Fast excitation and ionisation in a laser-pumped barium vapour: experiments and calculations

Dense barium vapour (>10 20 m -3 ) produced in a heat pipe is irradiated by a laser light pulse (10 7 W m -2 , 1.5 mu s) tuned to the resonant transition at lambda =553.5 nm. The transient population densities of various Ba I and Ba II states are measured, time resolved (<10 ns) by means of the hook method. As one result the strong ionisation, as described by other authors, is observed in its temporal development. For a quantitative interpretation a rate equation model is developed including a simplified treatment of radiative transfer. Most of the observed phenomena are described sufficiently by the authors calculations. The ionisation, especially, is simulated satisfactorily, showing that two-photon ionisation is the dominant process.