Abstract
Electron-impact excitation may occur either directly, or by a resonant process involving a doubly excited autoionising compound state. The contribution of the resonances is computed here by a perturbation method involving the principle of detailed balance and the calculation of autoionisation and radiative-decay transition probabilities. In ions of the Be I isoelectronic sequence, resonances are found to be of negligible importance for the strong 2s2 1S-2s2p 1P degrees excitation, but to provide 25 to 50% of the total excitation rate for the weak 1S-3P degrees excitation. For atoms more than 3 or 4 times ionised, this method-combined with distorted-wave calculation of the direct excitation process-should, in most cases, give total rates comparable in accuracy to detailed close-coupling calculations, and would appear to be particularly useful when departures from pure LS coupling are great or when radiative decay of the compound state is important.
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