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Reaction Dynamics of Excited States of Helium and Magneto-Optical Trapping of Helium Metastable Atoms.

Xinxin Zhao

Published Jan 1, 1993• 0 citations
OpenAlex

Abstract

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 $\sim$10$\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.

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The temperature dependence of conversion of He(2\sp3S\sb1)\rm He(2\sp3S\sb1)He(2\sp3S\sb1) metastable atoms to He\sb2(a\sp3Σ\sbspu+)\rm He\sb2(a\sp3\Sigma\sbsp{u}{+})He\sb2(a\sp3Σ\sbspu+) metastable molecules in the three-body reaction He(2\sp3S\sb1)+2He(1\sp1S\sb0)→He\sb2(a\sp3Σ\sbspu+)+He(1\sp1S\sb0)\rm He(2\sp3S\sb1)+ 2He(1\sp1S\sb0)\to He\sb2(a\sp3\Sigma\sbsp{u}{+}) + He(1\sp1S\sb0)He(2\sp3S\sb1)+2He(1\sp1S\sb0)→He\sb2(a\sp3Σ\sbspu+)+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 He(2\sp3S\sb1)−He(1\sp1S\sb0)\rm He(2\sp3S\sb1)-He(1\sp1S\sb0)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\sbs(\sb{\rm s}(\sbs(T) is accurately described by k\sbs(T)=[87.4 T exp(−750/T)+4.1T exp(−200/T)]×10\sp−37 cm\sp−6sec\sp−1\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}k\sbs(T)=[87.4 T exp(−750/T)+4.1T exp(−200/T)]×10\sp−37 cm\sp−6sec\sp−1. The first activation energy, 750 ±\pm± 70K (63 ±\pm± 6meV), is equal to the known He(2\sp3S\sb1)\rm He(2\sp3S\sb1)He(2\sp3S\sb1)-He(1\sp1S\sb0)\rm He(1\sp1S\sb0)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 He(2\sp3P\sbJ,m\sbJ)\rm He(2\sp3P\sb{J,m\sb{J}})He(2\sp3P\sbJ,m\sbJ) levels, and for conversion of He(2\sp3\sp3\sp3P) atoms to He\sb2(b\sp3Π\sbg)\rm He\sb2(b\sp3\Pi\sb{g})He\sb2(b\sp3Π\sbg) molecules in the three body reaction He(2\sp3P)+2He(1\sp1S)→He\sb2(b\sp3Π\sbg)+He(1\sp1S)\rm He(2\sp3P) + 2He(1\sp1S)\to He\sb2(b\sp3\Pi\sb{g}) + He(1\sp1S)He(2\sp3P)+2He(1\sp1S)→He\sb2(b\sp3Π\sbg)+He(1\sp1S) have been investigated over the range 1.4∼\sim∼300K. The measured thermally-averaged cross section for He(2\sp3P\sbJ,m\sbJ)\rm He(2\sp3P\sb{J,m\sb{J}})He(2\sp3P\sbJ,m\sbJ) mixing in collisions with ground state helium atoms are described by the function σ\sbpm(T)=(4.4+20.6/T\sp1/3)×10\sp−15cm\sp2,\rm\sigma\sb{pm}(T) = (4.4 + 20.6/T\sp{1/3})\times 10\sp{-15}cm\sp2,σ\sbpm(T)=(4.4+20.6/T\sp1/3)×10\sp−15cm\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, k\sbp(T)=(0.04+2.18/T)×10\sp−30 cm\sp6⋅s\sp−1,\rm k\sb{p}(T) = (0.04 + 2.18/T)\times 10\sp{-30}\ cm\sp6{\cdot}s\sp{-1},k\sbp(T)=(0.04+2.18/T)×10\sp−30 cm\sp6⋅s\sp−1, which suggests that, unlike conversion of He(2\sp3S\sb1)\rm He(2\sp3S\sb1)He(2\sp3S\sb1) to He\sb2(a\sp3Σ\sbspu+),\rm He\sb2(a\sp3\Sigma\sbsp{u}{+}),He\sb2(a\sp3Σ\sbspu+), there is no activation energy required for this reaction. A magneto-optical trap for helium 2\sp3\sp3\sp3S metastable atoms has been designed and constructed, utilizing superconducting magnet gradient coils and a Ti:Sapphire ring laser for pumping the helium 2\sp3\sp3\sp3S-2\sp3\sp3\sp3P transition. He(2\sp3\sp3\sp3S) 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\sp3\sp3S) 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 ∼\sim∼10\sp6\sp6\sp6 atoms are trapped, with a trap lifetime of about 0.2 sec limited by He(2\sp3\sp3\sp3S) - He(2\sp3\sp3\sp3P) 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 \sp4,3He(2\sp3S)\rm \sp{4,3}He(2\sp3S)\sp4,3He(2\sp3S) - \sp4,3\sp{4,3}\sp4,3He(2\sp3\sp3\sp3S) and resonantly-enhanced He(2\sp3\sp3\sp3S) - He(2\sp3\sp3\sp3P) Penning reactions in the ultra-cold quantum regime, and perhaps the He(2\sp3\sp3\sp3S) natural lifetime.
About 10 4 triplet metastable helium atoms have been trapped in a magneto-optical trap, based on the 2 3 S 1 –2 3 P 2 transition at 1.083 μm (volume 0.1 mm 3 , temperature 1 mK). The losses of the trap are analysed by monitoring the ion current due to Penning collisions either with background gases, or between trapped atoms. The rate coefficient for these collisions between trapped metastable helium atoms has been found to be unusually large, and the crucial role of the quasi-resonant laser light has been experimentally demonstrated. The rate coefficient of the dominant process (He*(2 3 S 1 ) + He*(2 3 P 2 ) Penning collisions) has been measured by two different methods, and found to be K sp ≃ 10 -7 cm 3 s -1 within an estimated factor of 4. The corresponding cross-section is 6 · 10 6 Å 2 .
We have studied the dynamic properties of the He(23S)\mathrm{He}(2^{3}S)He(23S) 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 He(23S)\mathrm{He}(2^{3}S)He(23S) 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 He(23S)\mathrm{He}(2^{3}S)He(23S) 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 He(23S)\mathrm{He}(2^{3}S)He(23S) 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.
A magneto-optical trap for He(2{sup 3}S) metastable atoms has been constructed, utilizing superconducting magnetic gradient coils and a Ti:Sapphire ring laser for pumping the helium 2{sup 3}S-2{sup 3}P transition. The He(2{sup 3}S) atoms are produced by a weak rf discharge in helium gas at a temperature of 1.4K. The discharge products flow through a small orifice into the trap cell, where a fraction of the He(2{sup 3}S) atoms are trapped and ground state helium atoms are rapidly cryopumped by zeolite pellets that cover most of the cell bottom. Preliminary experiments suggest that {approximately}10{sup 6} atoms are trapped in a small volume at {approximately}1 mK, with a trap lifetime of 10-100 msec limited by resonantly-enhanced He(2{sup 3}S)-He(2{sup 3}P) Penning reactions. Ultimately, it is estimated that a substantial number of atoms can be held at ultra-low temperature in near-perfect vacuum, in a dc magnetic trap. Measurements of decay times of the trapped atoms should yield rate coefficients in the quantum regime for He(2{sup 3}S)-He(2{sup 3}P) and He(2{sup 3}S)-He(2{sup 3}S) Penning reactions, and perhaps the He(2{sup 3}S) radiative lifetime.
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{}${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2 at 500\ifmmode^\circ\else\textdegree\fi{}K to less than ${10}^{\ensuremath{-}17}$ 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.
We have constructed a magneto-optical trap (MOT) for metastable triplet helium atoms utilizing the $2{}^{3}{S}_{1}\ensuremath{\rightarrow}3{}^{3}{P}_{2}$ line at 389 nm as the trapping and cooling transition. The far-red-detuned MOT (detuning $\ensuremath{\Delta}=\ensuremath{-}41\mathrm{MHz})$ typically contains few times 107{10}^{7}107 atoms at a relatively high $(\ensuremath{\sim}{10}^{9}{\mathrm{cm}}^{\ensuremath{-}3})$ density, which is a consequence of the large momentum transfer per photon at 389 nm and a small two-body loss rate coefficient $(2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}<\ensuremath{\beta}<1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}).$ The two-body loss rate is more than five times smaller than in a MOT on the commonly used $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}$ line at 1083 nm. Furthermore, laser cooling at 389 nm results in temperatures somewhat lower than those achieved using 1083 nm. The 389-nm MOT exhibits small losses due to two-photon ionization, which have been investigated as well.
Energy distributions of He+{\mathrm{He}}^{+}He+ and He2+{\mathrm{He}}_{2}^{+}He2+​ ions produced in ultracold He(23S1)+He(23P2)\mathrm{He}(2{}^{3}{S}_{1})+\mathrm{He}(2{}^{3}{P}_{2})He(23S1​)+He(23P2​) collisions are presented. These helium metastable atoms are trapped and cooled in a magneto-optical trap, and have a temperature of $1.1\ifmmode\pm\else\textpm\fi{}0.2 \mathrm{mK}.$ Initial kinetic energies of the product He+{\mathrm{He}}^{+}He+ and He2+{\mathrm{He}}_{2}^{+}He2+​ ions are analyzed in energy, using a weak electrostatic extraction field in combination with a time-of-flight technique. A pronounced structure is observed in the He+{\mathrm{He}}^{+}He+ ion distribution, which can be well explained by a theoretical model that considers the avoided crossings between only three metastable ${}^{3}{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$ states of the transient He2{\mathrm{He}}_{2}He2​ molecule. The measured He2+{\mathrm{He}}_{2}^{+}He2+​ distribution has a width that is consistent with the maximum recoil energy of 1.1 meV.