50, 413. (7) Day, R. L.; Suto, M.; Lee, L. C. J. Phys. B 1982, 15, 4403. (8) Horani, M.; Leach, S.;Rostas, J.; Berthier, G. J. Chim. Phys. 1966, 63, 1015. (9) Toyoda, M.; Ogawa, T.; Ishibashi, N. Bull. Chem. Soc. Jpn. 1974, 47, 95. (10) Ajello, J. M.; Srivastava, S. K. J. Chem. Phys. 1981, 75, 4454. (11) Taylor, G. W.; Setser, D. W. J. Mol. Specrrosc. 1972, 44, 108. (12) Taylor, G. W. J. Phys. Chem. 1973, 77, 124. (13) Coxon, J. A,; Marcoux, P. J.; Setser, D. W. Chem. Phys. 1976,17, 403. (14) Yencha, A. J.; Wu, K. T. Chem. Phys. 1980,49, 127. (15) Tsuji, M.; Matsuo, M.; Nishimura, Y. Int. J. Mass Spectrom. Ion Phys. 1980,34,273. (16) Tsuji, M.; Obase, H.; Matsuo, M.; Endoh, M.; Nishimura, Y. Chcm. Phys. 1980, 50, 195. (17) TabchbFouhaile,A,;Hubin-Franskin, M.-J.;Delwiche, J. P.; Fr(lhlich, H.; Ito, K.; Guyon, P.-M.; Nenner, I. J. Chem. Phys. 1983, 79, 5894. (18) Sekiya, H.; Tsuji, M.; Nishimura, Y. Chem. Phys. Lett. 1983,100, 494. (19) Wu, K. T. J. Phys. Chem. 1985,89,4617. (20) Cossart, D. J . Chim. Phys. 1981, 78, 711. (21) Xu, D.; Li, X.; Shen, G.; Wang, L.; Chen,H.; Lou, N. Chem. Phys. Lett. 1993, 210, 315. (22) Simons, J. P.; Tasker, P. W. Mol. Phys. 1973, 26, 1267. (23) Tokue, I.; Kudo, M.; Kusakabe, M.; Honda, T.; Ito, Y. J . Chcm. Phys. 1992, 96, 8889. (24) Bergeman, T.; Cossart, D. J. Mol. Spectrosc. 1981, 87, 119. (25) Hara, K.; Phillips, D. J. Chem. Soc. Faraday Trans. 2 1978, 74, 1441. (26) Herzberg, G. MolucularSpectraandMolecularStructwcI. Spectra of Diatomic Molecules; Van Nostrand Reinhold: New York, 1950. (27) Carlson, T. A.; Copley, J.; Duric, N.;Erman, P.; Larrson, M. Chcm. Phys. 1979, 42, 81. (28) de Vries, M. S.;Tyndall, G. M.; Colb, C. L.; Martin, R. M. J . Chem. Phys. 1987,86, 2653. (29) Holdy, K. E.; Klotz, L. C.; Wilson, K. E. J. Chem. Phys. 1970,52, 4588. (30) Heidrich, F. E.; Wilson, K. E.; Rapp, D. J . Chcm. Phys. 1971, 54, 3885. (31) Borst, W. L.; Zipf, E. C. Phys. Reu. A 1970, I , 834. (32) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; Van Nostrand-Reinhold New York, 1979. . (33) Chase, M. W., Jr.; Davies, C. A,; Downey, J. R., Jr.; Frurip, D. J.; mcDonald, R. A,; Syverud, A. N . J . Phys. Chem. ReJ Data 1985,14, Suppl. 1. (34) Bush, G. E.; Wilson, K. R. J . Chem. Phys. 1972, 56, 3655. (35) Nan, G.; Burak, I.; Houston, P. L. Chem. Phys. Lett. 1993, 209, 383.
