SELECTION-RULES IN RESONANT 4F PHOTOEMISSION FOR RARE-EARTHS

The difference in 4f photoemission spectrum (PES) for rare earths (RE) Nd, Gd and Dy under off-resonance and various on-resonance conditions near the 3d core threshold is discussed on the basis of an ionic model assuming trivalence. For the resonant process passing through a higher-energy intermediate state of both 3d(5/2) and 3d(3/2) regions, the spectral intensity in higher (lower) binding energy multiplets of 4f PES is shown to be enhanced (suppressed) compared with off-resonance spectra. This tendency is especially remarkable for Gd and heavier rare earths. An intuitive interpretation for this tendency is presented and a control of photoexcitation by tuning photon energy, which may be compared to that using a polarized photon, is shown to be feasible.