David R. Mullins

Normal raman scattering from pyridine adsorbed on the low-index faces of silver

Abstract We find no enhancement of the Raman scattering cross section for pyridine adsorbed at 120 K on the (100), (110) and (111) faces of silver in ultrahigh vacuum. The frequencies we observe are essentially unshifted from those of liquid pyridine, intensity ratios are similar to the liquid, the signal intensity is linear in coverage from submonolayer to multilayer, and the depolarization ratio is low. Since these observations are in marked contrast to those associated with surface-enhanced Raman scattering (SERS), we conclude that our spectra result from normal Raman scattering. Our results support the hypothesis that special adsorption sites are responsible for a substantial fraction of the total enhancement of the Raman cross section for the pyridine - silver system.

Unenhanced Raman scattering from pyridine chemisorbed on a stepped silver surface: Implications for proposed sers mechanisms

Abstract In an effort to separate chemical from electromagnetic sources of enhancement in surface-enhanced Raman scattering, we have investigated the adsorption

Isotope effects in surface diffusion: Hydrogen and deuterium on Ni(100)

The diffusion of hydrogen and deuterium on Ni(100) has been studied by laser-induced thermal desorption. The temperature dependence has been measured between 211 and 263 K. Throughout this range hydrogen diffuses more rapidly than deuterium. The diffusion activation energy is less for hydrogen than deuterium (3.5 kcal mol −1 versus 4.4 kcal mol −1 ). To some extent this activation energy difference is compensated by the pre-exponential factor which is larger for deuterium, 8.5×10 −3 cm 2 s −1 , than for hydrogen, 2.5×10 −3 cm 2 s −1 . Coadsorption of D and H had no effect on the diffusion constants of the individual species as their relative concentrations were varied from 30% to 100%

Unenhanced raman scattering from pyridine adsorbed on stepped and kinked silver surfaces under ultrahigh vacuum

Abstract As part of a study to elucidate chemical contributions to the total enhancement factor in surface-enhanced Raman scattering (SERS), the adsorption and Raman scattering from pyridine on several stepped and kinked silver surfaces in ultrahigh vacuum was examined. On Ag (521) and Ag (987), which are kinked surfaces comprising (111) terraces of different widths, pyridine does not chemisorb or exhibit SERS. On the Ag (540) surface, however, pyridine chemisorbs, but does not exhibit SERS. These observations are discussed in relation to current theories of the chemical enhancement mechanism.

Potassium on Ni(100) probed by photoemission of coadsorbed xenon

Abstract Valence band photoemission from Xe coadsorbed with controlled coverages of K on Ni(100) identifies three different kinds of Xe adsorption sites. These are attributed to: (1) Xe on Ni(100) with no nearest-neighbor K, (2) Xe on Ni(100) with K in the nearest-neighbor coordination shell, and (3) Xe on potassium. Short range XeK interactions and longer range through metal effects of K are discussed.

Normal (Unenhanced) Raman Scattering from Pyridine Adsorbed on the Low-Index Faces of Silver

Despite several years of intense investigation by a number of laboratories, the origin of surface-enhanced Raman scattering (SERS) remains the subject of a lively debate. Several review articles and a book summarize the current state of thinking on the subject [1–3]. The debate focuses upon the relative importance of two classes of enhancement mechanisms, which are termed electromagnetic (classical) and molecular (sometimes referred to as chemical or local). In the former, enhanced scattering arises from an increase in the local electric field of the exciting light at metal surfaces of high curvature, e.g., submicroscopically rough surfaces or diffraction gradings. In the latter, the enhancement is thought to arise from an increase in the molecular polarizability derivative that could occur when a molecule is chemisorbed to a metal surface. New optical transitions could result from charge transfer excitations which may be resonant with the exciting laser frequency, thus leading to resonance Raman scattering. Although there is general agreement as to the existence of both classes of enhancement mechanisms, the separation of their relative contributions, even for the best-studied case, pyridine on silver, remains elusive.