  Molecular beam experiments that use low-temperature bolometers as (energy-)
detectors are well suited to the study of physisorption and recombination of
hydrogen on low-temperature surfaces. Experiments where this technique is
combined with mass spectrometry to examine atoms and molecules released from
the surface are summarised and reviewed with reference to astrophysical
implications. Hydrogen atoms physisorbed on polycrystalline water ice are shown
to be sufficiently mobile to recombine efficiently even at surface temperatures
as low as 3 K. Molecules are formed with substantial internal energy, probably
of the order of 35000 K, and are immediately released when formed. Coverage by
molecular hydrogen plays an important role in determining overall recombination
efficiency and may self-regulate recombination in interstellar clouds: on
hydrogen-free grains recombination is limited by the low sticking coefficient
of hydrogen atoms, while on grains covered by molecular hydrogen the binding
energy is reduced so that recombination is limited by the rapid evaporation of
physisorbed atoms.

Chemical Physics;Astrophysics