Thermal compression of two-dimensional atomic hydrogen gas

We describe experiments where two-dimensional (2D) atomic hydrogen gas is compressed thermally at a small 'cold spot' on the surface of superfluid helium and detected directly with electron-spin resonance. We reach surface densities up to 5x10{sup 12} cm{sup -2} at temperatures {approx_equal}100 mK corresponding to the maximum 2D phase-space density {approx_equal}1.5. By independent measurements of the surface density and its decay rate we make a direct determination of the three-body recombination rate constant and get the upper bound L{sub 3b} < or approx. 2x10{sup -25} cm{sup 4}/s which is an order of magnitude smaller than previously reported experimental results.