Physical Review

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

L. D. Schearer

Published Jul 5, 1968• 11 citations

Abstract

The effects of spin-exchange collisions between thermal electrons and optically oriented metastable helium (${2}^{3}{S}_{1}$) 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{}${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$.

DOI: 10.1103/physrev.171.81

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Temperature Dependence of Helium Metastability Exchange Cross Section

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Physical Review

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

L. D. Schearer

Published Jul 5, 1968• 11 citations

Publisher

Abstract

The effects of spin-exchange collisions between thermal electrons and optically oriented metastable helium (

{2}^{3}{S}_{1}

) 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}

{\mathrm{cm}}^{2}

DOI: 10.1103/physrev.171.81

Related papers

Temperature Dependence of Helium Metastability Exchange Cross Section

F. D. Colegrove, L. D. Schearer, G. K. Walters

The cross section for exchange of metastability in collisions between ${\mathrm{He}}^{3}$ atoms in the ground ( $1^{1}S_{0}$ ) state and metastable ( $2^{3}S_{1}$ ) state has been measured as a function of temperature by optical pumping techniques. The exchange cross section decreases from about 5.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 500\ifmmode^\circ\else\textdegree\fi{}K to less than ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 4.2\ifmmode^\circ\else\textdegree\fi{}K. These results suggest a thermally activated exchange process in qualitative agreement with the predicted existence of a long-range repulsive interaction between metastable and ground-state helium atoms as proposed by Buckingham and Dalgarno.The cross section for exchange of metastability in collisions between ${\mathrm{He}}^{3}$ atoms in the ground ($1^{1}S_{0}$) state and metastable ($2^{3}S_{1}$) state has been measured as a function of temperature by optical pumping techniques. The exchange cross section decreases from about 5.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at 500\ifmmode^\circ\else\textdegree\fi{}K to less than ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 4.2\ifmmode^\circ\else\textdegree\fi{}K. These results suggest a thermally activated exchange process in qualitative agreement with the predicted existence of a long-range repulsive interaction between metastable and ground-state helium atoms as proposed by Buckingham and Dalgarno.

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A spin-exchange optical pumping experiment to study collisions between cesium atoms and quasifree electrons is reported. In this experiment, electrons in a weak magnetic field were polarized through spin-exchange collisions with optically pumped cesium atoms. The cesium-electron collisions were the principal source of the electron-resonance linewidth, and they also gave rise to a shift in the center frequency of the electron resonance. The magnitudes of the linewidth and frequency shift depend upon the two-body scattering amplitude for elastic collisions, the cesium polarization, and the cesium density. From measurements of the electron linewidth and the frequency shift, a value is derived for the electron-cesium spin-flip cross section. The spin-flip cross section at 20\ifmmode^\circ\else\textdegree\fi{}C is found to be approximately 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$ . The expressions for the linewidth and frequency shift due to spin-exchange collisions of electrons with alkali-metal atoms are generalized to cover the case where there is a spin-orbit interaction between the electron-alkali-metal-atom systems.A spin-exchange optical pumping experiment to study collisions between cesium atoms and quasifree electrons is reported. In this experiment, electrons in a weak magnetic field were polarized through spin-exchange collisions with optically pumped cesium atoms. The cesium-electron collisions were the principal source of the electron-resonance linewidth, and they also gave rise to a shift in the center frequency of the electron resonance. The magnitudes of the linewidth and frequency shift depend upon the two-body scattering amplitude for elastic collisions, the cesium polarization, and the cesium density. From measurements of the electron linewidth and the frequency shift, a value is derived for the electron-cesium spin-flip cross section. The spin-flip cross section at 20\ifmmode^\circ\else\textdegree\fi{}C is found to be approximately 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$. The expressions for the linewidth and frequency shift due to spin-exchange collisions of electrons with alkali-metal atoms are generalized to cover the case where there is a spin-orbit interaction between the electron-alkali-metal-atom systems.

Physical ReviewOct 5, 196431 citations

The effects of spin-exchange collisions between thermal electrons and optically oriented metastable helium (

{2}^{3}{S}_{1}

{10}^{\ensuremath{-}14}

{\mathrm{cm}}^{2}

The cross section for exchange of metastability in collisions between

{\mathrm{He}}^{3}

atoms in the ground (

1^{1}S_{0}

) state and metastable (

2^{3}S_{1}

) state has been measured as a function of temperature by optical pumping techniques. The exchange cross section decreases from about 5.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$

{\mathrm{cm}}^{2}

at 500\ifmmode^\circ\else\textdegree\fi{}K to less than ${10}^{\ensuremath{-}17}$

{\mathrm{cm}}^{2}

at 4.2\ifmmode^\circ\else\textdegree\fi{}K. These results suggest a thermally activated exchange process in qualitative agreement with the predicted existence of a long-range repulsive interaction between metastable and ground-state helium atoms as proposed by Buckingham and Dalgarno.

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.

{\mathrm{cm}}^{2}

. The expressions for the linewidth and frequency shift due to spin-exchange collisions of electrons with alkali-metal atoms are generalized to cover the case where there is a spin-orbit interaction between the electron-alkali-metal-atom systems.