J. Phys. Chem. l%M,98, 765-767
765
CARS Detection of Cl02 Jian-Xiang Zbang and Pamela M. Aker’ Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania I5260 Received: September 17, 1993; In Final Form: November 8, 1993’
Coherent anti-Stokes Raman scattering (CARS) spectroscopy has been used to detect the V I fundamental of gas-phase Cl02. CARS detection limits are reported. The results show that the CARS technique is ideally suited for gas-phase Cl02 photodissociation dynamics studies.
Introduction C102 is an important molecule in the kinetic scheme of stratospheric ozone depletion.’ From an atmospheric chemistry perspective, Cl02 is “schizophrenic”. It can act as a C1 sink if its near-UV photodissociation products are solely 0 and C10. But if C1 and 0 2 are produced, then the molecule will contribute to ozone depletion as the chlorine radical is an effective ozone destroyer. Recent experimentson thegas-phase photodissociation dynamics have been unable to resolve which personality is manifest. Conflicting results have been obtained in separate laboratories.1-1 In the near-UV, four photodissociationpathways are available.
c10, + hv
-
-
+
o(3~)OCI(~II,)
Cl(2P)
+ 0,(32,-)
Cl(’P)
+ 02(’Ag)
c1(2P) + 02(12g+)
(1)
(3) (4)
Symmetry arguments suggest that channel 3 is the result of a direct CZ,dissociation mechanism from the C102(2B2) excited state, while channel 2 arises from formation and subsequent unimolecular dissociationof the ClOO (A’ or A”) isomer. Matrix (Ar, Nz, and sulfuric acid) isolation studies show that the ClOO isomer is formed, with nearly unit quantum efficiency, when a UV excitation wavelength of 365 nm is ~ s e d . l ~ -Studies ’~ of solution-phase C102 photodissociation dynamics show that channels 1, 2, and 3 are active at a photolysis wavelength of 355 nm.l5-I8 The branchingratiosare 0.900:0.095:0.005, respectively, in water solution.lE The presence of channel 2 shows that the ClOO isomer is formed in the solution-phase photolysis. Conflicting results have been obtained for the gas-phase photodissociation dynamics. In a high-pressure study, Apkarian et al. estimated the quantum yield for C1 atom production to be 0.95.” These experiments also showed that the quantum yields were strongly mode specific. In particular, it was discovered that the total C1 + 0 2 yield increased, by a factor as large as 10, when symmetric stretching or symmetric stretching + bending modes in the upper electronic state were excited. At wavelengths less than 370 nm, however, this group showed that the combined quantum yield for channels 2 4 was less than 2 X 10-3. The results of the three Abstract published in Aduance ACS Absfructs, December 15, 1993.
0022-365419412098-0765$04.50/0
studies just mentioned suggest that isomerization to form ClOO does not occur in the gas phase at wavelengths to the blue of 370 nm. In molecular jet/REMPI (resonance-enhanced multiphoton ionization) studies, Vaida et al.1+7 showed that channel 1 dominates the photodissociation dyanamics at wavelengths between 335 and 400 nm. Exact quantum yields were not reported, but the studies showed that the C1+ REMPI signal intensitywas typically
