Photoabsorption Assignments for the C¹B₂ ← X¹A₁ 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 C¹B₂ ← X¹A₁ 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 C¹B₂ 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 C¹B₂ state, we compute vibrational energy levels and wave functions, and compare with a photoabsorption calculation obtained using the same PES and corresponding C¹B₂ ← X¹A₁ 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.