Interband electronic transitions and scattering of excess free carriers from a surface in nonlinear spectroscopy of ultrathin metal films

Based on degenerate four-photon spectroscopy of ultrathin (10 – 20-nm thick) Ni, Au, and Pt films, a conclusion is made about a dominating role of interband electronic transitions in the formation of a nonlinear response of the metals in the visible spectral range. It is shown that an excited electron subsystem in films of thickness less than the mean free path is thermalized and cooled due to rapid inelastic scattering of excess free carriers on the film surface. The estimates performed for the Ni film showed that up to 10% of 20-ps pump-pulse energy could be scattered in this way producing the surface deformation. Upon spatially nonuniform excitation, the efficient 'direct' generation of surface acoustic waves is possible.