Time-resolved excitation and ionization of a laser-pumped Ba vapor

The excitation and ionization of barium vapor by resonant laser irradiation was investigated in detail. Measurements were taken on a moderately dense vapor (${10}^{19}$ to ${10}^{21}$ ${\mathrm{m}}^{-3\mathrm{}}$) that was illuminated by an intense (7 GW ${\mathrm{m}}^{-2\mathrm{}}$) 1-μs-long dye-laser pulse. The laser was tuned to the Ba resonance line at $\lambda =553.5\mathrm{}$ nm, and burned through the 0.25-m-long vapor column in less than 50 ns. The excitation histories of twelve Ba and ${\mathrm{Ba}}^{+}$ levels were determined with a time resolution of 5 ns by use of hook-interferometry spectra. The emission, which reflected the population of high-lying levels, was also analyzed. For the higher densities investigated, almost complete ionization was achieved within 400 ns. Ionization was highly density-dependent and this pointed to collision-dominated ionization mechanisms. Strontium atoms—present as an impurity in the Ba vapor, yet not in resonance with the laser radiation—were also found to ionize rapidly. This provided further confirmation of the dominance of collisions. We established that seed electrons were heated in superelastic collisions with laser-excited atoms, and that subsequent electron-impact excitation and ionization, as well as photoionization of high-lying levels lead to the creation of more electrons. Electron density and concomitant ionization then increase rapidly. The observed transfer of the excitation energy to the electrons by superelastic collisions requires contributions not only from the laser-pumped resonance level, but also from the lower-lying metastable Ba levels. These are, in fact, populated very rapidly and efficiently via the resonance level.