Oscillator strength of the α‐ and β‐band in CaF₂

Abstract

During the last decade research on color centers in alkaline earth fluorides has shown a rapid increase. Although most of the coloration bands have been studied fairly extensively many of the phenomena a r e still much less well understood a s in the alkali halides, a group of compounds closely related to the alkaline earth fluorides. Up to now no physical explanation has been given for the fact that the subtractive colorability is extremely low and probably zero at room temperature for very pure crystals (1). At low temperatures it seems that colorability increases but a direct proof that intrinsic color centers are produced has not yet been given.In contrast to the low subtractive colorability we could obtain very high color center densities by heating the crystals in C a vapor (additive coloration). There i s no explanation for the anomalously high color center concentrations in C a F with respect to the vapor pressure and the activation energy for the formation of color centers in CaF has not been explained satisfactorily either (2 to 4). Of all coloration bands observed in the alkaline earth fluorides only the assignation of the F-band can be regarded as well established (5 to 7). 2In crystals which were quenched very quickly after additive coloration we found two absorption bands, the OL -band at 3700 x and the p -band at 5200 8 . These bands have approximately the same intensity. It was found earlier that the OL -band corresponds with the F-center ESR signal observed in additively colored CaF For the P-band several models have been proposed, but none of them has been able to explain the vast amount of experimental facts that has been published in the literature (2, 6 to 13). A more detailed discussion of the properties of the second main band will be given in a forthcoming paper (14). 2'It i s the purpose of this note to report the results of measurements that enable 2'us to calculate the oscillator strength of both the K -and p -band in CaF