Existence of near-zero minima in elastic photon–atom scattering

We consider near-zero minima in the photon–atom elastic scattering cross section, possible signatures of the presence of a particular element in a target, using the S-matrix approach. However, the usual formulation of this approach performs an averaging over magnetic substates at the level of the scattering amplitude that is approximate (except for the case of fully filled subshells). We consider whether the predicted near-zero minima persist in a more proper treatment. We find that near-zero minima seen below thresholds in scattering from ground-state atoms generally persist, but near-zero minima seen above thresholds in scattering from excited states may be spurious.