NaK photoabsorption and resonant KLL Auger spectra in naf and NaCl

The nature of Na Is photoabsorption in NaF and NaCl is investigated using the resonant Auger decay of Na Is core excitations. The appearance of new peaks in Auger spectra, when the energy of excitation coincides with the photoabsorption thresholds, shows that the core excitons are created in the threshold region of the Na 1s photoabsorption spectrum. The core excitons around 1074.2 eV originate from the dipole-forbidden 1s(-1)3s final state, the population of which is made possible by the symmetry disruption at the photoabsorption site. Another core exciton, excited at photon energies of 1076.8 eV in NaF and 1076.3 eV in NaCl, originates from 1s(-1)3p states similar to atoms of Ne and Na. Resonant Auger spectra show also that in NaCl the first sharp photoabsorption maximum at 1076.6 eV has only partially excitonic character.