Journal of Russian Laser Research, Volume 21, Number 6, 2000 GENERATION OF NONEQUILIBRIUM PHONONS, SUPPRESSION OF HEAT TRANSPORT, AND RAPID DISINTEGRATION OF A METAL LATTICE Yu. V. Afanasiev, 1 B. N. Chichkov, 1,2 V. A. Isakov, 1 A. P. Kanavin, 1 and S. A. Uryupin 1 1 P. N. Lebedev Physical Institute, Russian Academy of Sciences, Leninskii Pr. 53, Moscow 117924, Russia e-mail: uryupin@sci.lebedev.ru 2 Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany Abstract A self-similar description of electron and phonon kinetics in metals with a sharp gradient of the electron temperature is developed. A nonequilibrium distribution of the phonon number density is derived, which is peaked near the ion plasma frequency and is extended along the heat-flow axis. It is shown that the Cherenkov generation of nonequilibrium phonons results in suppression of the electron heat flux and rapid disintegration of the metal lattice. 1. Introduction The interaction of high-power ultrashort laser pulses with metals is a subject of great interest due to both the new physics involved and the demonstrated applications for the microstructuring of metal surfaces (see, for example, [1–7]). In this field, investigations of electron heating dynamics and heat transport play an important role. A theoretical basis for these investigations is provided by the two-temperature diffusion model [8, 9]. At moderate laser fluences, heat transport and energy exchange between the electron and lattice subsystems are described within the framework of conventional scattering mechanisms, such as electron– electron and electron–phonon collisions, assuming electron and phonon distribution functions to be close to the equilibrium Fermi and Planck distributions, respectively. As a rule, the assumption on the equilibrium electron distribution is usually satisfied, whereas for phonons the situation is different. There are some experimental results that can be explained by the generation of intense acoustic oscillations in metals irradiated by high-power laser pulses. One of the important reasons for the formation of a stongly nonequilibrium phonon distribution is the large heat flux into the metal target. When the drift velocity of electrons exceeds the phase velocity of acoustic waves, the Cherenkov generation of phonons becomes possible. In this case the phonon distribution can deviate from the Planck distribution. The nonequilibrium phonons, in turn, produce a backward action on the electron heat flux via a growing electron collision frequency. This mechanism is responsible for a modification of the metal-surface cooling rate and, as a result, of the laser ablation process. Another important feature of the nonequilibrium-phonon generation is fast energy transfer to the lattice vibrations, which accelerates disintegration of the metal lattice. The goal of the present paper is to study these phenomena. In Sec. 2, general equations for the electron and phonon distribution functions that are valid in the case of a strong electron heat flux into the target material are presented. It is assumed that, due to absorption of a short-pulse laser radiation in the skin-layer, a sharp gradient of the electron temperature is formed. This temperature gradient generates a heat flux propagating with a velocity higher than the phonon velocity. Under this condition, the Cherenkov generation mechanism produces strongly nonequilibrium phonon distribution. The growing phonon number density increases the electron collision frequency, which, in turn, produces Translated from a manuscript submitted May 3, 2000. 1071-2836/00/2101-0505$25.00 c  2000 Kluwer Academic/Plenum Publishers 505