Laser-induced collisional ionisation of Na vapour at the 3P1/2and 3P3/2resonances

The authors report simultaneous measurements of both Na + and Na 2 + ion yields when Na vapour in the 10 12 -10 13 cm -3 density range is irradiated by a pulsed laser tuned to one of the 3S 1/2 -3P 1/2,3/2 resonances with intensities in the 10 3 -10 6 W cm -2 range. In the studied vapour density and laser intensity ranges, electron impact ionisation and multiphoton ionisation processes are negligible; the ion yields are hence interpreted in terms of atom-atom collisions. The influence of associative ionisation on the Na 2 + yield is analysed on the basis of the experimental data and a calculated potential curve for the 1 Sigma g molecular state correlated to Na(3p)-Na(3p). A critical discussion of possible laser-induced (laser-assisted and/or laser-modified) ionising processes is given. A good account of the data is obtained only if laser-induced associative ionisation and laser-induced Penning ionisation (as the most probably processes) are assumed to take place in the vapour. The distribution over the vibrational levels of the Na 2 + ions thus formed and the photodissociation of these levels are discussed as well as the existence of independent Na + ion formation.