The valence shell excitations for some noble atoms studied by fast electron impact

The differential cross sections and apparent generalized oscillator strengths for the valence-shell excitations of the noble atoms of He, Ne, Ar, and Kr are reviewed. The asymptotical behaviors of the apparent generalized oscillator strengths show that the first Born approximation is more easily reached for the dipole-allowed transition than the dipole-forbidden one of 1S0 →1 S0 type, in which the final state has the same term symbol of the initial state. Furthermore, the experiments reveal that the generalized oscillator strengths for the transitions, which are formed by the same electronic configuration through the intrachannel interaction such as (n - 1)p5ns[3/2]1 and (n - 1)p5ns'[1/2]1 of Ne (n=3), Ar (n=4), and Kr (n=5), are nearly parallel. This phenomenon is elucidated by the fact that the LS coupling singlet nature of the ground state selects the LS coupling singlet component from the excited wave function, i.e., the contribution from the singlet component in the excitation is dominant in the generalized oscillator strength.