TRANSIENT GRATING EXCITATION OF INTERFACIAL ACOUSTICS - TREATMENT OF MULTILAYER STRUCTURES

Observation of acoustics generated by a surface restricted transient o ptical grating can be used as a sensitive probe of interface structure that can influence near-surface chemical interactions. In this paper, a theoretical analysis is presented detailing the interface acoustic waves generated by this optical grating technique for a structure cons isting of a liquid/solid interface with an interposed thin solid layer . Tn the absence of the layer, the predicted response consists of cont ributions from both the Stonely and the Leaky-Rayleigh wave interface modes, consistent with experimental observations. Thin intermediate la yers, those with thickness much smaller than the grating period, were found to have little influence on the acoustic response indicating tha t the spatial sensitivity of this technique is not sufficient for prob ing the short range order in liquids in contact with surfaces. However , the addition of a layer with thickness on the order of the grating s pacing affects the dominate period of the response as expected from th e dispersion relation for the liquid/solid/solid structure. For these thick layers, bulk waves propagating across the layer thickness also i nitially contribute to the response. (C) 1995 American Institute of Ph ysics.