Approximation of the electron excitation cross sections for triplet states excited from the 23 S 1 metastable state in helium

Abstract

The method of approximated four-parameter representation of the electron-impact cross sections for a helium atom excited from the 23 S 1 metastable state into higher triplet states is applied and discussed. The approximation consists in interpolation over the whole set of the cross section values for each helium atomic level measured in our experiments and reported by other researchers. The approximation parameters and the cross sections calculated using these parameters for the maxima of the corresponding excitation functions are presented for 19 triplet levels of the S, P, and D HeI series with n=2–10. The interpolated values are compared to the theoretical cross sections. The serial regularities were investigated for the S, P, and D levels studied and a decrease in the cross sections for excitation from the given metastable state within each series, described by the approximate law Q=Cn −5, was revealed. Validity of the similarity relationship and the Bethe approximation for cross sections in the 23 S−n 3 P series was verified. It is shown that the cross sections for a triplet level excitation from the 23 S 1 metastable state exceed the corresponding values for excitation from the ground state of helium by a factor of approximately 103 for n=2 and 3 and 10 2 for the higher levels. It is concluded that the proposed method of representation of the cross sections for the electron-impact excitation of triplet levels from the metastable state increases accuracy and more importantly, reliability of the final results.