Channel interaction and threshold behavior of photoionization

Channel interaction is shown to have a substantial effect of "spectral repulsion" that distorts the photoionization spectrum near the threshold of a weak channel. This effect is illustrated by calculating numerically the cross section of the $3s\to \epsilon p$ transition of Ar, taking into account its coupling with the $3p\to \epsilon d$ transition. The result is very different from that obtained by an independent-particle approximation. Examples calculated by other authors on He, Ne, and Xe serve as additional illustrations.