Competitive channels in the photodissociation of thionyl chloride

Abstract

The photodissociation dynamics of thionyl chloride are investigated by a one-dimensional resonance enhanced multi-photon/time-of-flight technique (REMPI/TOF) at a dissociation wavelength around 235 nm. Nascent sulfur monoxide molecules and chlorine atoms were detected state-specifically under collision-free conditions for parallel and perpendicular polarisation geometries. Dissociation and detection were performed using the same laser. Polarisation-dependent and state-specific TOF profiles were converted into kinetic energy distributions using a least squares fitting method taking into account velocity-dependent spatial fragment anisotropies. Kinetic energy distributions are narrow and structureless for SO and bimodal for both spin–orbit states of Cl. The bimodality reflects the competition between two- and three-body decay: SOCl₂ + hν → SOCl + Cl and SOCl₂ + hν → SO + 2 Cl. Chlorine atoms are preferentially released along the polarisation vector of the dissociation laser. The spatial distribution of Cl is characterised by an anisotropy parameter of 0.8 ± 0.2 for the two-body decay and 0.2 ± 0.2 for the three-body decay. SO molecules are isotropically released with low kinetic energy together with two ground state chlorine atoms. These products are most likely to be generated in a sequential three-body decay on a potential energy surface of A′ symmetry. Another surface of A″ symmetry instantaneously produces fast chlorine atoms in a two-body decay process to equal amounts in the ground and the excited spin–orbit states. The corresponding SOCl fragment carries about 50% of the available energy as internal energy.