Soviet Journal of Low Temperature Physics

Kinetics of oriented triplet metastable helium states in gas and liquid phases

D. A. Myshkis, E. G. Luzan

Published Dec 1, 1980• 0 citations

Abstract

The kinetics of oriented metastable states is examined in a helium plasma afterglow at T ≈ 10 °K and in superfliud helium. The initial conditions given by preliminary optical orientation under total mixing conditions are extended for both the cases under consideration. Their total spin is conserved in bulk processes of destruction of metastable states and the orientation reaches unity. Results are presented for a computation of the kinetics on an electronic computer. The time to achieve complete orientation is less than 10 msec for concentrations of He(23S1) and He2(23Σu+) ≈ 1012 cm3. In the gas phase, the behavior of the inverse concentration is linear in time for a zero initial orientation. In a liquid the features of the kinetics and the optical orientation are related to the formation of microbubbles around the metastable states and electrons. This results in nonmonotonic behavior of the concentration components with zero spin projection.

DOI: 10.1063/10.0030186

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Soviet Journal of Low Temperature Physics

Kinetics of oriented triplet metastable helium states in gas and liquid phases

D. A. Myshkis, E. G. Luzan

Published Dec 1, 1980• 0 citations

Publisher

Abstract

DOI: 10.1063/10.0030186

Related papers

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

L. D. Schearer

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{}${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$ .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{}${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$.

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Victor A. Kartoshkin, G. V. Klement’ev

OptSpMar 1, 19960 citations

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}

We have studied the dynamic properties of the

\mathrm{He}(2^{3}S)

and the ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ atomic and molecular metastable states of helium that are produced in electron-bombarded superfluid helium. The steady-state concentration of ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules is shown to be limited by collisions between pairs of these molecules. Measurements of the rate coefficient for this bimolecular reaction as a function of temperature and pressure show that the reaction is diffusion limited and that the diffusion coefficient for the metastable molecules in the superfluid is determined by ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$-roton collisions. The bilinear destruction of metastable molecules has been monitored for times as long as 0.1 sec after the initial population of the state, and this establishes a lower bound of 0.1 sec on the radiative lifetime of the $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ state in superfluid helium. The lifetime and decay mode of the

\mathrm{He}(2^{3}S)

atoms are distinctly different from that of the corresponding ${\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecule. In particular, the characteristic lifetime of the

\mathrm{He}(2^{3}S)

state in superfluid helium is essentially independent of the temperature between 2.08 and 1.4 \ifmmode^\circ\else\textdegree\fi{}K and is measured to be about 15 \ensuremath{\mu}sec. Our experimental observations indicate that the destruction of metastable atoms results in the population of high vibrational levels of the $a^{3}\ensuremath{\Sigma}_{u}^{+}$ molecular state. However, it is shown that the production of $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules responds immediately to the electron beam (response time 3 \ensuremath{\mu}sec) and thus the formation of this state in the liquid does not depend upon the destruction of metastable atoms. Despite the fact that the lifetimes and decay modes of the metastable

\mathrm{He}(2^{3}S)

atoms and $(v=0) {\mathrm{He}}_{2}(a^{3}\ensuremath{\Sigma}_{u}^{+})$ molecules are not related, the production rates for these states are nearly equal and correspond to about 450 and 500 per incident 160-keV electron, respectively.

We consider the decay kinetics of a trapped spin-polarized gas of metastable triplet helium

^{4}\mathrm{He}(2^{3}S

at ultralow temperatures. The sample lifetime is found to be determined by spin relaxation and Penning ionization, both induced by spin-dipole interaction in pair collisions. The rates of these processes are calculated. The Penning ionization proves to be 5 orders of magnitude slower than in the unpolarized case. The results indicate that spin-polarized triplet helium is a promising candidate for Bose-Einstein condensation.

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.