THERMAL-PROPERTIES OF HE-4 SURFACES AND INTERFACES

A first-principle quantum statistical mechanical theory is used to stu dy the properties of thick liquid He-4 films absorbed to the weakly bi nding substr ates: Li, Na, and Cs. Values for the liquid-gas and solid -liquid surface energies are determined By fitting, at long wavelength s, the film's lowest energy mode with the standard expression for the ripplon energy, which depends on the liquid-gas surface energy, we obt ain excellent agreement with rite liquid-vacuum surface energy from re cent experiments and also the one previously extracted from quantum li quid droplet calculations. The full spectrum of excitations for wave v ectors less than 0.50 Angstrom(-1) is calculated using a dynamical cor related basis function theory developed in earlier work, which include s multi-phonon scattering processes. Particle currents and transition densities are used to elucidate the nature of the excitations. At a co verage of 0.40 Angstrom(-2). the lowest mode shows no significant subs trate dependence, and is recognized as being a ripplon propagating in the liquid film at the liquid-gas surface. A new effect is observed fo r the Cs substrate; rite second lowest mode is qualitatively different than found on the other substrates and is identified as interfacial r ipplon. In the other substrates the second mode is a volume mode alter ed 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 studie d as function of the nature of the excitations, the number of modes, a nd variations in the substrate potentials.