Excitation-autoionization cross section of alkali atoms by electron impact

Abstract

The excitation-autoionization (EA) cross section of Na atoms was obtained experimentally in the form of normalized intensities of autoionizing spectra measured in the ejected-electron energy region between 25.5 and 33.7 eV for electron impact energies from the 2p6 excitation threshold at 30.77 up to 400 eV. The cross section exhibits the strong resonance behaviour in the near-threshold region and reaches the maximum value of 1.2 × 10−17 cm2 at 200 eV. By assigning the lines in ejected-electron spectra, the relative role of the 2p6 and 2s2 subshells as well as quartet and doublet levels in formation of the EA cross section was estimated. In particular, the autoionizing doublets in 2p53sns(n = 3 − 6), np(n = 3 − 6), nd(n = 3 − 4) configurations provide approximately 80% of the EA cross section at 200 eV impact energy. A comparative analysis of the EA cross sections for whole alkali group Li–Cs indicates that the near-threshold resonance behaviour of all cross sections is caused exclusively by the negative-ion resonances formed on the basis of the levels 1sn1l1n2l2 in Li and np5n1l1n2l2 in Na–Cs atoms. The relative EA contribution to the total single ionization cross section does not exceed 2% in Li and 5% in Na, and reaches 26% in K, 32% in Rb and 39% in Cs atoms.