The ultrafast laser excitation of matters leads to nonequilibrium states with complex solid-liquid phase-transition dynamics. We used electron diffraction at mega-electron volt energies to visualize the ultrafast melting of gold on the atomic scale length. For energy densities approaching the irreversible melting regime, we first observed heterogeneous melting on time scales of 100 to 1000 picoseconds, transitioning to homogeneous melting that occurs catastrophically within 10 to 20 picoseconds at higher energy densities. We showed evidence for the heterogeneous coexistence of solid and liquid. We determined the ion and electron temperature evolution and found superheated conditions. Our results constrain the electron-ion coupling rate, determine the Debye temperature, and reveal the melting sensitivity to nucleation seeds.
We describe experimental observations and theoretical analysis of the coarsening of distributions of twodimensional nanoclusters, either adatom islands or vacancy pits, on metal surfaces. A detailed analyses is provided for Ag(111) and Ag(100) surfaces, although we also discuss corresponding behavior for Cu(111) and Cu(100) surfaces. The dominant kinetic pathway for coarsening can be either Ostwald ripening (OR), i.e., growth of larger clusters at the expense of smaller ones, or Smoluchowski ripening (SR), i.e., diffusion and coalescence of clusters. First, for pristine additive-free surfaces, we elucidate the factors which control the dominant pathway. OR kinetics generally follows the predictions of mesoscale continuum theories. SR kinetics is controlled by the size-dependence of cluster diffusion. However, this size-dependence, together with that of nanostructure shape relaxation upon coalescence, often deviates from mesoscale predictions as a direct consequence of the nanoscale dimension of the clusters. Second, we describe examples for the above systems where trace amounts of a chemical additive lead to dramatic enhancement of coarsening. We focus on the scenario where "facile reaction" of metal and additive atoms leads to the formation of mobile additivemetal complexes which can efficiently transport metal across the surface, i.e., additive-enhanced OR. A suitable reaction-diffusion equation formulation is developed to describe this behavior. ReceiVed: July 19, 2008; ReVised Manuscript ReceiVed: December 25, 2008 We describe experimental observations and theoretical analysis of the coarsening of distributions of twodimensional nanoclusters, either adatom islands or vacancy pits, on metal surfaces. A detailed analyses is provided for Ag(111) and Ag(100) surfaces, although we also discuss corresponding behavior for Cu (111) and Cu(100) surfaces. The dominant kinetic pathway for coarsening can be either Ostwald ripening (OR), i.e., growth of larger clusters at the expense of smaller ones, or Smoluchowski ripening (SR), i.e., diffusion and coalescence of clusters. First, for pristine additive-free surfaces, we elucidate the factors which control the dominant pathway. OR kinetics generally follows the predictions of mesoscale continuum theories. SR kinetics is controlled by the size-dependence of cluster diffusion. However, this size-dependence, together with that of nanostructure shape relaxation upon coalescence, often deviates from mesoscale predictions as a direct consequence of the nanoscale dimension of the clusters. Second, we describe examples for the above systems where trace amounts of a chemical additive lead to dramatic enhancement of coarsening. We focus on the scenario where "facile reaction" of metal and additive atoms leads to the formation of mobile additivemetal complexes which can efficiently transport metal across the surface, i.e., additive-enhanced OR. A suitable reaction-diffusion equation formulation is developed to describe this behavior.
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