PHOTODISSOCIATION DYNAMICS OF OCLO

Photofragment translational energy spectroscopy was used to study the dissociation dynamics of a range of electronically excited OClO(A (2)A (2)) vibrational states. For all levels studied, corresponding to OClO (A (2)A(2)<--X B-2(1)) excitation wavelengths between 350 and 475 nm, the dominant product (>96%) was ClO((2) Pi)+O(P-3). We also observed p roduction of Cl+O-2 with a quantum yield of up to 3.9+/-0.8% near 404 nm, decreasing at longer and shorter wavelengths. The branching ratios between the two channels were dependent on the OClO(A (2)A(2)) excite d state vibrational mode. The Cl+O-2 yield was enhanced slightly by ex citing A (2)A(2) levels having symmetric stretching + bending, but dim inished by as much as a factor of 10 for neighboring peaks associated with symmetric stretching+asymmetric stretching. Mode specificity was also observed in the vibrationally state resolved translational energy distributions for the dominant ClO((2) Pi)+O(P-3) channel. The photoc hemical dynamics of OClO possesses two energy regimes with distinctly different dynamics observed for excitation energies above and below si milar to 3.1 eV (lambda similar to 400 nn). At excitation energies bel ow 3.1 eV (lambda>400 nm), nearly all energetically accessible ClO vib rational energy levels were populated, and the minor Cl+O-2 channel wa s observed. Although at least 20% of the O-2 product is formed in the ground (X (3) Sigma(g)(-)) state, most O-2 is electronically excited ( a (1) Delta(g)). At E<3.1 eV, both dissociation channels occur by an i ndirect mechanism involving two nearby excited states, (2)A(1) and B-2 (2). Long dissociation time scales and significant parent bending befo re dissociation led to nearly isotropic polarization angular distribut ions (beta similar to 0). At excitation energies above 3.1 eV (lambda< 400 nm), the Cl+O-2 yield began to decrease sharply, with this channel becoming negligible at lambda<370 nm. At these higher excitation ener gies, the ClO product was formed with relatively Little vibrational en ergy and a large fraction of the excess energy was channeled into ClOO translational energy. The photofragment anisotropy parameter (beta) also increased, implying shorter dissociation time scales. The sharp c hange in the disposal of excess energy into the ClO products, the decr ease of Cl+O-2 production, and more anisotropic product angular distri butions at E>3.1 eV signify the opening of a new ClO+O channel. From o ur experimental results and recent ab initio calculations, dissociatio n at wavelengths shorter than 380 nm to ClO+O proceeds via a direct me chanism on the optically prepared A (2)A(2) surface over a large poten tial energy barrier. From the ClO((2) Pi)+O(P-3) translational energy distributions, D-0(O-ClO) was found to be less than or equal to 59.0+/ -0.2 kcal/mol. (C) 1996 American Institute of Physics.