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Astron. Astrophys. 334, 363-375 (1998)

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5. Conclusion

For the first time, a complete molecular dynamics simulation has been done to understand the catalytic formation of molecular hydrogen on a graphite surface from a microscopic approach. With the empirical potential model used, it appears that the recombination process favours the formation of highly vibrationally excited molecules with a large translational kinetic energy. The formation efficiency by collision is found as large as 0.5 in the range of 0.1 eV collision energy. As a result of the effect of a small activation barrier in the entrance channel, the onset of the reactivity occurs at around 100 K depending on the potential model. The simulations have revealed the role of both direct and indirect processes, whose relative efficiencies depend primarily on the collision energy and on the surface coverage. The corrugation of the graphite surface plays an important role in the appearance of indirect trajectories because it allows an efficient transfer from normal to tangential components of the incoming H atom linear momentum. An alignment of the rotational angular momentum of the H2 molecule has been also characterized which seems independent of the nature (direct or indirect) of the collision. Inclusion of the zero-point energy using a semi-classical approach does not alter the results on the formation rate. Its only effect is a slight increase of the mean energy in the translation, rotation and vibration of the newly formed molecules.

Some important astrophysical consequences are outstanding from this study: in particular the possibility to heat the interstellar gas by the recoil energy of the newly formed H2 molecules, and the hope that the vibrational excitation resulting from this catalytic process becomes a route to recognize regions of formation in space through observations like the vibrational emission of the H2 molecule. In order to progress in that direction, refinements of the theoretical approach are necessary, together with confrontation with experiments. On the theoretical side, the first need is the improvement of the interaction, since these phenomena are expected to be strongly dependent on the potential energy surface. We are indeed working now on calculations of the reactive potential energy surface in the framework of the density functional theory. On the experimental point of view detailed dynamical behaviours revealed by the theoretical study, such as of course kinetic energy release and vibrational distribution but also angular distribution and alignment effects, would provide severe tests of the physical reality.

Furher aspects, of direct astrophysical relevance, of the H2 recombination process which are planned to be studied theoretically in the near future include deuteration effects (important also for experimental reasons), the sticking probability on a non rigid surface, and the Langmuir-Hinshelwood mechanism in which migration of H atoms on the surface dominates the dynamics. The ortho versus para distribution can also be reached through a quantum mechanical treatment. It would allow, through a full astrophysical modelling, to explore in detail the role of physical conditions such as the grain temperature [FORMULA], the gas temperature [FORMULA], the density of the gas [FORMULA], the density of grains [FORMULA] and size distribution, as well as grain velocities and interstellar radiation field.

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© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998