The Effect of Ionization on Cluster Formation in Laser Ablation Plumes

Ablation plumes caused by short-pulse laser irradiation provide conditions which are well suited to the formation of nanoclusters. The high saturation ratios and presence of ionization lead to extraordinarily high nucleation rates and small critical radii. We have explored the homogeneous nucleation and heterogeneous growth of condensate from Si targets expanding into a low-pressure He ambient using a Nd:YAG laser with pulse length of 8 ns, wavelength of 532 nm and intensities in the range of 5¥10 to 5¥10 W/cm. Clusters in the range of 5–50 nm have been produced. In the highly dynamic, non-linear regime of short-pulse laser-matter interactions, plume evolution and condensation processes are strongly coupled and difficult to predict accurately from modeling alone. Both numerical predictions and experimental results were used to quantify the competing effects of ionization and supersaturation. The results suggest a dominant influence of ionization for nearly all intensities above the ablation threshold.