Femtosecond Laser Interaction with Metallic Tungsten and Nonequilibrium Electron and Lattice Temperatures

Multiphoton photoelectron emission from metals has been the subject of experimental and theoretical investigation for several years [1]. The development of high intensity ultrashort pulse laser sources has made possible the extension of these studies into the picosecond time regime [2,3]. For intense pulses of short enough duration, it has been postulated that a transient nonequilibrium between the electrons and phonons may be generated [4]. This phenomenon has been termed anomalous heating and is predicted when the laser pulse durations are comparable to or shorter than the electron-phonon energy relaxation time. Because of the smaller heat capacity of the electron gas, heating of the electrons to temperatures in excess of the lattice melting temperature would then be possible. Previous experimental investigations of photoelectron emission have been performed with picosecond pulses in attempts to observe anomalous heating [2,3]. To date, none have achieved the temporal resolution necessary for such an observation. However, very recently, indirect eveidence of transient heating has been reported using picosecond reflectivity measurements in copper [5].