Analytical formula for interpretation of time-of-flight distributions for neutral particles under pulsed laser evaporation in vacuum

Abstract

A theoretical study of the time-of-flight (TOF) distributions of neutral species produced by low-fluence pulsed laser evaporation in a vacuum has been performed. A database of TOF distributions has been calculated by the direct simulation Monte Carlo method. The calculated TOF signals have been fitted by a modification of the shifted Maxwell–Boltzmann distribution function with the only parameter being the shift velocity. The dependence of the shift velocity on the number of evaporated monolayers has been described by an analytical expression, derived based on the gas-dynamic analysis of pulsed gas expansion into a vacuum. This expression allows the derivation of a new formula for TOF distributions for neutral particles with the only parameter being the surface temperature. This new formula has been used for the analysis of experimental data on pulsed laser ablation of niobium, copper, graphite and gold. The evaluated surface temperature agrees well with the results of the thermal model calculations and available experimental data with an error of 10% instead of 50–150% for commonly used formulas.