La formation de O+ et N+ par impact d'électrons de faible énergie sur l'hémioxyde d'azote.

peer reviewedThe O+ and N+ dissociation channels in N2O have been investigated by electroionization using ion kinetic energy and mass analysis. Appearance enegies are measured as a function of the translational energy of the fragment ions. The different dissociation mechanisms are discussed

Electronic, rotational and vibrational predissociations in the decay of electronically excited molecular ions

A review is made of experimental and theoretical studies about the occurrence of electronic predissociation processes either allowed or forbidden, in the dissociation of CH+, [Math], CO+, [Math], NO+ [Math] and N2O+. The importance of rotational predissociation is emphasized in the dissociation of diatomic ionized hydrides and its occurrence in the decay of [Math] brings attention to the possibility that such a process could play an important role in the apparance of metastable ions in the mass spectra of complex molecules, especially when the loss of H or H2 is involved. The discussion, with the help of correlation diagrams, of some dissociation channels of C2H3F+ and CH3OH+, shows the coexistence in these mechanisms of adiabatic processes, principally due to the occurrence of "avoided crossings” or of "conical intersections" and of non-adiabatic processes such as electronically forbidden predissociations. The role of autoionization in the population of dissociation processes has been emphasized each time it was involved

L'ionisation dissociative du fluorure de méthyle. La formation de CH2+ et CH3+.

peer reviewedThe decay of ionized CH3F was investigated in the CH2+ and CH3+ dissiciation channels by electronionization and fixed wavelength photoionization using HeI and NeI resonance lines. The translational energy spectra, ionization efficiency curves and kinetic energy-versus-appearance enegy diagrams of both ions were measured. Although dissociative autoionization is the prominent dissociation mechanism for the appearance of CH2+ and CH3+, direct dissociative ionization is observed for CH2+ at 23.4 eV and for CH3+ at 16.5 eV

Les spectres d'énergie cinétique des ions fragmentaires. Une comparaison entre l'électroionisation et la photoionisation à longueur d'onde fixe.

The HeI and NeI translational energy spectra have been recorded for NO+/N2O and CH3+/CH3F. The marked difference between the two spectra for NO+ is related to the NeI-A2Sigma+ photoelectron spectrum. Both spectra of CH3+ show two components, i.e. a thermal peak and a high energy distribution. As the energetic ions are ascribed to the decomposition of the CH3F+(2E+2A1) states the thermal ions could be interpreted by the dissociation of the CH3F+(X2E) state

L'électroionisation dissociative de NH3 et ND3. Une étude détaillée des cinq canaux de dissociation.

A general view of the results obtained for the dissociative ionization by electron impact on NH3 are presented. All five dissociation channels for the decomposition of the singly ionized NH3+ and the doubly ionized (NH3)2+ are reported.Laboratoire de Dynamique Moléculair

Ionisation dissociative par impact d'électrons de faible énergie. Distribution d'énergie de N++ à partir de N2 et O++ à partir de O2.

peer reviewedThe kinetic energy distribution of the doubly ionized species N++/N2 and O++/O2 , as produced by electron impact, is reported and discussed

L'apparition des ions CH3+ à partir des halogénures de méthyle par photoionisation résonante. Une analyse de la surprise d'énergie translationnelle. 1ère Partie: CH3F.

A surprisal analysis has been applied to the translational energy distribution (TED) of CH3++F from CH3F giving rise to a new and deeper insight into the dissociation dynamics involved in the process. For both the 16.5 eV and 21.22 eV photon energies used, the direct population at different energy levels of the B2E state of CH3F+ which is radiationlessly coupled to the repulsive CH3F+(A2A1) state, gives rise to wide and negatively surprised total TEDs. At both energies they have to be ascribed to severe limitations in the vibrational excitation of CH3+. Additionally, in the case of 21.22 eV photons, a non-statistical expression has to be used to account for the density of translational states. At both photon energies, the population of the CH3F+ electronic states through autoionization of Rydberg states accounts for (i) the concurrent appearance of TEDs governed by a pure RRKM behavour and (ii) the positively surprised TEDs where the angular momentum conservation plays an important role. At 16.85 eV photon energy the distributions are assigned to CH3F+ (X2E)->CH3+ (X1A'1)+F. At 21.22 eV photon energy, doubly excited electronic states of CH3F+ are assumed to take part and they are correlated with CH3+ (3A")+F channel

L'autoionisation dissociative comme mécanisme de formation du proton à partir du Méthane et du Méthane-d4 par impact d'électrons de faible énergie.

peer reviewedThe ion energy distribution and the electroionization efficiency curves of H+ and D+ from methane and methane-d4 have been examined. The existence of thermal ions is unambiguously evidenced and the two lowest appearance thresholds of H+ are measured at (21.3+/-0.3) eV and (22.17+/-0.1) eV. At both energies the protons have to be formed through dissociative autoionization. The present results closely parallel those of the dissociative excitation experiments on methane. The Jortner-Rice model for dissociation of polyatomic molecules is invoked to account for both the appearance of H+ and high-Rydberg H atoms from CH4

L'ionisation dissociative du CH4 et CD4 par impact d'électron de faible énergie. La formation du proton.

peer reviewedThe kinetic energy distributions of H+(D+)/CH(D)4 are produced at variable electron impact energies. Two different features are observed and are interpreted in terms of dissociative autoionization