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

Abstract

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 (${\ensuremath{\epsilon}}_{a}$) of H\ensuremath{\downarrow} on $^{4}\mathrm{He}$ are $\frac{{K}_{s}{B}^{2}}{{T}^{\frac{1}{2}}}=(2.6\ifmmode\pm\else\textpm\fi{}0.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{2}$ ${\mathrm{T}}^{2}$ ${\mathrm{K}}^{\ensuremath{-}1/2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ and $\frac{{\ensuremath{\epsilon}}_{a}}{{k}_{\mathrm{B}}}=0.89\ifmmode\pm\else\textpm\fi{}0.07$ K. An estimate of $\frac{{\ensuremath{\epsilon}}_{a}}{{k}_{\mathrm{B}}}=0.59$ K is obtained for H\ensuremath{\downarrow} on a dilute $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ mixture.