Magnetic resonance studies of atomic hydrogen at zero field and low temperature: recombination and binding on liquid helium

Magnetic resonance studies at zero field are reported for atomic hydrogen gas confined in a closed glass bulb with helium-coated walls for T < 1 K in a dilution refrigerator. Low-energy r.f. discharge pulses have been used to produce H atoms at temperatures as low as T = 0.06 K. The atom density nH (109 < nH < 1013) measured by the strength of the free induction decay signal, follows a second-order rate equation dnHdt = −KnH2. At the lowest temperatures recombination is dominated by the process H + H+ wall → H2 + wall. From the temperature dependence of the rate constant K we have determined the binding energy of H on liquid 4He and 3He, and also the cross section for recombination on the surface.