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.

Excitation and ionization of outer shells in Rb by electron impact

“…As shown previously [23,24], the relative EA contribution to the total single ionization cross section of Rb atom reaches 32%. No contributions from the 4s 2 or 3d 10 excitations were observed.…”

“…The poor agreement of all above data with the experimental total single ionization cross sections [3,5,6] (especially above the p 6 excitation threshold) is a direct consequence of the primary difficulty of all calculations, namely, the accurate consideration of complex coupling effects, target polarization, inner-shell excitation and ionization. The last two processes are particularly important in heavy potassium, rubidium and cesium atoms as the relative contribution from the excitation-autoionization reaches in these atoms the values of 26, 32 and 39% respectively [23]. As was shown by the recent calculations of the EA cross sections in relativistic distorted wave (RDW) [24][25][26] and R-matrix [27,28] approximations, the relativistic and configuration interaction effects are also of great importance in these atoms if the electron impact excitation of the outer p 6 shell is considered.…”

“…It is obvious that the certain conclusion with regard to the applicability of the above approximations can be drawn objectively only by the direct comparison of the calculated EA and ionization cross sections respectively with the experimental absolute EA aut s and the total direct ionization dir s cross sections. The former are now available for all alkali atoms in a broad impact energy range (see [23] and references therein). As regards the latter, it can be determined as dir s = tot s − aut s where tot s is the experimental total single ionization cross section [3,5].…”

Classification of the 5p5nln'l' LSJ energy levels of Cs excited by 30 eV electrons

“…Theoretical investigation of the 5p-core-excited autoionizing states of Cs is a complex task as the relativistic effects play a more important role compared with Na [1,2,3,4], K [5] or Rb [6]. The first attempt to perform the classification of Cs spectrum registered in an ultraviolet absorption experiment was made by Connerade [7] in jK notations.…”

Electron-impact ionization of the 4p6 subshell in strontium

“…The ionization of the 4p 6 subshell (1) and (2) followed by the Auger decay ( 5) is an indirect ionization process that essentially contributes to the total double ionization of Sr atoms [6]. As shown by studies of autoionization in alkali atoms [24], measuring the spectra of ejected (Auger) electrons in a wide range of impact energies is the only experimental method for directly studying both the excitation cross sections of individual autoionizing states and their total contribution (the EA contribution) to the direct ionization. The single-channel decay (5) of the 4p 5 nln ′ l ′ states makes their cross sections proportional to the intensity of the corresponding Auger lines.…”