title: Cold collision frequency shift in two-dimensional atomic hydrogen.
abstract: 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.
authors.affiliation: Wihuri Physical Laboratory, Department of Physics, University of Turku, 20014 Turku, Finland.
authors.forename: J
authors.initials: J
authors.lastname: Ahokas
authors.forename: J
authors.initials: J
authors.lastname: Järvinen
authors.forename: S
authors.initials: S
authors.lastname: Vasiliev