Ellipsometery to Probe prewetting and superfluid transitions of thin liquid helium films on rubidium

Helium films adsorbed on intermediate binding strength substrates, like rubidium, exhibit markedly different behavior than on more common strongly binding substrates, or weask substrates like cesium. Previous experiments on Rb using a quartz crystal microbalance(QCM) have shown that the prewetting and superfluid transitions occur at virtually the same chemical potential. The superfluid transition seems to be coupled to the prewetting transition and hysteretic, which would be an apparent disagreement with Kosterlitz-Thouless paradigm. By using a modulated null Ellipsometer in conjunction with the QCM we are able to simultaneously measure both the total coverage and the normal component of the helium film. This allows us to separate the prewetting and superfluid transitions and document how the binding potential of the substrate may lead to deviations from a standard Kosterlitz-Thouless transition.