Theoretical study of photodissociation dynamics on the lowest-lying Rydberg state of ketene.

In the present study, an attempt is made to reveal the main mechanism of photodissociation on the lowest-lying Rydberg state 1B1 of ketene, referred to as the second singlet excited state S2, by means of the complete active space self-consistent field and the second-order multiconfigurational perturbation theory methods. The located S2/S1/T1 three-surface intersection plays an important role in the dissociation process. It is shown that the intersection permits an efficient internal conversion from S2 to S1 state, but prohibits the intersystem crossing from S2 to T1 state because of the small spin-orbital coupling value of 0.136 cm-1. The main photodissociation process could be described as follows: after one photon absorption to the S2 state, ketene preferentially relaxes to the minimum S2_C2v, and undergoes a transition state S2_TS with small potential barrier along the Cs-I (out-of-plane bent) symmetry, and passes through the S2/S1/T1 intersection to reach S1 surface, then arrives at the transition state S1_TS along the minimum energy path. As is well known, S1→S0 internal conversion around the Franck-Condon region is expected to be very efficient, and eventually the hot S0 molecule has accumulated enough energy to yield the CH2 (ã 1A1) and CO (X 1Σ+) products. [ABSTRACT FROM AUTHOR]