The multielectron, hidden crossings method for inelastic processes in slow ion/atom–atom collisions

Abstract

A new method is described for studying collision dynamics in slow ion/atom–atom collisions. It is a generalization of the single-electron, two-center hidden crossings method to multielectron systems. This approach derives from the analytic properties of energy surfaces and wave functions of the adiabatic electronic Hamiltonian when the internuclear distance is extended into the complex plane. The collision dynamics in the adiabatic limit is determined by the topology of the unique multivalued electronic energy surface, particularly by its singular points, the square-root branch points. The surfaces described here have been studied using a complex version of the unrestricted Hartree–Fock and configuration interaction method with all single electron excitations, based on a bivariational principle. Although various inelastic processes can be calculated, the method is especially useful for the description of ionization. We have illustrated this through the calculation of cross sections for ionization of helium by proton and antiproton impact, as well as for the collision of two hydrogen atoms.

Ionization of Helium and Argon by Very Slow Antiproton Impact

“…Following this progress, Bent et al [21] developed the multielectron hidden crossing theory, which uses the adiabatic electron Hamiltonian but with the internuclear distance extended into the complex plane. The Hamiltonian is of Hartree-Fock type with configuration interaction, and sequential double ionization is assumed.…”

Effect of static electric field on cross sections in antiproton impact ionization of atomic helium

“…1. In the same figure, the calculated results of ionization cross sections are compared with the various experimental [3,4,6] as well as with the theoretical [10][11][12][13][14][15][16][17][18][19][20][21][22][23] results. From the experimental side, the present calculated data for ionization of the helium atom by antiproton are compared with experimental data of Andersen et al [4] who performed their experiment in the energy range from 40 keV to 3 MeV, Hvelpund et al [6] performed their experiment in the energy range from 13 to 500 keV and Knudsen et al [3] performed their experiment in the energy range from 3 to 25 keV.…”

“…As antiproton beams become available at CERN for atomic collision experiments in 1986, a number of measurements [1][2][3][4][5][6][7] have been carried out at different energy ranges (from keV to MeV) for antimatter and matter collisions. At the same time, many theoretical studies [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] of antiproton and atom collision have also been started. Although collisions between antiproton and helium have been studied during the last decade, it is still a challenge to understand in detail even for the simplest collisional processes.…”

“…From the theoretical point of view, the collision between antiproton and the two-electron helium atom is a fundamental problem, which includes both static and dynamic correlations. There are many theoretical studies [10][11][12][13][14][15][16][17][18][19][20][21][22][23] on the single ionization by various approximations. But in those studies, investigators have been studied collisional ionization cross sections in the absence of the electric fields.…”

Electrostatic ultra-low-energy antiproton recycling ring

“…Total ionisation cross section measurements for antiproton incident energies from 40 to 3,000 keV were reported over a decade ago [10,11]; above 100 keV incident energy, theory and experiment are in good agreement [7]. Several theoretical studies have predicted cross sectional trends in the low energy region (down to 1 keV) [12][13][14][15][16][17]. Whilst these theories all agree on the general trend towards increasing cross section from 1 to 70 keV, they vary considerably in the details of the shape and absolute value.…”