Autoionization cross-sections of cesium and barium atoms for 5p<sup>6</sup> shell excited by electron impact

Abstract

Background : The presence of a closed valence shell in alkaline-earth atoms makes the processes of their excitation and ionization as a whole much more complicated than for neighboring alkali atoms. In order to find patterns and differences in the course of autoionization in these two groups of atoms, it is interesting to compare their autoionization cross sections. Studies of the latter for alkaline-earth atoms have not been conducted until recently. In this paper, the 5p 6 autoionization cross sections of cesium and barium atoms excited by electrons were investigated in the impact-energy range from the excitation threshold of the 5p 6 subshells to 600 eV. Methods : The measurements of the ejected-electron spectra of Cs and Ba 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 Cs and Ba atoms were determined as the normalized total intensities of ejected-electron spectra measured at different impact energies. Results : The autoionization cross sections for 5p 6 subshells in Cs and Ba atoms were determined in an incident-electron energy range from the lowest autoionization thresholds up to 600 eV. The energy behavior of both cross sections is characterized by the presence of the strong resonance structure in the threshold region of 12-23 eV and a broad maximum at approximately 100 eV. The cross sections reach their maximum value of 4.2×10 -16 сm 2 for cesium and 6.7×10 -16 сm 2 for barium at 14.8 eV and 17.4 eV, respectively. An analysis is made of the excitation and decay processes of the 5p 5 n 1 l 1 n 2 l 2 (Cs) and 5p 5 n 1 l 1 n 2 l 2 n 3 l 3 (Ba) autoionizing states that determine the magnitude and the energy behavior of the autoionization cross sections. Conclusions : The strong resonant excitation of the lowest levels in 5p 5 6s 2 , 5p 5 5d6s configurations in cesium and 5p 5 5d6s 2 , 5p 5 5d 2 6s,6p,6d,7d, 5p 5 6s 2 6p,7s,7d and 5p 5 5d6s6p;7s,7d configurations in barium determines the resonant behavior of the measured autoionization cross sections at low impact energies. The shape and magnitude of the cross sections at high impact energies are determined by the total contribution from doublet autoionizing states in cesium and dipole-allowed autoionizing states in the lowest configurations 5p 5 5d6s 2 and 5p 5 5d 2 6s in barium.