J.C. Brenot

Absolute detection efficiency of a microchannel plate detector for neutral atoms

The absolute detection efficiency (ADE) of microchannel plates for neutral sodium and potassium atoms is measured in the low keV energy range. It is shown that ADE is primarily a function of the particle energy. This result is compared to measurements made by other authors for ionic particles.

Vector correlations in dissociative photoionization of O2 in the 20–28 eV range. I. Electron-ion kinetic energy correlations

Imaging and time resolved coincidence techniques are combined to determine ion-electron (VO+,Ve,P) velocity vector correlations in dissociative photoionization (DPI) of the O2 molecule induced by linearly polarized synchrotron radiation (P). The ion-electron kinetic energy correlations identify each DPI process by its reaction pathway, intermediate molecular state and dissociation limit. The 4π collection of ions and electrons provides their branching ratios. Up to 12 DPI processes are identified in the 20–28 eV range. Photoionization into the O2+(B 2∑g−) in the Franck–Condon (FC) region populating the [O+(4S)+O(3P)] limit is the dominant process. In the 22.3–24 eV region excitation of the O2*(3 2∏u,nsσg) Rydberg series, followed by dissociation and atomic autoionization to the [O+(2D)+O(3P)] limit reaches about 10% of the DPI flux. A new DPI process is identified in the same energy range, which populates the [O+(4S)+O(1D)] limit. At higher energies the relative weight of the four distinct processes which...

Vector correlations in dissociative photoionization of O2 in the 20–28 eV range. II. Polar and azimuthal dependence of the molecular frame photoelectron angular distribution

A combined experimental and theoretical study of the polar and azimuthal dependence of the molecular frame photoelectron angular distributions (MFPADs) for inner-valence-shell photoionization of the O2 molecule into the O2+(B 2Σg−,3 2Πu,c 4Σu−) states is reported. The measured MFPADs, for each orientation of the molecular axis with respect to the linear polarization of the synchrotron radiation, are derived from the spatial analysis of the (VO+,Ve,P) vector correlation, where the nascent ion and electron velocity vectors VO+ and Ve are determined for each dissociative photoionization (DPI) event using imaging and time of flight resolved coincidence technique as described in the companion paper of this series [J. Chem. Phys. 114, 6605 (2001)]. Expressed in the general form of four FLN(θe) functions which contain all the dynamical information about the photoionization processes, they are compared with the MFPADs computed using the multichannel Schwinger configuration interaction method. A very satisfactory ...

Molecular frame photoelectron angular distributions in dissociative photoionization of H2 in the region of the Q1 and Q2 doubly excited states

Dissociative photoionization of H2 induced by VUV linearly polarized synchrotron radiation P has been studied using the (VH+,Ve,P) vector correlation method. The ion–electron kinetic energy correlation diagrams obtained for the three photon excitation energies hν = 20, 28.5 and 32.5 eV enable us to identify and select the dominant dissociative photoionization processes. The Iχ(θe,e) molecular frame photoelectron angular distributions for any orientation χ of the molecular axis with respect to the polarization are reported for direct photoionization of H2 into the H2+(2Σg+) ionic ground state at hν = 20 eV and for the dominant DPI processes involving autoionization of the H2(Q1 1Σu+(1)) and H2(Q2 1Πu(1)) doubly excited states into the H2+(2Σg+) and H2+(2Σu+) continua at hν = 28.5 and 32.5 eV. They show the dominant excitation of a p σu partial wave in autoionization of the Q1(1Σu+(1)) state into the H2+(1s σg) ionic state and that of a d πg partial wave in autoionization of the Q2(1Πu(1)) state into the H2+(2p σu) continuum. A molecular frame forward–backward electron emission anisotropy is observed when ionization takes place at large internuclear distance.

Collision induced fragmentation of small ionic sodium clusters: Competition between electronic and impulsive mechanisms

Collision induced fragmentation of small Nan+ (n=3–9) clusters with He atoms is investigated in the 100 eV center-of-mass collision energy range. The experiment is based on the determination of the velocity vectors of the fragments using a multicoincidence technique. The relative populations of the various fragmentation pathways are determined. Fragmentation mechanisms are discussed in detail. The most important pathways are primarily populated via momentum transfer in elastic binary collisions between the He atom and a Na+ core. Direct release of fast Na atoms is observed at variance with what is usually assumed at eV energies. However most of the fragmentation involves multistep dynamics with energy redistribution inside the cluster via Na–Na collisions. In contrast, production of Na+ fragments comes dominantly from electronic transitions towards repulsive potential energy surfaces of the cluster. The role of electron pairing is emphasized.

The dynamics of Cl− + H2 reactive collisions

Abstract A multicoincidence analysis of the crossed beam Cl − + H 2 system in the 5.6–12 eV energy range has shown the existence of four different product channels: reaction (R), reactive detachment (RD), simple detachment (SD) and dissociative detachment (DD). For the whole energy range both R and RD channels give rise to HCl molecules at a unique and common center-of-mass scattering angle whereas the vibrational excitation probability of HCl obeys completely different rules for each channel: v ≤3 in channel R and equal probability in all possible vibrational levels in RD. A Thomas-type collision model joined to curve crossing with an intermediate autodetaching HCl − state accounts well for all of the experimental findings.

Complete analysis of the Na3+ fragmentation in collision with He atoms

Abstract An experimental investigation of the fragmentation mechanisms of Na3+ cluster ions in collision with He atoms at 263 eV centre-of-mass energy is presented. The relative populations of the three fragmentation pathways are determined. In particular, the kinematics of the three-body breakup is studied in detail. The analysis of the correlation between the velocity vectors of the fragments allows one to estimate the relative role of the electronic excitation or momentum transfer in the population of each pathway. The discussion of the fragmentation dynamics is based on a concomitant theoretical study.

Collision induced fragmentation of small ionic argon clusters

The mechanisms of collision induced fragmentation of small Arn+ (n=2–9) clusters are investigated in the 100 eV center-of-mass energy range. The velocity vectors of the fragments are measured in a multicoincidence experiment for two- and three-body fragmentation. The relative role of the two basic dynamics, electronic transitions, and momentum transfer in binary collisions is evaluated. The structure of the clusters deeply influences the type of mechanism. This is clearly the case of Ar3+ for which a specific impulsive process called “diatom” mechanism plays an important part in the fragmentation of one isomer.

Collision induced fragmentation of small ionic sodium clusters. II. Three-body fragmentation

Multiple fragmentation of small Nan+ clusters (n=3–5) induced by collision with He atoms is investigated in the 200 eV collision energy range. The Nan+→Nan−2++Na+Na channels are studied using a multicoincidence technique allowing for the determination of the velocity vectors of the three fragments. The relative contributions of the two basic mechanisms, namely the electronic excitation and the momentum transfer in elastic binary collisions between the He atom and a Na core are estimated. For Na4+ and Na5+ clusters the momentum transfer mechanism appears as a two-step process: The ejection of a fast Na atom in a binary Na–He collision followed by the evaporation of an additional slow Na atom. However the angular distribution of the slow Na atom is not isotropic showing that a partial memory of the initial binary collision appears in the evaporation process.

Reactive and detachment processes in halide ion-H2 collisions

Abstract A systematic investigation of the competition between reactive and detachment channels in halide ions-H2 collisions has been undertaken using a multicoincidence technique. Contour maps for detaching and non-detaching processes are obtained. It is found that the F−-H2 system behaves differently from that involving the heavier halide ions. The competition between reaction and reactive detachment processes for all systems is well accounted for by considering a non-adiabatic interaction between the two lowest ionic potential surfaces as previously discussed. From a kinematic point of view, this study suggests that at low energy (