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

The bimolecular reactions of the high recombination energy cations Ar\(^+\), F\(^+\) and Ne\(^+\) with four fully saturated (CF\(_4\), C\(_2\)F\(_6\), C\(_3\)F\(_8\) and n-C\(_4\)F\(_{10}\)) and three unsaturated (C\(_2\)F\(_4\), C\(_3\)F\(_6\) and 2-C\(_4\)F\(_8\)) perfluorocarbons (PFCs) are reported. The cation branching ratios obtained from these reactions, and from the reactions with O\(_2^+\), H\(_2\)O\(^+\), N\(_2\)O\(^+\), 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 of the reagent cation, are generally considered to be sufficient 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.