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Some experiments in optical pumping

Rodney C. Greenhow

Published Jan 1, 1963• 0 citations
Publisher

Abstract

The work reported in this thesis consists of an investigation into three aspects of optical pumping in Helium. Optical pumping in He⁴ This work was initially directed towards repeating the observations of Colegrove and Franken (ref. 2), who first reported experiments to produce atomic alignment by optical pumping in He⁴. This was achieved, and the theory and experiments were extended to cover the case of polarisation. The 2³S₁ term in He⁴ is metastable and is used as the ground level in an optical pumping experiment. The metastable atoms are created by exciting a weak electrodeless glow discharge in low pressure He⁴ gas. [To continue reading the abstract please see the pdf]

DOI
10.5287/ora-xrgzoo98z

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The work reported in this thesis consists of an investigation into three aspects of optical pumping in Helium. Optical pumping in He⁴ This work was initially directed towards repeating the observations of Colegrove and Franken (ref. 2), who first reported experiments to produce atomic alignment by optical pumping in He⁴. This was achieved, and the theory and experiments were extended to cover the case of polarisation. The 2³S₁ term in He⁴ is metastable and is used as the ground level in an optical pumping experiment. The metastable atoms are created by exciting a weak electrodeless glow discharge in low pressure He⁴ gas. [To continue reading the abstract please see the pdf]
The alignment of He4{\mathrm{He}}^{4}He4 atoms in the (n=2n=2n=2, metastable) 3S1^{3}S_{1}3S1​ state is described. Metastable atoms are produced by an rf discharge in a glass tube containing a few mm of pure helium, and the one micron pumping light ($2^{3}P\ensuremath{-}2^{3}S$) is provided by a helium lamp. A resonance signal is obtained from radio frequency disorientation by monitoring the transmitted pumping light. The double maximum line shape of this signal for strong rf magnetic fields is discussed. Included also is a discussion of the angular dependence of the signal when unpolarized light is used and an explanation of the inversion of the resonance signal for certain densities of the metastable helium atoms.The measured relaxation time of the oriented metastable atoms in the discharge is about 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ second and the pumping time is about a millisecond. A method is proposed and initial measurements are given for the cross section for destruction of metastable helium atoms by collision with foreign gas atoms. The application of optical pumping in helium to the measurement of weak magnetic fields is also discussed.
The 3P2^{3}P_{2}3P2​ metastable atoms of neon have been optically oriented. This represents the first case of the optical pumping of non-SSS state atoms. The optical-pumping process is described, and depolarization cross sections for collisions with ground-state neon and ground-state helium atoms measured. The cross section for collisional depolarization with helium is 4.30 \ifmmode\pm\else\textpm\fi{} 0.22 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2, and with neon is 166 \ifmmode\pm\else\textpm\fi{} 8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}17}$ cm2{\mathrm{cm}}^{2}cm2 at 300\ifmmode^\circ\else\textdegree\fi{}K. The metastable 3P2^{3}P_{2}3P2​ atoms of Ar and Xe have also been optically pumped. As the polarizability of the noble-gas buffer increases, the collisional depolarization cross section approaches the diffusion cross section.
At low magnetic field, the efficiency of metastability-exchange optical pumping of helium-3 is known to be optimal for pressures around 1 mbar. We demonstrate on several examples (up to 32 mbar) that operating in a higher magnetic field (up to 0.12 T) can significantly increase the nuclear polarisations achieved at higher pressures. Since polarisation measurements cannot be made with the standard technique, we use a general optical method based on absorption measurements at 1083 nm to measure the polarisation of the atoms in the ground state.
Transverse birefringence has been observed in an aligned spin assembly consisting of helium 4 atoms in the metastable state 2 3 S 1 . The alignment of the spins was produced by optical pumping with unpolarized light. Using a cross-beam of off-resonant radiation, for example from a helium 3 lamp, together with a suitable combination of polarizer and analyser, birefringence signals could be displayed analogous to the absorption signals conventionally used for the optical monitoring of magnetic resonance. An analysis of the phenomenon is given, showing quite generally that birefringence monitors separately the degree of alignment of the spin assembly, even when orientation is also present; the latter can be monitored by the magnetic rotation of the plane of polarization for axial light. The analysis also includes the effect of the spectral characteristics of the monitoring lamp, i.e. the central frequencies of emission and the Doppler width, on the strengths of absorption and birefringence signals displayed by optically thin samples.
Advances in metastability exchange optical pumping (MEOP) at high laser powers, but also at high gas pressures and high magnetic field strengths, has provided strong motivation for revisiting the understanding of the limitations of this powerful technique. A comprehensive model has been developed for improved description of the combined effects of OP, ME, and relaxation, and of detailed MEOP features observed over the broad range of operating conditions. A brief description is provided, with illustrative comparisons of computed and experimental results. This improved tool is used to explain the excellent photon efficiency of OP obtained at all field strengths. It is combined with an angular momentum budget approach to quantitatively investigate the newly discovered strong OP-enhanced polarisation losses that currently limits MEOP performance.