J.A. Fayeton

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.

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.

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 (

A zero dead-time, multihit, time and position sensitive detector based on micro-channel plates

We have developed a fast multihit position and time sensitive detector with zero dead-time for heavy particles in the keV energy range. This new type of detector makes use of a micro-channel plates (MCP) assembly and combines a detection based on delay line anode with a simultaneous particle imaging with a CCD-camera. The time pickup accuracy is enhanced by digitalizing the MCP biasing signal. This detector, operating at kHz repetition rate, allows a position resolution better than 100μm and a time resolution better than 100ps to be achieved.

Fast multichannel time correlator for coincidence experiments in atomic physics

Use of multichannel plate detectors has considerably extended the experimental techniques in atomic physics. In this context, a coincidence circuitry able to determine 64 simultaneous coincidence spectra is described. Use of fast electronic devices and separated amplifiers allows a high counting rate capability (5×105 events per second) with a time resolution of 3.1 ns. UV photon‐scattered particle coincidence in He–He collisions is given as an example of the application of the present apparatus.

Dalitz plot analysis of three-body fragmentation of Na3+ excited by He impact

Three-body fragmentation of Na3+ ions to Na++Na(3s)+Na(3s) following excitation by He is studied experimentally and theoretically. The three reduced kinetic energies of the products in the center-of-mass are determined for each fragmentation event, and the results are displayed in a Dalitz plot. The fragmentation involves three adiabatic 1A′ electronic states of Na3+ that become degenerate at the detector. It is possible to determine the final electronic state for each event, and here we show that each of the three product states appears in a particular sector of the Dalitz plot. Theoretical and experimental Dalitz plots for the three-body fragmentation of Na3+ are presented, and the results are related to various mechanisms for three-body fragmentation of this system.

The S−-H2 collision system revisited

Abstract It was found that the experimental results on the S − -H 2 collisional system previously published were affected at low collision energy by an experimental artefact. The correction primarily concerns the shape of the angular distributions of the three competing channels: simple detachment, reactive detachment and reactive charge transfer. The main conclusion of the previous paper stays unaffected, namely that, as for halide-H 2 systems, the trajectories leading at low collision energy to these three channels have a common path in the reaqctive SH-H − “valley” after an adiabatic passage through the crossing seam between the two S − HH and SHH − surfaces (diabatic with regard to the active electron). This conclusion is now reinforced in the light of the newly measured angular distributions obtained for the three channels. Their evolution with collision energy implies the existence of a unique short lived SH − 2 species explored by three types of trajectories leading to R, RD and SD channels. The role of the intermediate complex (SHH) − is investigated using a combination of the symmetric top model and the oscillating complex model.

Dynamics of competing channels in the S−/H2 collisional system

Abstract A dynamical study of the various channels competing in the S − /H 2 collisional system has been performed in a crossed beam experiment with multicoincidence detection of two products. Energy range extends from the threshold of reactions to 20 eV. Different dynamical behaviours are observed as a function of collisional energy for reactive detachment and simple detachment channels. Part of them were already observed in halide ion-H 2 systems and interpreted in term of adiabatic or diabatic behaviour at the crossing seam between two ionic surfaces. This seam is the locus where the active electron changes core. In the S − /H 2 system, the number of surfaces is greater than in the halide ion-H 2 systems. The main originality lies in the existence of a well in the ground state continuum corresponding to the formation of SH 2 molecule. An attempt to explain the mechanism of associative detachment is given. The dissociation of the H 2 S molecule is observed at threshold. The SH product is sideway scattered at 90°, this behaviour should reflect a large rotational excitation of the molecule.

Dynamical competition between reactive and reactive detachment channels in X− + H2 colliding systems (X = Cl, Br, I)

Abstract Two reactive processes are observed: X − + H 2 → XH + H − (R) and X − + H 2 → XH + H + e − (RD). The angular and energy distributions of the molecular XH products are measured at collision energies varying from 5 to 10 eV center of mass. These distributions obey identical rules in the three systems: (a) XH molecules formed by both R and RD processes are scattered at the same c.m. angle, respectively 55° ±10 for ClH, 80° ±20 for BrH and 90° ±20 for IH. (b) The rovibrational energy of the XH molecules, when formed by R processes, is limited to a small amount: ⩽ 2 eV for ClH, ⩽ 1.5 eV for BrH, ⩽ 1 eV for IH, whereas when formed by RD it extends to the highest amount available from the collision energy, up to the dissociation limit. The RD process is not observed experimentally in the I − /H 2 system. This dynamical behaviour is fully understood in terms of non-adiabatic interaction between the two lowest [XH 2 ] − ionic surfaces, but the reason of the angular anisotropy is still not well understood.