Thermal properties ofHe surfaces and interfaces

A first-principle quantum statistical mechanical theory is used to study the properties of thick liquid He films absorbed to the weakly binding substrates: Li, Na, and Cs. Values for the liquid-gas and solid-liquid surface energies are determined. By fitting, at long wavelengths, the film's lowest energy mode with the standard, expression for the ripplon energy, which depends on the liquid-gas surface energy, we obtain excellent agreement with the liquid-vacuum surface energy from recent experiments and also the one previously extracted from quantum liquid droplet calculations. The full spectrum of excitations for wave vectors less than 0.50 Å is calculated using a dynamical correlated basis function theory developed in earlier work, which includes multi-phonon scattering processes. Particle currents and transition densities are used to elucidate the nature of the excitations. At a coverage of 0.40Å, the lowest mode shows no significant substrate dependence, and is recognized as being a ripplon propagating in the liquid film at the liquid-gas surface. A new effect is observed for the Cs substrate; the second lowest mode is qualitatively different than found on the other substrates and is identified as interfacial ripplon. In the other substrates the second mode is a volume mode altered somewhat by the high density inner liquid layers. The linewidths of these modes are also calculated. The dynamic excitations provide the input for the thermodynamic theory and the effects on the free energy, heat capacity, and thermal surface broadening of our films are studied as function of the nature of the excitations, the number of modes, and variations in the substrate potentials.