F. Tommasini

A simple and flexible atom-surface potential with easy-to-handle vibrational spectrum

Abstract A highly flexible empirical potential Vp(z) suitable to describe the main features of the laterally averaged atom-surface interaction is presented and an accurate expression for the vibrational spectrum of the atom in Vp(z) is given. This establishes a simple method of inversion of the atom-surface vibrational spectra particularly suitable for levels located in the deepest 9/10 of the potential well. The present method is complementary to the one introduced by Le Roy [5].

The ALOISA end station at Elettra: a novel multicoincidence spectrometer for angle resolved APECS

Abstract Coincidence measurements have been extensively performed in atomic and molecular physics since early 1970s. To apply this methodology to solids and surfaces has been a major target since early days, but the long average time needed to complete a coincidence experiment has hampered its attainment. In particular the coincidence technique has not been yet applied in an angle resolved way such for studying the momentum correlation in the ejection of electron pairs from solid surfaces. The experimental chamber at the ALOISA beamline at Elettra, by means of a set of seven homemade electron analyzers, is the first apparatus able to perform Angle Resolved – Auger Photoelectron Coincidence Spectroscopy (AR-APECS) from solid surfaces. In the typical setup ten different pairs of coincident electrons can be measured simultaneously, so reducing the acquisition time by one order of magnitude.

Surface resonant phonons of Ag(110)

Abstract A study of the dynamics of the Ag(110) surface in the ḡGȲ direction is presented. The experimental TOF spectra show the existence of two low frequency modes with high scattering intensity and a dispersionless high frequency mode with a smaller intensity. The theoretical analysis based on a force constant model that contains long-range central interactions and short-range angular forces explains accurately both the dispersions and the scattering intensities of the three modes in terms of resonant and localized vibrations related to the modifications of the surface force constants caused by the different role played by the many-body forces in the surface region.

The ALOISA end station at Elettra

Abstract Coincidence measurements have been extensively performed in atomic and molecular physics since early 1970s. To apply this methodology to solids and surfaces has been a major target since early days, but the long average time needed to complete a coincidence experiment has hampered its attainment. In particular the coincidence technique has not been yet applied in an angle resolved way such for studying the momentum correlation in the ejection of electron pairs from solid surfaces. The experimental chamber at the ALOISA beamline at Elettra, by means of a set of seven homemade electron analyzers, is the first apparatus able to perform Angle Resolved – Auger Photoelectron Coincidence Spectroscopy (AR-APECS) from solid surfaces. In the typical setup ten different pairs of coincident electrons can be measured simultaneously, so reducing the acquisition time by one order of magnitude.

He beam study of deconstruction and roughening of Au(110)(1×2)

Abstract The deconstruction of the Au(110) surface, i.e., the (1 × 2) → (1 × 1) phase transition, has been studied by He beam scattering. A critical temperature of 700 ± 5 K is indicated by the temperature dependence of the half-integral diffraction intensities, and the incoherent elastic intensity. Above 700 K the peak width variation is consistent with Ising-like behaviour, where the main contribution to lateral disorder comes from domain walls. It is also demonstrated that the roughening temperature is substantially larger than the deconstruction temperature. The tails of the specular peak decay as a function of parallel momentum according to a power law with exponent −1.55 ± 0.1 at 723 K, indicating that the roughening temperature is higher.

First results from the new optical configuration for a synchrotron radiation monochromator applied to the ALOISA beamline

The new beamline ALOISA is now working at the Elettra synchrotron facility. It is equipped with a monochromator of new design to cover the 200 eV - 8 KeV energy range: the monochromator has two interchangeable dispersive systems, one for the low energies up to 2 KeV using reflection gratings and another with crystals for the higher energies. The monochromatic beam is finally re-focused from the fixed exit slit to inside the experimental chamber by a toroidal mirror. In this paper the results of the first resolution measurements are reported for the low energy section of the monochromator: a resolving power higher than 5000, with peaks of 10000 in the 400 - 500 eV region, is obtained while using all the optical elements at full illumination.

He scattering from Rh(110)

Abstract High resolution He diffraction and bound state resonance measurements carried out with energy selection of the scattered beam from a Rh(110) surface are reported and analyzed. As for Ag(110), Cu(110) and Au(110), the lateral average of the electron density of Rh(110) is found to be in close agreement with that obtained by the superposition of atomic electron densities calculated within the degeneracy-dependent self-interaction corrected (D-SIC) local density approximation (LDA). The spread of the atomic electron densities in the surface plane exhibits behaviour similar to that observed for noble metals. The He-surface dispersion coefficient C is found experimentally to be 280 ± 20 meV A 3 .

Quantitative structure determination of (1 × 2)-Rh(110) by helium scattering

Abstract A new method for determining the structure of open surfaces is described and applied to the (1 × 2)-Rh(110) surface. Measurements of helium diffraction and specular reflectivity are analysed by close-coupling calculations, using a model potential given by the superposition of pseudo-pairwise terms. The measurements show that the (1 × 2)-Rh(110) surface has a missing row structure with a relaxation of the first interlayer spacing of 21%. The electronic charge of the topmost atoms as seen by He is about 7% lower for the (1× 2) structure than for the unreconstructed one.

Observation of acoustic and optical surface phonons in LiF(001) by inelastic He scattering

The vibrational spectrum of the LiF(001) surface measured by inelastic He scattering with time-of-flight detection along the and crystal directions is reported. The acoustic and optical modes are described in the whole surface Brillouin zone up to energies of 50 meV.

Temperature behavior of the (√3 × √3)R30°-1CO and the (2 × 2)-3CO overlayers on Rh(111): a combined HAS and LEED investigation

Abstract The order-disorder phase transition of the Rh(111)-(√3 × √3)R30°-CO phase was investigated by He scattering. The critical temperature turned out to be 330 ± 5K, while the critical exponents are consistent with a phase transition belonging to the 3-state Potts universality class. The temperature dependence below the critical temperature of both CO phases was studied using elastic He scattering and LEED. These measurements demonstrate that a simple Debye-Waller evaluation of the diffraction data is not appropriate to provide information on the vibrational properties of the CO molecules. From LEED measurements at low temperatures (