Three-body recombination in spin-polarized hydrogen has been observed. Doubly polarized hydrogen was compressed by a piston into a thin cell and the temperature and pressure were independently monitored. Measurements were carried out in the temperature range 0.3 to 0.45 K in a magnetic field of 8.0 T at densities up to 1.5 xlO 18 cm" 3 . The results are in good agreement with the theory of Kagan, Vartanyants and Shlyapnikov.

This paper presents, in tabular form, the electronic factor of the electric monopole conversion probability as a function of the atomic charge number Z and transition energy k for K, LI, and LII shells. The table lists factors over a range of k/mc2 from 0.1 to 5.0 units in increments of 0.1 units and from Z = 40 to Z = 102 in steps of ΔZ = 2. The factors are calculated relativistically and include corrections for finite nuclear size and bound state atomic screening.

%e 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~=8.9(8))&10 ' cm s ' and 1. , =1.2{4})(10 cm s '. They decrease slowly with field and I. , 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 6 = 1.03(7) X 10 cm's 'exp( -E~/k~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 'He-He surfaces indicate that as little as one monolayer of 3He on the liquid He surface appreciably decreases the adsorption energy of atomic hydrogen. Densities achieved include 4.5~10' atomscm 3 at 0.55 K (pure He walls}, and 1.4&(10' atoms cm ' at 0.19 K (8 at. % 'He).(K&& --0 since a b bcollision is in -a pure triplet state. )

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) x 10~3 9 cm 6 s _1 and L s = 2.0(6) x 10~2 4 cm 4 s _1 . The large value of L s can account for effects previously attributed to an anomalously large surface two-body nuclear relaxation rate.PACS numbers: 67.40.Fd, 67.70. + n, 68.10JyThe three-body dipole recombination process in spin-polarized hydrogen predicted by Kagan, Vartanyantz, and Shlyapnikov 1 and recently observed experimentally 2,3 places severe limits on the densities which can be achieved in this quantum gas. We have measured how this process varies with temperature and magnetic field. The temperature dependence permits the gas and surface three-body rate constants to be determined separately. The magnetic field dependence of these constants is found to differ from that predicted theoretically. A troublesome problem to the entire study of spinpolarized hydrogen is that the two-body nuclear relaxation rate on a pure 4 He surface reported by several groups, including our own, is about a factor of 50 larger than theories predict. We find that the surface three-body recombination can explain effects which previous work may have erroneously attributed to surface two-body relaxation.The experimental technique is identical to that described earlier 2 ; however, we have achieved an improved signal-to-noise ratio which allows more precise data to be taken over a wider range of temperatures and densities. Electron-and nuclearpolarized atomic hydrogen is compressed into a pancake-shaped cell by a piston. The flat 4 Hecoated walls of the cell are perpendicular to the magnetic field. For these experiments the separation was reduced to 7.5(10) xl0~3 cm. After the compression, the pressure and temperature of the gas are recorded as the hydrogen sample decays at constant volume. The ideal gas law is then used to compute the density n(t) and its time derivative n(t).Measurements were carried out between 0.25 and 0.60 K. Thermal explosions 3,4 limit the compressions at temperatures above this region; heating problems complicate the interpretation of results below it. The highest density achieved in these experiments was 4.5x!0 18 atoms cm -3 at a temperature of 0.57 K.Under our experimental conditions the observed three-body recombination rate constant L is related to the gas and surface rate constants L g and L s by L=L g + (A/V)L s (a/n)\(1)where A / V is the area to volume ratio for the cell and a is the surface density of adsorbed hydrogen atoms. A similar expression relates the two-body relaxation rate constantsFor an isothermal decay under ideal experimental conditions -hln 2 =2G+nL.The intercept at n =0 on such a plot gives 2G and the slope gives L. Figure 1 shows experimental measurements of -n/n 2 as a function of n for two di...

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