Ho Folkerts

ION-IMPACT-INDUCED FRAGMENTATION OF WATER-MOLECULES

Abstract The multiple ionization and fragmentation of H 2 O by fast H + , He + , O 6+ and O 7+ ions was studied utilizing a position-and time-sensitive multi-particle detector which allows the coincident measurement of the momenta of correlated fragments. Thereby, the fragmentation energy and angular correlations for each individual event can be derived. Of special interest are the processes H 2 O → H + + H + + O q + , where a simple Coulomb-explosion model is insufficient to explain the measured energy and angular spectra. Better agreement was achieved with ab initio MCSCF calculations which take into account several molecular states of the fragmenting highly charged H 2 O ( q +2)+ ion.

DISSOCIATION OF CO INDUCED BY HE2+ IONS : I. FRAGMENTATION AND KINETIC ENERGY RELEASE SPECTRA

The dissociation of ions produced in collisions of ions with CO has been studied by time-of-flight measurements. Both singles and coincidence time-of-flight techniques have been used to determine the kinetic energy release of the dissociating CO molecules. We describe the method to transform the singles and coincidence time-of-flight spectra into total kinetic energy distributions and discuss these distributions. They represent kinetic energy release distributions which clearly exhibit various contributions associated with different dissociation channels. In comparison with other ionization methods similarities but also clear differences are noted.

He2+-He collisions: one-electron capture and target-ion excitation

By means of photon emission spectroscopy the authors have studied state selective one-electron capture and target-ion excitation in collisions of He2+ with He. The collision energy has been varied from 1 to 75 keV amu-1. Four-body classical trajectory Monte Carlo calculations have been performed in the energy range of 50-300 keV amu-1. In the energy range where experiment and theory overlap there is in general fair agreement, although the l-distributions within a principal quantum shell exhibit differences. At energies below 50 keV amu-1 the experiments confirm the results of atomic orbital calculations, presented in an accompanying paper by Fritsch (1994). Combining all the experimental and theoretical data the authors have determined a reliable cross section data base for use in neutral helium beam based plasma diagnostics on large tokamaks.

Dissociation of CO induced by He2+ ions: II. Dissociation pathways and states

The dissociation of ions produced in collisions of ions with CO has been studied by time-of-flight methods. From the time-of-flight spectra the energy released in the dissociation process is determined. Our results for the kinetic energy release of and ions are compared with the results of other ionization methods. Clear differences are observed. We observe a counterintuitive dependence of the kinetic energy release on the velocity of the ions, i.e. the kinetic energy release increases with decreasing velocity.

HE-2+-H2 COLLISIONS - NONDISSOCIATIVE AND DISSOCIATIVE ONE-ELECTRON CAPTURE

Electron-redistribution processes in collisions of He2+ ions on H2 are studied for energies from 1 to 25 keV amu-1. One-electron capture and target excitation cross sections are determined by photon-emission spectroscopy. At energies exceeding approximately 5 keV amu-1 capture into excited states is the dominant charge-transfer process while at lower energies one-electron capture into the ground state dominates. The latter process is found to be accompanied by dissociation of the target and excitation of the resulting hydrogen atom. This can be explained qualitatively with the classical overbarrier model under the assumption that during the collision binding energy is exchanged between the electrons, which is a kind of auto-excitation process.

ELECTRON-CAPTURE FROM LI BY B-5+, N-5+ AND BE-4+ IONS

The authors present state selective experimental and theoretical results for electron capture in B5+ and N5+-Li collisons at energies in the range of 1-10 keV amu-1. Experimentally they have used photon emission spectroscopy in the VUV and visible spectral range and theoretically they have performed classical trajectory Monte Carlo (CTMC) calculations. The influence of projectile-core electrons on electron capture processes in such highly charged ion-Li collisions has been investigated by comparing experimental data for He-like N5+ ions with both experimental and theoretical results for the fully stripped B5+ ions. They find that in the B5+ and N5+ cases the projectile-core electrons are of no direct importance for electron capture from Li by these ions. This allows them to compare the CTMC results for Be4+-Li electron capture collisions with atomic orbital calculations as well as with experimental data for He-like C4+ colliding on Li.

He2+-He collisions: one-electron capture versus electron removal and target-ion excitation

The authors have studied electronic redistribution processes, associated with production of excited He+ ions, in He2+-He collisions at energies ranging from 1 to 300 keV amu-1. Excited He+ ionic states can be formed (i) in the projectile by one-electron transfer, and (ii) in the target by removing one of the target electrons and simultaneously exciting the residual ion. At the highest energies between approximately 10 and 60 keV amu-1 the first process is the most important one. However, for low energies below 10 keV amu-1 they obtain the striking result that both processes are equally likely.

Electron capture and excitation in He2+-Na collisions

State-selective charge transfer and target excitation in collisions of He2+ ions with Na has been studied by means of far ultraviolet and visible light spectroscopy. Generally, the experimental results confirm the theoretical predictions by Shingal et al. (1987) for electron capture and for the dominant excitation channel, Na(3p). However in the lower half of the impact energy range studied, 2-9 keV amu-1, discrepancies in energy dependence and absolute magnitude (up to a factor of approximately 6) are found for the Na(4p) and Na(4d) excitation channels.

DISSOCIATION OF H-2 AND CO BY SLOW MULTIPLY-CHARGED IONS

Abstract We have studied the time of flight spectra of molecular ions and charged fragments resulting from collisions of 2 keV/amu He 2+ and 1 keV/amu O q + ( q = 1–5) ions with CO and H 2 molecules respectively. We found that the energy release from dissociation of CO after He 2+ impact is smaller than what one calculates assuming pure Coulomb explosion. For the O q + + H 2 system we can identify two processes leading to H + formation, one with an energy gain for the two protons of around 14 eV, the other yielding very slow fragments. The latter is probably due to one-electron transfer processes and cannot be explained assuming pure Frank-Condon type transitions.

Kinetic energy release of dissociating CO3+ ions produced in collisions of multiply charged ions with CO

We investigate fragmentation of CO molecules by collisions of He2+ ions at energies between 2 and 11 keV/amu by means of a reflectron time-of-flight (TOF) spectrometer. The kinetic-energy-release (KER) in the center of mass system of the molecule can be determined from the flight times of these particles. Different dissociation processes leading to different amounts of released energy and different fragments can be identified. The KER differs from the one expected from the pure Coulomb repulsion of two point charges, starting at the CO equilibrium distance and depended strongly on the collision energy. This will be discussed in terms of the classical overbarrier model. Furthermore, there is a strong influence of the projectile type on the KER. A feature of the experiments, namely the strong dependence of the KER on the projectile kinetic energy, is not yet fully understood.