Na⁺L_2,3 Absorption Spectra of Sodium Halides

Abstract

The Na⁺L_2,3 absorption spectra of sodium halides, NaF, NaCl, NaBr and NaI, have been obtained at 300°K using a grazing incidence spectrograph with a concave grating of 2 m radius and a vacuum spark source of Vodar type as a quasi-continuum background. Sharp absorption bands due to X-ray excitons at the Γ₁ point were found at the long-wavelength edge. The doublet structure of these exciton bands was analysed, according to the theory of Onodera and Toyozawa, by the spin-orbit splitting of Na⁺L_2,3 levels and the exchange interaction between the electron and the hole of an exciton. The structure of the absorption spectra in the energy region above the exciton doublet was tentatively assigned to the absorptions arising from the formation of excitons at the point other than Γ₁ and from the transition from the Na⁺L_2,3 level to the conduction band. The possibility of the presence of the two-electron excitation was suggested.