[en] Diatomic systems are considered from the points of view of their electronic structure and the dynamics of motion of the heavy particles (nuclei) upon collision. The electronic and nuclear motions are treated formally by expressing the Schroedinger equation for the nuclei and electrons in independent variables in the body-fixed (BF), center of mass of the nuclei, frame, and then introducing an expansion in terms of a complete set of electronic states at each internuclear separation, R. Born-Oppenheimer and Coulomb couplings between the electronic and nuclear motions are pointed out and discussed

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[en] A discretization procedure has been applied to the continuum of electronic states encountered in Penning ionization (PI) and associative ionization (AI) of an atom B by an excited (metastable) atom A*, where A*+B→A+B⁺+e^- (PI) and A*+B→AB⁺+e^- (AI). Within the context of discretization, a procedure has been introduced which treats the collision dynamics of PI and AI processes in terms of a small number of two-state coupled equations, each associated with a particular amount of kinetic energy epsilon removed by the emitted electron. Application has been made to PI of Ar by He* (1s2s,³S). Calculated results of partial ionization cross sections as a function of epsilon are reported and show excellent agreement with experimental measurements of the energy distribution of emitted electrons in PI and AI of Ar by He* (1s2s,³S). (Auth.)

[en] A semiclassical approach based on the propagation of classical trajectories on potential surfaces analytically continued in to the complex plane, together with a discretization procedure, has been developed for the problem of collisional ionization. Based on Franck-Condon considerations the formalism is reduced to that of the two-state approximation. Preionization loss and tunnelling beyond turning points have not been considered. Calculated partial ionization cross sections for the Ar-He system show good agreement with a fully quantum mechanical treatment. (Auth.)

[en] Results of calculations based on quantum-mechanical theory are presented for ionizing collisions of He* and Ar in the presence of a strong electromagnetic field

[en] Transitions among discrete and continuum electronic states of diatomics induced by atom-atom collisions and leading to ionization, are described by rigorously making the continuous electron energies epsilon discrete. An expansion in functions of epsilon leads to a set of coupled-channel scattering equations which may be solved to obtain differential and integral cross sections per unit of electron energies. This procedure is applied to Penning ionization in He* (1s2s,³S) + Ar collisions by choosing a suitable expansion basis and numerically integrating a selected set of coupled equations. Results confirm the Franck-Condon nature of electron emission used in previous theoretical analyses. They were obtained with a parametrized exponential coupling potential between the states of HeAr* and HeAr + e^-. The calculations include total integral cross sections per unit electron energies and the contribution of individual partial waves at a given final energy. These cross sections show prominent features that may be related to the potential of the product heavy particles

[en] We present a quantum-mechanical formalism for treating ionizing collisions occurring in the presence of an intense laser field. The theory rigorously takes into account both the intense laser radiation and the internal electronic continuum states associated with the emitted electrons. We accomplish this essentially by combining discretization techniques, used in a recent study of fieldfree collisional ionization, with expansions in terms of so-called electronic-field representations for the quasi-molecule-plus-photon system. This leads to a coupled-channel description of the heavy-particle dynamics which involves effective electronic-field potential surfaces and continua. We also discuss qualitatively characteristic features of ionizing collisions accompanied by intense lasers, drawing comparisons with their fieldfree counterparts. Our remarks are designed to encourage experimental investigation of collisional ionization in the presence of intense lasers, and to stimulate further theoretical work. Because the electronic continuum meets requirements of exact energy resonance for absorption of a photon over large ranges of the internuclear separations, collisional ionization in an intense field should occur much more readily than other field-influenced inelastic collisions, in which photon absorption is resonant only near potential surface pseudocrossings. We therefore suggest laser-influenced ionizing collisions as very good candidates for experimental verification of the effects of intense laser radiation on inelastic collisional processes. We describe the anticipated behavior of the energy distribution of electrons emitted due to radiative coupling. Our comments are based on some physically reasonable assumptions about the electronic transition dipole matrix elements between discrete and continuum electronic states. Actual calculations of such matrix elements involve special electronic structure considerations, and these are outlined in some detail in an Appendix

[en] We investigate forms of the molecular system Hamiltonian valid for rigorous quantum-mechanical treatments of inelastic atom--diatom collisions characterized by exchange of energy between electronic, vibrational, and rotational degrees of freedom. We analyze this Hamiltonian in terms of various choices of independent coordinates which unambiguously specify the electronic and nuclear positions in the context of space-fixed and body-fixed reference frames. In particular we derive forms of the Hamiltonian in the context of the following four sets of independent coordinates: (1) a so-called space-fixed set, in which both electronic and nuclear positions are relative to the space-fixed frame; (2) a so-called mixed set, in which nuclear positions are relative to the body-fixed frame while electronic positions are relative to the space-fixed frame; (3) a so-called body-fixed set, in which both electronic and nuclear positions are relative to the body-fixed frame; and (4) another mixed set, in which nuclear positions are relative to the space-fixed frame while electronic positions are relative to the body-fixed frame. Based on practical considerations in accounting for electronic structure and nonadiabatic coupling of electronic states of the collision complex we find the forms of the Hamiltonian in the context of coordinate sets (3) and (4) above to be most appropriate, respectively, for body-fixed and space-fixed treatments of nuclear dynamics in collisional transfer of electronic, vibrational, and rotational energies