The spectrally resolved low-level absorption of thin films and of solid and liquid surfaces was measured by cavity-enhanced photothermal spectroscopy. The technique is ultrasensitive and can determine surfacespecific absorbances alpha(omega) ~ 10(-6) at a power density of 10(4)W/cm(2). Both cases of continuous wave and modulated laser light absorption were studied experimentally and are interpreted theoretically. It is shown that it is possible to achieve a spatial resolution of absorbance variations in the few-micrometer range. The thermal diffusivity can also be simultaneously measured by observing the time evolution of the surface temperature during laser irradiation.
