Desorption of cryogenic layers of the solid hydrogens by electron bombardment: The role of the metal substrate

For solid hydrogenic films in the thickness range from similar to 50 ML to similar to 500 ML the desorption yield falls off inversely proportional to the thickness for both H-2 and D-2 films. This behavior is common for data obtained at CERN for solid H-2 and at Riso National Laboratory for solid D-2 at temperatures below 4.2 K. The thickness range is comparable to the range of the electrons for energies between 0.3 and 2 keV. For these energies less energy is deposited in the metal substrate with increasing film thickness. We have explored how the behavior of the desorption yield may be explained in terms of the energy dissipated in a copper substrate or as the surface value of the energy deposited in electronic excitations in copper, but not found convincing arguments for a close correlation between the desorption yield and these quantities. The decreasing desorption yield for film thicknesses that greatly exceed the electron mean penetration depth evaluated from uniform films, may be explained by nonuniform growth of the hydrogen films on the metal substrate. (C) 2008 Elsevier B.V. All rights reserved.