Resonant Charge-Transfer in Grazing Collisions of H− with Vicinal Nanosurfaces on Cu(111), Au(100) and Pd(111) Substrates: A Comparative Study

We compare the electron dynamics at monocrystalline Cu(111), Au(100) and Pd(111) precursor substrates with vicinal nanosteps. The unoccupied bands of a surface superlattice are populated via the resonant charge transfer (RCT) between the surface and a H − ion that flies by at grazing angles. A quantum mechanical wave packet propagation approach is used to simulate the motion of the active electron, and time-evolved wave packet densities are used to visualize the dynamics through the superlattice. The survived ion fraction in the reflected beam generally exhibits modulations as a function of the vicinal terrace size and shows peaks at those energies that access the image state subband dispersions. Differences in magnitudes of the ion-survival as a function of the particular substrate selection and the ion-surface interaction time, based on the choice of two ion-trajectories, are examined. A square well model, producing standing waves between the steps on the surface, explains the energies of the maxima in the ion survival probability for all the metals considered. This indicates that the primary process of confinement induced subband formation is robust. The work may motivate measurements and applications of shallow-angle ion-scattering spectroscopy to access electronic substructures in periodically nanostructured surfaces.

Theoretical modeling of voltage effects and the chemical mechanism in surface-enhanced Raman scattering

Theoretical approaches can provide insight into the mechanisms and magnitudes of electromagnetic and chemical effects in surface-enhanced Raman scattering (SERS), properties that are not readily available experimentally. Here, we model the SERS spectra of two geometries of the prototypical Ag20–pyridine cluster using a semiempirical INDO/SCI approach that allows a straightforward decomposition of the enhancement factors at each wavelength into electromagnetic and chemical terms, with proper treatment of resonant charge-transfer contributions to the enhancement. The method also enables us to determine the dependence of the enhancement on the electrochemical potential. We show that the electromagnetic enhancements for the Ag20 cluster are <10 far from resonance but can increase to 102 to 103 on resonance with plasmon excitation in the cluster. The decomposition also shows that for the systems studied here, the chemical enhancements are primarily due to resonance with excited states with significant charge-transfer character. This term is typically <10 but can be >102 at electrochemical potentials where the charge-transfer excited states are resonant with the incoming light, leading to total enhancements of >104.