Y. J. Picard

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.

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.

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.

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.

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.

Flexible attosecond beamline for high harmonic spectroscopy and XUV/near-IR pump probe experiments requiring long acquisition times

We describe the versatile features of the attosecond beamline recently installed at CEA-Saclay on the PLFA kHz laser. It combines a fine and very complete set of diagnostics enabling high harmonic spectroscopy (HHS) through the advanced characterization of the amplitude, phase, and polarization of the harmonic emission. It also allows a variety of photo-ionization experiments using magnetic bottle and COLTRIMS (COLd Target Recoil Ion Momentum Microscopy) electron spectrometers that may be used simultaneously, thanks to a two-foci configuration. Using both passive and active stabilization, special care was paid to the long term stability of the system to allow, using both experimental approaches, time resolved studies with attosecond precision, typically over several hours of acquisition times. As an illustration, applications to multi-orbital HHS and electron-ion coincidence time resolved spectroscopy are presented.

A zero dead-time multi-particle time and position sensitive detector based on correlation between brightness and amplitude

A new multi-particle time and position sensitive detector using only a set of microchannel plates, a waveform digitizer, a phosphor screen, and a CMOS camera is described. The assignment of the timing information, as taken from the microchannel plates by fast digitizing, to the positions, as recorded by the camera, is based on the COrrelation between the BRightness of the phosphor screen spots, defined as their integrated intensity and the Amplitude of the electrical signals (COBRA). Tests performed by observing the dissociation of HeH, the fragmentation of H3 into two or three fragments, and the photo-double-ionization of Xenon atoms are presented, which illustrate the performances of the COBRA detection scheme.

Dissociative charge transfer and collision induced dissociation of Ar2+ and Ar3+ clusters in collisions with argon atoms at keV energies

The dynamics of dissociative charge transfer and collision induced dissociation of Ar(2) (+) and Ar(3) (+) clusters colliding with Ar atoms at 4.8 keV has been investigated using a novel multifragment detection scheme that maps the postcollision vectors of all particles simultaneously. Estimation of internal energies and measurement of pre- and postcollision vectors enables a full description of reaction dynamics. The prominence of electronic excitation in defining the dynamics of these collision systems is demonstrated. The dissociation dynamics of Ar(3) (+) clusters is distinctly different from that of Ar(2) (+). This is attributed to a combination of lower internal energies and predominantly triangular T-shape structure of the Ar(3) (+) ion.