Theory of transient reflecting grating in fluid/metallic thin film/substrate systems for thin film characterization and electrochemical investigation

A general theory was developed to calculate the diffraction signals from a transient reflecting grating (TRG) at metallic thin film/substrate interfaces and liquid/solid interfaces. The TRG signals for an Au film/soda lime glass system were experimentally measured and theoretically calculated to examine their agreement. It was found theoretically and experimentally that longitudinal acoustic pulses and echoes with ultrahigh-frequency components (> 10 GHz) propagating normal to the sample surfaces are generated and can be detected at the surfaces of thin films, using the TRG technique, along with surface acoustic waves. By using the time intervals of two neighboring longitudinal acoustic echoes, thicknesses or longitudinal acoustic wave velocities for opaque thin films were determinable. Furthermore, it was shown that this method is applicable to the analysis of electrochemical interfaces in situ, through changes in the thermal and longitudinal acoustic properties of electric double layers with electrochemical potentials.