Pascale J. Marchalant

Second-order effects in (e, 2e) excitation-ionization of helium to ( n = 2)

First and second Born (e, 2e) calculations are presented for excitation-ionization of ground-state helium to . Results for ionization to the ground-state ion (1s) are also given. The physical content of the approximations is discussed, in particular, the two-step mechanism which appears in the second-order term for excitation-ionization. The second Born term is calculated in the closure approximation using a new numerical method based on prolate spheroidal coordinates. Comparison is made with absolute experimental data from Paris and Rome in very asymmetric coplanar geometry - scattered electron energies of 5500, 1500 and 570 eV and ejected electron energies of 5, 10, 20, 40 and 75 eV. For excitation-ionization the second Born approximation generally gives improved agreement with the experimental data in the recoil region and second-order effects are found to be still significant at 5500 eV. The importance of the second-order term decreases with increasing ejected energy for the cases studied here.

Triply differential single ionization cross sections in coplanar and non-coplanar geometry for fast heavy ion-atom collisions

We have performed a kinematically complete experiment and calculations on single ionization in 100 MeV/amu C6+ + He collisions. For electrons ejected into the scattering plane (defined by the initial and final projectile momentum vectors) our first- and higher-order calculations are in good agreement with the data. In the plane perpendicular to the scattering plane and containing the initial projectile axis a strong forward-backward asymmetry is observed. In this plane both the first-order and the higher-order calculations do not provide good agreement neither with the data nor amongst each other.

First and second Born calculations of (e, 2e) excitation-ionization of helium

We report detailed calculations of the first Born triple-differential cross section for (e, 2e) excitation-ionization of ground state He to He+ (n = 2). These are in accord with the very recent work of Kheifets et al (1999 J. Phys. B: At. Mol. Opt. Phys. 32 L433) and confirm that the first Born amplitude is now known very accurately. We illustrate the sensitivity of the first Born cross section to the choice of initial and final state wavefunctions. We combine our accurate first Born amplitude with an estimate of the second Born term evaluated in the closure approximation. As in previous work (Marchalant et al 1998 J. Phys. B: At. Mol. Opt. Phys. 31 1141) we find that second-order effects are significant even up to energies as high as 5.5 keV. Agreement with experiment generally remains not very satisfactory.

Excitation — Ionization and Excitation — Autoionization of Helium

In this article we survey some new theoretical results on (e, 2e) excitation — ionization and excitation — autoionization of ground state helium. A fuller account may be found in references [1–3]. Our interest lies in the high energy regime and in geometries for which the ejected electron is very much slower than the incident and scattered electrons. Under these conditions we may think of the collision as exciting a continuum state ψ f of the isolated atom. In the case of excitation — ionization this state consists of the ejected electron, momentum κ, moving in the field of an excited He+ ion, for example, the system {He+(2s or 2p) + e− (κ)}. In excitation — autoionization the helium atom makes a transition to an unstable doubly excited bound state He**(nl, n′l′) (nl ≠ ls, n′l′ ≠ 1s), ie, a resonant state of the atom, which then ejects an electron leaving a residual He+ ion. The particular case we consider in this article is where this ion is left in its ground state He+(1s). The autoionization reaction then interferes with ordinary direct ionization to He+(1s) since both processes lead to the same final state {He+(1s) + e−(κ)}

On the Importance of the Second Order Term for Double Excitation Processes of Helium by Charged Particles

n this article we survey some new theoretical results on double excitation processes of ground state helium by electrons and positively charged particles such as positrons (e+) and carbon nuclei (C6+). Our interest lies in the high energy regime and in geometries for which the ejected electron is very much slower than the incident and scattered charged particle, which in our approximation are represented by plane waves. Under these conditions we may think of the collision as exciting a continuum state ???f of the isolated atom. In the case of excitation-ionization this state consists of the ejected electron, momentum K, moving in the field of an excited He+ ion, for example, the system {He+(2s or 2p) + e-(K)}.

New Results for Double Excitation Processes with Helium Targets

In this article we report on some new theoretical results on (e, 2e) excitation — ion-ization and (e,3e) of the ground state of helium. These results are a direct continuation of those presented in [1]. Our interest lies in the high energy regime and in geometries for which the incident and scattered electrons are very much faster than any ejected electron. Under these conditions, we may think of the collision as exciting a continuum state ψf of the isolated atom. In the case of excitation — ionization this state consists of the ejected electron, momentum K, moving in the field of an excited He+ ion, for example, the system He+(2s or 2p) + e-(k). In (e,3e) we have a double continuum state of the atom. In the following, we use atomic units (au) in which ħ = m e = e = 1.

Electron Impact Ionization of Atoms with Two Active Target Electrons

The ionisation of atoms with two target electrons is reviewed. The use of perturbative and non-perturbative methods is discussed

The double ionisation of helium by both photon and electron impact

In this paper we are concerned with the double ionisation of helium by both electron and photon impact. The latter can be viewed as a special case of the former in the limit of zero momentum transfer. The ability to accurately evaluate the (γ,2e) cross section is thus a necessary requisite to be able to describe the (e,3e) cross section. In this paper we will be concerned only with the (γ,2e) cross section and the first Born approximation to the (e,3e) cross section. Once we have converged (γ,2e) results with no gauge discrepancies than we can have some confidence in the quality of the choice of the initial ground and final double continuum state wave function of the Helium atom.