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Physical Review

Optical Pumping of Helium in the S13 Metastable State

F. D. Colegrove, P. A. Franken

Published Jul 15, 1960• 118 citations
Publisher

Abstract

The alignment of ${\mathrm{He}}^{4}$ atoms in the ($n=2$, metastable) $^{3}S_{1}$ 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.

DOI
10.1103/physrev.119.680

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Depolarization of Light Scattered by Aligned 2S3 and 2P3 Helium Atoms at Resonance

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The $2^{3}S_{1}$ (metastable) and $2^{3}P_{1,2}$ excited-state atoms of ${\mathrm{He}}^{4}$ are aligned by an unpolarized beam of resonance radiation. The alignment is detected by the depolarization of the resonance fluorescence at 1\ensuremath{\mu}, which occurs when either the $2^{3}S_{1}$ level or the $2^{2}P_{1,2}$ levels are saturated by a rf magnetic field at the Zeeman frequency. The observed signals are several orders of magnitude larger than the corresponding signals observed by monitoring intensity changes in the transmitted beam. Considerably smaller signals are observed even in the absence of the pumping beam. Expressions for the intensity of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ components of the scattered radiation are derived in terms of the alignment and density of the metastable atoms. The alignment of the $2^{3}S_{1} (F=\frac{3}{2})$ and the $2^{3}P_{2} (F=\frac{5}{2})$ levels of ${\mathrm{He}}^{3}$ is also observed. The excited-state linewidth as a function of pressure leads to a radiative decay time of $\ensuremath{\tau}\ensuremath{\approx}0.96\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ sec and a mixing cross section of 53\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$. The alignment of the $3^{3}P$ level of ${\mathrm{He}}^{4}$ is also observed by monitoring polarization changes of the 3889 \AA{} line.

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Spin Exchange between Optically Oriented Metastable Helium Atoms and Thermal Electrons in a Flowing Afterglow

L. D. Schearer

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Physical Review•Jul 5, 1968•11 citations
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 (23P\ensuremath−23S2^{3}P\ensuremath{-}2^{3}S23P\ensuremath−23S) 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{10}^{\ensuremath{-}4}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 23S12^{3}S_{1}23S1​ (metastable) and 23P1,22^{3}P_{1,2}23P1,2​ excited-state atoms of He4{\mathrm{He}}^{4}He4 are aligned by an unpolarized beam of resonance radiation. The alignment is detected by the depolarization of the resonance fluorescence at 1\ensuremath{\mu}, which occurs when either the 23S12^{3}S_{1}23S1​ level or the 22P1,22^{2}P_{1,2}22P1,2​ levels are saturated by a rf magnetic field at the Zeeman frequency. The observed signals are several orders of magnitude larger than the corresponding signals observed by monitoring intensity changes in the transmitted beam. Considerably smaller signals are observed even in the absence of the pumping beam. Expressions for the intensity of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ components of the scattered radiation are derived in terms of the alignment and density of the metastable atoms. The alignment of the 23S1(F=32)2^{3}S_{1} (F=\frac{3}{2})23S1​(F=23​) and the 23P2(F=52)2^{3}P_{2} (F=\frac{5}{2})23P2​(F=25​) levels of He3{\mathrm{He}}^{3}He3 is also observed. The excited-state linewidth as a function of pressure leads to a radiative decay time of $\ensuremath{\tau}\ensuremath{\approx}0.96\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ sec and a mixing cross section of 53\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2. The alignment of the 33P3^{3}P33P level of He4{\mathrm{He}}^{4}He4 is also observed by monitoring polarization changes of the 3889 \AA{} line.
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.
It has been observed experimentally that the energy spacing between the magnetic sublevels of the 23S12^{3}S_{1}23S1​ state of helium as detected by optical pumping varies with the intensity and character of the resonant radiation. The relative shift as a function of the intensity, polarization, and spectral distribution are observed and measured. The energy shifts are on the order of 50-60 cps and become smaller as the intensity of the light decreases. A second lamp containing He3{\mathrm{He}}^{3}He3 incident on the absorption cell in the same manner as the pumping lamp also produces an energy shift. An explanation of the light shift in metastable helium is discussed.
We have experimentally detected a signal of magnetic resonance (MR) from 85Rb atoms in the 52 S 1/2 state by measuring the absorption of light from a helium lamp producing optical orientation of metastable 23 S 1 4He atoms in a cell containing alkali metal vapor and gaseous helium at a pressure of 1 Torr. The ratio of MR signal amplitudes in Rb and He atoms did not exceed 10–2. Peculiarities of the formation and observation of the collisional polarization of Rb atoms in alkali–helium plasma are discussed.
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 effects of spin-exchange collisions between thermal electrons and optically oriented metastable helium (23S1{2}^{3}{S}_{1}23S1​) atoms have been observed in a flowing helium afterglow, and the exchange cross section has been measured. The source of electrons derives from Penning collisions between the 19.8-eV metastable atoms and impurity atoms which are titrated into the flow region. The presence of thermal electrons shortens the metastable-spin-state lifetime and reduces the magnitude of the optical-pumping resonance signal. The electron density is determined from the reduction of the metastable density. The exchange cross section is found to be 1.6\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ cm2{\mathrm{cm}}^{2}cm2.