Hydrogen Recombination and Dimer Formation on Graphite from Ab Initio Molecular Dynamics Simulations.

We studied Eley-Rideal molecular hydrogen formation on graphite using ab initio molecular dynamics, in the energy range relevant for the chemistry of the interstellar medium and for terrestrial experiments employing cold plasma (0.02-1 eV). We found substantial projectile steering effects that prevent dimer formation at low energies, thereby ruling out any catalytic synthetic pathways that form hydrogen molecules. Ortho and para dimers do form efficiently thanks to preferential sticking, but only at energies that are too high to be relevant for the chemistry of the interstellar medium. Computed reaction cross sections and ro-vibrational product populations are in good agreement with available experimental data and capable of generating adsorbate configurations similar to those observed with scanning tunneling microscopy techniques.

Compact MCTDH wave functions for high-dimensional system-bath quantum dynamics.

We employ a simple multiconfiguration time-dependent Hartree (MCTDH) ansatz tailored to an effective-mode transformation of environmental variables that brings the bath into a linear chain form. In this form, important (primary) degrees of freedom can be easily identified and treated at a high correlation level, whereas secondary modes are left uncorrelated. The resulting approach scales linearly with the bath dimensions and allows us to easily access recurrence times much longer than usually possible, at a very small computational cost. Test calculations for model atom-surface problems show that the system dynamics is correctly reproduced in the relevant time window, and quantitative agreement is attained for energy relaxation and sticking, particularly in non-Markovian environments. These results pave the way for tackling realistic system-bath quantum dynamical problems on the picosecond scale.

A few simple rules governing hydrogenation of graphene dots.

We investigated binding of hydrogen atoms to small polycyclic aromatic hydrocarbons (PAHs)--i.e., graphene dots with hydrogen-terminated edges--using density functional theory and correlated wavefunction techniques. We considered a number of PAHs with three to seven hexagonal rings and computed binding energies for most of the symmetry unique sites, along with the minimum energy paths for significant cases. The chosen PAHs are small enough to not present radical character at their edges, yet show a clear preference for adsorption at the edge sites which can be attributed to electronic effects. We show how the results, as obtained at different levels of theory, can be rationalized in detail with the help of a few simple concepts derivable from a tight-binding model of the π electrons.

Hot-atom versus Eley-Rideal dynamics in hydrogen recombination on Ni(100). I. The single-adsorbate case.

We compare the efficiency of the Eley-Rideal (ER) reaction with the formation of hot-atom (HA) species in the simplest case, i.e., the scattering of a projectile off a single adsorbate, considering the Hydrogen and Hydrogen-on-Ni(100) system. We use classical mechanics and the accurate embedded diatomics-in-molecules potential to study the collision system over a wide range of collision energies (0.10-1.50 eV), both with a rigid and a nonrigid Ni substrate and for impact on the occupied and neighboring empty cells. In the rigid model metastable and truly bound hot-atoms occur and we find that the cross section for the formation of bound hot-atoms is considerably higher than that for the ER reaction over the whole range of collision energies examined. Metastable hot-atoms form because of the inefficient energy transfer to the adsorbate and have lifetimes of the order 0.1-0.7 ps, depending on the collision energy. When considering the effects of lattice vibrations we find, on average, a consistent energy transfer to the substrate, say 0.1-0.2 eV, which forced us to devise a two-step dynamical model to get rid of the problems associated with the use of periodic boundary conditions. Results for long-lived HA formation due to scattering on the occupied cell at a surface temperature of 120 K agree well with those of the rigid model, suggesting that in the above process the substrate plays only a secondary role and further calculations at surface temperatures of 50 and 300 K are in line with these findings. However, considerably high cross sections for formation of long-lived hot-atoms result also from scattering off the neighboring cells where the energy transfer to the lattice cannot be neglected. Metastable hot-atoms are reduced in number and have usually lifetimes shorter than those of the rigid-model, say less than 0.3 ps. In addition, ER cross sections are only slightly affected by the lattice motion and show a little temperature dependence. Finally, we find also that absorption and reflection strongly depend on the correct consideration of lattice vibrations and the occurrence of trapping.