The 4p6 autoionization cross section of Rb atoms excited by low-energy electron impact

Abstract

The autoionization cross section of rubidium atoms was obtained by measuring the total normalized intensities of ejected-electron spectra arising from the decay of the 4p5n1l1n2l2 autoionizing levels. The electron impact energy range from the 4p6 excitation threshold at 15.31 up to 50 eV was investigated. The cross section reaches the maximum value of (2.9 ± 0.6) × 10−16 cm2 at 21.8 eV impact energy. The general behaviours of the cross section and the role of particular autoionizing configurations in its formation were considered on the basis of large-scale configuration interaction calculations of energies, cross sections, autoionization probabilities in 5snl(n ⩽ 7; l ⩽ 4) and 4d nl(n ⩽ 5; l ⩽ 2) configurations as well as the measured excitation functions for the lowest levels in 5s2 and 4d5s configurations. The resonance behaviour of the cross section between 15.3 and 18.5 eV impact energy is caused exclusively by the negative-ion resonances present close to the excitation thresholds of the (5s2)2P and (4d5s)4P autoionizing levels. At higher impact energies, the autoionization cross section is composed of contributions from the high-lying quartet and doublet levels in 4d5s, 5p and 5s5p, 5d, 6s, 6p configurations. From the comparison of the present data with available experimental and calculated ionization cross sections, the 5s + 4p6 direct ionization cross section of rubidium atoms was determined with the maximum value of (7.2 ± 2.2) × 10−16 cm2 at 36 eV. It was also found that the 4p6 excitation–autoionization is the dominant indirect ionization process contributing over 30% of the total single ionization of rubidium atoms by electron impact in the 15.3–50 eV energy range.