Energy-Dependence of SO₂ Photodissociation with λ=212-220 nm: Transient IR Spectroscopy of Spin-Resolved SO Product States and <i>Ab Initio </i>Calculations

The UV photodissociation of the SO₂ C-state was investigated as a spectroscopic target for probing exoplanet atmospheres with potential to support life. Photodissociation was initiated with tunable, pulsed UV light with λ=220-212 nm and nascent SO photoproducts were detected using high-resolution transient IR absorption spectroscopy. Doppler profiles, rotational distributions, and energy partitioning were measured for individual SO spin-rotation states. Doppler profiles established the initial vibrational energy in the SO₂ X-state, yielding the C-state energy and total product energy. The product energy partitioning favors translation over rotation by a factor of four, indicating the equilibrium C-state bent geometry must become more linear prior to dissociation. Multireference ab initio calculations performed as a function of SO₂ geometry show that the dissociation barrier height is energetically accessible for the experimental energies only if the transition state has a linear geometry and includes singlet and triplet coupling to the C-state. The experimental results and ab initio calculations show that SO₂ dissociates with a linear geometry via coupling to a repulsive triplet near its dissociation threshold.