Ellipsometry of Liquid Helium Films on Gold, Cesium, and Graphite

We have developed a modulated null ellipsometer with sub-monolayer resolution to measure adsorbed liquid helium thin films at temperatures below 4 K. Adsorption isotherms of ⁴He on gold, cesium, and graphite are presented. For Au and Cs substrates, the reflecting surface for our ellipsometric measurements is the metallic electrode of a quartz crystal microbalance(QCM). Performing both types of measurements simultaneously on the same substrate provides a direct method of converting the ellipsometric signal into an absolute film thickness without constructing a detailed model of the refractive index of the substrate. Isotherms on gold above and below T¸ show that the ellipsometric signal is unaffected by the superfluid transition; the ellipsometer measures the total film thickness independent of the superfluid fraction. Isotherms on cesium above the wetting temperature show a prewetting step. Isotherms on clean graphite show steps due to layering.