Evidence is presented that an electric double layer is observable with hypersonic waves. Potential-dependent behavior of hypersonic waves was investigated with the transient reflecting grating (TRG) method for a gold film (1 μ) electrode supported on a substrate in acidic solutions. Shapes of initial parts of the TRG-responses were dependent on the potential and composition of the solution. Theoretical calculation showed that longitudinal hypersonic echoes and heat diffusion dominate the initial part for the gold film. The acoustic wavelength of 22 nm is comparable to the thickness of the electric double layer whose structure is potential-dependent. Reflectivity of the hypersonic echoes depends on the potential.
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