Recent Progress in Theoretical Methods for Atomic Collisions

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.

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.

Research in atomic and molecular collision processes and spectral line broadening has been very active since our last report (Schultz & Stancil 2007, Allard & Peach 2007). Given the large volume of the published literature and the limited space available, we have attempted to identify work most relevant to astrophysics. Since our report is not comprehensive, additional publications can be found in the databases at the web addresses listed in the final section. Elastic and inelastic collisions among electrons, atoms, ions, and molecules are included and reactive processes are also considered, but except for charge exchange, they receive only sparse coverage.

In 2-dimensional SCBF (Spherically Convergent Beam Fusion) device, the effect on neutron production rate of the grid cathode geometry was simulated. The motion of Particles was tracked using Monte Carlo Method including the atomic and molecular collision processes and potential distribution was calculated by Finite Element Method, Main processes of the discharge were the ionization of by fast . As the number of cathode rings was small and the size of grid cathode decreased, the ion current increased and neutron production rate will also increase. The star mode discharge which is a very important characteristic in SCBF device, was confirmed by the ionization position.

We review the development of the time-dependent close-coupling method to study atomic and molecular few body dynamics. Applications include electron and photon collisions with atoms, molecules, and their ions.

Introductory paper 1. The role of atomic and molecular processes in fusion research

Multi-electron processes in atomic collisions – theory

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 relationship between vacuum and atomic collisions in solids

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.

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 Physics from Atmospheric and Astrophysical Studies

S-matrix theory is used to analyze different atomic and molecular processes in a linearly polarized few-cycle laser field. The energy spectra of high-order above-threshold ionization (HATI) are presented. Electron-atom potential scattering assisted by a few-cycle laser pulse is also analyzed. It is shown that the plateau structures in the energy spectra of the electron-atom potential scattering are dependent on the carrier-envelope phase (CEP) of the laser pulse, so that the cutoff positions of the plateaus can be controlled by changing the CEP. Regarding our analysis of the molecular HATI process, the angle-resolved spectra, obtained by different theoretical approaches, are also presented.

Atomic Collision Processes in Controlled Thermonuclear Fusion Research