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, Lg=8.9(8)×{}10^-39 cm⁶ s^-1 and Lₛ=1.2(4)×{}10^-24 cm⁴ s^-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)×{}10^-15 cm³ s^-1exp(-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 ³-⁴He 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×{}10¹⁸ atoms?^-3 at 0.55 K (pure ⁴He walls), and 1.4×{}10¹⁸ atoms?^-3 at 0.19 K (8 at. ? sup 3---He).