Further Selection Rules for Excitation by Electron Impact at Small Momentum Changes

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Abstract
At kinetic energies in the range 300–500 eV, there is an accumulation of experimental evidence that the Born approximation does not hold when term symbols are the same in the initial and final states. In the present research, it is shown that deviations from the Born approximation at small momentum changes are proportional to a series of matrix elements of operators, the first (and dominant) term involving an operator which is invariant under the operations of the symmetry group of the scatterer. This accounts for the observation in the first sentence of this Selection rules for the next term in the series are also discussed.