Time-resolved detection of laser-ablated particles based on intensity decrease of cw probe laser beam

A prototype system was constructed for the detection of laser-ablated particles. The second harmonic of a picosecond Nd:YAG laser was used for the ablation of a polycrystalline copper target in vacuum. The typical laser intensity was 1011 W cm−2. A probe HeNe laser beam which passed through the ablation plume was scattered by the ablated particles. The intensity of the probe beam was measured as a function of the elapsed time from the ablation laser pulse, and its dip was observed to exhibit a Maxwell-Boltzmann-like distribution. The delay time of the dip depended linearly on the distance between the target and the probe beam. The velocity of the expansion at the peak of the dip was derived as8.3 × 103cm s−1, and this reflected the expansion velocity of the plume. When helium, neon, argon, and nitrogen gases up to 40 kPa were introduced, the expansion velocity decreased and a long tail of the dip was observed.