TWO-STEP AUTOIONIZATION OF STRONTIUM AND BARIUM ATOMS EXCITED BY ELECTRON IMPACT

Abstract.

Background: The presence in alkaline earth atoms of a closed valence shell makes the processes of their electronic excitation and ionization more complex than those in other atoms. This also applies to the influence of autoionization processes on electron-impact ionization of alkaline earth atoms. The fast rise of the single ionization cross section immediately beyond the excitation threshold of the outer p6 shell, the resonant shape of the cross section in this energy region, as well as its sharp decline at impact energies just 5-6 eV above the p6 shell excitation threshold and, at the same time, a sharp rise of the double ionization cross section – all this indicated the existence of additional electron decay channels for high-energy atomc autoionizing states np5nln1l1n2l2 in Sr (n = 4) and Ba (n = 5) atoms.This work is devoted to the study of these decay channels. Methods: The measurements of the ejected-electron spectra of Sr and Ba atoms were carried out at an impact energy value of 100 eV by using an electron spectrometer consisted of a source of the incident electron beam, an electron energy analyzer (of 127 electrostatic type) and a resistively heated atomic beam source. The incident and ejected-electron energy resolutions were about 0.4 eV and 0.07 eV, respectively. Results: By comparative analysis of experimental ejected-electron spectra and photoexcitation spectra with theoretical data on excitation cross sections and decay probabilities of autoionizing states in Sr and Ba atoms, it is shown that two-step and two-electron decay channels are most preferable for p5nln1l1n2l2 atomic autoionizing states. Conclusions: The two-step and two-electron decay processes of the high-energy states p5nln1l1n2l2 in Sr and Ba atoms, studied in this work, are the main reason for the high formation efficiency of Sr2+ and Ba2+ ions, which leads to an abrupt increase in the doule-ionization cross section of these atoms by electron impact

Keywords: electron, atom, subshell, state, ionization, autoionization

https://doi.org/10.24144/2415-8038.2020.47.74-81