Decay of the Na 2pcore exciton in sodium halides studied by photoelectron spectroscopy

The decay of the Na 2p core exciton in NaF, NaCl, NaBr, and NaI was investigated by photoelectron spectroscopy with synchrotron radiation. It was observed that the intensities of the valence band and the Auger-electron peak are resonantly enhanced around the excitation photon energy of the Na 2p core exciton in NaCl, NaBr, and NaI, while only the Auger-electron peak shows resonant behavior in NaF. These resonant enhancements are interpreted in terms of the nonradiative decay of the Na 2p core exciton through the $L_2$,3(exciton)-V and $L_2$,3(exciton)-VV processes, where $L_2$,3(exciton) denotes the initial $L_2$- or $L_3$-exciton state and V denotes the final state with a hole in the valence band. The decay probabilities of the Na 2p core exciton through these two processes were estimated from the constant-initial-state spectra and the absorption spectra. It was found that the decay through the $L_2$,3(exciton)-VV process, where the Na 2p core hole forming a core exciton recombines with a valence electron transferring energy to another valence electron, is the predominant nonradiative decay of the Na 2p core exciton. The correlation between two valence holes and one bound electron produced in the final state of this core-exciton decay was confirmed by the energy shift of the Auger-electron peak. The decay probabilities of the excited states higher than the core-exciton states were also estimated from the constant-final-state spectra. The result indicates that these higher excited states decay dominantly through the Auger process due to the recombination of the Na 2p core hole with the valence electron.