Positronium formation in the n=1 and n=2 states in e+-He scattering

Positronium formation in the n=1 and n=2 levels in e+-He collision has been studied using a second-order Born approximation. Differential and integrated capture cross sections are reported in the energy range 100 eV--2 keV. The predicted total positronium-formation cross sections differ from measured values, the theoretical results being lower. The first-order Born results are not expected to be valid in the energy range considered. A Thomas mechanism is found to be valid in the case of the ground as well as the 2s excited state. A similar structure in the differential cross section for 2p-state capture has not been found