Super-elastic effects in laser-induced ionisation of Na vapour

Abstract
The authors report the observation and the measurement of an intenses atomic ion yield when Na vapour in the density range 1013-1015 cm-3 is irradiated by a laser tuned to 3S1/2 -3P3/2 resonance with intensity 3*105 W cm-2. It is shown that super-elastically heated electrons are present in the vapour and play a determining role in the ionisation process. A numerical analysis following the LIBORS code of Measures et al. (1979) is given which emphasises the requirement of a large cross section for the electron seeding mechanism: 5*10-15 cm2 at the given intensity. Laser-induced collisional ionisation discussed in the preceding paper (Carre et al. 1981) is assumed to initiate further efficient ionisation.