Relaxation and recombination in spin-polarized atomic hydrogen

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, $L_g$=8.9(8)×${10}^{\mathrm{-}39}$ ${\mathrm{cm}}^6$ ${\mathrm{s}}^{\mathrm{-}1}$ and $L_s$=1.2(4)×${10}^{\mathrm{-}24}$ ${\mathrm{cm}}^4$ ${\mathrm{s}}^{\mathrm{-}1}$. They decrease slowly with field and $L_s$ 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 $G^{\mathrm{bc}}$=1.03(7)×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$exp(-$E_{\mathrm{bc}}$/$k_B$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 ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He surfaces indicate that as little as one monolayer of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ on the liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ surface appreciably decreases the adsorption energy of atomic hydrogen. Densities achieved include 4.5×${10}^{18}$ atoms${\mathrm{?}}^{\mathrm{-}3}$ at 0.55 K (pure ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ walls), and 1.4×${10}^{18}$ atoms${\mathrm{?}}^{\mathrm{-}3}$ at 0.19 K (8 at. ? sup 3—He).