Autoionization of strontium atom during excitation of 4p<sup>6</sup> shell

Abstract

Background : Autoionization is an indirect ionization process that significantly increases the total ionization cross section of the atom. Measurements of electron autoionizing spectra in a broad range of impact energies and the determination of their total intensity allow obtaining the absolute value of the autoionization contribution (the autoionization cross section) and its energy behavior. Methods : The study of the ejected-electron spectra of strontium atoms were carried out by using an electron spectrometer. It consists of a source of the incident electron beam, an electron energy analyzer (of 127° electrostatic type deflector) 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 was determined as the normalized sum of total intensities of ejected-electron spectra measured at different impact energies. Results : Autoionization cross section for 4p 6 shell of strontium atoms was determined in an incident-electron energy range from the lowest autoionization threshold (20,98 eV) up to 600 eV. 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. Cross section reaches its maximum value of 1.6×10 -16 сm 2 at 24.8 eV. We analyze the excitation and decay processes of the 4p 5 n 1 l 1 n 2 l 2 n 3 l 3 autoionizing states. They determine the magnitude and the energy behavior of the autoionization cross section. Conclusions : The strong resonant excitation of the levels in 4p 5 4d5s 2 , 4p 5 4d 2 5s, and 4p 5 5s 2 5p configurations determines the resonant behavior of the measured autoionization cross section at low impact energies. 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 4p 5 4d5s 2 , 4p 5 4d 2 5s.