Photodissociation dynamics of OCS at ∼210 nm: The role of c(2A″) state.

Photodissociation dynamics of carbonyl sulfide (OCS) in the deep ultraviolet region is investigated using a time-sliced ion velocity map imaging technique. The measured total kinetic energy release spectra from the photodissociation of OCS at ∼210 nm shows three dissociation channels to the fragment S(D), corresponding to low, medium, and high kinetic energy release (E), respectively. The high E channel is found to be a new dissociation channel opening with photolysis wavelength at ∼210 nm. Based on the a(p) polarization parameters as well as the anisotropy parameters β determined from the images of S(D), the dissociation of OCS to S(D) + CO at 210 nm is concluded to involve a direct vertical excitation of the triplet c(2A) state from the ground state, followed by processes as: the low E component arises from a non-adiabatic transition from the repulsive A(2A) state to the electronic ground state X(1A); the medium E component arises from a simultaneous excitation to two repulsive excited states; and the high E component arises from the intersystem crossing from the triplet c(2A) state to the repulsive A(2A) state. The present study shows that, due to the strong spin-orbit coupling between the triplet c(2A) state and the repulsive A(2A) state, a direct excitation to c(2A) significantly contributes to the photodissociation dynamics of OCS in the deep-UV region.