<title>Evolution of plasma plume generated during laser ablation of solid targets</title>

Abstract

Plasma plume generated during laser ablation of Y, YBaCuO, GaAs, and InAs targets was investigated by laser-induced fluorescence technique. Time-of-flight method was used. The velocity distribution functions of atoms, ions, and atoms in metastable states were obtained. Two regimes of plasma expansion were observed. In the first regime the plasma expands in vacuum as ideal gas, and in the second one the acceleration of ions by inner plasma electrical field takes place. The bimodal velocity distribution of plasma particles is the main feature of the second regime. Also two regions in plasma plume were observed: 'collisional' and 'collisionless'. The first region is placed in vicinity of a target. Its dimension is about 30 mm and does not depend on laser fluence. In this region recombination processes and transformation of velocity distribution takes place. In the second region velocity distribution functions are not changing. Stable shapes of velocity distribution functions is a feature of the second region. It is well known that during laser ablation the drops appear in plasma plume. The concentration and velocity of drops generated during laser ablation of target were measured. The drops velocity is about 10 times less than atoms. So it is possible to separate drops from atomic component in laser plasma. The mechanical chopper was used for drops removing. The drops' concentration on films deposited with chopper was negligible.