An impulsive stimulated scattering study of surface acoustic waves under high pressure

Impulsive stimulated scattering (ISS) offers an accurate spectroscopic method of obtaining surface acoustic velocities of materials. High-pressure ISS results are obtained for polycrystalline aluminum in a diamond anvil cell (with fluid nitrogen as the pressure-transmitting medium) and these results are compared with analytical solutions for an isotropic (polycrystalline) solid in contact with an isotropic fluid. The choice of an isotropic solid–isotropic fluid interface permits a straightforward analysis of the effect of pressure on both the Scholte and generalized Rayleigh (GR) surface acoustic modes intrinsic to such a system. The present study is the first to measure and analyze both the Scholte and GR modes under high pressure. Pressure dependent surface acoustic wave (SAW) velocities are demonstrated as a spectroscopic probe of the equation of state of materials under extreme conditions. In addition, directionally resolved ISS measurements on single crystal surfaces of Ni are compared with numerical calculations to determine the origin of dramatic changes in SAW velocities on crystal surfaces of slightly lower symmetry. The calculations show that the SAW velocity becomes strongly mixed with the bulk transverse acoustic mode for deviations of just a few degrees away from the (100) surface.