Rotational excitation of CO molecules colliding at low energy with a cold graphite surface

The CO molecule is of interest in various environments because, being IR active, it is often used as a tracer of inactive IR molecules. This is particularly important in the interstellar medium, where CO is the most abundant molecule after H2. Aims. We adopted our computational set-up to obtain insights into the surface elementary processes promoted by the interaction of gaseous CO molecules with graphitic grain surfaces and to resolve the rotational spectra of CO scattered at low collision energies by the cold substrate. Methods. Molecular dynamics simulations are based on a chemical state-to-state treatment,which, by explicitly including the coupling with substrate phonons and the role of long-range interactions, is suitable for controlling the rotational excitation of the incident molecules. Results. Sticking and state-resolved reflection coefficients for CO interacting in its ground roto-vibrational state and two low-lying rotational states with a cold graphite surface are determined. For collision energies lower than 0.01 eV, CO adsorbs on a graphitic substrate with high probability, while reflected molecules populate excited rotational states up to level 7. The two atoms in the molecule probe a torque effect, arising from the asymmetric position of the C and O atoms in CO and from the different strengths of their interactions with graphite, influencing the interaction dynamics mainly for extremely low collision energies. In addition, a sort of resonance, more evident at the lowest energies, between the energy of impinging molecules and the phonons of the cold surface that promotes rotational excitation is highlighted for the lowest initial rotational state. Conclusions. Rotational energy transfer undergone by gaseous CO molecules in the ISM is not uniquely induced by CO–H2 collisions but can also be a consequence of the CO–graphitic grain interactions, and hence the amount of H2 inferred from IR spectroscopic measurements is presumably slightly overestimated.