Experimental and theoretical exploration of photodissociation of SO₂ via the C¹B₂ state: Identification of the dissociation pathway

The photodissociation reaction of SO₂ via the C¹B₂ state, SO₂ (C¹B₂) → SO(³Σ(~)) + O(³P) was investigated by experimental and theoretical approaches cooperatively to clarify its dissociation mechanism. We measured the laser induced fluorescence (LIF) spectrum of the C-X band in the short UV wavelength region (210-200 nm) under jet-cooled conditions. The fluorescence quantum yields and the dissociation rates of individual vibronic levels were determined in the 220-200-nm region using (i) the LIF spectrum measured in the present study, (ii) that measured previously by Yamanouchi et al. (J. Mol. Struct. 352/353 (1995) 541) in the longer wavelength region above 210 nm, and (iii) the high-resolution 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-X vibronic transitions. It was found that the dissociation rates determined through two different procedures were consistent with each other, and that the rate increases almost exponentially as an excess energy above the dissociation threshold increases though there is a certain fluctuation of the dissociation rates reflecting a mode specificity. We also performed theoretical ab initio calculations to derive potential energy surfaces (PESs) of the electronic ground states and low-lying electronically excited states of SO₂ within the MCSCF and MRCI levels. The theoretical calculations showed that (i) the PES of the 2¹A' state (the C¹B₂ state in C(2v) symmetry), correlating with the SO(¹Δ) + O(¹D) asymptote, crosses with the repulsive singlet (3¹A') state, correlating with the SO(³Σ^-) + O(³P) asymptote, to form a pseudo-seam, (ii) the crossing pseudo-seam of these two PESs is located near the equilibrium bent angles for the X and C states along the energy contour of ~9700 cm^-1 measured from the SO(³Σ^-) + O(³P) dissociation limit, and (iii) the crossing seam between the 2¹A' (C¹B₂) and repulsive 2³A' states is located in a lower energy region than the singlet seam; at ~6700 cm^-1 measured from the SO(³Σ^-) + O(³P) dissociation limit. On the basis of the above experimental and theoretical results together with the previous experimental evidence, we propose that (i) the photodissociation reaction via the C state proceeds mainly through the vibronic mixing between the C state vibronic levels with the quasi-bound dissociation continuum of the electronic ground X¹A₁ state, and (ii) the additional dissociation channels may be open through the crossing seam with the repulsive singlet (3¹A') state and that with the repulsive triplet (2³A') state in their narrow crossing energy regions.