  Recent experimental results about the formation of molecular hydrogen on
astrophysically relevant surfaces under conditions close to those encountered
in the interstellar medium are analyzed using rate equations. The parameters of
the rate equation model are fitted to temperature-programmed desorption curves
obtained in the laboratory. These parameters are the activation energy barriers
for atomic hydrogen diffusion and desorption, the barrier for molecular
hydrogen desorption, and the probability of spontaneous desorption of a
hydrogen molecule upon recombination. The model is a generalization of the
Polanyi-Wigner equation and provides a description of both first and second
order kinetic processes within a single model. Using the values of the
parameters that fit best the experimental results, the efficiency of hydrogen
recombination on olivine and amorphous carbon surfaces is obtained for a range
of hydrogen flux and surface temperature pertinent to a wide range of
interstellar conditions.
