Measurements of the time of the onset of melting of self-implantation amorphized (a) Si, during an incident laser pulse, have been combined with modified melting model calculations and measurements of surface melt duration to demonstrate that the thermal conductivity, Ka, of a-Si is very low (≃0.02 W/cm-K). Ka is also shown to be the dominant parameter determining the dynamical response of ionimplanted Si to pulsed laser radiation; the latent heat and melting temperature of a-Si are relatively unimportant. Cross-sectional transmission electron micrographs on implantation-amorphized Si layers of several different thicknesses show that for energy densities less than the threshold value for complete annealing there are usually two distinct regions in the re-solidified a-Si, consisting of fine-grained and large-grained polycrystalline Si, respectively. The presence of the fine-grained poly-Si suggests that bulk nucleation occurs directly from the highly undercooled liquid phase. Thermal melting model calculations suggest that the nucleation temperature, Tn is ≃1200°C.