Capillary waves on an asymmetric liquid film of pentane on water

Surface light-scattering measurements have been carried out on thin asymmet ric films of pentane on water. We vary the film thickness l over a wide ran ge (10(-9) m < l < 10(-5) m). Compared with the ripplons wavelength 1/q, th ick films of pentane (l much greater than 1/q) display the same power spect rum as a pure pentane-vapor interface: a single peak. When thinning the fil m down to l approximate to 1/q, hydrodynamic coupling between the layers of the interface is revealed by the appearance of a second peak beside the fi rst one. We describe the thickness dependence of the two coupled modes thro ugh variations of the positions, the widths, and the amplitudes of both pea ks. (C) 2001 Optical Society of America.