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Short laser pulse in wide range of wavelengths, from infrared to X-ray, disturbs electron-ion equilibrium and rises pressure in a heated layer. The case where pulse duration τ L is shorter than acoustic relaxation time t s is considered in the paper. It is shown that this short pulse may cause thermomechanical phenomena such as spallative ablation regardless to wavelength. While the physics of electron-ion relaxation strongly depends on wavelength and various electron spectra of substances: there are spectra with an energy gap in semiconductors and dielectrics opposed to gapless continuous spectra in metals. The paper describes entire sequence of thermomechanical processes from expansion, nucleation, foaming, and nanostructuring to spallation with particular attention to spallation by X-ray pulse.

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Key words Two-temperature warm dense matter, action of ultrashort laser pulse, pump-probe technique.We combine theoretical and experimental methods to study the processes induced by fast laser heating of metal foils. These processes reveal themselves through motion of frontal (irradiated) and rear-side foil boundaries. The irradiated targets are 0.3-2 micron thick aluminum foils deposited on much thicker (150 microns) glass plate. The instant boundary positions is measured by pump-probe technique having ∼ 40 − 150 fs time and ∼ 1 nm spatial resolutions. Ultrashort laser pulse transforms a frontal surface layer with thickness dT into two-temperature (Te Ti) warm dense matter state. Its quantitative characteristics including its thickness are defined by poorly known coefficients of electron-ion energy exchange α and electron heat conductivity κ. Fast laser heating rises pressure in the dT -layer and therefore produce acoustic waves. Propagation and reflection from the frontal and rear boundaries of these waves causes the displacement Δx(t) of boundary positions. Pressure wave profiles, and hence functions Δx(t), depend on thickness dT . This is why the experimental detection of Δx(t) opens a way to accurate evaluation of the coefficients α and κ.