VATTUONE , Luca

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We discuss the influence of elastic reflectivity of the electrons for the analysis of dissociation phenomena on surfaces and adsorbate-induced reconstructions of the substrate. Two situations corresponding to the enhancement of the reflectivity induced by low-energy-electron-diffraction fine structures and by multiple scattering for O2...

We demonstrate the feasibility of a new type of electrostatic cylindrical analyzer terminated with plates with focus at 90degrees also in the presence of significant space charge. The transmission function is determined by numerical simulation of the trajectories after a full three-dimensional solution of the Poisson problem with boundary...

Many reactions like the ethylene epoxidation which work readily in industrial reactors can not be reproduced under controlled laboratory conditions. This failure is ascribed to so-called "pressure" and "structure" gaps. The latter is possibly determined by an active role of defects, since surface science experiments are performed on nearly...

Oxygen adsorption on Ag(110) is investigated as a function of coverage by high resolution electron energy loss spectroscopy. We find for the O-Ag(110) stretching frequency a value of 39.1 meV from the limit of low coverage up to the (4 x 1) structure. The frequency is then gradually shifted upwards, whereby an increase of 0.9 meV is observed...

The surface plasmon dispersion of ion bombarded Ag(001) is investigated by angle resolved high resolution electron energy loss spectroscopy. We find it to be parabolic and dominated by the quadratic term, contrary to the case of flat Ag(001) where the linear term is dominant. The case is reminiscent of surface plasmon dispersion on the oxygen...

The coverage dependence of electron-energy loss intensity (EELS) and of the frequency of the O-Ag(110) stretching vibration are re-examined and compared with other EELS and infrared reflection absorption spectroscopy (IRAS) data. The analysis of the EELS intensity in our experiment yields a charge transfer q=0.51+/-0.03 e(-), which compares...

The frequency shift in the stretching mode of peroxo-like O2 on Ag(110) is studied by EELS. The anharmonicity parameter and an estimate of the dissociation energy are obtained from the frequency of the 0 --> 2 vibrational transition. A comparison with already published data for free and physisorbed oxygen is given. We find that chemisorption...

We demonstrate by vibrational spectroscopy that open steps are the active site for O-2 dissociation on Ag(410), and that the barrier to adsorption at defects can be measured by energy and angle resolved investigation of the dynamics of the gas-surface interaction. We identify a molecular adsorption channel with considerably reduced activation...

The active sites for catalytic reactions in heterogeneous catalysis are often minority sites related to defects at the surface. However, in most studies performed so far under controlled conditions, nearly perfect low Miller index surfaces were used, giving rise to the so-called structure gap in the surface science approach to catalysis. In...

We show with supersonic molecular beams and surface vibrational spectroscopy that, contrary to the case of Ag(100) and Ag(110), O-2 undergoes total dissociation on Ag(210) at 105 K. Moreover, metastable subsurface sites can be accessed either directly or indirectly. For the direct channel, the final configuration of the oxygen atoms depends on...

The role of defects in catalytic reactions, especially those involving low rates and high degrees of selectivity, has been debated ever since the very early days of surface science. However, most studies on gas-surface interaction and chemisorption performed under controlled laboratory conditions have dealt so far with nearly perfect...

In the present paper we review our findings on ethylene adsorption on clean and oxygen covered Ag(001) surfaces investigated by dosing the gas with a Supersonic Molecular Beam and analysing the adsorption state either by High Resolution Electron Energy Loss Spectroscopy or by High Resolution X Rays Photoemission Spectroscopy. The final...