Flow of 2D atomic hydrogen over the surface of liquid 4He

Experimental evidence has been obtained for the hydrodynamic flow of a 2D gas of hydrogen atoms adsorbed on the surface of liquid helium. The observed flow manifestations are consistent with the concepts of the quantum hydrodynamics of the helium surface. This circumstance allows both investigation of the interaction of 2D hydrogen with ripplons and surface ³He quasiparticles and possible future observation of the superfluidity of the 2D Bose gas of hydrogen atoms. The experimental results on thermal compression make it possible to estimate the characteristic times of the transfer of longitudinal momentum between the subsystems of hydrogen and ³He atoms bound to the surface (τ_H3), as well as from ripplons to the substrate (τ_R). The value τ_H3 ∼ 4 × 10⁻⁸ s agrees with a value calculated using the mean-field parameter U ₃₀ for the interaction of hydrogen atoms with the ground surface state of ³He. At the same time, τ_R is more than an order of magnitude shorter than the value obtained in experiments by Mantz et al. Phys. Rev. Lett. 44, 66 (1980) [Erratum: Phys. Rev. Lett. 44, 1094 (1980)]. This discrepancy can be attributed to the dependence of the ripplon momentum relaxation rate on the substrate roughness scale.