Magnetic resonance lineshape of atomic hydrogen on liquid helium surfaces: the sticking probability of H on 4He and 3He

High resolution magnetic resonance studies of the 1420 MHz transition in atomic hydrogen confined in a liquid helium coated container have been performed at temperatures 0.06 < T < 1 K. The lineshape of the free induction decay signal is determined by the relative number of H atoms on the liquid helium surface, by the exchange rate between atoms in the gas and atoms on the surface, and by the hyperfine frequency shift of an H atom in the bound surface state. Using a model with a Poisson distribution for the residency times, it is possible to interpret both the hyperfine frequency shift and the width of the resonance line as a function of temperature for H on 4He. The analysis gives the binding energy, the frequency in the surface state, and the sticking probability α for H on 4He. We obtain α = 0.035 ± 0.004. The results for H on 3He are more complicated, and possible explanations are discussed.