Two-bath model for activated surface diffusion of interacting adsorbates

Probing the non-pairwise interactions between CO molecules moving on a Cu(111) surface

The coverage dependent dynamics of CO on a Cu(111) surface are studied on an atomic scale using helium spin-echo spectroscopy. CO molecules occupy top sites preferentially, but also visit intermediate bridge sites in their motion along the reaction coordinate. We observe an increase in hopping rate as the CO coverage grows; however, the motion remains uncorrelated up to at least 0.10 monolayers (ML). From the temperature dependence of the diffusion rate, we find an effective barrier of 98 ± 5 meV for diffusion. Thermal motion is modelled with Langevin molecular dynamics, using a potential energy surface having adsorption sites at top and bridge positions and the experimental data are well represented by an adiabatic barrier for hopping of 123 meV. The sites are not degenerate and the rate changes observed with coverage are modelled successfully by changing the shape of the adiabatic potential energy surface in the region of the transition state without modifying the energy barrier. The results demonstrate that sufficient detail exists in the experimental data to provide information on the principal adsorption sites as well as the energy landscape in the region of the transition state.

Quantum Zeno and anti-Zeno effects in surface diffusion of interacting adsorbates

Surface diffusion of interacting adsorbates is here analyzed within the context of two fundamental phenomena of quantum dynamics, namely the quantum Zeno effect and the anti-Zeno effect. The physical implications of these effects are introduced here in a rather simple and general manner within the framework of non-selective measurements and for two (surface) temperature regimes: high and very low (including zero temperature). The quantum intermediate scattering function describing the adsorbate diffusion process is then evaluated for flat surfaces, since it is fully analytical in this case. Finally, a generalization to corrugated surfaces is also discussed. In this regard, it is found that, considering a Markovian framework and high surface temperatures, the anti-Zeno effect has already been observed, though not recognized as such.

A theoretical study of rotational and translational diffusion dynamics of molecules with a six-fold point symmetry adsorbed on a hexagonal lattice by neutron scattering

A complete analytical model for the rotational and translational diffusion of molecules with a six-fold point symmetry on a hexagonal lattice is presented. It can be applied, in particular, to the diffusion of benzene molecules adsorbed flat on the basal plane of graphite in the case of incoherent scattering. Under the weak hindered approximation, the classical mechanics framework and making use of the van Hove formalism of correlation functions, the intermediate scattering function and its Fourier transform, the scattering law, are both obtained. They can be expressed as sums of exponential decays or Lorentzian functions, respectively, containing the contribution of each of the dynamical processes taking place. In the case of benzene lying flat on the substrate we expect translational diffusion, continuous rotations of isolated molecules and hindered rotations of molecules within clusters. Each particular diffusive mechanism can be recognized owing to its particular signature in the dependence of the quasi-elastic broadening on the momentum transfer.

Phonon lineshapes in atom-surface scattering

Phonon lineshapes in atom-surface scattering are obtained from a simple stochastic model based on the so-called Caldeira-Leggett Hamiltonian. In this single-bath model, the excited phonon resulting from a creation or annihilation event is coupled to a thermal bath consisting of an infinite number of harmonic oscillators, namely the bath phonons. The diagonalization of the corresponding Hamiltonian leads to a renormalization of the phonon frequencies in terms of the phonon friction or damping coefficient. Moreover, when there are adsorbates on the surface, this single-bath model can be extended to a two-bath model accounting for the effect induced by the adsorbates on the phonon lineshapes as well as their corresponding lineshapes.