Structure generated by interchannel coupling in high-energy photoionization

The 3$d$ core level photoemission of metallic Ag and In was measured as a function of photon energy over a wide range including the 2$p$ ionization thresholds. The intensities of the 3$d_{5/2}$ and 3$d_{3/2}$ lines were observed to modulate significantly with photon energy, both absolute and relative. The modulation of the photoionization cross section is most pronounced in the vicinity of the 2$p$ thresholds, i.e., at photon energies about an order of magnitude above the 3$d$ thresholds. Theoretical calculations based on the relativistic-random-phase approximation show that this effect is due to interchannel coupling of the 3$d$ photoionization channels with the 2$p$ channels affecting the cross section over a wide range of energies. It is argued that this is a general phenomenon in high-energy photoionization throughout the periodic table as well as for molecules, clusters and condensed matter.