Studies of HD formed in excited vibrational states from atomic recombination on cold graphite surfaces

Sticking of atomic hydrogen on graphene

Recent years have witnessed an ever growing interest in the interactions between hydrogen atoms and a graphene sheet. Largely motivated by the possibility of modulating the electric, optical and magnetic properties of graphene, a huge number of studies have appeared recently that added to and enlarged earlier investigations on graphite and other carbon materials. In this review we give a glimpse of the many facets of this adsorption process, as they emerged from these studies. The focus is on those issues that have been addressed in detail, under carefully controlled conditions, with an emphasis on the interplay between the adatom structures, their formation dynamics and the electric, magnetic and chemical properties of the carbon sheet.

Full quantum dynamical investigation of the Eley-Rideal reaction forming H2 on a movable graphitic substrate at T = 0 K

The dynamics of the Eley-Rideal abstraction reaction of hydrogen atoms on a movable graphitic surface is investigated for the first time in a numerically exact fully quantum setting. A system-bath strategy was applied where the two recombining H atoms and a substrate C atom form a relevant subsystem, while the rest of the lattice takes the form of an independent oscillator bath. High-dimensional wavepacket simulations were performed in the collision energy range 0.2-1.0 eV with the help of the multi-layer multi-configuration time-dependent Hartree method, focusing on the collinear reaction on a zero-temperature surface. Results show that the dynamics is close to a sudden limit in which the reaction is much faster than the substrate motion. Unpuckering of the surface is fast (some tens of fs) but starts only after the formation of H₂ is completed, thereby determining a considerable substrate heating (∼0.8 eV per reactive event). Energy partitioning in the product molecule favors translational over vibrational energy, and H₂ molecules are vibrationally hot (∼1.5 eV) though to a lesser extent than previously predicted.

Quantum dynamical investigation of the isotope effect in H2 formation on graphite at cold collision energies

The Eley-Rideal abstraction of hydrogen atoms on graphitic surfaces at cold collision energies was investigated using a time-dependent wave packet method within the rigid-flat surface approximation, with a focus on hydrogen-deuterium isotopic substitutions. It is found that the marked isotope effect of collinear collisions disappears when the full dimensionality of the problem is taken into account, thereby suggesting that abstraction is less direct than commonly believed and proceeds through glancing rather than head-on collisions. In contrast, a clear isotope effect is observed for "hot-atom" formation, which appears to be strongly favored for heavy projectiles because of their higher density of physisorbed states. Overall, the dynamics is essentially classical and reasonably well described by quasi-classical trajectory methods at all but the lowest energies (≲10 meV). A comparison of the results obtained in the (substrate) adiabatic and diabatic limits suggests that the reaction is only marginally affected by the lattice dynamics, but highlights the importance of including energy dissipation processes in order to accurately describe the internal excitation of the product molecules.

Free-electron laser induced processes in thin molecular ice

Intermolecular reactions in and on icy films on silicate and carbonaceous grains constitute a major route for the formation of new molecular constituents in interstellar molecular clouds. In more diffuse regions and in protoplanetary discs, energetic radiation can trigger reaction routes far from thermal equilibrium. As an analog of interstellar ice-covered dust grains, highly-oriented pyrolytic graphite (HOPG) covered with D2O, NO, and H atoms is irradiated by ultrashort XUV pulses and the desorbing ionic and neutral products are analysed. The yields of several products show a nonlinear intensity dependence and thus enable the elucidation of reaction dynamics by two-pulse correlated desorption.

Insights into H2 formation in space from ab initio molecular dynamics

Hydrogen formation is a key process for the physics and the chemistry of interstellar clouds. Molecular hydrogen is believed to form on the carbonaceous surface of dust grains, and several mechanisms have been invoked to explain its abundance in different regions of space, from cold interstellar clouds to warm photon-dominated regions. Here, we investigate direct (Eley-Rideal) recombination including lattice dynamics, surface corrugation, and competing H-dimers formation by means of ab initio molecular dynamics. We find that Eley-Rideal reaction dominates at energies relevant for the interstellar medium and alone may explain observations if the possibility of facile sticking at special sites (edges, point defects, etc.) on the surface of the dust grains is taken into account.