A study of the ablation process of an amorphous Si target by a picosecond Nd:YAG laser is presented. Both spatial and temporal resolution were used to characterize the laser-induced plasma expansion. The presence of Si 2+ and Si 3+ ions in the plasma shows a high degree of ionization and a collisional excitation of the ejected particles. The velocities of the neutral atoms and ions are (1–13) x 10 6 cm/s and depend on their charge, excitation energy and spatial position. The structures of the deposited films are affected by the presence of the energetic ions.