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

Abstract

First and second Born (e, 2e) calculations are presented for excitation-ionization of ground-state helium to He + (n = 2). Results for ionization to the ground-state ion He + (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. † Whereas the 2s → 1s transition is not 'optically allowed', the 2s state can decay radiatively to 1s by emission of two photons. These photons will have a distribution of energies and will therefore not appear as a single spectral line like the optically allowed 2p → 1s decay.‡ And also with collisions of other projectiles such as Li q+ , B q+ , C q+ , H + 2 and H +

Three-dimensional imaging of atomic four-body processes

“…Such a good reproduction of the data by theory in the scattering plane is quite typical for single ionization of He by high-energy electron impact as well, even when compared to less sophisticated models such as the first Born approximation. 17 In that sense, our results are consistent with high-energy electron-impact data. In the perpendicular plane, in contrast, dramatic discrepancies between experiment and theory are quite obvious.…”

“…Such contributions would also explain small, but systematic underestimations of the height of the recoil peak by theory both for electron and ion impact. 1,13,17 This process represents a potentially important few-body effect involving all collision products. In principle, it is contained in the final state wavefunction in our calculation.…”

Second-order Born model for two-electron atomic ionization by fast charged-particle impact

“…III A we test our model by calculating electron-impact ionization of He leading to the n =2 excited state. We compare our results with available experimental data [21,22] as well as the earlier second Born calculations [5,6]. As our model is restricted to dipole interaction in the second Born term, we also compare our results with analogous data obtained by the R-matrix method with pseudostates (RMPS) with all the multipoles included in the second Born term and the dipole contribution only [23].…”

“…The first Born term has been investigated in great detail and the benchmark results had been established for the electron-impact ionization excitation of He to the n = 2 state [3,4]. There is also a good agreement between different close-coupling theories for the second Born contribution as far as ionization excitation is concerned [5,6]. However, there is a broad variation of the first and second Born results for the double ionization by electron impact.…”

Electron-Impact Ionization and Excitation of Helium to then=1–4Ionic States

“…Such experiments were performed by Dupré et al [13] and later by Avaldi et al [14] and Rouvellou et al [15]. For their conditions of highly asymmetric energy sharing between the two outgoing electrons, hybrid methods based upon a perturbative treatment, up to second order, of the projectile-target interaction and an R matrix (closecoupling) approach for the ejected-electron-residual-ion interaction [16] were quite successful, especially when the latter part of the problem (e-He collisions) was systematically driven to convergence [17]. Except for a pioneering study by Dogan and Crowe [18], the TDCS experiments mentioned above were concerned with ionization and excitation to the n 2 residual ionic states.…”