EXPERIMENTAL AND THEORETICAL EXPLORATION OF PHOTODISSOCIATION OF SO2 VIA THE (C)OVER-TILDE(1)B(2) STATE - IDENTIFICATION OF THE DISSOCIATION PATHWAY

The photodissociation reaction of SO2 via the (C) over tilde(1)B(2) st ate, SO2 ((C) over tilde(1)B(2)) --> SO((3) Sigma(-)) + O(P-3) was inv estigated by experimental and theoretical approaches cooperatively to clarify its dissociation mechanism. We measured the laser induced fluo rescence (LIF) spectrum of the (C) over tilde-(X) over tilde band in t he short UV wavelength region (210-200 nm) under jet-cooled conditions . The fluorescence quantum yields and the dissociation rates of indivi dual vibronic levels were determined in the 220-200-nm region using (i ) the LIF spectrum measured in the present study, (ii) that measured p reviously by Yamanouchi et al. (J. Mol. Struct, 352/353 (1995) 541) in the longer wavelength region above 210 nm, and (iii) the high-resolut ion absorption spectrum measured by Freeman et al. (Planet. Space. Sci . 32 (1984) 1125). The dissociation rates were also derived in the 210 -200-nm region from the broadening of the rotational lines of the (C) over tilde-(X) over tilde vibronic transitions. It was found that the dissociation rates determined through two different procedures were co nsistent with each other, and that the rate increases almost exponenti ally as an excess energy above the dissociation threshold increases th ough there is a certain fluctuation of the dissociation rates reflecti ng a mode specificity. We also performed theoretical ab initio calcula tions to derive potential energy surfaces (PESs) of the electronic gro und states and low-lying electronically excited states of SO2 within t he MCSCF and MRCI levels. The theoretical calculations showed that (i) the PES of the 2(1)A' state (the (C) over tilde(1)B(2) state in C-2r symmetry), correlating with the SO((1) Delta) + O(D-1) asymptote, cros ses with the repulsive singlet (3(1)A') state, correlating with the SO ((3) Sigma(-)) + O(P-3) asymptote, to form a pseudo-seam, (ii) the cro ssing pseudo-seam of these two PESs is located near the equilibrium be nt angles for the X and C states along the energy contour of similar t o 9700 cm(-1) measured from the SO((3) Sigma(-)) + O(P-3) dissociation limit, and (iii) the crossing seam between the 2(1)A' ((C) over tilde (1)B(2)) and repulsive 2(3)A' states is located in a lower energy regi on than the singlet seam; at similar to 6700 cm(-1) measured from the SO((3) Sigma(-)) + O(P-3) dissociation limit. On the basis of the abov e experimental and theoretical results together with the previous expe rimental evidence, we propose that (i) the photodissociation reaction via the (C) over tilde state proceeds mainly through the vibronic mixi ng between the (C) over tilde state vibronic levels with the quasi-bou nd dissociation continuum of the electronic ground X(1)A(1) state, and (ii) the additional dissociation channels may be open through the cro ssing seam with the repulsive singlet (3(1)A') state and that with the repulsive tiplet (2(3)A') slate in their narrow crossing energy regio ns. (C) 1997 Elsevier Science B.V.