Core excitons in Na $K$ photoabsorption of NaF: Resonant Auger spectroscopy

The nature of Na $1s$ photoabsorption in NaF is investigated by using the resonant Auger decay of Na $1s$ core excitations. The appearance of peaks in the Auger spectra, when the energy of excitation coincides with the photoabsorption thresholds, shows that, despite the absence of pronounced structures in photoabsorption spectra, core excitons are created in the threshold region of the Na $1s$ x-ray-absorption spectrum. The core exciton excited at a photon energy of 1076.8 eV originates from ${1s}^{-1}3p$ states similarly to the atoms of Ne and Na. The role of the dipole-forbidden transitions to the ${1s}^{-1}3s$ and ${1s}^{-1}3d$ states in the formation of the double structure at 1073.7 and 1074.7 eV is discussed.