Adsorption and two-body recombination of atomic hydrogen on ${}^3$He-${}^4$He mixture films

We present the first systematic measurement of the binding energy $E_a$ of hydrogen atoms to the surface of saturated ${}^3$He-${}^4$He mixture films. $E_a$ is found to decrease almost linearly from 1.14(1) K down to 0.39(1) K, when the population of the ground surface state of ${}^3$He grows from zero to $6\times {10}^{14}$ cm${}^{-2}$, yielding the value $1.2(1)\times {10}^{-15}$ K cm${}^2$ for the mean-field parameter of H-${}^3$He interaction in 2D. The experiments were carried out with overall ${}^3$He concentrations ranging from 0.1 ppm to 5 % as well as with commercial and isotopically purified ${}^4$He at temperatures 70…400 mK. Measuring by ESR the rate constants $K_{aa}$ and $K_{ab}$ for second-order recombination of hydrogen atoms in hyperfine states $a$ and $b$ we find the ratio $K_{ab}\mathrm{/}{K}_{aa}$ to be independent of the ${}^3$He content and to grow with temperature.