Abstract

Background. Autoionization is an indirect ionization process that significantly increases the total ionization cross section of the atom above the excitation threshold of the corresponding electron shell. For strontium atom, the autoionization contribution to the total ionization cross section by electron impact is more than 16%. Measurements of electron autoionizing spectra in a broad range of impact energies and the determination of their total intensity allow obtaining with an acceptable accuracy not only the absolute value of the autoionization contribution (the autoionization cross section), but also its energy behavior. In this paper, the 4p⁶ autoionization cross section of strontium atoms is investigated in the impact-energy range from the excitation threshold of the 4p⁶ shell to 600 eV.
Methods. The study of the ejected-electron spectra of strontium atoms were carried out by using an electron spectrometer consisted of a source of the incident electron beam, an electron energy analyzer (of 127° electrostatic type) and an atomic beam source. To minimize the influence of the anisotropy of the angular distribution of ejected electrons, the measurements were carried out at a "magic" observation angle of 54.7°. The incident and ejected-electron energy resolutions were about 0.4 eV and 0.07 eV, respectively. The increment step of the incident electron energy was 0.1 eV in the near-threshold energy region. The autoionization cross section of strontium atoms was determined as the normalized total intensities of ejected-electron spectra measured at different impact energies.
Results. The autoionization cross sections for 4p⁶ shell of strontium atoms were determined in an incident-electron energy range from the lowest autoionization thresholds up to 600 eV. The energy behavior of the cross section is characterized by the presence of the strong resonance structure in the threshold region of 21-30 eV and a broad maximum at approximately 100 eV. The cross section reaches its maximum value of 1.6×10^-16 сm² at 24.8 eV. An analysis is made of the excitation and decay processes of the 4p⁵n₁l₁n₂l₂n₃l₃ autoionizing states that determine the magnitude and the energy behavior of the measured autoionization cross section.
Conclusions. The strong resonant excitation of the levels in 4p⁵4d5s², 4p⁵4d²5s, and 4p⁵5s²5p configurations determines the resonant behavior of the measured autoionization cross section at low impact energies. The shape and magnitude of the cross section at high impact energies are determined by the total contribution from dipole-allowed and singlets autoionizing levels in the configurations 4p⁵4d5s², 4p⁵4d²5s

Keywords: electron, strontium atom, shell, state, configuration, autoionization, cross section

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