TIME-AND-SPACE RESOLVED X-RAY-ABSORPTION SPECTROSCOPY OF LASER-ABLATED SI PARTICLES

We have performed time-and-space resolved X-ray absorption spectroscopy with a time resolution of 10 ns to study laser-ablated Si particles in a time scale ranging from 0 ns to 120 ns. Neutral and charged particles produced by laser ablation are observed through X-ray absorption spectra. Assignments of transitions from 2s and 2p initial states to higher Rydberg states of Si atom and ions are achieved, and we experimentally determine the L(II,III) absorption edges of neutral Si atom (Si0) and Si ions such as Si+, Si2+, Si3+ and Si4+. The main ablated particles are found to be Si atom and Si ions in the initial stage of 0 ns to 120 ns. The relative amounts depend strongly on times and laser energy densities. We find that the spatial distributions of particles produced by laser ablation are changed with supersonic helium gas bombardment, but no cluster formation takes place. This suggests that a higher-density region of helium gas is formed at the top of the plume of ablated particles, and free expansion of particles is restrained by this helium cloud, and that it takes more than 120 ns to form Si clusters.