Two-temperature thermal conductivity coefficient κ, and electron-ion coupling parameter o: are obtained using
Boltzmann kinetic equation in the relaxation time approximation. These coefficients are necessary for the quantitative description of the two-temperature state with hot electrons Te much greater than Ti created as result of absorption
of femtosecond laser pulse. Simple, noble, and transition metals are considered. An influence of d- band electrons, which play a significant role, has been evaluated for two latter groups of metals.
Interaction of ultrashort laser pulse with metals is considered. Ultrafast heating in our range of absorbed fluences Fabs > 10 mJjcm2 transfers matter into two-temperature (2T) state and induces expressed thermomechani cal response. To analyze our case, where 2T, thermomechanical, and multidimensional (formation of surface
structures) effects are significant, we use density functional theory (DFT), solutions of kinetic equations in τ- approximation, 2T-hydrodynamics, and molecular dynamics simulations. We have studied transition from light absorption in a skin layer to 2T state, and from 2T stage to hydrodynamical motions. We describe (i) formation of very peculiar (superelasticity) acoustic wave irradiated from the laser heated surface layer and (ii) rich com plex of surface phenomena including fast melting, nucleation of seed bubbles in hydrodynamically stretched fluid, evolution of vapor-liquid mixture into very spatially extended foam, mechanical breaking of liquid membranes in foam (foam disintegration), strong surface tension oscillations driven by breaking of membranes, non-equilibrium freezing of overcooled molten metals, transition to nano-domain solid, and formation of surface nanostructures.
The paper is devoted to experimental and theoretical studies of ablation of condensed matter by optical (OL),
extreme ultraviolet (EUV) and X-ray lasers (XRL). Results obtained at two different XRL are compared. The
first XRL is collision Ag-plasma laser with pulse duration τL = 7 ps and energy of quanta hv=89.3 eV, while
the second one is EUV free electron laser (EUV-FEL) and has parameters τL = 0.3 ps and energy of quanta 20.2
eV. It is shown that ablation thresholds for these XRL at LiF dielectric are approximately the same. A theory is
presented which explains slow growth of ablated mass with fluence in case of XRL as a result of transition from
spallative ablation near threshold to evaporative ablation at high fluencies. It is found that the metal irradiated
by short pulse of OL remains in elastic state even in high shear stresses. Material strength of aluminum at very
high deformation rates V/V ~ 109 s-1 is defined.
The motion of both Lennard-Jones solids and metals induced by ultrashort laser irradiation near the ablation threshold is
investigated by molecular dynamics simulation. The universality of the ablation threshold fluence with respect to the
cohesion energy of solids irradiated by femtosecond laser pulses is demonstrated for Lennard-Jones solid and metals
simulated by many-body EAM potentials.
Theoretical consideration of the ablation of laser heated metal target based on two-temperature hydrodynamic calculation
is performed for aluminum and gold targets. Concurrent with the hydrodynamic calculation the molecular dynamics
simulation of the ablation was carried out in the case of aluminum. The initial state of matter for the molecular dynamics
is taken as a final state of hydrodynamic calculation. Molecular dynamics simulation is extended to cover late stages of
the evolution of two-phase foam placed between the crater and spalled cupola. Theoretical results are in a good
agreement with the experimental data obtained by the microinterferometer diagnostics of the femtosecond laser ablation
both for aluminum and gold.
In the present work phenomena are considered related to the interaction of ultra-short laser pulses, τL~0.1 ps, with metallic targets. The absorption of laser pulse results in formation of thin layer of hot electrons strongly superheated (Te>>Ti) relative to the ion temperature, Ti. Initial thickness of the layer dheat is small, dheat~δ, where δ~10 nm is the skin layer thickness. Subsequent developments include the following stages: (1) Propagation of electron thermal wave which expands the hot layer dheat; (2) Cooling of electrons due to energy transfer to cold ions; (3) Onset of hydrodynamic motion that constitutes the rarefaction wave with positive pressure; (4) Further expansion of target material leading to the appearance of negative pressure; and (5) Long separation process which begins with nucleation of voids and goes on to the total separation of spallation plate. The thickness of the plate is ~10 nm (we call it nanospallation). Theoretical model involves two-temperature hydrodynamic equations with semiempirical EOS for a metal, electron heat conduction and electron-ion energy exchange. The decay of metastable strongly stretched matter is described by molecular dynamics (MD) simulation with extremely large number of atoms. The experimental setup includes femtosecond chromium-forsterite laser operating in the pump-probe regime. The experiments are performed with gold target. Measured ablation threshold for gold is 1.35 J/cm2 of incident pump light at inclination 45°, p-polarization. Calorimeter measurements give for the absorbed fluence Fabs=0.3Finc, therefore the threshold value of Fabs is 0.4 J/cm2.
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