Both electron thermal conductivity and thermal exchange with the lattice can cool an electron distribution initially heated on a metallic surface with an ultrashort laser pulse. The interplay between the two processes allows the electron-lattice coupling parameter to be determined. We report measurements of optical damage to molybdenum and copper. Damage caused by pulses have a duration TL ^ 1 nsec can be understood only with a two-temperature model of metals.PACS numbers: 72.15.Eb, 78.47.+p A reliable value of the electron-phonon coupling constant g in metals ^ has wide implications, perhaps even in the field of superconductivity.^ Recent investigations^'^ have deduced or inferred experimental values of g. However, a large uncertainty remains because of the difficulty in firmly relating observations to theory. The experimental approach has been to diagnose the electron temperature either by photon-assisted electron emission^ or by the change in reflection^'^ (or transmission^) of visible light near the rf-state resonance. These processes are all limited to relatively weak excitation, the former because of space-charge effects and the latter because of nonlinear saturation processes. ^ Moreover, in the case of thick samples, the initial reduction of the electron temperature that is measured at the surface is mainly due to the fast electron diff*usion process,^ and unless correctly treated leads to an incorrect value of g. Thin samples, on the other hand, can have anomalously fast electron cooling due to electron-impurity and electron-surface scattering.This paper reports a new method of determining g. It relies on competition between two fast processes: electron-lattice energy exchange and electron thermal conduction. The former cools electrons heated with an ultrashort 10-/im pulse (penetration depth -200 A) by transferring their heat to the lattice. The latter removes energy from the surface.The paper has a second purpose. A theory is developed to describe the pulse-duration dependence of optical damage to metals, and experimental results are presented in support of the theory.The heat transport inside the metal can be described with the following one-dimensional, two-temperature model ^:Ce-l-Te-^^K^Te-g(Te-Ti) +A(x,t) (l) at ox ox andQ^Ti=g(Te-Ti),(2) ot where x is the direction perpendicular to the surface.The electron heat capacity Ce is given by Ce =CeTe, Ce being a constant,^ and K is the heat conductivity. According to Sommerfeld's model,^ KozTe/v, where v = Vee + Vei and Vee and Vei are the electron-electron and electron-phonon collision frequencies, respectively. Te and Ti are, respectively, the electron and lattice temperatures and d is the lattice heat capacity. A{xj) is a source term and g is the electron-phonon coupling constant.As a guide, we will derive some approximate scaling laws for the response of a metal to an ultrashort pulse. These scaling laws have two essential roles. First, they allow quantities that are necessary to understand the following measurements to be introduced. Second, they...

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Photon absorption by Al, Ta, and Bi between 3 and 30 MeV was measured using as a photon spectrometer a photoneutron time-of-flight detector and a liquid deuterium target, The atomic cross sections of Ta and Bi at the lowest energies (and of Al at higher energies) agree with calculated values appearing in published tabulations but exceed them at 25 MeV by about 2% in Ta and 3% in Bi. Calculations by others using empirical Coulomb corrections and improved screening corrections to the cross section for pair production by the nucleus agree with experiment to within (0.5+0.4)%, Best experimental values of the combined correction for Bi are given. NUCLEAR REACTIONSAl,

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