The photodissociation dynamics of the SO2 C-state was investigated using high-resolution transient IR absorption spectroscopy and multireference potential energy calculations. Rotational energy distributions of the SO 3Σ- products were measured following UV excitation at five wavelengths between 212 and 220 nm from a tunable, pulsed UV optical parametric oscillator. The nascent appearance of individual SO(v=0,1) product rotational states was probed using transient IR overtone absorption near 4.4 μm. Transient IR Doppler profiles establish the initial SO2 X-state vibrational energy, C-state vibrational energy and available product energy. The spin-rotation manifolds of the SO(v=0) products have nearly equal branching ratios. The product translational energy is four times greater than the rotational energy, indicating that SO2 undergoes substantial bond angle opening from a bent equilibrium geometry during the dissociation. Photodissociation quantum yields show structure near the photolysis threshold at 220 nm, resulting from competition between predissociation and C-state fluorescence. UV action spectra show that the SO(v=0) product yield is fairly constant for photolysis wavelengths of 220-212 nm, including at the threshold for SO(v=1) products near 214.6 nm. The SO(v=1) products account for 2% of the photodissociation yield at 213.7 nm and also have energy partitioning associated with a near-linear transition state. Potential energy curves were calculated using multireference configuration interaction (MRCI-F12+Q) as a function of SO2 geometry and show that the dissociation barrier height is energetically accessible when the transition state has a linear geometry and includes C-state coupling to singlet and triplet states. The experimental results and the theoretical calculations show that SO2 C-state dissociates with a near-linear geometry via coupling to a repulsive triplet near its dissociation threshold.

Energy-dependent photodissociation of the SO2 C-state with 220-212 nm light: Transient product state spectroscopy and multireference electronic state calculations

ACS Spring Meeting 2021

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