Radiative decay of 4d94fn+1excited states inLaB6andCeB6

The $N_4$,5 emission spectra of ${\mathrm{LaB}}_6$ and ${\mathrm{CeB}}_6$ were measured to investigate the radiative decay of the 4$d^9$4$f^{n+1\mathrm{}}$ excited states, where n is the number of 4f electrons in the ground state. These spectra show an intense and broad emission peak above the $N_4$,5 threshold in addition to sharp peaks below the threshold. The energy positions of these emission peaks are compared with those of the peaks in the absorption spectra and the existing results of resonant photoemission. The comparison shows that the fine structure below the threshold coincides in energy with the multiplet structures in the absorption spectrum, and that the intense emission peak is located in the energy region where the electron-emission peak due to the direct recombination of the 4f electron with the 4d hole has been observed in resonant photoemission. The peaks observed in the energy region below and above the $N_4$,5 threshold are attributed to the radiative decay of the 4$d^9$4$f^{n+1\mathrm{}}$ excited states.