【正文】 ?2, interesting phenomena appear both for basic investigations and technological applications. Ina very small volume, calculated as the laser spot size times skin depth, theelectrons can reach temperatures of some tens of thousands Kelvin during thelaser pulse while the lattice basically remains thermally undisturbed. Photoemission of electrons starts if the electron kinetic energy bees larger thanthe work function. After the termination of the laser pulse the phonon temperature can rise rapidly close to or even above the critical temperature caused bythe phonon emission during the energy relaxation of the electrons. The metalis then in an unstable nonequilibrium state leading to ejection of materialdriven by the high critical pressure followed by a rapid quenching. Therefore,a great part of the absorbed laser energy can be removed from the bulk duringthe ablation process by the ejected material. This is the origin of one of thegreat advantages of femtosecond laser applications pared to nanosecondlasers, since the dissipation of energy due to thermal conduction plays a minorrole. The remaining heat a?ected zone therefore bees quite small and onlyvery little collateral damage may appear. Depending on the process conditions,this can be used for the processing of smooth and sharp structures in practically any material. Moreover, the optimum ablation e?ciency, de?ned as theratio of the volume of ablated matter to the laser pulse energy, was found forfemtosecond lasers [1]. What is Di?erent Compared to Longer Pulses? The ElectronElectron Scattering TimeThe phenomena of femtosecond laser interaction with metals are drasticallydi?erent from those related to laser pulses longer, say, than 100 ps. There is,however, in the following no sharp boundary between the two pulse rangesbecause it depends on the material and the property under investigation.Nevertheless, it will be assumed that long pulse behaviour is established if alocal thermal equilibrium exists between the electron and phonon subsystemsduring the laser pulse, equivalently, both systems have the same local temperature. This implies that for ultrashort laser pulses the electron temperaturesare higher than the phonon temperatures because the electromagnetic laser?elds couple only with the electrons. Indeed, the electron temperature canincrease to values far above the critical temperature of metals. In this way,the electronelectron scattering, usually of less importance than the electronphonon scattering time as a result of Pauli’s exclusion principle [2], beessigni?cant or even dominant. In the frame of the Fermi liquid theory, the318 Bernd H168。uttnerFig. . Electronelectron scattering time of gold for three laser energies as afunction of electron temperature.electronelectron scattering time is de?ned bywhere the constants a and b are independent of the electron energy, E, andthe electron temperature, Te. For normal transport, remembering T = 300 Kcorresponds to eV, the electron energy is very close to the Fermi energy,EF, leading to a rather large value for the electronelectron scattering time.In the case of laser excitation, however, the electron energy is approximatelyFermi energy plus photon energy. Consequently, the denominator may beelarge and, therefore,τe?e small. Furthermore, since electron temperatures ofsome thousand degrees Kelvin are quite normal for femtosecond laser metalinteraction, the second temperature dependent part also contributes remarkably to the reduction of the scattering time. Physically speaking, far abovethe Fermi energy the phase space for scattering is huge and is essentially notrestricted by Pauli’s principle since there are a lot of empty places availablefor scattering events. Figure shows, as an example, the electronelectronscattering time of gold for three energies above the Fermi energy as a functionof the electron temperature。 the value for the constant β has been taken fromParkins et al. [3].10 Femtosecond Laser Pulse Interactions with Metals 319From Fig. it can be concluded that the electronelectron scatteringtime for high temperatures bees quite short and has, therefore, been takeninto account explicitly in the consideration of the interaction of femtosecondlaser pulses with metals. In the following, many examples will be found ofchanges in the behaviour of properties that can be traced back to the enhancede?ect of the electronelectron scattering time.Although laser energy is absorbed by the electrons in a thin layer for bothlong and ultrashort pulses with an optical penetration depth that is typicallyof the order of 10–20 nm, there is a large di?erence in the energy densitynear the surface. There are two quite di?erent causes. First, it is wellknownthat absorption increases slightly with temperature in metals for frequencies far away from interband transitions. For aluminium at wavelengths ofabout 800 nm, however, the greatest part of the absorption is governed by theinterband transition, which is decreasing with the phonon temperature dueto band broadening [5]. This e?ect, however, has to be taken into account forlong pulses only because, on the femtosecond scale, the phonon temperatureis changed little or not at all. On the other hand, the increase of the electrical resistivity caused by stronger electronelectron scattering for femtosecondpulses can even induce an increase in absorption [6, 7]. Experimentally, suchchanges can be seen in transient thermal re?ectivity pump and probe measurements in which the metal is heated with a strong pump laser and the changeof re?ectivity is detected。 in most cases this is done at another wavelength, bya weak probe laser at the surface. For very thin ?lms, one can also determinethe transient thermal transmiss