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Physical Review

Temperature Dependence of Helium Metastability Exchange Cross Section

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

Published Jul 20, 1964• 51 citations
Publisher

Abstract

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.

DOI
10.1103/physrev.135.a353

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Spin polarization transfer in ground and metastable helium atom collisions

D. Vrinceanu, H. R. Sadeghpour

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New Journal of Physics•Jun 28, 2010•7 citations
The cross section for exchange of metastability in collisions between He3{\mathrm{He}}^{3}He3 atoms in the ground (11S01^{1}S_{0}11S0​) state and metastable (23S12^{3}S_{1}23S1​) 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{10}^{\ensuremath{-}16}10\ensuremath−16 cm2{\mathrm{cm}}^{2}cm2 at 500\ifmmode^\circ\else\textdegree\fi{}K to less than 10\ensuremath−17{10}^{\ensuremath{-}17}10\ensuremath−17 cm2{\mathrm{cm}}^{2}cm2 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 temperature dependence of conversion of $\rm He(2\sp3S\sb1)$ metastable atoms to $\rm He\sb2(a\sp3\Sigma\sbsp{u}{+})$ metastable molecules in the three-body reaction $\rm He(2\sp3S\sb1)+ 2He(1\sp1S\sb0)\to He\sb2(a\sp3\Sigma\sbsp{u}{+}) + He(1\sp1S\sb0)$ has been investigated over the temperature range 65K-700K. This reaction is thermally activated as a consequence of a long range repulsive barrier in the $\rm He(2\sp3S\sb1)-He(1\sp1S\sb0)$ interaction potential. The data reveal that there are two reaction channels with distinctly different activation energies. The temperature dependence of the measured rate coefficient k$\sb{\rm s}($T) is accurately described by $\rm k\sb{s}(T) = \lbrack 87.4\ T\ exp({-750}/T) + 4.1 T\ exp({-200}/T)\rbrack \times 10\sp{-37}\ cm\sp{-6}sec\sp{-1}$. The first activation energy, 750 ±\pm± 70K (63 ±\pm± 6meV), is equal to the known $\rm He(2\sp3S\sb1)$-$\rm He(1\sp1S\sb0)$ repulsive barrier height. The second activation energy is 17 ±\pm± 2 meV. The temperature dependences of the rate constants for collision-induced mixing among $\rm He(2\sp3P\sb{J,m\sb{J}})$ levels, and for conversion of He(2$\sp3$P) atoms to $\rm He\sb2(b\sp3\Pi\sb{g})$ molecules in the three body reaction $\rm He(2\sp3P) + 2He(1\sp1S)\to He\sb2(b\sp3\Pi\sb{g}) + He(1\sp1S)$ have been investigated over the range 1.4$\sim$300K. The measured thermally-averaged cross section for $\rm He(2\sp3P\sb{J,m\sb{J}})$ mixing in collisions with ground state helium atoms are described by the function $\rm\sigma\sb{pm}(T) = (4.4 + 20.6/T\sp{1/3})\times 10\sp{-15}cm\sp2,$ and can be understood in terms of Langevin theory. The measured rate coefficients for the three body reaction exhibit a strong inverse temperature dependence, $\rm k\sb{p}(T) = (0.04 + 2.18/T)\times 10\sp{-30}\ cm\sp6{\cdot}s\sp{-1},$ which suggests that, unlike conversion of $\rm He(2\sp3S\sb1)$ to $\rm He\sb2(a\sp3\Sigma\sbsp{u}{+}),$ there is no activation energy required for this reaction. A magneto-optical trap for helium 2$\sp3$S metastable atoms has been designed and constructed, utilizing superconducting magnet gradient coils and a Ti:Sapphire ring laser for pumping the helium 2$\sp3$S-2$\sp3$P transition. He(2$\sp3$S) atoms are produced by a weak discharge in helium gas at temperature 1.3K. The discharge products flow through an orifice into the trap cell, where the He(2$\sp3$S) atoms are trapped and ground state helium atoms are rapidly cryopumped by zeolite pellets that cover most of the cell bottom. Preliminary experimental results suggest that $\sim101010\sp6$ atoms are trapped, with a trap lifetime of about 0.2 sec limited by He(2$\sp3$S) - He(2$\sp3$P) Penning reactions. Ultimately, it is estimated that a substantial number of atoms can be trapped and cooled for much longer times in a near-perfect vacuum. Measurements of decay times of the trapped atoms should yield rates for $\rm \sp{4,3}He(2\sp3S)$ - $\sp{4,3}$He(2$\sp3$S) and resonantly-enhanced He(2$\sp3$S) - He(2$\sp3$P) Penning reactions in the ultra-cold quantum regime, and perhaps the He(2$\sp3$S) natural lifetime.
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.
Previous semiempirical determinations by Fitzsimmons et al. of the asymptotic form of the potentials Vg{V}_{g}Vg​ and Vu{V}_{u}Vu​ for the helium molecular states $^{3}\ensuremath{\Sigma}_{g,u}^{+}$ are refined to include new experimental data. The resulting potentials yield cross sections for elastic scattering, diffusion, and metastability exchange in $\mathrm{He}(2^{3}S_{1})\ensuremath{-}\mathrm{H}\mathrm{e}(1^{1}S_{0})$ collisions in agreement with various experiments from 15 to 500 \ifmmode^\circ\else\textdegree\fi{}K.
The cross section for the transfer of excitation from the 23P02^{3}P_{0}23P0​ level of helium to the 23P12^{3}P_{1}23P1​ and 23P22^{3}P_{2}23P2​ levels as a result of collisions with the ground-state helium atom (11S01^{1}S_{0}11S0​) has been measured with a conventional resonance-fluorescence technique. A He4{\mathrm{He}}^{4}He4 source excited the $2^{3}S_{1}\ensuremath{-}2^{3}P_{0}$ transition in a He3{\mathrm{He}}^{3}He3 absorption cell. The scattered light was examined with a high-resolution grating spectrometer and the ratio of the $2^{3}P_{0}\ensuremath{\rightarrow}2^{3}S_{1}$ to the $2^{3}P_{1,2}\ensuremath{\rightarrow}2^{3}S_{1}$ intensities was measured as a function of cell pressure from 0.05 to 10 Torr. The cross section for collision-induced transitions out of the 23P02^{3}P_{0}23P0​ level is found to be (68\ifmmode\pm\else\textpm\fi{}3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2 at room temperature.