The three-body recombination rate in spin-polarized hydrogen has been measured at temperatures from 0.25 to 0.60 K in magnetic fields from 5 to 9 T. In contrast to theoretical expectation, the three-body rate is a weakly decreasing function of field in this region. At 7.6 T the gas and surface three-body rate constants are ${L}_{g}=7.5(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}39}$
cm6 ${\mathrm{s}}^{\ensuremath{-}1}$ and ${L}_{s}=2.0(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$
cm4 ${\mathrm{s}}^{\ensuremath{-}1}$. The large value of
Ls can account for effects previously attributed to an anomalously large surface two-body nuclear relaxation rate.
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=8.9(8)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}39}$
cm6 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and
Ls=1.2(4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}24}$
cm4 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. They decrease slowly with field and
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=1.03(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$exp(-
Ebc/
kBT). 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 on the liquid
4He surface appreciably decreases the adsorption energy of atomic hydrogen. Densities achieved include 4.5\ifmmode\times\else\texttimes\fi{}
1018 atoms${\mathrm{?}}^{\mathrm{\ensuremath{-}}3}$ at 0.55 K (pure
4He walls), and 1.4\ifmmode\times\else\texttimes\fi{}
1018 atoms${\mathrm{?}}^{\mathrm{\ensuremath{-}}3}$ at 0.19 K (8 at. ? sup 3---He).
We calculate the surface dipolar recombination rate
Ls for spin-polarized hydrogen adsorbed on
4He surfaces at temperatures in the 0.2- to 0.6-K regime and for magnetic fields up to 30 T. For a magnetic field of 7.6 T normal to the surface and 0.4 K we find ${L}_{s}=1.3(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}$
cm4 ${\mathrm{s}}^{\ensuremath{-}1}$ increasing by 10%/T in the range of experimental interest. The anisotropy with the direction of the magnetic field is considerably smaller than in the case of the surface dipolar relaxation.
Measurements of the two-body recombination of spin-polarized atomic hydrogen in a magnetic field of 40 kG have been extended to temperatures above 0.5 K. The rate constant for the formation of parahydrogen shows an unexpected increase with temperature, which is explained by inverse predissociation into the
v=14,
J=4 level of
H2. Data indicate the level is bound by 0.7\ifmmode\pm\else\textpm\fi{}0.1 K.
In sufficiently large magnetic fields, the weakly bound levels of H2 acquire finite lifetimes owing to hyperfine predissociation and become manifest as scattering resonances in spin-polarized atomic hydrogen (H↓). In this work we investigate in detail the contribution to the reaction kinetics in H↓ due to the resonance, denoted [Formula: see text], arising from the predissociation of the (v,J) = (14,4) level of H2. To characterize the formation and disintegration of [Formula: see text], we calculate the predissociation as a function of both the applied magnetic field and the dissociation energy, D, of the level. Recombination occurs when [Formula: see text] is stabilized in collisions that cause transitions to lower molecular levels. For low H↓, densities, collisions with atoms of the saturated 4He vapour dominate; we calculate the cross section in the distorted wave Born approximation and also use a coupled-channels approach. At higher densities, stabilization occurs in collisions with H↓ as well; the effect of proton exchange between the atom and the molecule is derived. Previous experiments, analyzed neglecting proton exchange, are reanalyzed including this; the dissociation energy D is not as well determined by the data as was previously thought. Prospects for future experiments are discussed.
We determine the rate of b+b+b surface dipole recombination for hydrogen atoms adsorbed on a
4He film. The calculation is based on the Kagan mechanism adapted to a 2(1/2-dimensional initial state. We find a rate constant ${L}_{s}^{\mathrm{eff}=2.7(7)\ifmmode\times\else\texttimes\fi{}{10}^{\mathrm{\ensuremath{-}}25}}$
cm4 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at B=7.6 T and T=0.4 K, which is roughly a factor of 6 smaller than the experimental results. The magnetic-field dependence also disagrees with experiment. A scaling approach, although leading to the correct order of magnitude, is discussed and shown to be unsatisfactory. The reliability of some of our approximations is estimated. We conclude that the dipole-exchange mechanism is probably responsible for the discrepancies with experiment.