Theoretical study of isotope effects in the quenching of electronically excited halogen atoms by H2, HD and D2

Abstract

The quenching of X (XF, Cl, Br, I) from its ²P_½ to its ²P spin-orbit electronic state due to collisions with H₂, HD and D₂ is investigated theoretically. The potential interaction is expressed as two adiabatic potential energy surfaces correlating asymptotically to X(²P) and X(²P_½), respectively. Within the diatomics-in-molecules formulation, two model nonreactive surfaces are derived together with nonadiabatic coupling terms. Rigourous quantum mechanical calculations are carried out for collisions restricted to collinear configurations to yield transition probabilities between asymptotic spin-orbit states of the atom and vibrational states of the molecule. These results are then used in a simple three-dimensional kinematic model for predicting cross sections. For fluorine and chlorine collisons, H₂ is the most efficient and D₂ the least efficient quencher. While D₂ is still the least efficient quencher for bromine and iodine collisions, HD is the most efficient. The results are explained in terms of a resonance effect in electronic-to-vibrational energy transfer, which dominates the behaviour of the bromine and iodine collisions but is absent from the fluorine and chlorine collisions.