| Abstract
| - Transient grating experiments performed with evanescent fields resulting from total internal reflection at aninterface between a polar absorbing solution and an apolar transparent solvent are described. The time evolutionof the diffracted intensity was monitored from picosecond to millisecond time scales. The diffracted signaloriginates essentially from two density phase gratings: one in the absorbing phase induced by thermal expansionand one in the transparent solvent due to electrostriction. A few nanoseconds after excitation, the latter gratingis replaced by a thermal grating due to thermal diffusion from the absorbing phase. The speed of sound andthe acoustic attenuation measured near the interface are found to be essentially the same as in the bulk solutions.However, after addition of a surfactant in the polar phase, the speed of sound near the interface differssubstantially from that in the bulk with the same surfactant concentration. This effect is interpreted in termsof adsorption at the liquid/liquid interface. Other phenomena, which are not observed in bulk experiments,such as acoustic echoes and a fast oscillation of the signal intensity, are also described.
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