Spin-polarized atomic hydrogen adsorbed on the
surface of liquid helium form a weakly interacting two-dimensional (2d) gas.
Hydrogen is a bosonic atom and is prediced to undergo a Kosterlitz-Thouless
(KT) transition  to superfluidity when cooled to low temperature. Deauterium
is simlar but being a Fermion it can be conrasted with its bosonic counterpart.
The KT transition is accompanied by a transition that can be regarded as
the two-dimensional analog of Bose-Einstein Condensation (2-d BEC). People
have been aware of the possibility of the possibility of 2-d BEC in hydrogen
right from the start of cold H research, but only since 1991 has it been
actively analyzed (see e.g. ) and pursued experimentally by groups in
the field. In 1998 the first experimental evidence was obtained for the existence
of such a phase by a Finish-Russian collaboration . In Amsterdam we study
the adsorbate of H atoms on top of bulk helium using resonant fluorescence
at the Lyman-alpha transition of the hydrogen atom  as well as in the
deuterium atom . In atomic hydrogen the 2-d gas is in equilibrium with
a three dimensional gas above the surface that is so dense that photo-association,
the formation of molecules mediated by light absorption, could be directly
 J.M. Kosterlitz and D.J. Thouless,
Ordering, metastability, and phase transitions in two-dimensional systems,
J. Phys. C 6, 1181 (1973).
 B.V. Svistunov, T.W. Hijmans, G.V. Shlyapnikov,
and J.T.M. Walraven,
Resonant light absorption and the problem of observing the Kosterlitz-Thouless transition in spin-polarized hydrogen adsorbed on a liquid-He surface,
Phys. Rev. B 43, 13412 (1991).
 A.I. Safonov, S.A. Vasiliev, I.S. Yasnikov, I.I. Lakashevich, and
Observation of a quasi-condensate in two-dimensional atomic hydrogen,
Phys. Rev. Lett. 81, 4545 (1998).
 A.P.Mosk, M.W. Reynolds, T.W. Hijmans, and J.T.M. Walraven,
Optical excitation of atomic hydrogen bound to the surface of liquid helium,
Phys. Rev. Lett. 81, 4440-4444 (1998).
 A.P.Mosk, M.W. Reynolds, and T.W. Hijmans,
Atomic Deuterium adsorbed on the surface of liguid helium,
Phys. Rev. A 64, 022901 (2001).
 A.P. Mosk, M.W. Reynolds, T.W. Hijmans, and J.T.M. Walraven,
Photoassociation of ultra-cold atomic hydrogen,
Phys. Rev. Lett 82 307-311 (1999).
Atomic hydrogen home page