  Aims. Interstellar dust grains, because of their catalytic properties, are
crucial to the formation of H2, the most abundant molecule in the Universe. The
formation of molecular hydrogen strongly depends on the ability of H atoms to
stick on dust grains. In this study we determine the sticking coefficient of H
atoms chemisorbed on graphitic surfaces, and estimate its impact on the forma-
tion of H2. Methods. The sticking probability of H atoms chemisorbed onto
graphitic surfaces is obtained using a mixed classical-quantum dynamics method.
In this, the H atom is treated quantum- mechanically and the vibrational modes
of the surface are treated classically. The implications of sticking for the
formation of H2 are addressed by using Kinetic Monte Carlo simulations that
follow how atoms stick, move and associate with each other on dust surfaces of
different temper- ature. Results. In our model, molecular hydrogen forms very
efficiently for dust temperatures lower than 15 K through the involvement of
physisorbed H atoms. At dust temperatures higher than 15 K and gas temperatures
lower than 2000 K, H2 formation differs strongly if the H atoms com- ing from
the gas phase have to cross a square barrier (usually considered in previous
studies) or a barrier obtained by DFT calculations to become chemisorbed. The
product of sticking times efficiency can be increased by many orders of
magnitude when realistic barriers are considered. If graphite phonons are taken
into account in the dynamics calculations, then H atoms stick better on the
surface at high energies, but the overall H2 formation efficiency is only
slightly affected. Our results suggest that H2 formation can proceed
efficiently in photon dominated regions, X-ray dominated regions, hot cores and
in the early Universe when the first dust is available.
