M. Farizon

Structure and energetics of hydrogen clusters. Structures of H+11 and H+13. Vibrational frequencies and infrared intensities of the H+2n+1 clusters (n=2–6)

Ab initio self‐consistent‐field (SCF) Hartree–Fock and configuration interaction (CI) calculations have been carried out for H+2n+1 (n=1–6) clusters using a triple‐zeta plus polarization basis set. Fully optimized structures and energies of H+11 and H+13 are presented. These structures can be thought as the addition of H2 molecules to a deformed H+9. Dissociation energies as a function of cluster size follow the pattern established experimentally by Hiraoka and Mori. Nevertheless, our energy results on the biggest clusters suffer from the lack of size consistency of CI with single and double substitutions (CISD) calculations. Analytic gradient techniques have been used to locate stationary point geometries and to predict harmonic vibrational frequencies and infrared intensities at the two levels of theory SCF (n=1–6) and CISD (n=1–4) both with triple‐zeta polarization basis sets. Of special interest are the new vibrational modes of H+11 and H+13, which have no counterpart in the H+9 cluster. Our predicted...

Ab initio structure calculations of hydrogen ionic clusters

Abstract Ab initio molecular electronic-structure theory has been used in an attempt to characterize the low-lying stationary points on the potential energy hypersurface of positive ionic hydrogen clusters. Using triple-zeta-plus-polarization basis sets, self-consistent-field Hartree-Fock calculations, configuration-interaction calculations with single + double substitutions and fourth-order Moller-Plesset calculations have been carried out for H+nn = 3 to 9 (odd). Dissociation energies as a function of cluster size follow the most recent experimental data. To our knowledge, the present study represents the best theoretical prediction for H+7 and H+9.

Fragmentation of high energy mass-selected ionic hydrogen clusters

Abstract First results on fragmentation of fast ionic hydrogen clusters ( n = 5−31, odd; 60 keV/amu) by single collision with a helium atom have been obtained with the new beams delivered by the variable energy post-accelerator of RFQ type at Institut de Physique Nucleaire de Lyon. The fragmentation apparatus and preliminary results are presented. The inclusive mass distributions of the charged fragments measured are strongly different from those resulting from lower energy collision. An important production of intermediate mass fragments is observed.

Gas-jet target for cluster fragmentation induced by high velocity collision

Abstract A gas-jet target for cluster fragmentation experiments has been studied in the inlet pressure range of 0.1–5.5 Torr corresponding to a target thickness range of 1013–1.2 × 1015 at. cm−2. The total flow has been found to be in quantitative agreement with the Poiseuille formula in the pressure range of 1.0–5.5 Torr. The profile of the jet is in good agreement with the (cos φ) 5 2 law expected in the viscous flow regime in the inlet pressure (P0) range of 0.1–5.5 Torr. From cross section measurements using the inlet pressure range of 1.5-5.5 Torr, the target thickness has been found to be proportional to P02 as expected. The target thickness characterization has been extended in the range 1013 to 1014 at. cm−2 from measurements with cluster beams.

Velocity of a Molecule Evaporated from a Water Nanodroplet: Maxwell–Boltzmann Statistics versus Non-Ergodic Events

The velocity of a molecule evaporated from a mass-selected protonated water nanodroplet is measured by velocity map imaging in combination with a recently developed mass spectrometry technique. The measured velocity distributions allow probing statistical energy redistribution in ultimately small water nanodroplets after ultrafast electronic excitation. As the droplet size increases, the velocity distribution rapidly approaches the behavior expected for macroscopic droplets. However, a distinct high-velocity contribution provides evidence of molecular evaporation before complete energy redistribution, corresponding to non-ergodic events.

Multifragmentation after multi-ionization of hydrogen clusters in high energy cluster–atom and cluster–cluster collisions

Abstract We report on a cluster fragmentation study involving collisions of high-energy hydrogen cluster ions with atomic helium or fullerenes. The experimental characterization of the cluster fragmentation, not only by the average fragment size distribution but also by a statistical analysis of the fragmentation events, has become possible because of a recently developed multicoincidence technique in which all the fragments of all the collisions occurring in the experiment are mass analyzed on an event by event basis. From the breakup in two fragments to the complete disintegration of the cluster, the fragmentation phenomenon exhibits a transition with an increase of the fluctuations. The fragmentation events with more than one H 3 + fragment evidence the cluster multifragmentation process. An important aspect of these results is that high-energy cluster collision can induce a reaction in the cluster.

Absolute total and partial cross sections for ionization of nucleobases by proton impact in the Bragg peak velocity range

We present experimental results for proton ionization of nucleobases (adenine, cytosine, thymine, and uracil) based on an event-by-event analysis of the different ions produced combined with an absolute target density determination. We are able to disentangle in detail the various proton ionization channels from mass-analyzed product ion signals in coincidence with the charge-analyzed projectile. In addition we are able to determine a complete set of cross sections for the ionization of these molecular targets by 20-150 keV protons including the total and partial cross sections and the direct-ionization and electron-capture cross sections.

DIRECT OBSERVATION OF MULTI-IONIZATION AND MULTI-FRAGMENTATION IN A HIGH-VELOCITY CLUSTER-ATOM COLLISION

Abstract We report the direct observation of the multi-ionization of the H 21 + hydrogen cluster in a single collision with a helium atom at 60 keV/u. Up to quadruple ionization of the cluster was observed and new multi-fragmentation channel were detected. Moreover, the results show two different fragmentation processes of doubly charged H 21 2+ clusters: emission of an H 2 + dimer, or emission of an H 3 + trimer after rearrangement in the cluster prior to fragmentation.

Neutral fragment production induced by collision of 60-keV u−1-H25+ clusters with helium

We report on measurements of the neutral fragment yields resulting from the collision of 60-keV u−1-H25+ clusters with helium. The sum of the mass numbers of all the neutral fragments (Sneutral) coming from each cluster is measured. The histogram obtained can be roughly divided into three Sneutral value ranges corresponding to evaporation (2u ⩽ Sneutral ⩽ 10 u, even), electron capture (Sneutral = 25 u) and ionization (11 u ⩽ Sneutral ⩽ 23 u, mainly). The ionization process which corresponds to both odd and even values of Sneutral appears to be involved in about 83% of the dissociation events observed. These results, and their comparison with the inclusive ionic fragment distributions previously measured, show the importance of multi-ionization of the incident cluster during the collision. Moreover, the fragmentation of the unstable multicharged cluster leads to the production of several light Hp+ fragments (p ⩽ 5).

Ionic hydrogen clusters : structure and fragmentation

The structure of Hn + hydrogen clusters which are a nucleation of H2 molecules around a H3 + core has been theoretically studied for n=5−13 (odd) using ab initio methods. For n larger than 9, the predicted structures can be thought as an addition of H2 molecules around a weakly deformed H9 + core. Besides, mass-selected ionic hydrogen cluster beams of high energy are now available at the cluster acceleration facility at the Institut de Physique Nucleaire de Lyon. Fragmentation of 60-keV/amu — Hn + clusters induced by single collision on helium atoms is investigated for n = 9, 25 and 31. The deduced inclusive mass distributions of the charged fragments are measured from mass 3 to n−2. These distributions appear strongly different from those obtained at lower energy. An important production of intermediate-mass fragments is observed.