The Coulomb Ochkur-Rudge approximation applied to excitation of triplet levels in helium-like ions

Abstract

Bely's modification of the Ochkur-Rudge exchange approximation (1966, 1967) which allows for Coulomb distortion has been used to obtain total cross sections for electron impact excitation of some 11S to n3L transitions in helium-like positive ions. A comparison with the recent Li+ (11S to 23P) measurement of Rogers, Olsen and Dunn (1978) as well as with other theoretical results, enables an assessment to be made of the merits of this simple approximation. The authors conclude that the Coulomb Ochkur-Rudge approximation probably provides a reasonable estimate (i.e. to within about a factor of 2) for excitation of triplet P levels but considerably overestimates (underestimates) excitation to n3S(n3D). Furthermore, since it differs from the exact non-relativistic result when Z= infinity one can argue that the validity of this approximation does not necessarily improve with increasing Z.