Quantum dynamics of H2 formation on a graphite surface through the Langmuir Hinshelwood mechanism. 
We have studied the formation of the H2 molecule on a graphite surface, when both H atoms are initially physisorbed.  The graphite surface is assumed to be planar, and a model potential is obtained in a semiempirical way to reproduce the experimental properties of H physisorption on graphite.  The reaction probability has been computed in the case when the angular momentum of the relative H-H motion lies parallel to the surface plane.  Three-dimensional wave packet calculations have been performed for collision energies ranging from 2 to 50 meV. It is shown that the reaction occurs with a significant probability and produces the H2 molecule with a considerable amount of vibrationnal energy.  A simple mechanical model is presented, where desorption of the nascent H2 molecule results from two successive binary elastic collisions.