Reaction dynamics of excited states of helium and magneto-optical trapping of helium metastable atoms

[摘要] The temperature dependence of conversion of $m He(2sp3Ssb1)$ metastable atoms to $m Hesb2(asp3Sigmasbsp{u}{+})$ metastable molecules in the three-body reaction $m He(2sp3Ssb1)+ 2He(1sp1Ssb0)o Hesb2(asp3Sigmasbsp{u}{+}) + He(1sp1Ssb0)$ 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 $m He(2sp3Ssb1)-He(1sp1Ssb0)$ 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$sb{m s}($T) is accurately described by $m ksb{s}(T) = lbrack 87.4 T exp({-750}/T) + 4.1 T exp({-200}/T)brack imes 10sp{-37} cmsp{-6}secsp{-1}$. The first activation energy, 750 $pm$ 70K (63 $pm$ 6meV), is equal to the known $m He(2sp3Ssb1)$-$m He(1sp1Ssb0)$ repulsive barrier height. The second activation energy is 17 $pm$ 2 meV.The temperature dependences of the rate constants for collision-induced mixing among $m He(2sp3Psb{J,msb{J}})$ levels, and for conversion of He(2$sp3$P) atoms to $m Hesb2(bsp3Pisb{g})$ molecules in the three body reaction $m He(2sp3P) + 2He(1sp1S)o Hesb2(bsp3Pisb{g}) + He(1sp1S)$ have been investigated over the range 1.4$sim$300K. The measured thermally-averaged cross section for $m He(2sp3Psb{J,msb{J}})$ mixing in collisions with ground state helium atoms are described by the function $msigmasb{pm}(T) = (4.4 + 20.6/Tsp{1/3})imes 10sp{-15}cmsp2,$ and can be understood in terms of Langevin theory. The measured rate coefficients for the three body reaction exhibit a strong inverse temperature dependence, $m ksb{p}(T) = (0.04 + 2.18/T)imes 10sp{-30} cmsp6{cdot}ssp{-1},$ which suggests that, unlike conversion of $m He(2sp3Ssb1)$ to $m Hesb2(asp3Sigmasbsp{u}{+}),$ 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 $m sp{4,3}He(2sp3S)$ - $sp{4,3}$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.

[发布机构] Rice University

[效力级别] physics