The charge transfer and collision-induced dissociation cross sections of state-selected H +2 and D +2 ions

Abstract

The collision-induced dissociation (CID) and charge transfer (CT) cross sections for H + 2 (υ = O to υ = 10) reactants with H 2 have been measured by threshold photoelectron-photoion coincidence (TPEPICO)_at center-of-mass translational energies of 4, 8, 12 and 16 eV. The relative cross sections, normalized by using the known absolute charge transfer cross sections for H + 2 (υ = 0 and 1), exhibit a marked dependence on the H + 2 vibrational energy. The CID cross sections increase from less than 1 Å 2 for υ = 0 to more than 6 Å 2 for υ = 10, while the CT cross section decreases from about 11 to 3 Å 2. One observes a much weaker variation with translational energy. The analysis of the H + product ion time of flight distribution shows that the H + ions are formed in a distribution of kinetic energies which peaks at both low and high laboratory kinetic energies. A direct dissociation model in which the projectile ion is excited to the repulsive 2Σ + u surface is consistent with the observation of fast H + ions. If the CID is preceded by a charge transfer step, then the same mechanism will give rise to the slow H + product ions. The ratio of fast to slow H + ions is unity for low υ but increases as υ increases. This is consistent with the observed decreasing charge transfer cross section with increasing υ.