Measurement of Pressure of Gaseous H↓: Adsorption Energies and Surface Recombination Rates on Helium

Pressure decay measurements are used to study adsorption and recombination of atomic hydrogen on liquid helium surfaces below 1 K. Recombination is suppressed by five orders of magnitude by applying a 10-T magnetic field. The surface-recombination-rate constant ($K_s$) and the adsorption energy (${\epsilon }_a$) of H↓ on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ are $\frac{K_s{B}^2}{T^{\frac{1}{2}}}=(2.6\mathrm{}\pm 0.8)\times {10}^{-7\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{T}}^2$ ${\mathrm{K}}^{-1/2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ and $\frac{{\epsilon }_a}{k_{\mathrm{B}}}=0.89\mathrm{}\pm 0.07$ K. An estimate of $\frac{{\epsilon }_a}{k_{\mathrm{B}}}=0.59\mathrm{}$ K is obtained for H↓ on a dilute ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixture.