An analytical continuum-based model of time-of-flight distributions for pulsed laser ablation

An analytical model for time-of-flight (TOF) distributions of particles produced by pulsed laser ablation in vacuum has been proposed. The model takes into account the hydrodynamic expansion stage of the ablation plume and is based on a ‘sudden freeze’ model developed previously for steady-state supersonic jets. It is assumed that a continuum-like expansion of the plume takes place until a freezing time moment t _free (or, alternatively, until a ‘freezing distance’ x _free) whereupon the collisionless expansion begins. The proposed model is applied for analysis of experimental data on graphite ablation with nanosecond laser pulses. For verification, the analytical distributions are compared with calculated results obtained using a hybrid model combining a thermal model of laser-induced material heating with calculations of the plume dynamics by the direct simulation Monte Carlo (DSMC) method. It is shown that the proposed model can accurately estimate the surface temperature for conditions when the common approach fails.