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Chinese journal of atomic and molecular physics

RESEARCH ON DOPPLER-FREE BACKWARD FLUORESCENCE IN SODIUM VAPORS

J. S. Fu

Published Jan 1, 1987• 0 citations
OpenAlex

Abstract

Spectral characteristics of forward and backward fluorescence in vapors are analysed. In the experiment, we observed backward fluorescence of the Sodium D line. The frequency separation between hyperfine transition and the intensity distribution of the fluorescence and the velocity distribution of atoms are measured. We show that our measurements are in full agreement with rate equation value.

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Fluorescence Intensity Ratio of Sodium Doublet Observed in the Optical Dissociation of Sodium Iodide Vapor

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Physical Review A•Jul 8, 2003•33 citations
Spectral characteristics of forward and backward fluorescence in vapors are analysed. In the experiment, we observed backward fluorescence of the Sodium D line. The frequency separation between hyperfine transition and the intensity distribution of the fluorescence and the velocity distribution of atoms are measured. We show that our measurements are in full agreement with rate equation value.
The intensity ratio D2/D1 for the sodium doublet has been measured for radiation from excited sodium atoms which result from the optical dissociation of sodium iodide vapor by ultraviolet light in the 2082→2500 A region. With shorter wavelength ultraviolet light as exciting source, the D2/D1 ratio is 0.98 and increases to above 1.5 with longer exciting wavelengths. Addition of foreign gases cause the observed D2/D1 ratio to increase to 1.89 for large foreign gas pressure. Cross sections for the transfer of excited sodium atoms from the 32P32 to the 32P½ state and vice versa due to collisions with foreign molecules have been measured as they depend on the relative velocity between excited sodium atom and foreign molecule. The Doppler broadening of the separate D lines emitted in the atomic fluorescence of sodium iodide has been estimated from absorption measurements. Both lines show equal broadening within the experimental error of the observations. Measurements were made with a photomultiplier tube. The sodium doublet was resolved with a reflection grating monochromator.
The phase relaxation of the resonance fluorescence from sodium vapor was studied by measuring the contrast of interference between the delayed fluorescence pulses. It is shown that the phase relaxation time can be measured by a conventional cw mode-locked pico-second pulsed laser without any sophisticated apparatus to actively lock the phases of exciting laser pulses. Using a modified Michelson type interferometer, and a homemade software based on FFT algorithm, we obtained a phase correlation time as 300 ps with good S/N ratio. A local minimum observed in the decay curve is attributed to the ground state hyperfine splitting and the temperature dependence of the phase relaxation time is explained by the change in the spread of atomic velocity distribution with temperature (Doppler broadening mechanism). Analytical calculation indicates that when the laser bandwidth is much broader than the linewidth of the fluorescence, as in the present case, the envelope of the interfering fluorescence signal can be represented by Fourier transform of the absorption spectra. The analysis based on this approximation are in good agreement with the results obtained in the experiment.
Literature mentions only incidentally a sub-Doppler contribution in the excitation spectrum of the backward fluorescence of a dense vapor. This contribution is here investigated on Cs vapor, both on the first resonance line (894 nm) and on the weaker second resonance line (459 nm). We show that in a strongly absorbing medium, the quenching of excited atoms moving towards a window irradiated under near normal incidence reduces the fluorescence on the red side of the excitation spectrum. Atoms moving slowly towards the window produce a sub-Doppler velocity-selective contribution, whose visibility is here improved by applying a frequency-modulation technique. This sub-Doppler feature, induced by a surface quenching combined with a short absorption length for the incident irradiation, exhibits close analogies with the narrow spectra appearing with thin vapor cells. We also show that a normal incidence irradiation is essential for the sub-Doppler feature to be observed, while it should be independent of the detection geometry.
The resonant excitation of the sodium D2 transition by laser radiation was investigated experimentally in this thesis. Line polarization measurement of the resonance fluorescence was employed to investigate the excitation process. The sodium atoms, which were collimated in a very narrow beam, were excited by a monochromatic laser beam with the frequency tuned to resonance with the 32S 112(F=2) - 32P312 hyperfine transition. The direction of the laser beam was perpendicular to the atomic beam and the fluorescence photons were detected along a direction perpendicular to both the laser and atomic beams. The experimental measurements were performed to determine the effect due to the variation of the laser intensity and frequency on the line polarization. A theoretical expression of the line polarization as a function of density matrix elements of the upper excited hyperfine sub -states was derived. A full quantum electrodynamic theory was used to derive an atomic operator equation of motion which was needed to calculate atomic operator elements. These elements then yielded the corresponding density matrix elements. The experimental results and the theoretical modelling were in a good agreement. The first experimental results for the line polarization of the highly excited states, 42D512 and 42D312, are also presented. The use of two lasers enabled these highly excited states to be excited. The line polarization of the fluorescence for transitions from the 4D level down to lower states is derived in terms of the density matrix elements of the upper excited states. Two limiting cases for the theoretical predictions of the line polarization are considered.
We present the results of a pump-probe laser spectroscopic investigation of the Doppler-broadened sodium D1D1D1 resonance line. We find 13 resonances in the resulting spectra. These observations are well described by the numerical predictions of a four-level atomic model of the hyperfine structure of the sodium D1D1D1 line. We also find that many, but not all, of these features can be understood in terms of processes originating in a two-level or three-level subset of the full four-level model. The processes we observed include forward near-degenerate four-wave mixing and saturation in a two-level system, difference-frequency crossing and nondegenerate four-wave mixing in a three-level V system, electromagnetically induced transparency and optical pumping in a three-level lambda system, cross-transition resonance in a four-level double-lambda system, and conventional optical pumping. Most of these processes lead to sub-Doppler or even subnatural linewidths. The dependence of these resonances on the pump intensity and pump detuning from atomic resonance are also studied.