Direct generation of ultrasound by electromagnetic radiation in metals: Effect of surface scattering

The usual theory of direct ultrasonic generation by electromagnetic radiation in metals is based on the assumption that the conduction electrons experience specular scattering at the boundary of the metal. The present study generalizes the theory of Reuter and Sondheimer for the anomalous skin effect in metals to take into account the motion of the lattice. We develop a procedure for the calculation of the conversion efficiency of electromagnetic to acoustic energy when the electrons are scattered diffusely by the surface of the metal. The results show that this efficiency is comparable in the two cases of specular and diffuse scattering. For example, for metallic potassium under the conditions of the experiments by Chimenti et al. (incident radiation of frequency equal to 9 MHz and electron mean free path of the order of 1.4 × ${10}^{-2\mathrm{}}$ cm) we find that the ratio of acoustic to electromagnetic fluxes is 4.5 × ${10}^{-12\mathrm{}}$ for specular scattering and 4.7 × ${10}^{-12\mathrm{}}$ for diffuse scattering.