Refractive index of LiF films as a function of time

The optical properties and thickness of evaporated LiF films on silver or aluminum were determined by attenuated total reflection techniques as a function of time after deposition. With the LiF films maintained in vacuum, the refractive index increased from ~1.3 to ~1.4, or close to the crystalline value, in the course of a few days. This implies that the LiF, loosely packed when first deposited, anneals over a period of time to the more closely packed crystalline configuration. The LiF films were found to be optically isotropic in contrast to earlier reports of anisotropy.