Charge transfer from neutral perfluorocarbons to various cations: long-range versus short-range reaction mechanisms

摘要

The bimolecular reactions of the high recombination energy cations Ar~+, F~+, and Ne~+ with four fully saturated (CF_4, C_2F_6, C_3F_8, and n-C_4F_(10)) and three unsaturated (C_2F_4, C_3F_6, and 2-C_4F_8) perfluorocarbons (PFCs) are reported. The cation branching ratios obtained from these reactions, and from the reactions with O_2~+, H_2O~+, N_2O~+, O~+, CO_2~+, CO~+ N~+, and N_2~+ [reported by us, Jarvis et al., J. Phys. Chem. 100 (1996) 17166], are compared with those determined from the threshold photoelectron-photoion coincidence spectra of the PFCs at the recombination energies of the reagent cations. This comparison provides information that helps to interpret the dynamics of charge transfer, and whether it occurs via a long-range or a short-range mechanism. Energy resonance and good Franck-Condon factors connecting the ground electronic state of a reactant neutral molecule to one of its ionic states, at the recombination energy of the reagent cation, are generally considered to be sufficient criteria for long-range charge transfer to occur. However, the results from this study imply that good Franck-Condon factors are not critical in determining the efficiency of a long-range charge transfer. Instead, the results suggest that, in addition to the requirement for energy resonance, the electron taking part in the charge-transfer process must be removed from a molecular orbital which is unshielded from the approaching reagent cation. This enables the cation to exert an influence on the electron at large impact parameters.