  Our understanding of both structure and dynamics of adsorbed liquids heavily
relies on the capillary wave Hamiltonian, but a thorough test of this model is
still lacking. Here we study the capillary wave fluctuations of a liquid film
with short range forces adsorbed on a solid exhibiting van der Waals
interactions. We show for the first time that the measured capillary wave
spectrum right above the first order wetting transition provides an interface
potential consistent with independent calculations from thermodynamic
integration. However, the surface tension exhibits an oscillatory film thick
dependence which reveals a hitherto unnoticed capillary wave broadening
mechanism beyond mere interfacial displacements.
