  We report on the effect of intermolecular forces on the fluctuations of
supported liquid films. Using an optically-induced thermal gradient, we form
nanometer-thin films of wetting liquids on glass substrates, where van der
Waals forces are balanced by thermocapillary forces. We show that the
fluctuation dynamics of the film interface is strongly modified by
intermolecular forces at lower frequencies. Data spanning three frequency
decades are in excellent agreement with theoretical predictions accounting for
van der Waals forces. Our results emphasize the relevance of intermolecular
forces on thermal fluctuations when fluids are confined at the nanoscale.
