The influence of quantum reflection in Eley-Rideal hydrogen formation on graphite at interstellar cloud conditions

Interaction of hydrogen atoms with graphitic surfaces is currently subject of intense reasearch 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 ISM1. In this work we present results of accurate, time-dependent quantum scattering calculations at cold collision energies relevant to the interstellar conditions (i.e. down to 10-5 eV). Calculations make use of a recently devoloped two-wave approach2 which a priori solves the asymptotic problem affecting the traditional intial state-selected wave packet method at cold collision energies. In addition, an efficient Fourier mapping tecnique3 is used to introduce transmission-free absorbing potentials4 of length appropriate to the interesting energy regime. Overall, these calculations represent the first realistic application of time-dependent wave packet dynamics at low collision energy. Results shows that Eley-Rideal(ER) cross-sections for hydrogen formation decrease substantially as the De Broglie wavelength of the incident atom gets larger than the range of the H-H interaction, that is for collision energies in the range 1-100 K typical of diffuse interstellar clouds, as a consequence of the fact that quantum reflection prevents the atoms to reach the exit channel where they would form the product molecule. 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.