Coherent and incoherent multiple-harmonic generation from metal surfaces

A theory is presented for multiple-harmonic generation from metal surfaces with the laser field polarized in the plane of incidence. The theory is applied to the analysis of a recent experiment on a gold surface. It is shown that the relatively slow decrease in the measured efficiency (10(-10)-10(-13)) for the second, third, fourth, and fifth harmonics is related to the stepwise nature of the excitation process. While for the second harmonic the intensities of the coherent and the incoherent component are comparable, for the higher harmonics the incoherent component dominates.