Gallium transformation under femtosecond laser excitation: Phase coexistence and incomplete melting

The reversible phase transition induced by femtosecond laser excitation of Gallium has been studied by measuring the dielectric function at 775 nm with ~ 200 fs temporal resolution. The real and imaginary parts of the transient dielectric function were calculated from absolute reflectivity of Gallium layer measured at two different angles of incidence, using Fresnel formulas. The time-dependent electron-phonon effective collision frequency, the heat conduction coefficient and the volume fraction of a new phase were restored directly from the experimental data, and the time and space dependent electron and lattice temperatures in the layer undergoing phase transition were reconstructed without ad hoc assumptions. We converted the temporal dependence of the electron-phonon collision rate into the temperature dependence, and demonstrated, for the first time, that the electron-phonon collision rate has a non-linear character. This temperature dependence converges into the known equilibrium function during the cooling stage. The maximum fraction of a new phase in the laser-excited Gallium layer reached only 60% even when the deposited energy was two times the equilibrium enthalpy of melting. We have also demonstrated that the phase transition pace and a fraction of the transformed material depended strongly on the thickness of the laser-excited Gallium layer, which was of the order of several tens of nanometers for the whole range of the pump laser fluencies up to the damage threshold. The kinetics of the phase transformation after the laser excitation can be understood on the basis of the classical theory of the first-order phase transition while the duration of non-thermal stage appears to be comparable to the sub-picosecond pulse length.Comment: 28 pages, including 9 figs. Submitted to Phys. Rev. B 14 March 200

Структурні особливості вакуумних конденсатів заліза, легованих вольфрамом

The initial structure and the composition of Fe–W films which were obtained by vacuum deposition were studied by the methods of transmission electron microscopy, x-ray diffraction, energy-dispersive x-ray spectroscopy and x-ray fluorescence analysis. It is shown that microalloying of iron with tungsten results in dispersion iron matrix grain structure. The inhomogeneous distribution of tungsten within each grain was discovered. The observed regularities are explained by the formation of grain boundary segregation of alloying component during binary vapors condensation.Методами просвічуючої електронної мікроскопії, рентгенівської дифрактометрії, енергодисперсійної рентгенівської спектроскопії та спектрального рентгенівського флуоресцентного аналізу вивчена вихідна структура і склад плівок Fe-W, отриманих вакуумним осадженням. Показано, що мікролегування заліза вольфрамом призводить до диспергування зеренної структури залізної матриці. Виявлено неоднорідний розподіл вольфраму в межах кожного зерна. Спостережувані закономірності пояснюються формуванням зерногранічних сегрегацій легуючого компонента при конденсації двокомпонентного пара