Cold Target Recoil Ion Momentum Spectroscopy: a &momentum microscope' to view atomic collision dynamics

High-resolution recoil-ion momentum spectroscopy (RIMS) is a novel technique to determine the charge state and the complete final momentum vector of a recoiling target ion emerging from an ionizing collision of an atom with any kind of radiation. It offers a unique combination of superior momentum resolution in all three spatial directions of with a large detection solid angle of . Recently, low-energy electron analysers based on rigorously new concepts and reaching similar specifications were successfully integrated into RIM spectrometers yielding so-called `reaction microscopes'. Exploiting these techniques, a large variety of atomic reactions for ion, electron, photon and antiproton impact have been explored in unprecedented detail and completeness. Among them kinematically complete experiments on electron capture, single and double ionization in ion - atom collisions at projectile energies between 5 keV and 1.4 GeV have been carried out. Double photoionization of He has been investigated at energies close to the threshold up to . At the contributions to double ionization after photoabsorption and Compton scattering were separated kinematically for the first time. These and many other results will be reviewed in this paper. In addition, the experimental technique is described in some detail and emphasis is given to envisaging the rich future potential of the method in various fields of atomic collision physics with atoms, molecules and clusters.

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Recoil-ion momentum spectroscopy

Charge-exchange spectroscopy in fusion plasmas entails the use of optical transitions that follow electron transfer from a neutral atom into an excited state of an impurity ion. In most applications, the sources of neutral particles are high-energy beams employed either for heating or for the specific purpose of active plasma diagnosis. The transitions following charge exchange are particularly useful for determining the densities of fully stripped low-Z ions and for measuring ion temperatures and plasma rotation, although they have also been exploited for other purposes. In this review, the atomic physics considerations for interpreting the data, including the influence of the plasma environment, are reviewed, and examples of recent applications to fusion studies are presented.

https://iopscience.iop.org/article/10.1088/0741-3335/36/2/001/pdf

An overview of charge-exchange spectroscopy as a plasma diagnostic

Structure and dynamics of molecules in high charge states

An analytical procedure is presented which enables a fast estimate of collision-energy-dependent cross-section effects on thermal charge exchange spectra. The model is based both on experimental evidence and numerical simulations showing that the observed charge exchange (CX) spectra are essentially Gaussian in their shape. The collision-energy-dependent emission rate leads effectively to a lineshift (apparent velocity), usually to a reduction in linewidth (apparent temperature), and to a change in the effective emission rate averaged over the entire thermal velocity distribution function. It is demonstrated that the cross-section effect can be treated analytically introducing an approximated emission rate factor which retains the characteristics of a Maxwellian velocity distribution function using an exponential expression with only linear and quadratic velocity terms in its exponent. An algebraic deconvolution procedure is described, which enables the reconstruction of true temperature, velocity and intensities from measured CX spectra. Examples taken from a recent JET experimental campaign are used to illustrate the cross-section effects on low-Z impurity CX spectra for a comprehensive variety of neutral beams (deuterium, tritium or helium), target densities, temperatures and toroidal rotation speeds. An overview is given of representative correction factors established for high-power, high-temperature plasmas, as well as for plasmas with combined neutral beam and radiofrequency heating, and for the case of locked modes.

http://iopscience.iop.org/article/10.1088/0741-3335/37/2/002/pdf

Analytical approximation of cross-section effects on charge exchange spectra observed in hot fusion plasmas

Dissociation of CO molecules by collisions with He{sup 2+} and O{sup 7+} at keV energies has been investigated by measuring the charge states and kinetic energies of the ionized fragments in coincidence with each other. As opposed to earlier investigations with fast (MeV) projectiles we find that the kinetic energies of the fragment ions are strongly influenced by the projectiles{close_quote} charge and velocity; e.g., O{sup 7+} impact results in 50{percent} {ital less} kinetic energy release in the C{sup +}-O{sup +} fragmentation than He{sup 2+} impact. For a qualitative understanding of these effects we invoke the classical overbarrier model. {copyright} {ital 1996 The American Physical Society.}

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Velocity and Charge State Dependences of Molecular Dissociation Induced by Slow Multicharged Ions.

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.

He2+-H2 collisions : Non-dissociative and dissociative one-electron capture

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.

Dissociation of CO induced by He2+ ions : I. Fragmentation and kinetic energy release spectra

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.

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

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.

https://iopscience.iop.org/article/10.1088/0953-4075/30/24/020/pdf

H O Folkerts

Papers

Velocity and Charge State Dependences of Molecular Dissociation Induced by Slow Multicharged Ions.

He2+-H2 collisions : Non-dissociative and dissociative one-electron capture

Dissociation of CO induced by He2+ ions : I. Fragmentation and kinetic energy release spectra

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

Dissociation of co induced by he2+ ions : ii. dissociation pathways and states