Fourier transform heterodyne spectroscopy of liquid interfaces


This thesis describes the application of a novel Fourier transform heterodyne spectroscopy technique with an ultrahigh resolution of 200 mHz to the study of capillary waves at liquid-vapor interfaces. The apparatus uses a frequency-shifted local oscillator to separate signals from counter- propagating capillary waves of identical frequency. The main beam and local oscillator are aligned in such a way as to select capillary waves of a given, continuously adjustable frequency. This capability to separate counter-propagating waves was used to study the spectral asymmetry of light scattered from capillary waves at a nonequilibrium water surface in the presence of a temperature gradient. The observed asymmetries agree, in sign and order of magnitude, with the one predicted by linearized fluctuating hydrodynamics. This apparatus was also used to measure the spatial damping coefficients of capillary waves at a clean water surface and a water surface covered with a monolayer of pentadecanoic acid. For these measurements a double-beam heterodyne technique, which requires no calibration or deconvolution of instrumental functions, was used. The spatial damping coefficient of a clean water is in good agreement with the hydrodynamics theory. A sharp maximum in the spatial damping was observed at the end of the coexistence region of two phases of the pentadecanoic acid monolayer.