In this work concerted experiments and theoretical analysis are applied to conclusively demonstrate the vacancy generation during fast melting and re-growth of Si by laser irradiation. Experiments, based on the positron annihilation spectroscopy and designed to test the theoretical predictions, evidence a vacancy super-saturation after the laser process The dependence on the pulse energy and number of shots of the residual damage, after a multi shot laser irradiation process, is characterised by means of these measurements. Kinetic Monte Carlo simulations of the molten Si re-crystallization show trapping of vacancies in the re-crystallized region. The main outcome of this simulation is the dependence of the vacancy's generation efficiency on under-cooling. The knowledge of this dependence allows us to implement a continuum model (based on kinetics equations for the phase, the free defect and clustered …