Theory of electromagnetic generation of acoustic waves in metals

The theory of the anomalous skin effect in metals is used in order to study the direct generation of ultrasound by electromagnetic radiation incident on a metal surface, in the absence and in the presence of a magnetic field. The non-monotonic behaviour of the acoustic flux observed experimentally, as a function of the magnetic field can be explained within the framework of the free-electron model assuming that the electrons are scattered diffusely from the metal surface. The reason for this behaviour is traced to the variation of the relative phases of the collision drag force on the bulk ions and that on the surface with increasing magnetic field. The relations between existing theories of direct generation of ultrasound have been investigated within both the free-electron approximation and the effective-mass theory. We conclude that, besides the collision drag force, the Lorentz force and the Bragg reaction force, an additional ‘deformation’ force acts upon the positive ions. We find that the assumption of electron diffuse surface scattering is the only mechanism that can account for the observations on the variety of materials investigated. We also analyse, within a simple effective-mass approximation, possible band structure effects on acoustic wave propagation in metals and on the helicon-phonon interaction.