J . Phys. Chem. 1988, 92, 1163-1 164
1163
CS+(A2nrX22+) Chemiluminescence Produced in the Reaction of C+(2P) with CS, at Thermal Energy Masabaru Tsuji,* Kazumi Mizukami,+ Hiroshi Obase, and Yukio Nishimura Research Institute of Industrial Science and Department of Molecular Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 81 6, Japan (Received: April 29, 1987; In Final Form: September 18. 1987)
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CS+(A211i-X2Z+)chemiluminescence has been observed from the C+(,P) CS2reaction in a flowing afterglow. The vibrational distribution of CS+(A,u' = 1-8) was nearly exponential with an effective Boltzmann temperature of 2800 & 300 K. The average fraction of vibrational energy deposited into CS+(A) was estimated to be 0.1 1.
Introduction The reactivity of carbon cations in the ground state, C+(,P), upon collisions with small molecules has been a subject of active study because of its importance in atmospheric and astrophysical chemistry. We have studied ion-molecule reactions of C+(,P) with di- and triatomic molecules at thermal energy by observing UV and visible chemiluminescence from excited products in the flowing afterg1ow.l4 In a previous paper,3 CS+(A211,-X2Z+) chemiluminescence produced from the C+(,P) + OCS reaction has been reported. New emissions of CS+ (A-X) from high vibrational levels of u' = 10-14 to u" = 0-3 have been identified in the 460-560-nm region. The vibrational population of CS+(A211i)for u ' = 1-14 was approximately exponential with a Boltzmann temperature of 5000 & 300 K. In this communication, CS+(A-X) chemiluminescence from the C+(,P) + CS2 reaction has been investigated. The mechanism of energy disposal into CS+(A) vibration is discussed by comparing the observed vibrational distribution with a statistical prior one. To the best of our knowledge, the rate constant and the product distribution have not been measured for the C+(*P) + CS2 system.
Experimental Section The flowing-afterglow apparatus used in the present study has been described previously.'S2 Briefly, the H e buffer gas was introduced into a flow tube fitted with a high-capacity mechanical booster pump. He+ and He(23S) were generated by a microwave discharge and flowed downstream past an inlet of the C O source gas placed 10 cm downstream from the center of the discharge. Reactant C+(zP)ions were produced by the He+ + C O dissociative charge-transfer reaction and carried past another inlet of CSz located 10 cm downstream from the first inlet. In order to determine the excitation source of the resulting chemiluminescence, an experiment using a pair of ion-collector grids was undertaken. The pressure in the reaction zone was 0.4-1.2 Torr for He, 10-60 mTorr for CO, and about 20 mTorr for CS,, as measured by an MKS Baratron gauge. UV and visible chemiluminescence in the 200-800-nm region was dispersed by a Spex 1269 monochromator equipped with a cooled Hamamatsu Photonics R376 photomultiplier. The wavelength response was calibrated by a standard halogen lamp. Results and Discussion The reactions of He active species (He(2%), He+) with C O in the flowing afterglow give C+(,P), CO+(X2Z+),C(3P, 'D,'S), and O(3P, ID, IS)as secondary Figure l a shows the emission spectrum obtained from reactions of these reactants with CS, i,n the t00-800-nm region. The spectrum consists of the CS2+(AZIIu-XZII,)emission due to the CO+ (X2Z+) + CS2 reaction reported previously8 and the CS+(AZIIi-XZZ+)chemiluminescence on which the present work is focused. Although this chemiluminescence was very weak in the previous measurement,8 a significant enhancement of the emission intensity is 'Present address: Nippon Steel Co., Nakahara-ku, Kawasaki 21 1, Japan.
0022-3654/88/2092-1163$01.50/0
TABLE I: Observed and Calculated Vibrational Distributions of CS+(A) in the C+(*P) CS, Reaction at Thermal Energy -1n (Pd/P,,O) U' Pd" PdO 0 0.40 0.31 -0.25
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1 2 3 4
0.27 0.14 0.073 0.039 0.027 0.023 0.01 1 0.0082
5 6 7 8
0.23 0.16 0.1 1 0.076 0.048 0.029 0.016 0.0080
-0.16 0.13 0.41 0.67 0.59 0.23 0.37 -0.025
Uncertainty &lo%. achieved by increasing the [He+]l[He(23S)]concentration ratio. The absence of CS2+(82~u+-XZII,)and CS+(B2P-A211i) emissions observed in the H e afterglow reactions of CS28-11indicates that He(2%) and He+ are completely quenched before reaching the reaction zone and make no contribution to the formation of CS+(A). When C+(,P) ions produced from the He+ C O reaction were trapped with the ion-collector grids, the CS+(A-X) chemiluminescence disappeared almost completely as shown in Figure 1b. On the basis of these facts and the energetics, the CS+ (A-X) chemiluminescence was concluded to be excited from the ion-molecule reaction: C+('P) + CS2 CS+(A211i, U' = 0) + C S AHoo = -1.39 eV (1) The mean total available energy for reaction 1, (Etot),is 1.45 eV by adding the thermal energy of collision (5/2R7') to the heat of formation. This energy is capable of exciting the CS+(A) state up to u' = 12 based upon known spectroscopic constant^.^,^*^^ Although the CS+(A-X) che_miluminescence is partially superimposed upon the CS2+ (A-X) emission, a careful comparison between parts a and b of Figure 1 allows us to identify vibronic transitions from u' = 1-8 in the 535-800-nm region. The relative populations in the CS+(A,u'= 1-8) levels were determined from the relative intensity of each vibronic band by using the same analytical method as that reported previ~usly.~
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(1) Tsuji, M.; Susuki, T.; Endoh, M.; Nishimura. Y. Chem. Phys. Lett. 1982, 86, 41 1.
(2) Tsuji, M.; Susuki,T.; Mizukami, K.; Nishimura, Y. J . Chem. Phys. 1985, 83, 1677. (3) Mizukami, K.; Obase, H.; Tsuji, M.; Nishimura, Y. Chem. Phys. Letf. 1985, 116, 510.
(4) Tsuji, M.; Nagano, I.; Susuki, T.; Mizukami, K.; Obase, H.; Nishimura, Y. J. Phys. Chem. 1986, 90, 3998. ( 5 ) Hurt, W. B.; Grable, W. C. J . Chem. Phys. 1972, 57, 734. (6) Richardson, W. C.; Setser, D. W. J . Chem. Phys. 1973, 58, 1809. (7) Rakshit, A. B.; Stock, H. M. P.; Wareing, D. P.; Twiddy, N. D. J. Phys. B 1978, 11, 4237. (8) Tsuji, M.; Mizukami, K.; Sekiya, H.; Obase, H.; Shimada, S.; Nishimura, Y. Chem. Phys. Left. 1984, 107, 389. (9) Coxon, J. A.; Marcoux, P. J.; Setser, D. W. Chem. Phys. 1975,17,403. (IO) Yencha, A. J.; Wu, K. T. Chem. Phys. 1980, 49, 127. ( I I ) Tsuji, M.; Obase, H.; Matsuo, M.; Endoh, M.; Nishimura, Y. Chem. Phys. 1980, 50, 195. (12) Gauyacq, D.; Horani, M. Can. J . Phys. 1978, 56, 587.
0 1988 American Chemical Society
1164 The Journal of Physical Chemistry, Vol. 92, No. 5, 1988
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Tsuji et al.
c Si(x - I ) i
i i
CSYA-X)
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-A-
Figure 2. C, symmetry correlation diagram between (C
cs++ cs.
study such as measurements of angular distribution of products is required. The mean vibrational energy deposited into CS+(A), ( E d ) ,and were estimated to its fraction of the total available energy, 0.16 eV and 0.1 1, respectively, from the observed distribution. The corresponding values in the C+(2P) OCS reaction were = 0.13. evaluated from reported data:3 ( E d ) = 0.38 eV and Ud) value suggests that the viA reasonable agreement of the Vu() brational energy disposal into CS+(A) is similar between the two systems. If the nascent rotational distribution of CS+(A) is determined from the observed CS+(A-X) chemiluminescence, more detailed information about energy disposal and reaction dynamics is obtained. The rotational profile of CS+(A-X) is characterized by a Boltzmann temperature of -300 K. Although the radiative lifetime of CS+(A) has not been measured, very long lifetimes of 9.4-22.3 ps have been theoretically predicted for the six lowest vibrational 1 e ~ e l s . l ~It is, therefore, reasonable to assume that the rotational relaxation of CS+(A) is significant in the H e afterglow as in the case of CO+(A) with shorter radiative lifetimes of 2-4 ps.1,2 A further low-pressure experiment under singlecollision conditions is necessary in order to determine the nascent rotational distribution. Figure 2 shows the adiabatic correlation diagram between the entrance and exit channels constructed from Shuler's tableL6 assuming a planar symmetry (C,) for reaction intermediates. It is seen in Figure 2 that the entrance C+(*P) + CS,(X) channel does not correlate to the observed CS+(Aj + CS(X) product channel, but the lower lying C(3P) CS2+(R) reactant channel correlates to the product channel adiabatically through the 2A' + 2A" surfaces. One could imagine from this diagram that at least one nonadiabatic charge-transfer transition initially takes place from an entrance C+(2P) CS2 potential to a C(3P) CS2+(X) potential through the 2A' and/or 2A'' surfaces in the avoided crossing region, and the subsequent adiabatic dissociation of the charge-transfer intermediate provides the CS+(A) CS(X) products through the other 2Afand/or 2Af' surfaces. A similar reaction process was expected for the CS+(A) formation from the C+ + OCS CS+(A) + C O reaction on the basis of the correlation diagram. A direct excitation process through an adiabatic pathway is not open and at least one nonadiabatic transition between an entrance C+(2P) OCS potential and a C(3P) OCS+(R) potential must be involved in the reaction. Theoretical calculations of potential surfaces and trajectory are required to obtain detailed information about the reaction dynamics. In summary, the chemiluminescent study of the C+(2P) CS2 reaction has shown that the CS+(A) + CS(X) product channel is open at thermal energy. The energy disposal into CS+(A) vibration was found to be similar to that in the C+(2P) OCS reaction. The adiabatic correlation diagram suggested that nonadiabatical transitions are involved for the formation of CS+(A).
vd),
L
600
500
70 0
800 nm Figure 1. Chemiluminescence spectra produced from reactions of (a) Het, He(2%)/CO + CS, and (b) He(2'S)/CO + CS2. Stray He I lines are marked with a dot. A weak CO+(A-X) emission is a stray light from the He(23S)/C0 Penning ionization which strongly occurs around the CO gas inlet. 400
+ CS2)+and
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Since the CS+(A-X) emission produced from the He(2%) CS2 dissociative Penning ionization could be observed at wide operating pressure range in comparison with the present reaction system, it was used to determine the effect of vibrational relaxation of CS+(A) by collisions with He buffer gas. No significant pressure dependence was found for the CS+(A) vibrational distribution in the He pressure range of 0.2-1.0 Torr, indicating that the collisional relaxation was insignificant under operating conditions. The CS+(A) vibrational distribution in the C+(2P) CS2reaction is shown in Table I. It is represented approximately by a single Boltzmann temperature of 2800 f 300 K. This value is lower than that in the C+(*P) OCS reaction (5000 f 300 K),3 In Table I is listed a statistical prior distribution for comparison, which was evaluated from the relation,13
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Pd0 a ( I - f C , ) ' / 2 (2) Here h,is given by E , / (E,,,), Ed being the vibrational energy of CS+(A). The observed distribution is less vibrationally excited than the prior one. One reason for this discrepancy may be effects of angular momentum constraints which are ignored in the simple statistical distribution calculated here. The deviation from the prior distribution can be expressed in the form of surprisal -In (Pd/Pdo)and a linear relation has often been obtained between The surprisal calculated for the present -In (Pd/Pdo)and f*l3,I4 system is included in Table I. The surprisal changes signs twice, because PL,decreases more rapidly than Pdo for low u'levels, while the decrease in Pd becomes more slow than Pdo for high u'levels. The absence of linearity in the vibrational surprisal plot against f d implies an absence of an exponential gap behavior between the observed and prior distributions. A similar deviation from statistical prediction has been found for the vibrational distributions of CO+(A) in the C+(2P) + NO2 and N 2 0 reactions with high total available energies: (Etot)= 2.78 and 4.21 eV for the former and the latter, respectively.2 Thus the low vibrational excitation in the present system with a relatively high ( E t o t )value may be due to not only the effects of angular momentum constraints in the exit channel but also the fact that the lifetime of collision complex is shorter than expected; therefore, the intermediate complex does not sustain long enough to allow complete energy randomization. According to this explanation, the reaction would proceed via a direct mechanism rather than a complex-forming one. In order to confirm this prediction, a further experimental (13) Levine, R. D.; Kinsey, J. L. In Atom-Molecule Collision Theory; Bernstein R. B., Ed.: Plenum: New York, 1979. ( 14) Bernsdn, R. B. Chemical Dynamics uia Molecular Beam and Laser Techniques; Oxford University Press: London, 1982.
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Acknowledgment. We are indebted to Professor T. Ogawa for allowing us to use a Spex 1269 monochromator. Registry No. CS2, 75-15-0; C + , 14067-05-1; C S , 12351-95-0. ( 1 5) Larsson, M. Chem. Phys. Left. 1985, 1 Z7, 33 1. (16) Shuler, K. E. J . Chem. Phys. 1953, 21, 624
