  We reconsider H2 formation on grain surfaces. We develop a rate equation
model which takes into account the presence of both physisorbed and chemisorbed
sites on the surface, including quantum mechanical tunnelling and thermal
diffusion. In this study, we took into consideration the uncertainties on the
characteristics of graphitic surfaces. We calculate the H2 formation efficiency
with the Langmuir Hinshelwood and Eley Rideal mechanisms, and discuss the
importance of these mechanisms for a wide range of grain and gas temperatures.
We also develop a Monte Carlo simulation to calculate the H2 formation
efficiency and compare the results to our rate equation models. Our results are
the following: (1) Depending on the barrier against chemisorption, we predict
the efficiency of H2 formation for a wide range of grain and gas temperatures.
(2) The Eley-Rideal mechanism has an impact on the H2 formation efficiency at
high grain and gas temperatures. (3) The fact that we consider chemisorption in
our model makes the rate equation and Monte Carlo approaches equivalent.
