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Journal of Physics B Atomic and Molecular Physics

Quenching of resonant laser-driven ionisation at high buffer gas pressures

W. T. Hill

Published Feb 14, 1986• 8 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/19/3/019

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Journal of Physics B Atomic and Molecular Physics•Jun 28, 1980•11 citations
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.
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.
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.
Individual Ba ions are trapped in a gas-filled linear ion trap and observed with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage times of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ $(\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{min})$ are achieved using He (Ar) as a buffer gas at pressures in the range $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}--4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{torr}$. Trap dynamics in buffer gases are experimentally studied in the simple case of single ions. In particular, the cooling effects of light gases such as He and Ar and the destabilizing properties of heavier gases such as Xe are studied. A simple model is offered to explain the observed phenomenology.
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.