Abstract:

Carbonyl sulfide (OCS) was photoexcited at 230 nm so that it dissociated into a vibrationally cold but rotationally hot CO (X¹Σ_g⁺, v = 0, J = 42–69) fragment, which was eventually subjected to resonance enhanced multiphoton ionization. The kinetic energy release distribution and angular distribution of the CO fragment were obtained by detecting the time-sliced velocity map images of CO⁺ in various rotational states (J = 55–69), wherein both the singlet dissociation channel of S(¹D) + CO and the triplet pathway of S(³P_J) + CO were involved. For the triplet fragment channel, the total quantum yield of OCS dissociation at 230 nm was estimated to be 4.16%, based on the measured branching ratioin every rotational state. High-level quantum chemical calculations on the potential energy surface and the absorption cross section of OCS revealed the dissociation mechanism along the triplet channel of OCS, with photolysis at 230 nm. The ground state OCS (X¹A') is photoexcited to the bent A¹A' state at 230 nm, which then decays back to X¹A' in a bent structure via internal conversion and subsequently couples to the 2³A"(c³A") state by spin-orbit coupling, followed by direct dissociation along its potential energy surface.

Key words: Carbonyl sulfide (OCS), Photodissociation, Resonance enhanced multiphoton ionization, Branching ratio, Ion velocity imaging