Theoretical study of hydrogen adsorption and dynamics on graphitic surfaces

Interaction of hydrogen atoms with graphitic surfaces is currently subject of intense research activity because of its relevance in fields as diverse as nuclear fusion, hydrogen storage and astrophysics. In particular, the adsorption of H atoms on the graphitic dust grains of the interstellar medium (ISM) and their subsequent recombination reactions are considered to be key steps for H2 formation and its anomalous large abundance especially in diffuse regions of the ISM. In this contribution a number of processes concerning adsorption of hydrogen atoms on the (0001) graphite surface and their recombinative desorption are addressed by means of electronic structure and quantum dynamical calculations. Firstly, accurate first-principles calculations with the plane-wave, periodic Density Functional Theory (DFT) approach and a large 5x5 cell are used to investigate single, double and multiple adsorption of hydrogen atoms on graphite. Binding and barrier energies for chemisorption of hydrogen atoms are computed for a number of different configurations, the spin densities are analysed, and the results are rationalized in terms of the p-resonance theory of aromatic compounds [1]. Secondly, several model Polycyclic Aromatic Hydrocarbons (PAHs) are used both in DFT and Multi-Reference Perturbation Theory (MRPT) calculations to assess the quality of the DFT data, to investigate the adsorption properties on (H-saturated) edges of graphitic surfaces, and to validate the above p-resonance chemical picture [2]. This extends recent work on physisorption and diffusion of hydrogen atoms on graphite [3]. Thirdly, time-dependent wave packet calculations in the rigid, flat surface approximation are used to investigate a number of processes induced by collisions of projectile hydrogen atoms on target hydrogens previously adsorbed on graphite. The collision energy ranges from values where collision induced desorption is possible [4] to values typical of interstellar cloud conditions, i.e. from some eV down to 10-5 eV. In particular, converged results obtained with a novel two-wave packet approach [5] show that hydrogen formation by Eley-Rideal reaction in the cold collision energy regime is largely prevented by quantum reflection [6]. This result is true both for initially chemisorbed and for initially physisorbed H atoms on graphite, and does not depend on the presence of a barrier in the reaction path, thereby reducing the role that Eley-Rideal hydrogen recombination processes play at interstellar cloud conditions. We conclude by critically re-discussing the possibility of forming molecular hydrogen on graphite at low temperature in the light of the above results.