J. Phys. Chem. 1985, 89, 3609-361 1 the C-C bonds break, also in agreement with single crystal s t u d i e ~ . ~The * ~ species formed are predominantly single carbon atoms (no hydrogen attached) and a small amount of CH3, in contrast with the situation on P t ( l l 1 ) surfaces in single crystal studies, in which it has been proposed that the carbon is in C H
3609
or C H 2 groups. Acknowledgment. This research was supported in part by the Department of Energy, Division of Materials Sciences, Contract No. DE-AC02-76ER01198.
State-Resolved Photofragmentation of OCS Monomers and Clusters N. Sivakumar, I. Burak,t W.-Y. Cheung,*P. L. Houston,* Department of Chemistry, Cornell University, Ithaca, New York 14853
and J. W. Hepburn Center for Molecular Beams and Laser Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada (Received: June 3, 1985)
Dissociation of the OCS monomer at 222 nm yields S('D) and S(3P) in the ratio 0.85/0.15 and CO which is >98% in u = 0 but with very high rotational energy. The peak in the rotational distribution occurs at J = 55 and corresponds to about 56% of the available energy for the S('D) channel. These results support the contention that the upper OCS 'A state is bent. Dissociation of OCS clusters leads to a completely different photochemistry: at least some of the CO is formed rotationally cold (Trot= 50 K), and Sz is also produced.
Introduction State-resolved photofragmentation of both triatomic moleculesId and van der Waals is currently receiving much attention. In the former case, the dynamics of the dissociation process can be used to infer the shape of the dissociative potential surface, while in the latter case the distribution of energy in the fragments gives insight into the mechanism by which electronic and vibrational energy migrates between strong and weak bonds. In this paper we report the distribution of energy in the S and C O fragments formed from dissociation of OCS monomers and (OCS), clusters at 222 nm. Excitation of the monomer at this wavelength is through a weak IB' transition absorption band that has been assigned to a l A from the linear ground state to a bent upper Previous work has shown that both S(lD) and S(3P) are primary photoproducts; studies of the secondary photochemistry suggest that the branching ratio of S(')/S(3P) is 0.75/0.25.13314 N o direct measurements have been performed either on the distribution of the S atom states or on the distribution of internal energy in the C O fragment. Photodissociation of van der Waals clusters of OCS in this region of the ultraviolet has never before been reported. We find that dissociation of (OCS), yields significantly different products than dissociation of the monomer. Specifically, C O is formed in very low rotational levels (TrotE 50 K). S2is also detected as a product of the dissociation, suggesting that a chemical reaction occurs in the cluster as a result of the dissociation. To our knowledge, this is one of the few examples in which excitation has been observed to result in chemical reaction between the weakly bound components of a van der Waals c l ~ s t e r . ' ~
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Experimental Section Photolysis of the OCS monomers and clusters was performed in a pulsed supersonic jet apparatus similar to that described earlier.16 Mixtures were prepared by flowing helium over OCS which was held in a trap at a specified temperature. The seeded gas was then expanded from a total pressure of 1300 torr through a 0.5-mm-diameter pinhole by using a pulsed nozzle assembly Permanent address: Department of Chemistry, University of Tel Aviv, Tel Aviv, Israel. 'Current address: Geo-Centers, P.O. Box 523, Wharton, NJ 07885.
0022-3654/85/2089-3609$01 SO10
(Newport). A KrCl excimer laser (Lumonics TE861-4) dissociated the molecular jet roughly 1.25 cm from the nozzle source. The S(lD), S(3P), and CO(X'Z,v,J) products were probed by laser-induced fluorescence using a tunable vacuum-UV source based on four-wave mixing in magnesium vapor.17 S('D) and S(3P)were detected by the 'Pol +- ID2and 3D03 3P2or 'Do2 3P2transitions, respectively, while C O was detected by the A'II X'B transition. The vacuum-UV source has been described in detail in a recent study of the photodissociation of glyoxalIs and is very similar to that used previously for detection of Br and C0.1s24 The S2products were probed with a frequency-doubled
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+-
(1) R. Bersohn, J . Phys. Chem., 88, 5145 (1984). (2) H. Okabe, "Photochemistry of Small Molecules", Wiley-Interscience, New York, 1978. (3) S. R. Leone, Adu. Chem. Phvs., 50, 255 (1982) (4) R. Bersohn, ZEEE J . Quantum Electron.;QE-16, 1208 (1980). ( 5 ) C. H. Greene and R. N . Zare, Annu. Rev. Phys. Chem., 33, 119 (1982). (6) M. Shapiro and R. Bersohn, Annu. Reu. Phys. Chem., 33,409 (1982). (7) D. H. Levy, C. A. Haynam, and D. V. Brumbaugh, Faraday Discuss. Chem. SOC., 73, 137 (1982). (8) N. Halberstadt and J. A. Beswick, Faraday Discuss. Chem. SOC.,73, 357 (1982). (9) D. S. King, J . Chem. Phys., to be published. (10) W. H. Breckenridne and H. Taube. J . Chem. Phvs., 52. 1713 (1970). i l l i B. M. Ferro and BYG. Reuben. Trans. Faradav Soc.. 67.2847 f1971j. (12j J. W. Rabalais, J. M. McDonald, V. Scheh, andS.'P. McGlynn, Chem. Rev., 71, 73 (1971). (13) H. E. Gunning and 0. P. Strausz, Adu. Photochem., 4, 133 (1966). (14) W. H. Breckenridae and H. Taube, J . Chem. Phvs.,53, 1750 (1970). (15) For another receit example see S. Buelow, G: Radhakrishnan, J. Catanzarite, and C. Wittig, to be published. (16) R. D. Bower, R. W. Jones, and P. L. Houston, J . Chem. Phys., 79, 2799 (1983). (17) S. C. Wallace and G. Zdasiuk, Appl. Phys. Lett., 28, 449 (1976). (18) J. W. Hepburn, N. Sivakumar, and P. L. Houston in 'Laser Techniques in the Extreme Ultraviolet", S. E. Harris and T. B. Lucatorto, Eds., American Institute of Physics, New York, AIP Conf. Proc. No. 119, pp 126-1 34. (19) J. W. Hepburn, D. Klimek, K. Lui, J. C. Polanyi, and S. C. Wallace, J . Chem. Phys., 69, 4311 (1978). (20) J. W. Hepburn, D. Klimek, K. Liu, R. G. Macdonald, F. J. Northrup, and J. C. Polanyi, J. Chem. Phys., 74, 6226 (1981). (21) J. W. Hepburn, K. Liu, R. G. Macdonald, F. J. Northrup, and J. C. Polanyi, J . Chem. Phys., 75, 3353 (1981). (22) J. W. Hepburn, F. J. Northrup, G. L. Ogram, J. M. Williamson, and J. C. Polanyi, Chem. Phys. Lert., 85, 227 (1982).
0 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 68970
s
69045
69120
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x 0 095 1Po~-1D2
:
I
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35
Figure 1. Laser-induced fluorescence spectrum of the S and CO products formed following photodissociation of OCS monomers at 222 nm. The three most intense lines are identified as transitions in the S atom, while the remainder of lines correspond to high rotational transitions on the CO (3,O) A X band.
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dye laser pumped by a XeCl excimer laser (Lambda Physik 150ES, 2002E). The molecular jet, the dissociation laser, and the ultraviolet or vacuum-UV beam propagated in mutually orthof m a l directions. Fluorescence from the dissociation products w i s detected at 45O from each laser and 90° from the jet by an EMR solar-blind photomultiplier tube (541G-09-17) or, in the case of S2, by a Hamamatsu photomultiplier (R928). A second solar-blind photomultiplier detected a reflection of the tunable vacuum-UV light; its signal was used to normalize the spectra of S and CO for variations in the probe laser intensity.
Results and Discussion The S ( 1 D ) / S ( 3 PBranching ) Ratio for Dissociation of the Monomer. Figure 1 shows a portion of the laser-induced fluorescence spectrum of the products obtained when the OCS partial pressure behind the nozzle was 40 torr. The most prominent lines are due to laser-induced fluorescence of S atoms on the 3D02 3P2,3D03 3P2,and lPol ID2 transitions, as indicated in the figure. In bulb experiments to be described in detail elsewhere,25 we have quenched the S(lD) with N2 in order to determine experimentally the ratio of the 3D03 3P2to 'Pol 'D2 line strengths. The measured ratio was found to be 1.01 f 0.03 and is within the error limits of the values given in the literature.26 From the measured line strength ratio and the from the intensities of the lines in our unrelaxed spectra of Figure 1, we find that the S(lD) branching ratio for dissociation of the monomer at 222 nm is 0.85 f 0.05. This value is slightly higher than that of 0.75 obtained by using photochemical trapping methods following dissociation at 253.7 and 228.8 nm.13,14Reasons why these trapping methods might yield a low value for the S('D) yield have been discussed previously by van Veen et al.,27who used twophoton laser-induced fluorescence to probe the S(3P) and S('D) atoms following dissociation of OCS at 248 nm. While these latter authors argue that the quantum yield for S(lD) is 1.0 at 248 nm, their data admit the possibility of some S(3P)production. In any event, there is no reason a priori for the quantum yield of S('D) to be the same as 248 and 222 nm, so that our result of 0.85 0.05 at 222 nm is not at variance with their finding.
-
-
+-
O 0
45
55
65
75
Rotational Level (J) Figure 2. Rotational distribution of CO formed following the dissociation of OCS monomers at 222 nm. The peak in the distribution at J = 55 corresponds to disposal of 56% of the available energy into rotation, assuming that S('D) is also produced. Assuming that the subsidiary peak at J = 66 corresponds to the S(3P)channel, then 43% of the available energy is produced as CO rotation for the triplet channel.
675OC
s
677 *IO'20
Frequency 67625(cm-')
400 To-r OCS Pressdire
b
I
li
+
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(23) P. Ho and A. V. Smith, Chem. Phys. Lett., 90, 407 (1982). (24) D.J. Bamford, S. V. Filseth, M. F. Foltz, J. W. Hepburn, and C. B. Moore, J . Chem. Phys., 82, 3032 (1985). (25) N. Sivakumar, I. Burak, P. L. Houston, and J. W. Hepburn, in
preparation.
Band
-
0
67950
Frequency ( c m - ' )
-
c?
C O 3-0
0
67650
Letters
(26) W. L. Wiese, M. W. Smith, and B. M. Miles, Natl. Stand. Ref: Data Ser. ( U S . , Natl. Bur. Stand.), NSRDS-NBS 22, 134-135 (1969). (27) N. van Veen, P. Brewer, P. Das, and R. Bersohn, J . Chem. Phys., 79, 4295 (1983).
67532
67625
f775c
FreqLercy (cm-)
*IO*
Figure 3. (a) CO (2,O) band head region interspersed with high rotational lines of the (3,O) band; the OCS pressure behind the nozzle was 40 torr. (b) The same spectral region for a partial pressure of OCS behind the nozzle equal to 400 torr. Note the relative increase in the (2,O) band head intensity near 67637 cm-].
The CO(u,J) Distribution for Dissociation of the Monomer. The remaining lines in Figure 1 are assigned to laser-induced fluorescence of CO(u=O,J=51-63) on the (3,O) A X band. The rotational distribution obtained from the complete spectrum is shown in Figure 2. It is clear that very high rotational states of CO are formed in the dissociation of OCS, as might be expected for a linear-to-bent dissociative transition. The peak of the CO rotational distribution at J = 55 corresponds to an energy of 5948 cm-', which is 56% of the 10642 cm-l available for the S(lD) channel. The rotational distribution shows a subsidiary peak with a maximum a t J = 66 which contains roughly 15/85 of the intensity of the peak at J = 55. If this subsidiary peak corresponds to the S(3P) channel, then the rotational energy of 8535 cm-' represents 43% of the total available energy, 19 879 cm-l. Despite extensive searches, we were unable to observe any spectral features
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J . Phys. Chem. 1985, 89, 3611-3614 corresponding to CO(v=l). From the signal-to-noise ratio of our spectra, we place an upper limit of 2% on the amount of CO(v=l) produced by the dissociation. It thus appears that the C O product of OCS dissociation at 222 nm is formed in very excited rotational levels but almost entirely in V = 0. Dissociation of OCS Clusters. The most striking result of our study is the observation of state-resolved photodissociation of OCS clusters. Figure 3 shows the region of the C O (2,O)band head near 67 625 cm-', which is interspersed in the high rotational lines of the (3,O) band. The (2,O) band head is barely visible at -67 637 cm-' in Figure 3a, for which the partial pressure of OCS behind the nozzle is 40 torr, but it is clearly observed in Figure 3b, for which the partial pressure is 400 torr. The C O rotational distribution from the band head region is roughly Boltzmann with a temperature of 50 K. Several observations indicate that this rotationally cold C O results from dissociation of OCS clusters: (1) Detailed experimentsZSshow that the integrated intensity of the (2,O) band head increases as the square of the O C S partial pressure behind the nozzle, while the integrated intensity of the high rotational lines increases somewhat less than linearly. (2) The presence of clusters in our nozzle source has been confirmed in separate experiments using a quadrupole mass spectrometer. Large clusters, up to (OCS)8, were observed with 400 torr of OCS behind the nozzle, while only monomers and some dimers were observed with an OCS partial pressure of 40 torr. (3) Appearance times for lines in both the band head region and the high-J region were obtained by delaying the pulsed probe laser relative to the dissociation laser. In both cases the C O intensity appeared in less than 50 ns, the time required for our photomultiplier to recover from scattered laser light, and no subsequent rise was observed. Rotational relaxation of CO(J=55) to CO(J
