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Journal of Physics B Atomic and Molecular Physics

Resonant ionisation behaviour of laser-pumped barium vapour

Rainer Künnemeyer, Manfred Kock

Published Oct 28, 1983• 20 citations
Publisher

Abstract

Dense barium vapour (10 19 -10 21 m -3 ) produced in a heatpipe oven is irradiated by a laser light pulse (4.6*10 8 W m -2 , 300 ns, 30 pm) tuned to the resonant transition at lambda =553.55 nm. The transient population densities of various Ba I and Ba II states are measured and time resolved by means of the hook method. Simultaneously, the IR forward scattering (1.5 mu m; 1.13 mu m; 2.55 mu m) in the barium vapour is measured. When the laser pulse is tuned out of resonance the ion density decreases resonantly whereas the population densities of the low-lying Ba I states do not. This resonant ionisation behaviour is interpreted by means of radiative processes.

DOI
10.1088/0022-3700/16/20/002

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•Jan 1, 1982•0 citations
Dense barium vapour (10 19 -10 21 m -3 ) produced in a heatpipe oven is irradiated by a laser light pulse (4.6*10 8 W m -2 , 300 ns, 30 pm) tuned to the resonant transition at lambda =553.55 nm. The transient population densities of various Ba I and Ba II states are measured and time resolved by means of the hook method. Simultaneously, the IR forward scattering (1.5 mu m; 1.13 mu m; 2.55 mu m) in the barium vapour is measured. When the laser pulse is tuned out of resonance the ion density decreases resonantly whereas the population densities of the low-lying Ba I states do not. This resonant ionisation behaviour is interpreted by means of radiative processes.
Dense barium vapour (10 19 -10 21 m -3 ) produced in a heat-pipe oven is irradiated by pulsed laser light tuned to the Ba I, 6s 2 1 S 0 -6s6p 1 P 1 0 , resonance transition at 553.55 nm. The transient population densities of various Ba I and Ba II states are measured and time-resolved by means of the hook method. Simultaneously, the IR forward scattering (1.5, 1.13, 2.55 mu m) in the barium vapour is measured. Emphasis is laid on the investigation and the interpretation of the IR emission, the 'anomalous' 6p 3 P 2 0 level overpopulation, and of processes which are responsible for the initiation of ionisation. In addition, the radial diffusion of excitation and ionisation energy in a barium vapour pumped with a focused laser pulse is investigated, as well as the energy transfer in a Ba-Sr vapour.
For pt.I see ibid., vol.22, no.10, p.1691-704 (1989). The kinetic processes of a laser-pumped barium vapour are analysed by the adjustment of model calculations to various experimental data. The authors consider temporally resolved measurements of population densities of low-lying Ba states, the ionisation, and the stimulated infrared emission. Furthermore, the radiative transfer laterally out of the vapour column and the transmitted fraction of the laser pulse are investigated. In the analysis of the laser pulse are investigated. In the analysis of the relevant processes, emphasis is laid on the population transfer from the pumped state to lower-lying states and on a detailed description of the ionisation mechanism. The connection of experimental data with model calculations makes it possible for the first time to determine a number of collisional cross sections for the inelastic scattering of electrons, rare-gas atoms and barium atoms with excited barium atoms.
Time-resolved measurements of fluorescence and level populations have been made on barium vapour illuminated by an intense pulse of laser radiation (1 MW cm -2 , 50 ns FWHM) tuned to the resonance transition of Ba I, 6s 2 1 S 0 -6s6p 1 P 1 , at 5535 AA. Barium number densities between 10 13 -10 15 cm -3 were investigated, using a buffer gas of helium at 100 Torr. Fluorescence was measured with two different geometrical arrangements, one allowing partial spatial resolution. In neither case did the observed fluorescence saturate with increasing laser power, although the maximum flux was more than sufficient to burn through the vapour column, as confirmed by transmission measurements. The power loss from the laser beam was found to be compatible with the scattered radiation from the pumped column in the absence of radiation trapping. In contrast to other similar experiments, no significant ionisation was detected (<5%). Time-resolved population measurements by anomalous dispersion on the 6s6p 3 P 2 and 6s5d 1 D 2 metastable levels yielded quenching cross sections for both Ba-Ba and Ba-He collisions, but gave anomalous result for the peak populations. A qualitative explanation of the fluorescence anomaly is proposed on the basis of the AC Stark effect, and it is shown that the same effect can inhibit the onset of stimulated emission to the metastable D levels.
The modification of ion production efficiency via resonant laser-driven ionisation in the presence of high buffer gas atmospheres has been experimentally investigated in a barium vapour. The populations of several energy levels of neutral and singly ionised Ba were measured as a function of both helium and argon pressures by the time-resolved hook technique. The behaviours of He and Ar are quite distinct. The percentage of ionisation decreased monotonically from nearly 100% to less than 10% as the helium pressure was increased from 0.01 to 1 atm, while no quenching was observed in 1 atm of Ar. These observations are consistent with a quenching mechanism based in part on cooling of the 'hot' electrons through momentum-changing elastic collisions with the He atoms.
The behaviour of an optically thick column of barium vapour resonantly 10 -2 illuminated at 553.5m by a 50 ns, 10 Wm pulse from a flashlamp pumped dye 19 21 -3 laser system was investigated for barium number densities in the range 10 -10 m in the presence of He buffer gas at 100 Torr.A variety of optical saturation effects associated with the principal resonance transition were studied.In marked contrast to earlier work, saturation of the intensity of the scattered light near 553.5 nm with increasing laser power was found for a suitable geometry of the collection optics but at a laser intensity typically two orders of magnitude higher than that predicted by simple theory, despite the fact that this theory was found to describe correctly transmission measurements along the length of the vapour column.The apparent contradiction between the fluorescence and transmission results, which indicated that the scattered radiation intensity continued to rise with incident laser power, despite the population of the upper resonance level having reached a limiting saturation value, could be understood in terms of a simple model which took into consideration the spectral distribution of the scattered light and its transfer through the unexcited vapour.However, in common with other similar experiments, saturation of the scattered radiation intensity with laser power could not be totally achieved when a more usual collection optics geometry was employed.This was interpreted as a consequence of laser intensity variation across the face of the laser beam.A number of subsidiary experiments were performed.These included spatially resolved intensity measurements of the scattered radiation and time resolved metastable level population measurements.In addition, evidence for laser induced ionization was sought using two different methods and this enabled an upper limit of 10% ionization to be placed on the experiments in the present work.