Analysis of Atomic Collisions

Multi-electron processes in atomic collisions – theory

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.

Atomic Collision Processes in Controlled Thermonuclear Fusion Research

A micro-channel plate dynode type image intensifier whose output window is a fibre face plate coated with phosphor on its inner surface has been applied to a low f -number spectroscopic detector for studies of very weak light signals sent from the crossing between an electronic or ionic beam and an atomic or molecular beam. It can reduce long exposures extending over nights for optimum density spectrograms to be obtained in the conventional way, down to scores of minutes and to several seconds. The resolution and sensitivity are both nearly satisfactory, as far as atomic lines, continua, and diatomic band-heads are concerned.

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 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.

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.

The relationship between vacuum and atomic collisions in solids

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.

The use of a liquid ionization chamber for measurements in electron and photon beams with energies above 1 MeV has been investigated. The liquid in the chamber is 2,2,4-trimethylpentane. Fundamental properties such as the reproducibility of the measured charge per absorbed dose, the temperature dependence, and the angular dependence have been experimentally analysed. The general recombination losses in pulsed beams are shown to be dependent on the transport time of the ions in the liquid. For a longer transport time than the time between two consecutive pulses the losses depend on the mean dose rate and for a shorter transport time the losses depend on the absorbed dose per pulse. The Jaffe method is used for determination of the initial recombination losses, and compared with results from other authors. The chamber is particularly useful for absorbed dose determination in electron beams since the relation between measured charge and absorbed dose to water is almost energy independent. In photon beams the relation is somewhat quality dependent particularly at qualities below 10 MV. The change of the free ion yield caused by the small change of mean LET with the quality of the photon beam is probably the reason. Procedures for the calibration of the liquid chamber and the absorbed dose determination in electron and photon beams are described.

In the following sections, we will consider the problems connected with collisions of a radiating atom with atomic particles in external fields that have an influence on the elementary collision event itself. The influence of external fields is of importance for resonance processes where the external electromagnetic (E.M.) field may result in detuning the resonance or, oppositely, coming into resonance. Of principal importance are the collisions of atoms with particles in a resonant laser field ξ 0 cos ωt, whose frequency ω is close to the frequency ω 0 of the atomic transition. These conditions respond to the standard formulation of nonlinear spectroscopy problems with the essential difference that the field ω 0 influences not only relaxation kinetics, but the collision event itself. Accounting for multiparticle atomic collisions with perturbing particles in a resonant E.M. field is also of great importance because it is the case when the time of the collision and the time between collisions cannot be separated. The consideration of the problems mentioned has its foundation on [11.1–8].

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.

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.

Correlation in ion-atom collisions