Surface studies of spin-polarized atomic hydrogen

Cold collision frequency shift in two-dimensional atomic hydrogen

We report a measurement of the cold collision frequency shift in atomic hydrogen gas adsorbed on the surface of superfluid (4)He at T approximately < 90 mK. Using two-photon electron and nuclear magnetic resonance in 4.6 T field we separate the resonance line shifts due to the dipolar and exchange interactions, both proportional to surface density sigma. We find the clock shift Delta nu(c) = -1.0(1) x 10(-7) Hz cm(-2) x sigma, which is about 100 times smaller than the value predicted by the mean field theory and known scattering lengths in the three-dimensional case.

Adsorption and two-body recombination of atomic hydrogen on 3He-4He mixture films

We present the first systematic measurement of the binding energy E(a) of hydrogen atoms to the surface of saturated 3He- 4He mixture films at temperatures 70-400 mK. 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 3He grows from zero to 6x10(14) cm(-2), yielding the value 1.2(1)x10(-15) K cm(2) for the mean-field parameter of H- 3He interaction in 2D. 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)/K(aa) to be independent of the 3He content and to grow with temperature.