Resonant enhancement in photoemission from GaSe

A giant enhancement in the total photoyield of GaSe has been observed and investigated by measuring photoemitted energy-distribution curves, constant initial- and constant final-energy photoemission spectra using synchrotron radiation. We show that the enhancement, which occurs at the Se $3d$ threshold, can be attributed to decay of the $3d$ core hole into a two-hole excited valence state. Final-state effects in the $3d$ excitation mechanism close to threshold may be responsible for the magnitude of the effect. Above threshold we observe photoelectron-diffraction effects in azimuthal emission plots from both Ga and Se $3d$ showing that such effects are manifest in both forward- and back-scattering cases.