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Journal of Physics B Atomic Molecular and Optical Physics

Kinetics of a laser-pumped barium vapour. II. Experiments and calculations

A. Kallenbach, Manfred Kock

Published May 28, 1989• 44 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/22/10/022

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Physical Review A•Aug 1, 1994•20 citations
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.
A numerical model has been developed, which is well suited to describe the kinetics of a laser-pumped barium vapour. The numerous processes essential for the temporal evolution in the population of the various barium levels are outlined in detail. A number of approximations and simplifications have to be introduced to adapt the complex problem to a numerical solution at an economical expense. Since the authors consider all substantial features of the laser-pumped barium vapour like the population development of low-lying levels, the ionisation of the vapour, stimulated infrared emission and radiative transfer, the requirement for a quantitative interpretation of experimental data is fulfilled.
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.
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.
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 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.
Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p(3{(}^{3}(3P1{\mathit{P}}_{1}P1​) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s(1{(}^{1}(1D) and 5d6s(3{(}^{3}(3D) metastable levels. The densities of these low-lying excited states are measured by the absorption of lines from a Ba hollow-cathode lamp, and the states populated by energy-pooling collisions are studied by comparing their fluorescence intensity to that of the laser-excited level. The rate coefficients are on the order of gas-kinetic rates, and they are not strongly dependent on energy defect, spin changes, or angular-momentum changes.