Hydrogen molecules, physisorbed or condensed on cryosurfaces at liquid‐helium temperatures, can be desorbed by thermal radiation. The probability of the process depends on the nature of the cryosurface, the degree of H2 coverage, and the spectrum of the radiation. For a given spectrum the desorption rate is proportional to radiation intensity, and, with a few exceptions, to the absorptivity of the cryosurface. The desorption efficiency of the absorbed 300‐K radiant energy is approximately 7×10−6 for most of the 20 bakeable cryosurfaces tested. A decrease of radiation temperature below 130 K causes a progressive reduction of the desorption efficiency which amounts to a factor of three at 80 K. This variation of efficiency can be fitted to a curve obtained by assuming that a threshold wavelength for desorption exists in the radiation spectrum at 45 μm. The energy carried by a photon of this threshold wavelength is about three times that required for the desorption of a condensed H2 molecule. Precondensed sublayers of heavier gas also introduce a decrease of the desorption efficiency in the following order: HD; D2; Ne; and Ar, N2, Kr. For the last three gases the desorption efficiency is practically constant and independent of the substrate and the radiation temperature. Similar observations for the H2 isotopes, HD and D2, are also reported. A physical model for the desorption process and its agreement with the results obtained here is discussed.
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