Dynamic effects in small-angle electron differential cross sections for excitation of H 2 2P, He 2 1P and Li 2 2P levels

Abstract

Experimental and theoretical small-angle electron differential cross sections (DCSS) for excitation of H 2 2P, He 2 1P and Li 2 2P states from their respective ground states are investigated through the generalized oscillator strength (GOS) and compared with the GOS from the recent universal function in an attempt to delineate the behaviour of dynamic effects as a function of impact energy, E. The results cover the impact energy range from near threshold to the strong Born approximation regime. We find the surprising result that for the Li 2 2S-2P transition the dynamic effects first decrease as E increases from 10 to about 20 eV where they reach a minimum, manifested through reasonable agreement between measurement and the universal curve over a wide range of the momentum transfer squared, K2 values, 0.01<or=K2<or=0.1 au. Thereafter, for a given K2, away from the asymptotic region, their importance increases until the Born approximation regime is reached. In contrast, dynamic effects in H 2 2P and He 2 1P persist over a wide range of E values all the way to the Born approximation limit where it agrees with the universal function GOS, indicative of their diminished significance to zero.