FEMTOSECOND THERMIONIC EMISSION FROM METALS IN THE SPACE-CHARGE-LIMITED REGIME

We study femtosecond-laser-pulse-induced electron emission from W(100) , Al(110), and Ag(111) in the sub-damage regime (1-44 mJ/cm2 fluence) by simultaneously measuring the incident-light reflectivity, total ele ctron yield, and electron-energy distribution curves of the emitted el ectrons. The total-yield results are compared with a space-charge-limi ted extension of the Richardson-Dushman equation for short-time-scale thermionic emission and with particle-in-a-cell computer simulations o f femtosecond-pulsed-induced thermionic emission. Quantitative agreeme nt between the experimental results and two calculated temperature-dep endent yields is obtained and shows that the yield varies linearly wit h temperature beginning at a threshold electron temperature of approxi mately 0.25 eV. The particle-in-a-cell simulations also reproduce the experimental electron-energy distribution curves. Taken together, the experimental results, the theoretical calculations, and the results of the simulations indicate that thermionic emission from nonequilibrium electron heating provides the dominant source of the emitted electron s. Furthermore, the results demonstrate that a quantitative theory of space-charge-limited femtosecond-pulse-induced electron emission is po ssible.