Photoabsorption Assignments for the C̃1B2 ← X̃1A1 Vibronic Transitions of SO2, Using New Ab Initio Potential Energy and Transition Dipole Surfaces.

The high resolution spectroscopy of the SO2 molecule is of great topical interest, in a wide variety of contexts ranging from origins of higher life, to astrophysics of the interstellar medium, to environmental chemistry. In particular, the C̃1B2 ← X̃1A1 UV photoabsorption spectrum has received considerable attention. This spectrum exhibits a highly regular progression of ∼20 or so strong peaks, spaced roughly 350 cm-1 apart, which is comparable to the C̃1B2 bending vibrational frequency. Accordingly, they have for decades been largely attributed to the (1, v2', 2) ← (0, 0, 0) bend progression. Using a highly accurate new ab initio potential energy surface (PES) for the C̃1B2 state, we compute vibrational energy levels and wave functions, and compare with a photoabsorption calculation obtained using the same PES and corresponding C̃1B2 ← X̃1A1 transition dipole surface (TDS). We find that the above putative assignment is incorrect, contradicting even general qualitative trends-thus necessitating a very different dynamical picture for this highly unusual molecule.