Linear and nonlinear total-yield photoemission observed in the subpicosecond regime in Mo

The total charge emitted from a polycrystalline Mo sample by 500 fs laser p ulses at normal incidence is measured as a function of the laser peak inten sity. Total yield data are taken at wavelengths of 527 and 264 nm. In both cases, a nonlinearity higher than expected is measured. A thermally enhance d regime is clearly observed when using 264 nm pulses for laser peak intens ity larger than 0.1-0.2 GW/cm(2). This effect is interpreted on the basis o f the nonequilibrium heating of the conduction electrons. Pump and probe ph otoemission data at 527 nm show a significant enhancement of the photoelect ric sensitivity when the probe pulse is delayed by 1 ps from the pump. This enhancement is related to the growth of the available electron density ind uced by the nonequilibrium heating. Single pulse photoemission at this wave length is not properly explained by a thermally assisted photoemission regi me. This may indicate that other processes have a role in determining the p hotoemission yield. [S0163-1829(99)10527-7].