Alessandro Ruocco

Electronic structure of copper phthalocyanine: An experimental and theoretical study of occupied and unoccupied levels

An experimental and theoretical study of the electronic structure of copper phthalocyanine (CuPc) molecule is presented. We performed x-ray photoemission spectroscopy (XPS) and photoabsorption [x-ray absorption near-edge structure (XANES)] gas phase experiments and we compared the results with self-consistent field, density functional theory (DFT), and static-exchange theoretical calculations. In addition, ultraviolet photoelectron spectra (UPS) allowed disentangling several outer molecular orbitals. A detailed study of the two highest occupied orbitals (having a(1u) and b(1g) symmetries) is presented: the high energy resolution available for UPS measurements allowed resolving an extra feature assigned to vibrational stretching in the pyrrole rings. This observation, together with the computed DFT electron density distributions of the outer valence orbitals, suggests that the a(1u) orbital (the highest occupied molecular orbital) is mainly localized on the carbon atoms of pyrrole rings and it is doubly occupied, while the b(1g) orbital, singly occupied, is mainly localized on the Cu atom. Ab initio calculations of XPS and XANES spectra at carbon K edge of CuPc are also presented. The comparison between experiment and theory revealed that, in spite of being formally not equivalent, carbon atoms of the benzene rings experience a similar electronic environment. Carbon K-edge absorption spectra were interpreted in terms of different contributions coming from chemically shifted C 1s orbitals of the nonequivalent carbon atoms on the inner ring of the molecule formed by the sequence of CN bonds and on the benzene rings, respectively, and also in terms of different electronic distributions of the excited lowest unoccupied molecular orbital (LUMO) and LUMO+1. In particular, the degenerate LUMO appears to be mostly localized on the inner pyrrole ring.

Electronic states of CuPc chains on the Au(110) surface

The electronic properties of Cu-phthalocyanine (CuPc) molecules flat lying along the channels of the Au(110) reconstructed surface have been investigated by means of ultraviolet and x-ray photoelectron spectroscopy. The ordered chains give rise to a highly ordered single-layer structure with a (5x3) symmetry. Although from the core-level analysis not any significant charge transfer between the molecules and the underlying Au surface is observed, the valence band photoemission data bring to light CuPc-induced features localized at the interface. In particular, energy versus momentum dispersion of an interface state reveals a bandwidth of about 90 meV along the enlarged Au channels, where the CuPc chains are formed, with a defined fivefold symmetry well fitting the CuPc intermolecular distance.

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.

Relevance of the electron energy-loss spectroscopy for in situ studies of the growth mechanism of copper phthalocyanine molecules on metal surfaces: Al(100)

Reflection electron energy loss spectroscopy (EELS) in specular and off specular geometry has been employed to study the early stage of the copper phthalocyanine (CuPc) growth on Al (100) substrate. EEL spectroscopy has been a useful tool in order to study electronic structure of molecular films also in the submonolayer regime. The electronic structure of the first deposited layer of CuPc is strongly influenced by charge transfer from the Al substrate to the lowest unoccupied molecular orbital (LUMO). The strong molecule-substrate interaction gives rise to a coverage dependent frequency shift of the Al surface plasmon. Successive layers have essentially the electronic structure of the molecular solid. Momentum resolved EELS measurements reveal that, in the case of the thicker film investigated (22 \AA), the plane of the molecule is almost perpendicular to the surface of the substrate.

Interplay of the volume and surface plasmons in the electron energy loss spectra of C60

The results of a joint experimental and theoretical investigation of the C60 collective excitations in the process of inelastic scattering of electrons are presented. The shape of the electron energy loss spectrum is observed to vary when the scattering angle increases. This variation arising due to the electron diffraction of the fullerene shell is described by a new theoretical model which treats the fullerene as a spherical shell of a finite width and accounts for the two modes of the surface plasmon and for the volume plasmon as well. It is shown that at small angles the inelastic scattering cross section is determined mostly by the symmetric mode of the surface plasmon, while at larger angles the contributions of the antisymmetric surface plasmon and the volume plasmon become prominent.

HYDROGEN-INDUCED DERELAXATION ON A MBE GROWN GAAS(110) SURFACE

Abstract Up to now only indirect evidences of hydrogen induced derelaxation of the GaAs(110) surface have been obtained. In order to gain direct information on the surface structure MBE prepared GaAs(110) surfaces have been studied after H chemisorption (∼ 1 monolayer) by means of grazing incidence X-ray diffraction (GIXD). Because of the weak interaction of X-rays with H the effect of the hydrogenation can only be addressed through the modifications induced in the GaAs outermost layer. In the absence of surface reconstruction, the information on the in-plane displacements of the Ga and As surface atoms has been derived from integer order peaks at Δq z = 0. Assuming a constant bond length between surface and bulk atoms, an estimate of the normal atomic coordinates could also be obtained. The main result is a change in the surface structure after hydrogen chemisorption corresponding to a derelaxation of the hydrogenated GaAs(110) surface.

Contribution of surface plasmon decay to secondary electron emission from an Al surface

Spectra of secondary electrons (SE) emitted from a polycrystalline Al surface have been measured in coincidence with 500 eV‐electrons for energy losses between 10 and 155 eV. The spectra for a given energy loss are qualitatively similar, consisting of surface and volume plasmon decay and a contribution attributable to direct electron–electron scattering. The similarity of the contribution of surface and volume plasmon decay in the SE spectra proves directly that electron multiple scattering is governed by a Markov‐type process. The average value of the surface plasmon decay contribution to the SE spectrum amounts to ∼25%.

Experimental and theoretical study of the PL2,3VV Auger lineshape of GaP(110)

Abstract A joint experimental and theoretical investigation of the PL 2.3 VV Auger transition on GaP(110) is presented. The theoretical data, calculated within a pure band-like picture, are compared to the experimental spectra. The angular momentum character of the five features constituting the lineshape is identified. Strong contributions to the experimental data from many-body effects may be ruled out due to the good overall agreement with the theoretical lineshape therefore supporting a single particle description for this Auger feature. We discuss effects such as Coulomb correlation, initial state core hole screening, self-energy corrections, that may possibly contribute to the relative energy shifts observed in one of the pp components of the compared spectra.

Evidence for the collapse of short-range magnetic order in CoO at the Néel temperature

Auger-photoelectron coincidence spectroscopy is used for investigating the electronic properties of a CoO thin film above and below the magnetic transition temperature (TN). By using the dichroic effect in angle-resolved measurements, we identify and assign well-defined high-spin and low-spin structures in spite of the otherwise featureless Auger singles spectra, typically found for open-band systems. The disappearance of the dichroism for temperatures just above TN indicates a collapse of the surface short-range magnetic order, presumably due to a strongly reduced exchange field in the surface compared to that in the bulk.

Experimental and theoretical investigation of the pyrrole/Al(100) interface.

The electronic properties of the pyrrole/Al(100) interface have been investigated from both a theoretical and experimental point of view. Electron energy loss spectroscopy (EELS) in specular reflection geometry does not reveal modification of the electronic structure of the molecule when adsorbed on the Al surface. EELS results and the low desorption temperature of pyrrole indicate a weak molecule/metal interaction. Ab initio calculations in the framework of the single-particle density functional theory within the local density approximation was used to investigate the adsorption energy and geometry. The low adsorption energy, -0.51 eV per molecule, and the high N-Al distance, 1.98 A, confirm the weak interaction of pyrrole adsorbed on the Al surface.