M. Rocca

Low-energy EELS investigation of surface electronic excitations on metals

Recent progress in experimental and theoretical investigations of surface electronic excitations of metals is reviewed with an emphasis on surface plasmon dispersion. The experimental methods applied to these studies are critically discussed, highlighting present limitations and possible future developments. Available data on the dependence of surface plasmon frequency, dispersion and damping on crystal temperature and of surface plasmon damping on parallel momentum and energy are collected. Surface plasmon dispersion and Mie resonance shift as a function of cluster size are compared when possible. The implications of surface plasmon dispersion for the position of the centroid of the induced charge at the surface and in general for the optical surface properties are presented and discussed.

Carbon Dioxide Hydrogenation on Ni(110)

We demonstrate that the key step for the reaction of CO 2 with hydrogen on Ni(110) is a change of the activated molecule coordination to the metal surface. At 90 K, CO 2 is negatively charged and chemically bonded via the carbon atom. When the temperature is increased and H approaches, the H-CO 2 complex flips and binds to the surface through the two oxygen atoms, while H binds to the carbon atom, thus yielding formate. We provide the atomic-level description of this process by means of conventional ultrahigh vacuum surface science techniques combined with density functional theory calculations and corroborated by high pressure reactivity tests. Knowledge about the details of the mechanisms involved in this reaction can yield a deeper comprehension of heterogeneous catalytic organic synthesis processes involving carbon dioxide as a reactant. We show why on Ni the CO 2 hydrogenation barrier is remarkably smaller than that on the common Cu metal-based catalyst. Our results provide a possible interpretation of the observed high catalytic activity of NiCu alloys.

Initial sticking coefficient of O2 on Ag(110)

We investigated the dynamics of the adsorption of O2 on Ag(110) with the molecular beam technique combined with EEL spectroscopy and with the method of King and Wells. The initial sticking coefficient S0 is reported for molecules impinging along both high symmetry azimuthal directions 〈001〉 and 〈110〉 as a function of total energy and angle of incidence of the molecules and crystal temperature. The initial sticking coefficient is anisotropic for low as well as for room temperature adsorption. The dependence of the sticking coefficient on the temperature of the sample, Ts, indicates that adsorption takes place directly into the molecular well. Dissociation is eventually induced thermally for Ts≳150 K. The physisorbed state plays no role in the chemisorption process in the investigated impact energy range.

Low-energy acoustic plasmons at metal surfaces

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons, and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors. Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons. Rather, metallic surfaces should support only conventional surface plasmons—higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy and sub-wavelength optics, but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.

Apparatus for adsorption studies

In this paper a UHV apparatus for studying adsorption of gases on surfaces will be described. The system is specifically designed for measuring the sticking coefficient of a molecule on the surface and its dependence on energy and angle of incidence. The method is based on the use of an electron energy loss spectrometer which detects the amount of the adsorbed species as a function of the exposure of the surface to a supersonic nozzle beam which deposits the molecular species at different energies and angles of incidence. Measurements of the sticking probability of O2 on Ag(001) are presented showing the capability of the method.

O2 DISSOCIATION ON AG(001) : THE ROLE OF KINK SITES

Abstract The dynamics of the dissociative adsorption of O 2 on Ag(001) were investigated with a supersonic molecular beam source and electron energy loss spectroscopy versus surface temperature. Contrary to the case of Ag(110) where dissociation occurs at the atomic terraces and has a high probability, for Ag(001) we find that only 0.44% of the adsorbed molecules dissociate at room temperature. An Arrhenius analysis indicates that the process is thermally activated and the activation energy coincides with the energy for generating kinks, which are thus identified as the active sites. The interplay between a local geometry similar to a (110) site and enhanced charge transfer to the antibonding molecular orbitals is responsible for the pronounced reactivity of such sites. Molecules adsorbed at (001) terrace sites instead desorb with a high probability.

Energy and angle dependence of the initial sticking coefficient of O2 on Ag(001)

We investigated the dynamics of the adsorption of 02 on Ag(001) with the supersonic molecular beam technique combined with EEL spectroscopy, TDS and the reflection detector technique. The initial sticking coefficient S O was measured as a function of the impact energy and of the angle of incidence of the molecules at crystal temperatures of 100 and 300 K, corresponding to molecular and dissociative adsorption, respectively. Both regimes are characterized by the same distribution of energy barriers. This indicates that, in analogy to the case of our previous investigation of O2-Ag(110), the main reaction path involves activated adsorption into the molecular well, followed eventually by thermally induced dissociation for T~> 150K. Again in analogy to O2-Ag(II0), the physisorbed state plays no role in the chemisorption process.

Coverage dependence of sticking coefficient of O2 on Ag(110)

We report on the dependence of the sticking coefficient S of O2 on Ag(110) on oxygen coverage, for molecular and dissociative adsorption. S is found to diminish strongly with increasing coverage. The initial dependence is linear and for dissociative adsorption the slope depends on crystal temperature. The data are indicative of a coverage dependence of the adsorption barrier height, which is caused by the change of the surface work function with coverage. The analysis of the temperature variation of the dependence of S on coverage allows to extract the variation of the ratio of the pre‐exponential factors and of the height of the barriers for desorption and dissociation from the molecularly chemisorbed state.

Tuning surface reactivity by in situ surface nanostructuring

As recently demonstrated, the morphology of a surface can be modified on the mesoscopic scale by ion sputtering. Here we show by microscopy and spectroscopy that the chemical properties of the surface are strongly affected by nanostructuring and that surface reactivity can be tuned by changing surface morphology. For the otherwise inert Ag(001) surface significant O2 dissociation takes place on the nanostructured surface, thus allowing us to control the relative coverage of admolecules and adatoms. The dissociation probability is determined by the experimentally tunable density of kinks.

Oxygen adsorption on Ag(111)

Abstract We have investigated the interaction of oxygen with Ag(111) by using a supersonic molecular beam in the impact energy range 93–800 meV. At 105 K, contrary to the results of Carley et al. [Surf. Sci. 238 (1990) L467], we find no evidence for O 2 adsorption even after very high O 2 exposures (∼ 25000 L) indicating that for a clean surface the sticking probability S is lower than 6 × 10 −7 for the whole impact energy range. At room temperature dissociative oxygen adsorption occurs at E i = 0.80 eV, with S ≈ 9 × 10 −7 . The data show however evidence that the adsorption process is mediated also in this case by adsorbed OH so that S is even smaller for the clean surface.