Influence of Electronic Excitation in the Charge Reversal of halogenated Cations.

Charge reversal of high kinetic energy (8keV) cations during collisions with atomic target gases (G=Xe, Kr, Ne) has been studied for CCln+ (n=1 or 2) ions resulting from dissociative electroionization of CCl4. The influence of electronic excitation of the incident cation CCln+ and of the nature of the target gas G on the anion yield have been examined. In our experiments, the two-collision process CCln+->CCln+G+->(CCln-)+2G+ is predominant. By comparing the anion and cation yields as a function of the kinetic energy of the ionizing electrons, we found evidence, with some target gases, of the existence of long-lived excited states of CCl+ and CCl2+, which lead to an increase in the anion yield in comparison with charge reversal of the correponding ground state ions. For CCl+, we deduce an excitation energy of 4+/-1 eV with a lifetime longer than 6.2 µs. These results are compatible with the a3Pi long-lived state of CCl+ previously detected by optical spectroscopy and by charge-stripping experiments. Our experimental data on charge reversal and metastable dissociation of CCl2+ reveal the existence of a long-lived state (lifetime>8.2 µs) of CCl2+ lying at 4.3+/-1 eV above the ground state.Semi-empirical MNDO calculations show that the energy of the first quartet state, a4A2, is compatible with the observed excitation energy. The spin-orbit coupling-induced transition from the a4A2 state to the B2A2 state is suggested to play an important role in the metestable dissociation CCl2+*->(CCl+)+Cl. The origin of the increase in the anion yield when long-lived excited states of CCl+ and CCl2+ are charge-reversed is discussed. Step a is suggested to be responsible for this yield enhancement which is observed when an appropriate target gas makes this step quasi-resonant.