Excitation of autoionizing states of helium by 100 keV proton impact: II. Excitation cross sections and mechanisms of excitation

Abstract

Mechanisms of two-electron excitation of the (2s 2) 1 S, (2p 2) 1 D and (2s2p) 1 P autoionizing states of helium are studied both experimentally and theoretically. It is shown that an explicit introduction of a kinematic factor, with a process-specific phase leads to a productive parametrization of experimental cross sections of ionization, allowing one to extract cross sections of excitation of autoionizing states. Using a new fitting procedure together with the proposed parametrization made it possible to obtain the excitation cross sections and magnetic sublevel population from electron spectra as well as, for the first time, to resolve the contribution of resonance and interference components to resonance profiles. Interference with direct ionization is shown to contribute significantly into resonance formation even for backward ejection angles. We demonstrate theoretically that the excitation cross sections thus extracted from experimental electron spectra hold information about the interaction of autoionizing states with an adjacent continuum.

Zero-degree target electron spectroscopy: autoionizing resonances of helium excited by fast H+, He+and He2+impact

“…The doubly differential cross section (DDCS) of ionization as a function of electron emission energy E e and emission angle θ e in the vicinity of autoionizing resonances can be written as (Godunov et al 2000)…”

“…where A c int,µ (E i , ϑ e ), B c int,µ (E i , ϑ e ) and B c exc,µ (E i , ϑ e ) are the resonant parameters, ρ µ (ε µ ) and ϕ µ (ε µ ) are the kinematic factors, ε µ = 2(E e − E µ )/ µ is the relative energy deviation from the resonance position E µ , E e is the ejected electron energy and µ is the resonance width. The exact definitions for the resonance parameters, as well as the kinematic factors, can be found in Godunov et al (1997bGodunov et al ( , 2000. For an alternative representation, one can introduce a phase parameter δ µ (E i , θ e ) between the amplitudes of direct and resonant ionization as (Godunov et al 2000)…”

“…The exact definitions for the resonance parameters, as well as the kinematic factors, can be found in Godunov et al (1997bGodunov et al ( , 2000. For an alternative representation, one can introduce a phase parameter δ µ (E i , θ e ) between the amplitudes of direct and resonant ionization as (Godunov et al 2000)…”

“…Since the first works (Rudd 1964, Boredenave-Montesquieu and Benoit-Catten 1971, Stolterfoht 1971, Schowengerdt and Rudd 1972, electron spectroscopy has been considered as the most powerful method for studying autoionizing resonances in cross sections of ionization. Electron spectroscopy of autoionizing resonances has the important virtue of high energy resolution and the availability of a number of parametrizations for different kinematic regions (Fano 1961, Shore 1967, Balashov et al 1973, Godunov et al 2000.…”

“…However, these calculations did not get overall satisfactory agreement with experimental data, particularly for the forward electron emission angles where CIFS is considerable. Considerable progress has been made in a joint experimental and theoretical study of excitation of lowlying autoionizing (2s 2 ) 1 S, (2p 2 ) 1 D and (2s2p) 1 P states of helium by 100 keV proton impact (Moretto-Capelle et al 1997, Godunov et al 1997b, 2000. It was clearly demonstrated that an adequate solution must accurately account for all the important effects (electron correlation, Fano interference, CIFS, two-electron excitation).…”