Kinetic study of time-of-flight distributions during pulsed laser evaporation into vacuum

A numerical study of the dynamics of gas expansion into vacuum during nanosecond laser evaporation out based on the analysis of time-of-flight distributions of neutral particles along the normal to the evaporation surface has been carried out. The solution to the problem in the axisymmetric formulation is obtained by using two independent kinetic approaches: the direct simulation Monte Carlo method and the solution of the Bhatnagar-Gross-Krook model equation. The particle velocity distribution on the time-of-flight detector was analyzed. For a correct description of the experimental data with good accuracy, the optimal value of the velocity cone angle, which determines the fraction of particles arriving at the detector, is determined. It is shown that at intensive evaporation, the number of low-velocity molecules arriving at the detector increases with increasing size of the evaporation spot, which leads to a decrease in the average particle energy. Reliable data on the dependence of the particle energy at the time-of-flight detector on the number of evaporated monolayers and the size of the evaporation spot have been obtained. Good agreement with calculations of other authors and known experimental data is shown.