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Acta Physica Sinica (Overseas Edition)

Production of polarized7Li atomic beam in strong magnetic field by optical pumping

Ganghua Mei, Huang Gui-long, Jia‐Min Wang, Delin Yang, Zhang Yuan, Xiwen Zhu

Published Jul 1, 1993• 0 citations
Publisher

Abstract

The production of a polarized 7 Li atomic beam in a strong magnetic field was experimentally studied by laser optical pumping of a single ground hyperfine F level. It was shown that nearly complete negative and partial positive polarizations of 7 Li atoms could be realized under appropriate conditions by this pumping scheme, which was in agreement with the rate equation calculations. Based on the analyses of transition probabilities, the maximum polarizations for various transitions and light polarizations were given.

DOI
10.1088/1004-423x/2/7/004

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The production of a polarized 7 Li atomic beam in a strong magnetic field was experimentally studied by laser optical pumping of a single ground hyperfine F level. It was shown that nearly complete negative and partial positive polarizations of 7 Li atoms could be realized under appropriate conditions by this pumping scheme, which was in agreement with the rate equation calculations. Based on the analyses of transition probabilities, the maximum polarizations for various transitions and light polarizations were given.
Using rate equation formalism, we got an analytic expression describing the optical pumping of an alkali atomic beam in strong magnetic fields. By comparing with the traditional description in single velocity approximation, the influence of velocity distribution in a practical alkali atomic beam on the pumping effect was quantitatively discussed. It turned out that the rate equation treatment in single velocity approximation might result in an error within twenty percent. Laser optical pumping of 7 Li atomic beam in a strong magnetic field (0.15 T) was experimentally studied, and the results are in agreement with the theory in which the velocity distribution is taken into account.
The authors have succeeded in demonstrating laser isotope enrichment via magnetic deflection of a polarized atomic beam. By using a single dye laser with the frequency tuned to the two accidentally close transitions of the natural lithium (2 2 S 1/2 (F=1/2) to 2 2 P 3/2 for 6 Li and 2 2 S 1/2 (F=2) to 2 2 P 1 2/(F=1) for 7 Li), 6 Li and 7 Li atoms in a beam were positively and negatively polarized respectively. The 6 Li enrichment was achieved via magnetic deflection of the polarized beam through a hexapole magnet which focuses the positively polarized 6 Li atoms and defocuses the negatively polarized 7 Li atoms. The dependence of 7 Li polarization on laser intensity was experimentally examined, and the result was in good agreement with that calculated by rate equations. The abundance ratio of 6 Li to 7 Li was enhanced from 1 : 12.5 to about 1 : 3.1.
We describe optically pumping a beam of sodium atoms to >96% mS and >92% mS, mI state selection. (We have accurately measured the population of every mS, mI state in the optically pumped beam.) For the optical pumping both ground hyperfine states are pumped, using single-mode cw dye-laser radiation tuned to the 3S1/2–3P1/2 transition that is phase modulated in a LiTaO3 crystal to produce first-order sidebands at approximately the 1772-MHz hyperfine splitting of the ground state. The z-directed optical pumping is performed in a z-directed magnetic field of ~5 G. The state-selected atoms then move, in ~1 cm, into an ~200 G, z-directed field. The downstream probe laser beam is scanned through the 3S1/2 (mS, mI) → 3P3/2 (mS′, mI) transitions, which are spectroscopically resolved at 200 G, and the fluorescence intensities portray the residual populations in each of the eight 3S1/2 states.
A compact source which produces atomic alkali-metal beams in individual hyperfine sublevels has been developed. The methods are demonstrated with 23Na^{23}\mathrm{Na}23Na atoms. Initially nearly all atoms are transferred into one hyperfine sublevel (∣F,mF〉=∣2,2〉|F, {m}_{F}〉=|2, 2〉∣F,mF​〉=∣2,2〉) by a double-resonance technique using laser optical pumping and rf transitions. Subsequently the other hyperfine sublevels can be populated selectively by an adiabatic high-frequency transition. The hyperfine sublevel populations are determined by laser-induced fluorescence spectroscopy in a magnetic field.
Diode laser optical pumping to produce a highly spin-polarized metastable He beam to be used in a spin-polarized metastable atom deexcitation spectroscopy experiment on magnetized surfaces is described. Efficient pumping of the beam is performed by means of an SDL-6702 distributed Bragg reflector diode laser which yields 50 mW of output power in a single longitudinal mode at 1083 nm, the resonance wavelength for the 23 S→23 P0,1,2 (D0, D1, and D2) transitions of He*. The light is circularly polarized by a quarter-wave plate, allowing easy change of the sense of atomic polarization. The laser frequency can be locked to the atomic transition for several hours by phase-sensitive detection of the saturated absorption signal in a He discharge cell. Any of the three transitions of the triplet system can be pumped with the laser but the maximum level of atomic polarization of 98.5% is found pumping the D2 line.
The general solution of the Boltzmann kinetic equations obtained previously by the author for the atomic density matrix in a long cylindrical cell with an antirelaxation coating of the walls under conditions of laser pumping in a magnetic field is used for the calculation of the magneto-optical rotation of the light polarization plane. It is shown that, in accordance with experimental data, the dependence of the rotation angle of the polarization plane of the pumping light on a magnetic field ranging from −1 to 1 Oe represents a sum of two antisymmetric curves: a narrow curve with a spacing between extrema of about 2 μOe and a broad curve with a spacing of about 0.03 Oe. It is found that the characteristics (the width and the amplitude) of these two curves depend quite differently on the parameters of the problem—the pumping power, the coating Q factor, the pumping beam radius, and the cell radius. This difference is explained by the fact that the two curves are associated with different physical processes: the narrow curve characterizes the distortion caused by the magnetic field in the atomic polarization accumulated during multiple passages through the pumping beam after almost nondepolarizing collisions with the coating, while the broad curve corresponds to the distortion of the polarization formed during a single passage through the beam.