Recombination In Spin-polarized Hydrogen Research Articles

Surface recombination in spin-polarized hydrogen: Where does the heat go?

The recombination of two hydrogen atoms to the molecular state is the most energetic chemical reaction per unit mass known. The heat released from such recombination has so far made it impossible to cool a gas of spin-polarized hydrogen (H↓) into the regime of quantum degeneracy. To better understand this problem, we have studied 2nd-order surface recombination in an absorbed gas of H↓ from the standpoint of heat distribution. We have measured an upper bound on the fraction F of the total recombination energy which is deposited locally on the surface at the point of recombination.

Theory of surface spin-exchange recombination of spin-polarized hydrogen

Calculations of the two-atom surface recombination rate for spinpolarized atomic hydrogen are carried out. Both the plane-wave Born approximation (PWBA) and the\((2\tfrac{1}{2})\)-dimensional distorted-wave model are employed. The field dependence forB=4 to 10 Tesla, and the temperature dependence for 0.2<T<1 K are examined for systems initially consisting of equal numbers ofa- andb-state atoms. The rates are analyzed with respect to para- and ortho-H2 production, and to recombination to various vibrational and rotational states of H2. The PWBA gives reasonable agreement with experiment with respect to the field-dependence and ortho-para production, but predicts an overall rate a factor of 40 above experiment. The\((2\tfrac{1}{2})\)-dimensional model underestimates the rate by a factor of ∼ 103 and yields poorer agreement with experiment than the PWBA for the field-dependence and ortho-para production ratio. These results point out the need for a proper many-body treatment.

Theory of surface recombination of spin-polarized hydrogen

A theory is presented, based on the Faddeev equations, for direct two-body recombination of hydrogen atoms on a liquid helium surface. The equations developed are applicable to hydrogen or deuterium atoms in any spin state, but are applied in particular to dipolar recombination ofb state hydrogen atoms. The equations yield terms corresponding to one- and two-step processes. These terms are calculated for low temperatures (T-0.1 to 1.1 K) and high field strengths (B=4 to 14 T). The one-step term increases slowly withB, while the two-step term is rapidly decreasing. While the overall rate is quite small (∼5×10−18 cm2/s) compared to recombination by two-body spin-relaxation, the results have important consequences in understanding the experimentally measured three-atom dipolar surface recombination rates. In three-atom recombination, where the role of spin-relaxation and the two-atom one-step processes are repressed, the role of the underlying two-atom, two-step process is enhanced. The field dependence of the process relevant to the three-atom system is calculated and found to be in fairly good agreement with the experimental three-atom data. The role of possible liquid excitations in enhancing the contribution of the two-step processes is also discussed.

Temperature and Magnetic Field Dependence of Three-Body Recombination in Spin-Polarized Hydrogen

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 ${L}_{g}=7.5(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}39}$ ${\mathrm{cm}}^{6}$ ${\mathrm{s}}^{\ensuremath{-}1}$ and ${L}_{s}=2.0(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{4}$ ${\mathrm{s}}^{\ensuremath{-}1}$. The large value of ${L}_{s}$ can account for effects previously attributed to an anomalously large surface two-body nuclear relaxation rate.

Observation of Three-Body Recombination in Spin-Polarized Hydrogen

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 x 10/sup 18/ cm/sup -3/. The results are in good agreement with the theory of Kagan, Vartanyants, and Shlyapnikov.