DIRECT SPECTROSCOPIC OBSERVATION OF LOCALIZED BAND-GAP STATES IN LIQUID NA-NAI

We have studied the optical absorption of fluid NaxNaI1-x solutions at high temperatures up to 1073 K and in the energy range from 0.5 eV to 3.6 eV. The metal mole fraction x has been varied in situ by coulomet ric titration in the range from 10(-3) to 4 x 10(-2). This ensures a p recise control of changes in the spectra as a function of x. Besides t he well known F centre excitation and a Drude-like contribution the mo st striking characteristic of the spectra is a step-like intensity pro file in the UV. Its main features and variation with x are consistentl y described by excitations from the F centre-like trap states into sta tes of the conduction band tail above the Fermi energy.