Multi-electron processes in atomic collisions – theory

The literature on electronic polarization behavior in atomic collisions was relatively sparse, up to a few years ago. This situation has now changed, and there is no difficulty in encountering comprehensive review papers on this subject. Some of these are listed in reference 1. These proceedings also contain a review by Professor Kessler on polarized electron sources,2 which of necessity includes discussion of the physical processes in atomic collisions which underlie most of present polarized beam source technology.

Atomic Collision Processes in Controlled Thermonuclear Fusion Research

This paper presents the results of semi-classical calculations of rate coefficients of (n − n′)-mixing processes in collisions of Rydberg atoms H*(n) with H(1s) atoms. These processes have been modelled by the mechanism of the resonant energy exchange within the electron component of the H*(n) + H collisional system. The calculations of the rate coefficients, based on this model, were performed for the series of principal quantum numbers, n and n′, and atomic, Ta, and electronic, Te, temperatures. It was shown that these processes can be of significant influence on the populations of Rydberg atoms in weakly ionized plasmas (ionization degree ⪅10−4), and therefore have to be included in appropriate models of such plasmas.

We present accurate rates for spin exchange processes in collisions of hydrogen atoms for temperatures in the range 1 K < T < 300 K. A fully quantal, multichannel theory is employed in the calculations. We discuss the respective roles of spin exchange and long-range magnetic interactions in collisional redistribution of sublevel populations. We use the rates to calculate the spin temperature of hydrogen in the universe at 20 < z < 200 and explore its sensitivity to the collision data. We stress the important role of atomic collisions in enabling 21 cm tomography, recently proposed by Loeb & Zaldarriaga, of the dark age universe.

Results of an electron spectroscopy study of ionisation processes in collisions of F atoms with He, Ne, O2 and CO2 at low keV energies are presented. It is shown that in case of the He and Ne targets ionisation is partly due to population of singly-excited autoionising states of the 1D nl series and partly due to another mechanism which results in the production of a continuous electron energy distribution and becomes particularly important at high energies. A discussion in terms of the molecular orbital model is presented and indicates that quasimolecular Auger ionisation must be the other F+ production channel. In the case of molecular targets the authors observe a direct ionisation channel and also dissociative ionisation via the population of 2D nl autoionising states of fragment O atoms.

In this lecture I shall review recent developments which have occurred in theoretical methods for calculating atomic collision cross sections. The lecture is an up-dated (but self-contained) version of the review I gave at the previous NATO Advanced Study Institute on “Atomic and Molecular Processes in Controlled Thermonuclear Fusion”1. It is meant to provide a general introduction to subsequent lectures by other speakers, who will deal in more detail with the theoretical analysis of specific atomic and molecular collision processes of interest in fusion research.

The key cycles of atomic and molecular collision processes contributing to the formation and quenching of the excited molecular states in exciplex (such as KrF*) and excimer (such as Xe/sub 2/*) laser systems are delineated and discussed.

An algorithm for diagnosing the electron density and temperature of helium plasma at atmospheric pressure has been developed based on a revised helium collisional-radiative (CR) model. Atomic collision processes are included, and part of the atomic data of electron collision processes in the conventional CR model has been updated to expand its valid pressure. The algorithm uses eight emission lines in the visible-wavelength range as inputs to determine the electron density, electron temperature, and number density of the two metastable states by fitting the number density of the states corresponding to the emission lines. The algorithm has a considerably small theoretical error. In the microwave-discharged low-pressure helium plasma experiment, the results obtained with the algorithm agreed well with the results obtained with the probe method. The electron density and temperature of the atmospheric-pressure helium plasma obtained with the algorithm agreed well with the results of the continuum spectrum analysis.

An atomic collision process can be illustrated and classified by its geometry. An atomic particle \(A\) may collide with an atomic particle \(B\). Particle \(A\) propagates in a collimated aligned beam of atoms, ions, electrons or photons. It may then hit particle \(B\) of an atomic target. Often the target \(B\) is also produced by a collimated beam of particles (crossed-beam technique).

The coloration produced near room temperature in MgO by 1.0- to 4.8-MeV $^{20}\mathrm{Ne}^{+}$ bombardment has been investigated. Energetic ${\mathrm{Ne}}^{+}$ ions incident upon crystalline MgO induce point defects on the anion sublattice primarily through atomic collisions. An electron(s) trapped in the anion vacancy forms an $F$-type center producing a characteristic, intense optical-absorption band near 5 eV. This point-defect lattice damage is restricted primarily to a thin layer of material approximately 1 to 2 \ensuremath{\mu} thick located 2 to 4 \ensuremath{\mu} beneath the bombarded surface. Average $F$-type-center volume concentrations in this thin layer range from 4\ifmmode\times\else\texttimes\fi{}${10}^{18}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ resulting from a ${\mathrm{Ne}}^{+}$ dose range of ${10}^{13}$ to 3\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ions/${\mathrm{cm}}^{\ensuremath{-}2}$. $F$-type-center production is relatively inefficient; it is estimated that 5 keV are expended per center produced, in the low-dose range, assuming centers are created only by atomic collision processes. The peak $F$-type-center volume concentration saturates near ${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ resulting from a dose of ${10}^{16}$ ions/${\mathrm{cm}}^{2}$ at 3-MeV energy. This concentration level is followed by a decrease upon subsequent bombardment. A similar saturation effect is observed for a 574-nm absorption band. A quadratic dependence between the growth of the intensity of the $F$-type center and that of the 574-nm band suggests the latter is associated with a divacancy. Volume-concentration saturation levels for both bands are independent of many impurities commonly present in this material. Isochronal annealing results demonstrate that both bands have annealed out at 600 \ifmmode^\circ\else\textdegree\fi{}C. Comparison is made with previously reported fast-neutron irradiations and the present work. Point defects on the cation sublattice, such as the ${V}^{\ensuremath{-}}$ center, were sought for but determined not to be present in sufficient concentration to be detected by the presence of the well-known 2.3-eV optical-obsorption band.

An accurate knowledge of atomic collision processes is important for a better understanding of many astrophysical and laboratory plasmas. Collision databases which contain electron-impact excitation, ionization, and recombination cross sections and temperature dependent rate coefficients have been constructed using perturbative distorted-wave methods and non-perturbative R-matrix pseudo-states and time-dependent close-coupling methods. We present recent atomic collision results.

A summary of observations of collisions of alkali ions with atoms and molecules and collisions of H− and D− with alkali atoms is presented. The energy range of the experiments was 5<E<500 eV. Results on cross sections for collisional electron detachment and charge transfer of negative ions are discussed. (AIP)

Atomic collision processes that proceed through electron correlation are examined in the context of the electron-impact ionization of atomic ions. Contributions to the ionization cross section from simultaneous double excitation followed by autoionization are calculated for ${\mathrm{Li}}^{+}$ in a close-coupling approximation. Contributions from simultaneous double excitation with capture followed by simultaneous double autoionization, which proceed through triply excited states, are also calculated for ${\mathrm{Li}}^{+}$ using high-order perturbation theory. Both calculated cross sections are then compared with recent high-resolution crossed-beams experimental measurements.

Many areas of science today, like atomic and molecular physics, nuclear physics, astrophysics, laboratory plasma research etc., depend on data for ionic, atomic, and molecular collision processes. The purpose of the Special Issue “Atomic and Ionic Collisions with Formation of Quasimolecules” in Atoms is to engage a broad community of researchers to consolidate knowledge, make new discoveries, and to continue the exchange of ideas.

The relationship between vacuum and atomic collisions in solids