We have investigated the formation dynamics of silicon nanoparticles after laser ablation of silicon targets in H-2 gas and O-2 gas diluted with Ar gas by 2D imaging of time-resolved photoluminescence (PL). PL intensity from silicon nanoparticles is found to be enhanced in H-2 gas with partial pressures up to 400 Pa. In O-2 gas, the PL intensity drastically decreases with increasing partial pressures. A broad band green PL indicates that hydrogenated Si nanoparticles are formed in the H-2 gas. (C) 2002 Elsevier Science B.V. All rights reserved.