Theoretical Analysis of Generalized Oscillator Strengths for Helium by R-Matrix Method

The high-energy electronic-impact excitation cross section is directly proportional to the generalized oscillator strength (GOS) of the target atom. The generalized oscillator strengths of helium atom from the ground state to the excited states (2¹S, 2¹P and 3¹D) are calculated using the updated R-matrix codes within the first Born approximation. Our calculation results are in good agreement with the previous theoretical and experimental results at high incident energies. In order to treat the bound-bound and bound-continuum transitions in a unified manner, the GOS density is defined based on the quantum defect theory. We calculate the GOS densities of ¹S, ¹P and ¹D channels, namely the complete high-energy collision cross sections of electronic-impact excitations into all the n¹S, n¹P and n¹D excited states. In addition to high-energy excitation cross sections, a scheme to calculate the excitation cross sections for entire incident energy range is discussed.