Ellipsometric studies of thin liquid helium films

Ellipsometry is an in-situ technique for thin film measurement that uses the polarization shift of reflected light to monitor film thickness. We have used ellipsometry to measure adsorbed helium films on several substrates with sub-monolayer resolution. By simultaneously measuring the total film thickness with the ellipsometer and the normal helium thickness with a quartz crystal microbalance (QCM) we are able to accurately measure the superfluid fraction. The helium-helium interaction is so weak that at low temperatures the helium readily adsorbs to almost all surfaces, forming initially two dense, solid-like layers and subsequently thick films of liquid. The only exception to this is the Alkali metals, whose weak binding strength does not promote helium adsorption. Rubidium's intermediate binding strength offers a rare chance to test the substrate invariance of the onset superfluid density predicted by Kosterlitz-Thouless theory. Due to the lack of solidified helium first layers this is the only known substrate for which the liquid helium can interact directly with the substrate during the superfluid transition.; Before attempting measurements on evaporated alkali metal films, we successfully measured adsorption isotherms both above and below Tlambda using the ellipsometer to measure the helium film on the gold electrode of the QCM. Cold is a representative strongly binding substrate. These measurements helped calibrate the ellipsometer and demonstrate its invariance to the superfluid fraction. We then tested the limits of the ellipsometer's resolution by measuring layer by layer growth of the first four helium layers on graphite. This was the first optical measurement of this phenomena using liquid helium, whose refractive index is an extremely low 1.028.; We then evaporated films of cesium and rubidium directly onto the QCM and performed isotherms to probe the helium interaction with these weaker substrates. Our evaporation takes place in-situ while the experiment is kept below 8 K to ensure UHV conditions. Cesium and rubidium are highly reactive so ensuring clean, contaminant free films is critical. The ellipsometer was able to resolve a sharp prewetting step on the cesium surface. This data agreed well with our previous results using just the QCM. The rubidium surface, in contrast, produced surprising results.; Previous work on rubidium using only a QCM found a strong coupling between the prewetting and superfluid transitions, and hinted at possible deviations from K-T theory. (Abstract shortened by UMI.)...