Measurements and Calculations of the Helium Film Thickness on Alkaline Earth Fluoride Crystals
Accurate measurements have been made of the thickness of helium films adsorbed on alkaline earth fluoride crystals as a function of the van der Waals potential. The helium film thickness, as we have shown,1 is measured through the detection of simple standing wave patterns of a given frequency set up between the substrate which supports the film and the liquid-gas interface, the film thickness being determined by counting the number of wavelengths across the film and calculating the magnitude of the wavelength from the existing frequency. The van der Waals potential which is solely responsible for the adsorbed film is obtained by measuring the height of the crystal above the free liquid surface in the case of saturated films and by measuring the helium gas pressure at the position of the resonant peaks for unsaturated films. The substrates are cleaved single crystals of CaF2, SrF2, and BaF2 doped with 0.02 mole % of the paramagnetic ion divalent thulium.
KeywordsDielectric Function Helium Atom Resonant Peak Free Liquid Surface Helium Film
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