Impulsive Stimulated Thermal Scattering (ISTS) is a noninvasive optical technique that can be used as a powerful tool for high frequency acoustic measurements at solid surfaces. In surface ISTS, two interfering picosecond laser pulses absorbed at the surface launch counterpropagating surface acoustic waves (SAWs) detected via diffraction of a probe laser beam. In this paper, we introduce surface ISTS measurements in the frequency domain using a Fabry-Perot interferometer to analyze the spectrum of the diffracted light. This spectrally separates the ISTS signal from light scattered by surface imperfections and enables us to extend the acoustic frequency range beyond 1 GHz. We demonstrate thickness and density determination for a gold film on silica from measured SAW velocity dispersion. The results obtained show the potential of the technique for mechanical characterization of him as thin as 0.1 mu m or less.