Adsorption of hydrogen atoms on a single graphite sheet (graphene) has been investigated by first-principles electronic structure means, employing plane-wave based periodic density functional theory. A 5×5 surface unit cell has been adopted to study single and multiple adsorptions of H atoms. Binding and barrier energies for sequential sticking have been computed for a number of configurations involving adsorption on top of carbon atoms. We find that binding energies per atom range from ∼0.8 to ∼1.9 eV, with barriers to sticking in the range 0.0–0.15 eV. In addition, depending on the number and location of adsorbed hydrogen atoms, we find that magnetic structures may form in which spin density localizes on a 3×3R30° sublattice and that binding (barrier) energies for sequential adsorption increase (decrease) linearly with the site-integrated magnetization. These results can be rationalized with the help of the valence-bond resonance theory of planar π conjugated systems and suggest that preferential sticking due to barrierless adsorption is limited to formation of hydrogen pairs.

Non-Markovian processes can often be turned Markovian by enlarging the set of variables. Here we show, by an explicit construction, how this can be done for the dynamics of a Brownian particle obeying the generalized Langevin equation. Given an arbitrary bath spectral density \documentclass[12pt]{minimal}\begin{document}$J_{0}$\end{document}J0, we introduce an orthogonal transformation of the bath variables into effective modes, leading stepwise to a semi-infinite chain with nearest-neighbor interactions. The transformation is uniquely determined by \documentclass[12pt]{minimal}\begin{document}$J_{0}$\end{document}J0 and defines a sequence \documentclass[12pt]{minimal}\begin{document}$\lbrace J_{n}\rbrace _{n\in \mathbb {N}}$\end{document}{Jn}n∈N of residual spectral densities describing the interaction of the terminal chain mode, at each step, with the remaining bath. We derive a simple one-term recurrence relation for this sequence and show that its limit is the quasi-Ohmic expression provided by the Rubin model of dissipation. Numerical calculations show that, irrespective of the details of \documentclass[12pt]{minimal}\begin{document}$J_{0}$\end{document}J0, convergence is fast enough to be useful in practice for an effective Ohmic reduction of the dissipative dynamics.

Following up on our recent study of ultrafast charge separation at oligothiophene-fullerene interfaces [H. Tamura, I. Burghardt, and M. Tsukada, J. Phys. Chem. C 115, 10205 (2011)10.1021/jp203174e], we present here a detailed quantum dynamical perspective on the charge transfer process. To this end, electron-phonon coupling is included non-perturbatively, by an explicit quantum dynamical treatment using the multi-configuration time-dependent Hartree (MCTDH) method. Based upon a distribution of electron-phonon couplings determined from electronic structure studies, a spectral density is constructed and employed to parametrize a linear vibronic coupling Hamiltonian. The diabatic coupling is found to depend noticeably on the inter-fragment distance, whose effect on the dynamics is here investigated. MCTDH calculations of the nonadiabatic transfer dynamics are carried out for the two most relevant electronic states and 60 phonon modes. The electron transfer process is found to be ultrafast and mediated by electronic coherence, resulting in characteristic oscillatory features during a period of about 100 fs.

We study n\ifmmode\times\else\texttimes\fi{}n honeycomb superlattices of defects in graphene. The considered defects are missing pz orbitals and can be realized by either introducing C atom vacancies or chemically binding simple atomic species at the given sites. Using symmetry arguments and electronic-structure calculations we show that it is possible to open a band gap without breaking graphene point symmetry. This has the advantage that new Dirac cones appear right close to the gapped region. We find that the induced gaps have an approximate square-root dependence on the defect concentration x=1/n2 and compare favorably with those found in nanoribbons at the same length scale.