The dynamics of H2 formation on a graphite surface at low temperature

Abstract

The catalytic formation of H₂ on carbonaceous grains in interstellar conditions is studied theoretically. The grain is modelled by a coronene molecule. The coronene–H–H interaction is described at the DFT level. The dynamics study is limited to the collinear case, with two degrees of freedom (Cor–H and H–H) handled by a wavepacket description. The collision energy range extends between 0.3 meV and 0.5 eV. The main results are: (i) the collision time has the same order of magnitude as the lattice relaxation time; (ii) the vibrational excitation is very large; (iii) the reaction probability is very sensitive to the small potential barrier in the entrance valley predicted by the DFT calculation.