Department of Chemistry and Biochemistry
University of Notre Dame
Notre Dame, Indiana 46556

We present calculations of the bulk modulus, heat capacity, and the period of the breathing mode for spherical nanoparticles following excitation by ultrafast laser pulses. The bulk modulus exhibits relatively sharp transitions at both the surface-melting and bulk melting transitions, while the heat capacity shows only a relatively broad transition at the bulk melting temperature. Equilibrium calculations of the heat capacity show that the melting transition is sharper and occurs at a lower temperature than one would observe from an ultrafast experiment. We also observe an intriguing splitting in the low-frequency spectra of the nanoparticles and analyze this splitting in terms of Lamb's classical theory of elastic spheres. We conclude that the particles either 1) melt during the observation period following laser excitation, or 2) melt an outer shell while maintaining a crystalline core. Both mechanisms for melting are commensurate with our observations.