A. S. Schlachter

The ratio of cross sections for double to single ionization of helium by high energy photons and charged particles

Data and analysis for the ratio of double to single ionization in helium is reviewed for impact by photons and charged particles. In the case of photoionization there are two processes, namely, (i) photoionization where the photon is annihilated, and (ii) Compton scattering where the photon is inelastically scattered. In the case of charged particle scattering the ratio of total cross sections tends toward an asymptotic high energy value of 0.26% which is well below the value observed for photons of 1.7% at photon energies between 2 and 12 keV. Theoretical relations between various ratios have been predicted and to some extent confirmed by observations.

In situ measurement of rovibrational populations of H2 ground electronic state in a plasma by VUV laser absorption

Abstract Absorption spectroscopy employing laser-generated VUV has been employed to measure the populations and temperatures of H2 in individual rovibrational levels of the ground electronic state in a low-pressure plasma discharge. Vibrational states up to ν=5 and rotational states up to J=8 were observed. This is the first quantitative in situ measurement of vibrational levels with ν ⪢ 3 of the X1Σg+ state of H2 in a plasma.

In situ density and temperature measurements of vibrationally excited hydrogen molecules in ion source plasmas

The role of highly vibrationally excited hydrogen molecules has been postulated to be of great importance in H− ion sources. However the the difficulty of making in situ measurements has led to a paucity of direct determinations of these species within the plasmas of these sources. Recently, vacuum‐ultraviolet (VUV) laser absorption spectroscopy has been used to measure the H2 rovibrational populations up to v‘=5 and J‘=8 in a medium‐power hydrogen plasma. This work extends those measurements to v‘=8 and to J‘=13. The populations of the vibrational levels still appear to be almost Boltzmann. The theoretically predicted plateau is not observed up to the detection limit. The dependence of several vibrational levels on discharge current and filling pressure is shown.

VUV laser absorption spectrometer system for measurement of H0 density and temperature in a plasma

A system to determine the density and temperature of ground‐state hydrogen atoms in a hydrogen plasma by measurement of the absorption of Lyman‐beta or Lyman‐gamma radiation is described. The Lyman‐series radiation is generated by resonant four‐wave sum‐frequency mixing in mercury vapor. A wide range of hydrogen atom densities can be measured by employing these two transitions. A sample measurement on a H− ion‐source discharge is presented. Extensions to Lyman‐alpha and other vacuum‐ultraviolet wavelengths are discussed.

Charge transfer of hydrogen ions and atoms in metal vapors

Cross sections and equilibrium fractions for energetic H+, H−, and H0 in collisions with metal‐vapor targets have been compiled and evaluated. Both experimental and theoretical results are reported. Sources of errors are discussed, and recommended values for the data are presented.

Atomic hydrogen density measurements in an ion source plasma using a VUV absorption spectrometer

A system to determine the density and temperature of ground‐state hydrogen atoms in a plasma by vacuum ultraviolet laser‐absorption spectroscopy is described. The continuous tunability of the spectrometer allows for analysis at any of the Lyman transitions. The narrow bandwidth of the laser source allows for the accurate determination of the hydrogen absorption line shape and, hence, the translational temperature. The utility of the system is exemplified by data obtained on an ion source plasma. The measurements show the quality of the data as well as illustrating the behavior of these sources under varying discharge conditions.

Transfer ionisation in slow collisions of multiply-charged ions with atoms

Systematic experiments on transfer ionisation processes Aq++B to A(q-k)++Bi+(i-k)e+ Delta E have been performed where A and B are rare-gas atoms, q is the charge state of the incident ions (up to 7 for Ne, 9 for Ar, 12 for Kr and 15 for Xe), k is the number of captured electrons found with the projectile long after the collision (k=1, 2 or 3) and i is the charge state of the slow recoil ion (measured by a time-of-flight coincidence technique). The projectile energies range from 3q keV to 15q keV; however, with very few exceptions, the charge-state fractions Fi of recoil ions were found to be independent of the energy in the energy range studied. The ionisation of five additional target electrons in 7% of the two-electron-capture collisions of Xe15+ ions with Xe atoms shows the importance of transfer ionisation in such collisions. The observed charge-state fractions of the recoil ions strongly depend on k, the number of electrons captured, and on q, the charge state of the projectile; the mean charge state (i)= Sigma jjFj/ Sigma jFj increases strongly with increasing k and q. In general, the investigated processes are exoergic, and the charge states of the recoil ions produced are correlated with the potential energy available in the collision system. When the charge-state fractions Fi observed for a given target atom are plotted as a function of the maximum potential energy Delta E calculated for transitions between ground states they fall on common curves which are only specified by the charge state i and the number of electrons k captured indicating that Fi depend explicitly neither upon the projectile species nor upon the incident charge state.

Electron-capture and impact-ionisation cross sections for partially stripped iron ions colliding with atomic and molecular hydrogen

Cross sections for single-electron capture and for impact ionisation of Feq+ (q=10,15,20,25) ions incident on atomic hydrogen have been calculated in the energy range 50 to 1200 keV amu-1 using a classical-trajectory Monte Carlo method. Cross sections for the same processes for Feq+ ions incident on molecular hydrogen have been measured for q=11-22 at 1100 keV amu-1, for q=9 at 277 keV amu-1, and for q=12 and 14 at 262 keV amu-1. The experimental cross sections for molecular hydrogen divided by two are in excellent agreement with the calculated atomic hydrogen cross sections. Scaling laws for the cross sections with q, the charge state of the incident ion, are discussed. The scaling with q is not found to follow the q2 law predicted by the binary-encounter theory.

COLLISIONS OF FAST HIGHLY CHARGED IONS IN GAS TARGETS - IONIZATION, RECOIL-ION PRODUCTION, AND CHARGE-TRANSFER

Electron-capture, ionization, and recoil-ion-production cross-sections are measured and calculated for fast highly charged projectiles in hydrogen and rare-gas targets. Recoil-ion-production cross-sections are found to be large; the low energy and high charge states of the recoil ions make them useful for subsequent collision studies.

Single electron capture and loss cross sections for highly stripped Fe ions in hydrogen at 3.4 MeV/nucleon

Abstract Single-electron-capture and loss cross sections of 3.4 ± 0.1 MeV/nucleon iron ions in H 2 gas are reported for change states +20 through +25.