Optical Properties of NaF Single Crystals in VUV Region

Abstract

The results of the reflectivity and absorption measurements on NaF single crystals (Harshaw) at 78°K and at room temperature are presented. The fluorine doublet, and the side band structure which is located at the higher energy side of the former and is presumably due to the transition of simultaneous creation of exciton and phonon, are observed in the reflectivity spectrum at 78°K. The spectra of optical of constants derived from these reflectivities by means of the Kramers-Kroning relation are also presented. It is shown that the fluorine doublet can be decomposed into two asymmetric Lorentzian lines with the oscillator strengths 0.155 and 0.215 for the lower and higher energy components, respectively. The position of the first reflectivity peak of ( 2 S 1/2 , 2 P 2/3 ), determined in the temperature range of 10°K to 295°K, is found to be well fitted by the formula of Bose-statistics. The empirical relation which is capable of representing the thermal shift of the first exciton peak of all alkali halides is proposed, and is discussed from the viewpoint of exciton-phonon interaction. On the basis of this relation, the thermal and spectral dependences of intrinsic absorption tails of LiF are discussed.