Porosity of MgF2 Films—Evaluation Based on Changes in Refractive Index Due to Adsorption of Vapors

Abstract

The refractive index nf of evaporated MgF2 films changes almost reversibly with the relative vapor pressure p/po of water in the ambient atmosphere. If q is the porosity, or the fractional volume of the pores in the film, and ns and np are the mean indices of the skeleton of the film and the pore space respectively, it could be assumed that nf = ns(1 — q) + npq. If the further assumptions are admitted that (1) all the pores are accessible to vapor molecules, and (2) np = 1.000, if p = 0, and np = 1.333 (the pore space is filled with water) if p = po, then ns and q can be evaluated from measurements of nf at p = 0 and p = po. The values of q obtained in this way for a number of MgF2 films range from 0.02 to 0.06. Evaluations of q on the same principle have also been made from measurements of nf in a vacuum and in saturated vapor of ethyl alcohol.