OPTICAL STUDIES OF ALKALI FLUORIDES AND ALKALINE EARTH FLUORIDES IN VUV REGION

The results of a series of reflectivity and absorption measurements performed to clarify respectively the excitonic transitions in the intrinsic region to 13.5 eV and the thermal and spectral dependences of absorptions in the tail region of LiF, NaF, KF, CaF 2 SrF2 and BaF2, each being cleaved from Harshaw blocks of single crystal, are presented. The tails of Harshaw LiF, being expressed by exponential functions of energy in the range of 100 cm−1 ≲A≲5⋅102 cm−1 and 300°K≤ T ≤573°K, have turned out to be extrinsic to LiF. The present experiments could not provide sufficiently conclusive evidences in clarifying the question whether the tails of Harshaw alkaline earth fluorides, to Urbach rule is applicable for the range of 100 cm−1 ≲A≲102 cm −1 and 78°K≤ T ≤573°K, are intrinsic. In the first peak of reflectivity spectra, the fluorine doublet with ∼0.05 eV separation is detected for KF and quintet structure for SrF2 and BaF2. In the latter, the lower energy lying doublet was identified with the Γ-exciton (fluorine doublet) and the remaining triplet with the X -excitons. The apparent doublet in the first peak in CaF2 is inferred to result from the unresolved quintet lines. The oscillator strengths for the first peak as a whole were estimated by means of Kramers-Kroning transform of reflectivity spectra: they are 0.52 for CaF2, 0.47 for SrF 2 and 0.44 for BaF2. © 1969, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.