Abstract
A violation of the propensity rule for positive angular-momentumtransfer at small scattering angles in He 1 1S → 3 1Delectron-impact excitation (Cvejanović D, McLaughlin D T and Crowe A 2000 J. Phys.B: At. Mol. Opt. Phys. 33 3013) has been investigated. The analysis of results fromnumerical models with an increasing number of coupled states in a close-couplingexpansion shows that only the most sophisticated ‘convergent close-coupling’ and‘R-matrixwith pseudo-states’ models can reproduce the experimental data. This points tochannel-coupling effects as the principal reason for violation of the propensityrule, a conclusion supported by comparing predictions for the angle-integratedcross section from these models. Finally, the component of the charge cloudlocated along the direction perpendicular to the scattering plane becomesincreasingly important with increasing energy. This can also be seen in theaccompanying QuickTime movies.
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