Recoil velocity-dependent spin–orbit state distribution of chlorine photofragments

Abstract

Abstract We present the results of experimental and theoretical studies of the speed-dependent spin–orbit state distributions of chlorine photofragments produced in the photodissociation of thiophosgene (CSCl 2 ) at 235 nm. Three-dimensional imaging has been employed for observing chlorine photofragments in their ground (Cl) and excited (Cl * ) spin–orbit states. The kinetic energy distributions for Cl and Cl * fragments reflect excitation of several electronic states of the partner fragment CSCl. The spin–orbit branching ratio of P (Cl * )/[ P (Cl) + P (Cl * )] was found to depend on the kinetic recoil energy increasing from 0.1 for low kinetic energy to 0.8 for high kinetic energy. The theoretical interpretation is based on the computation of the CSCl 2 potential energy surfaces (PES) along the C–Cl bond. Two completely different methods of determination of the PES were applied for small and for large values of the C–Cl bond separation R . In case of small and intermediate R values time-dependent density-functional theory has been used. In case of large R values we used an asymptotic method of computation of the PES, which is a generalisation of the Heitler–London approach for many-electron systems. Basis molecular wavefunctions with definite values of the total spin S and the spatial and spin reflection symmetry σ v with respect to reflection of the total electronic wavefunction in the molecular plane were used. The developed theoretical approach was used for the assignment of the molecular states involved in the photodissociation and for the qualitative explanation of the non-statistical population of the spin–orbit states of the chlorine photofragments as function of the kinetic energy. The spin–orbit branching ratio of P (Cl * )/[ P (Cl) + P (Cl * )] predicted by the theory strongly depends on the quantum state of the CSCl fragment. It is large in case of the CSCl(X) + Cl and CSCl(A) + Cl channels and small in case of the CSCl(B) + Cl channel which explains the experimental results.