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Physical Review A

Inelastic decay processes in a gas of spin-polarized triplet helium

Peter O. Fedichev, M. W. Reynolds, U. M. Rahmanov, Georgy V. Shlyapnikov

Published Mar 1, 1996• 69 citations
Publisher

Abstract

We calculate the rates of elastic and inelastic collisions in a gas of spin-polarized triplet helium ${(}^{4}$${\mathrm{He}}^{\mathrm{*}}$\ensuremath{\uparrow}) in a wide range of temperatures (0T0.5 K) and magnetic fields (0B100 kG), which includes all values of T and B relevant for trapped $^{4}\mathrm{He}^{\mathrm{*}}$\ensuremath{\uparrow}. At densities n\ensuremath{\lesssim}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, where three-body recombination is not important, the main inelastic processes are spin relaxation and relaxation-induced ionization, both induced by the spin-dipole interaction in pair collisions. For ultralow temperatures T\ensuremath{\lesssim}10 mK and low magnetic fields B\ensuremath{\lesssim}100 G, the leading decay channel is relaxation-induced ionization. If either T or B is higher, the gas decays through spin relaxation. The ratio of elastic to inelastic rate is found to be large even in the nanokelvin regime, which is promising for evaporative cooling of $^{4}\mathrm{He}^{\mathrm{*}}$\ensuremath{\uparrow}. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.53.1447

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We calculate the rates of elastic and inelastic collisions in a gas of spin-polarized triplet helium {(}^{4}$${\mathrm{He}}^{\mathrm{*}}\ensuremath{\uparrow}) in a wide range of temperatures (0T0.5 K) and magnetic fields (0B100 kG), which includes all values of T and B relevant for trapped 4He∗^{4}\mathrm{He}^{\mathrm{*}}4He∗\ensuremath{\uparrow}. At densities n\ensuremath{\lesssim}1013{10}^{13}1013 cm\ensuremath−3{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}cm\ensuremath−3, where three-body recombination is not important, the main inelastic processes are spin relaxation and relaxation-induced ionization, both induced by the spin-dipole interaction in pair collisions. For ultralow temperatures T\ensuremath{\lesssim}10 mK and low magnetic fields B\ensuremath{\lesssim}100 G, the leading decay channel is relaxation-induced ionization. If either T or B is higher, the gas decays through spin relaxation. The ratio of elastic to inelastic rate is found to be large even in the nanokelvin regime, which is promising for evaporative cooling of 4He∗^{4}\mathrm{He}^{\mathrm{*}}4He∗\ensuremath{\uparrow}. \textcopyright{} 1996 The American Physical Society.
We consider the decay kinetics of a trapped spin-polarized gas of metastable triplet helium 4He(23S^{4}\mathrm{He}(2^{3}S4He(23S 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.
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We have studied relaxation and recombination processes in compressed, doubly polarized atomic hydrogen at temperatures from 0.13 to 0.60 K and magnetic fields from 3 to 9 T. The gas and surface dipole three-body recombination rate constants at a field of 7.6 T are measured to be, respectively, Lg{L}_{g}Lg​=8.9(8)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}39}$ cm6{\mathrm{cm}}^{6}cm6 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and Ls{L}_{s}Ls​=1.2(4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}24}$ cm4{\mathrm{cm}}^{4}cm4 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. They decrease slowly with field and Ls{L}_{s}Ls​ exhibits no significant angular dependence. The three-body recombination rate due to hyperfine mixing has also been measured. Electronic and nuclear relaxation rates have been measured; the b-c electronic relaxation rate constant in the gas is Gbc{G}^{\mathrm{bc}}Gbc=1.03(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$exp(-Ebc{E}_{\mathrm{bc}}Ebc​/kB{k}_{B}kB​T). The temperature and field dependence of the nuclear relaxation rate in the gas are observed to be in excellent agreement with recent theoretical calculations. Three-body surface recombination-rate measurements using $^{3}\mathrm{\ensuremath{-}}^{4}$He surfaces indicate that as little as one monolayer of 3He^{3}\mathrm{He}3He on the liquid 4He^{4}\mathrm{He}4He surface appreciably decreases the adsorption energy of atomic hydrogen. Densities achieved include 4.5\ifmmode\times\else\texttimes\fi{}1018{10}^{18}1018 atoms${\mathrm{?}}^{\mathrm{\ensuremath{-}}3}$ at 0.55 K (pure 4He^{4}\mathrm{He}4He walls), and 1.4\ifmmode\times\else\texttimes\fi{}1018{10}^{18}1018 atoms${\mathrm{?}}^{\mathrm{\ensuremath{-}}3}$ at 0.19 K (8 at. ? sup 3---He).
A calculation is presented of the relaxation rate ${{T}_{1}}^{\ensuremath{-}1}$ between the lowest two hyperfine states of a dilute gas of H atoms in high magnetic field at low temperatures. Dipole-dipole interactions dominate ${{T}_{1}}^{\ensuremath{-}1}$, leading to long relaxation times at low temperatures (${{T}_{1}}^{\ensuremath{-}1}\ensuremath{\approx}{10}^{\ensuremath{-}2}{n}_{\mathrm{H}}{T}^{\frac{1}{2}}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ at $H\ensuremath{\approx}100$ kG). The recombination rate due to wall collisions is much greater than ${{T}_{1}}^{\ensuremath{-}1}$ and can lead to a depletion of the lowest hyperfine state from thermal equilibrium and an effective recombination rate equal to ${{T}_{1}}^{\ensuremath{-}1}$.
We report a combined experimental and theoretical study of collision-induced dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use buffer gas cooling to create trapped samples of 14N^{14}\mathrm{N}14N and 15N^{15}\mathrm{N}15N atoms with densities $(5\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}{10}^{12} {\mathrm{cm}}^{\ensuremath{-}3}$ and measure their magnetic relaxation rates at milli-Kelvin temperatures. These measurements, together with rigorous quantum scattering calculations based on accurate ab initio interaction potentials for the ${}^{7}{\ensuremath{\Sigma}}_{u}^{+}$ electronic state of N2{\mathrm{N}}_{2}N2​ demonstrate that dipolar relaxation in N+N\mathrm{N}+\mathrm{N}N+N collisions occurs at a slow rate of $~{10}^{\ensuremath{-}13} {\mathrm{cm}}^{3}$/s over a wide range of temperatures (1 mK to 1 K) and magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are insensitive to small variations of the interaction potential and to the magnitude of the spin-exchange interaction, enabling the accurate calibration of the measured N atom density. We find consistency between the calculated and experimentally determined rates. Our results suggest that N atoms are promising candidates for future experiments on sympathetic cooling of molecules.
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