Influence de l'excitation électronique sur l'inversion de charge de cations halogénés.

peer reviewedCharge 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

L'ionisation dissociative de l'Azote.

Energy distributions and appearance potentials of N+ ions produced by low-energy electron impact on N2 have been measured. At least five dissociaton limits were detected either directly or by extrapolation of the 1/2 slope straight lines drawn through the kinetic energy versus appearance potential energy diagram. Fine structure observed at low energy has been interpreted by the predissociation of at least two N2+ excited states one of which being the well-known C2Sigmau/+ state. One of the others could be a 2Piu state predissociated to the dissociation limit at 24.34 eV. Close correlation between the results of the present work and the photoelectron spectroscopic observations could be done over the electron energy range between 24.3 eV and 36 eV and assignments are discussed in detail

Ionisation dissociative de N₂

Clearly resolved structures are observed in the appearance efficiency curve of N+ ions produced by the impact of monoenergetic electrons on N2. The observed structures between 24.3 and 25 eV are attributed to N+ ions arising from predissociation of the N2+ C2Σu+ state. Between 25 and 29.5 eV some new structures are reported. N+ ions resulting from collision induced dissociation of N2+ also appear in an energy region lower than the first N2+ limit. The structures observed in this energy range are attributed to autoionizing Rydberg states of N2. </jats:p

The rotational Predissociation of the CH4+ ion.

peer reviewedThe kinetic energy release distribution of the metastable transition corresponding to CH(D)4+->CH(D)3+ + H(D) dissociation has been measured using a duoplasmatron coupled with a tandem mass spectrometer. The metastable peak is modulated by discrete energy states assigned to vibro-rotational levels of CH(D)4+