Resonance effects in electron-impact ionization of helium

Abstract

We have extended our recent work on electron-impact ionization and ionizationexcitation of helium (Fang Y and Bartschat K 2001 J. Phys. B: At. Mol. Opt. Phys. 34 L19) to investigate resonance structures in the ejected-electronresidual-ion interaction. The calculations were performed using a second-order perturbative model for a 'fast' incident projectile together with a convergent R-matrix with pseudo-states close-coupling model for the initial bound state and the scattering of a 'slow' ejected electron in the field of the ion. The agreement with previous calculations by Marchalant et al using a similar model is satisfactory and confirms the importance of the 'two-step' mechanism in these processes. However, significant discrepancies remain with experimental data by Lower and Weigold and, to a lesser extent, by McDonald and Crowe.

Triple Coincidence(e,γ2e)Experiment for Simultaneous Electron Impact Ionization Excitation of Helium

“…These methods yield nearly identical first-order results [20], but they differ in the extent to which second-order effects in the projectiletarget interaction are incorporated. So far the CCC method accounts only for dipole interactions in second order [21], while the RMPS implementation includes monopole, dipole, and quadrupole terms [22,23]. Consequently, the second-order CCC approach is currently limited to small momentum transfers.…”

Ejected-electron spectroscopy of autoionizing resonances of helium excited by fast-electron impact

“…Therefore, besides further experimental studies, corresponding theoretical calculations are also absolutely essential. So far as we know, relative to the very few theoretical calculations about the DDCS of autoionization of He at low incident energy [19,36], many calculations have been developed for comparing and explaining the results of triple differential cross section (TDCS) of (e, 2e) experiments about autoionization of He, such as the first-order distorted-wave model [19,36,40], sixstate momentum space close-coupling approximation [41], and more sophisticated methods using the second-order theoretical model [33,34,[42][43][44][45]. While first-order models reach a fair agreement in some cases, the second-order models have performed better in most cases.…”

Second-order distorted wave calculation for electron-impact ionization of helium to He+(n= 1 and 2)

“…Since the only measurement which has been made for where there are no problems with resonances is the 40 eV case, we will compare with the existing data with the understanding that there are potential problems associated with the resonances for the 9.25 and 10 eV results. Fortunately, there are no resonances in this energy range for the case of direct ionization shown in figures 1 and 2 [24].…”