Structure and excitations of liquid-helium films

We study the structure and the spectrum of elementary excitations of films of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atoms adsorbed to plane substrates of varying strengths. An optimized variational description of the ground-state wave function provides the short- and long-ranged structure, the distribution functions, the dispersion relations, and the spatial shape of the collective modes.