Non-linearity observed in the direct sub-ps photoemission regime in Mo

The total charge emitted from a polycrystalline Mo surface by 500 fs-264 nm laser pulses has been measured. Though a one-photon photoelectric effect i s expected, a non-linear increase of the photoelectric yield was observed a s a function of laser peak intensity, confirming earlier observations on Au , W and Zr. The threshold intensity for this non-linearity is between 0.1 a nd 0.2 GW/cm(2). The linear and non-linear regimes were clearly discerned i n the experimental data. The non-equilibrium heating of the conduction elec trons is considered as the cause of the observed non-linear behaviour. (C) 1999 Elsevier Science B.V. All rights reserved.