Verification of the Lifshitz Theory of the van der Waals Potential Using Liquid-Helium Films

Accurate measurements are presented of the thickness of helium films on cleaved surfaces of alkaline-earth fluoride crystals at 1.38 ${}_{}{}^{\mathrm{o}}{}_{}{}^{}\mathrm{K}$. The films were measured between 10 and 250 Å using an acoustic interferometry technique. The data are in excellent agreement with calculations based on the Lifshitz theory of van der Waals forces. Calculations of the film thickness on a variety of other substrates are also given.