  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 $\tau_L$ 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 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.
