Influence of electronic excitation in the charge reversal of halogenated cations

Abstract

Charge reversal of high kinetic energy (8 keV) cations during collisions with atomic target gases (G = Xe, Kr, Ne) has been studied for CCl + n ( n = 1 or 2) ions resulting from dissociative electroionization of CCl 4. The influence of electronic excitation of the incident cation CCl + n and of the nature of the target gas G on the anion yield have been examined. In our experiments, the two-collision process ▪ 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 CCI + and CCl + 2, which lead to an increase in the anion yield in comparison with charge reversal of the corresponding 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 a 3Π 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 CCl + 2 reveal the existence of a long-lived state (lifetime > 8.2 μs) of CCl + 2 lying at 4.3 ± 1 eV above the ground state. Semi-empirical MNDO calculations show that the energy of the first quartet state, a 4A 2, is compatible with the observed excitation energy. The spin-orbit coupling-induced transition from the a 4A 2 state to the B 2A 2 state is suggested to play an important role in the metastable dissociation CCl + 2* → CCI + + Cl. The origin of the increase in the anion yield when long-lived excited states of CCl + and CCl + 2 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.