Higher-harmonic generation from a metal surface in a powerful laser field

We investigate the generation of higher harmonics from a metal surface in a powerful laser field. The metal is described by the Sommerfeld free-electron model. Solving the Schr\"odinger equation of the problem perturbatively in the Kramers-Henneberger frame, a simple expression for the harmonic production can be derived and evaluated numerically. Good agreement for the calculated harmonic-generation rates for a gold surface is found if compared with recent experimental data of Farkas et al. [Phys. Rev. A 46, R3605 (1992)].