PARTICLE NUMBER DENSITY AND VELOCITY DISTRIBUTIONS IN LASER PLUMES

A time-of-flight mass spectrometer with electron impact ionization fac ility was used in investigations of the laser plume structure. Densiti es and velocity distributions of positively charged and neutral specie s were measured 12 cm downstream of the target. Velocities of particle s in a plume were measured by the retarding potential method. The comb ination of a skimmer and declining electric field was used to suppress the influence of charged particles during the measurement of the neut ral component parameters. In the case of YBaCuO ceramic laser ablation , a strong variation of the laser-induced plume composition was observ ed from its head to its tail. It seems to be accounted for by the diff erence of the starting (phase transition) temperatures of various laye rs of a plume. Ions detected mainly in the head of a plume were follow ed by atoms, molecules and clusters in inverse succession to their app earance in the plume under the light intensity increase. The character istic of the number density dependence upon the laser spot diameter ma ke it clear that most of the molecules BaO and YO are the direct produ ct of ablation. In contrast, the detected clusters with masses up to 2 000 amu are the product of condensation in the expanding plume under t he conditions of the experiments.