Thermally coupled interfacial acoustic waves observed in transient surface-grating measurements

We report the observation of interfacial acoustic waves in the gas above opaque condensed media excited by pulsed laser radiation in transient surface-grating experiments. They differ from previously described interfacial waves in the coupling between the media: Stoneley, Scholte and others described elastically coupled interfacial waves, whereas our analysis, based on approximate solutions of the Navier-Stokes equations, shows that thermal coupling is dominant in this case. Our model relates the amplitude and phase velocity of these interfacial waves to the thermal, elastic and optical properties of the adjoining media. The new thermo-acoustic effect may be of use in the study of thermal and elastic properties of gases at frequencies above 1 MHz, where conventional ultrasonic techniques fail.