New Method for Treating Slow Multielectron, Multicenter Atomic Collisions

Abstract

The method of hidden crossings is generalized to treat multielectron systems utilizing molecular Hartree-Fock and configuration interaction methods, extended into the plane of complex internuclear distance. Diabatic promotion of low lying states to the continuum in a two-electron, two-center system via a series of localized transitions is shown for the first time. Excellent agreement with experiments is found regarding single ionization in 50 eV to 1 keV H1H collisions.[S0031-9007 (96)01249-5] PACS numbers: 34.50.-s, 34.10.+x, 34.30.+hSlow atomic collisions involving several electrons play a substantial role in astrophysical, fusion, and technical plasma processes, in chemical reactions, and in collision spectroscopy. For example, knowledge of the ionization cross section in the collision of hydrogen atoms is critical for modeling the collapse of protostellar material as well as the divertor and edge regions of tokamak plasmas. Still, few reliable results are available. Experiments involving collisions of slow neutral particles are difficult, while theoretical calculations suffer because of the enormous number of molecular states needed for the reliable description of ionization.We introduce in this Letter an alternative method to the coupled channels approach and, for the fundamental case of ionization in H 1 H collisions, demonstrate its relative simplicity by readily identifying the main reaction channels, obtaining an accurate estimate of the cross section, and thus creating a new method for treating multielectron heavy particle collisions.The essential foundations and inspiration for the present work come from the "hidden crossing theory" [1]. In this approach the electronic quasimolecular Hamiltonian and its eigenfunctions are analytically continued into the plane of complex internuclear distance R. The resulting eigenvalue problem is no longer Hermitian and the complex eigenenergies of states of the same symmetry cross at isolated points R c , which have the structure of square-root branch points. The physical significance of such branch points lies in the fact that the radial matrix element between the states has a sharp local maximum at Re͕R c ͖, causing localization of the electronic transition in a slow atomic collision. The hidden crossing theory may be considered as a generalization of the multichannel Landau-Zener method to include those regions of localized nonadiabatic transitions not conventionally recognized as avoided crossings of the electronic quasimolecular terms. The main triumph of this theory when applied to two-center, one-electron systems has been the identification of a very localized series of branch points at small internuclear distances. This series has been called the S series and it represents a strong channel for ionization by almost diabatic promotion of low lying quasimolecular states into the continuum [1,2]. A main objective of this work is to confirm the existence of such a series in a two-center, multielectron system.The principal obstacle in applying hidden crossing theory...

Excitation and ionization in H(1s)-H(1s) collisions

“…In Figure 4 we compare our results with the experimental measurements of Gealy and Van Zyl (1987) and McClure (1968). The lower energy predictions (4 keV and less) of Krstic et al (1996) and Bent et al (1998) Our calculations cannot predict the ionization cross section reliably when it is less than 10-17cm2 as the absolute error becomes too large (Riley and Ritchie (1999)) due to numerical resolution.…”

Excitation and ionization in H(1s)-H(1s) collisions: II. Inclusion of electron exchange

“…In Figure 3 we compare our results with the experimental measurements of Gealy and Van Zyl (1987) and of McClure (1968). The lower energy predictions (4 keV and less) of Krstic et al (1996) and Bent et al (1998) agree extremely well with the data at low energy, all the way down to 0,1 keV. The recent calculation of Shingal et al (1989) of the ionization cross section includes electron symmetry with a 22 state per center atomic basis with translation factors but without double excitation channels.…”

“…Strong coupling among those nearly degenerate levels leads to excitation as well as ionization. This has been exploited by Bent, et al (1998) and Krstic et al (1996) as a means of calculating ionization in H+H scattering. -The triplet united-atom limit at close encounters is difficult for us to solve by the purely numerical methods because of the great difference in size (a factor of eight)…”

Commission 14: Atomic and Molecular Data: (Donnees Atomiques et Moleculaires)

“…Notable examples of results for excitation include proton impact excitation for Be-like (Ryan et al 1998), B-like (Foster et al 1997), C-like (Ryan et al 1999), and Be-like to Cl-like ions . Regarding ionization, the following works are of particular interest: He* + H (Mihajlov et al 1996), H + H (Krstic et al 1996), H+ + 0 (Thompson et al 1997), H+, He 2 +, Li 3 + + H (Toshima 1997), and H+ + H (Shah et al 1998).…”