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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Low Vapor Density Measurements by Saturated Absorption

Michael S. Raab, James J. Snyder

Published Oct 19, 1983• 0 citations
Publisher

Abstract

We report an application of two saturated absorption techniques, saturation spectroscopy and polarization spectroscopy to low density measurements in sodium vapor. The sensitivity we achieved using the resonance lines D1 and D2, was enough to detect a minimum number of a few thousand atoms. We discuss principal limits of such experiments and the benefits of both of the techniques.

DOI
10.1117/12.936244

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We report an application of two saturated absorption techniques, saturation spectroscopy and polarization spectroscopy to low density measurements in sodium vapor. The sensitivity we achieved using the resonance lines D1 and D2, was enough to detect a minimum number of a few thousand atoms. We discuss principal limits of such experiments and the benefits of both of the techniques.
The aim of this work was to develop a high-resolution, ultra-sensitive laser method that makes use of a Faraday modulator in a polarization spectroscopy arrangement. Though saturation spectroscopy is not a sensitive technique, polarization spectroscopy is a useful technique for obtaining both high resolution and high sensitivity. However, spectral line shapes are distorted. Our proposed birefringence-modulated polarization spectroscopy has additional advantages in sensitivity and symmetry of the line shape over saturation and polarization spectroscopy. The spectra of the sodium D1 line have been calculated in order to compare the various spectroscopic techniques.
The method of atomic-resonance fluorescence has been applied to the detection of very small concentrations of radioactive 20Na atoms (<3 × 109 m−3). The 20Na isotope is produced by the reaction 20Ne(p, n)20Na by passing a 20 MeV proton beam through a neon gas target of density 3.5 × 1024 atoms/m3. A beam of a continuous-wave dye laser tuned to the D2 line of sodium is transmitted through the production region and the fluorescence light is detected by means of photon counting. A digital synchronous-detection technique has been applied to measure the time-dependent behavior of the 20Na atom density shortly after a proton irradiation. This behavior is determined both by radioactive decay and by diffusion of the atoms out of the production region. The absolute 20Na atom density has been estimated, using Rayleigh scattering of the laser beam on the neon gas for calibration of the optical system. The density of neutral 20Na atoms appeared to be an order of magnitude lower than the density produced. The detection method has also been used to measure the temperature dependence of saturated sodium-vapor density down to 3 × 1011 atoms/m3 (292 K).
A new method has been designed for obtaining the hyperfine optical pumping of Na vapor. The pumping lamp is placed in a magnetic field and the light emitted in the field direction is used for excitation after crossing a circular polarizer selecting one of the Zeeman lines. This method has two interesting features: (i) The atoms can be pumped either in the lower or in the upper hyperfine sublevel by simply rotating a polarizer by an angle of $\frac{1}{2}\ensuremath{\pi}$; (ii) the hyperfine pumping is obtained using both the D1{D}_{1}D1​ and D2{D}_{2}D2​ resonance lines without filtering losses. The pumping efficiency is good and it has been possible to measure the spin-exchange cross section between sodium atoms by studying the hyperfine relaxation rates as a function of the sodium density. The result is ${\ensuremath{\sigma}}_{\mathrm{ex}}=(1.109\ifmmode\pm\else\textpm\fi{}0.005)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2.
We analyze the excitation by an electromagnetic wave of a two-level molecule (or atom) within a rarefied gas. This process differs from the excitation of an isolated molecule because there is significant interaction between the molecule and the surrounding gas particles as they are all polarized by the electromagnetic wave. These interactions lead to corrections to the recoil and Doppler shifts of the molecule's transition frequency. We propose an experiment using saturation spectroscopy to observe sodium molecular transitions near the atomic DDD lines which could easily detect the correction to the first-order Doppler shift caused by the interaction between the sodium molecules and the surrounding atomic vapor.