Photodissociation dynamics of phosgene: New observations by applying a three-dimensional imaging technique

Abstract

The photodissociation dynamics of COCl2 has been studied by monitoring ground Cl(2P3/2) and spin–orbit excited Cl*(2P1/2) fragments by applying a novel technique where the three-dimensional momentum vector of a single reaction product is directly determined. The photodissociation at 235 nm produces exclusively three fragments: COCl2+hν→CO+2Cl. The kinetic energy distributions of Cl and Cl* are bimodal and exhibit a different behavior for the different spin–orbit states. Our attention was turned to the dependence of the anisotropy parameter β on the fragment velocity which was observed for the first time. For both spin–orbit states the anisotropy parameter differs clearly for slow and fast chlorine atoms, where a pronounced change from the value ∼0.7 to zero at about 20 kJ/mol is observed. Slow chlorine atoms are released isotropically and predominantly in the ground state Cl whereas fast chlorine atoms have an anisotropy parameter close to the theoretically limiting value and are distributed between ground and excited state Cl. These observations can be explained by a sequential decay where the first Cl fragment is released in a fast process characterized by the nonvanishing positive β parameter and a lifetime of ⩽210 fs, whereas the second Cl fragment is released after a period which is long on a rotational time scale. A significant contribution of a symmetric mechanism can be excluded.