In experiments where intense radiation penetrates into the bulk of a solid and causes ultrafast (femtosecond) heating, the superheated crystalline solid melts from within at a temperature above the equilibrium melting temperature. But what happens on the atomic scale as a solid loses crystalline order remains an open question. Molecular dynamics modelling allows the position of every atom to be traced at each instant, as a crystal transforms from solid to liquid. Here we use such detailed atomistic simulations, relevant for aluminium, to show that the thermal fluctuation initiating melting is an aggregate typically with 6-7 interstitials and 3-4 vacancies. This mechanism differs from those that have traditionally been proposed, which generally involve many more atoms at the initial melting stage.
The electronic vibrational damping rates of the CN and CO internal stretch modes on the (111) surfaces of Ag, Cu, Au, and Pt were calculated using density functional theory calculations. Our calculated damping rates are in excellent agreement with experimental data obtained from pump-probe laser spectroscopy. The striking difference in trends and magnitudes between the internal stretch modes of CN and CO is in part rationalized in terms of the adsorbate-induced electronic structure within the framework of a simple Newns-Anderson model.
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