A laser optoacoustic method of inducing high-energy states and the investigation of phase transitions in metals at high pressures

Results are given of investigations of high-energy states and phase transitions, using lead as an example, under the effect of nanosecond laser pulse in a wide range of intensities. The metal surface being irradiated is mechanically confined by a transparent dielectric plate in order to realize conditions of highly effective generation of pressure while maintaining local thermodynamic equilibrium. The dynamics of thermodynamic state of metal are analyzed by the shape and amplitude of pressure pulse propagating from the surface being heated and by the variation of reflectivity of the metal surface being irradiated. The measurements of pressure pulse make possible the recording of phase transitions in lead: melting on the sample surfaces being irradiated at a pressure up to $P_{\text{max}} \approx 0.1$ GPa. It is demonstrated that the pressure amplitude prior to melting is proportional to the intensity of laser radiation, and that upon passing the melting threshold – to the absorbed energy of laser pulse. The instant of time, at which the “deformation” of the leading edge of pressure pulse begins, corresponds to the beginning of melting. When high-energy states are realized in the thin surface layer of metal, the density decreases significantly; this results in a significant decrease in both reflectivity and electrical conductivity.