G. Stefani

High-energy photoemission in silver: resolving d and sp contributions in valence band spectra

We present high-resolution valence band and core level spectra of silver for photoelectron kinetic energies up to 8 keV. At these kinetic energies we estimate a surface contribution of less than 3%. Taking advantage of the favourable sp/d relative cross-sections, a comparison with the calculated density of states is presented. We observe an increasing photoemission intensity when approaching the Fermi level, which we assign to a free-electron-like character in the 5p-band, whereas the principal s-like contribution is located at the bottom of the d-band. The difference between measured and calculated values of the sp/d cross-section ratio is discussed.

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.

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.

Electron–electron coincidence spectroscopies at surfaces

Abstract In the past 20 years, a steadily increasing number of electron–electron coincidence experiments on atoms and molecules have contributed to a deeper understanding of electron–electron correlation effects. In more recent years this technique has been extended to the study of solid surfaces. This class of one photon IN two electrons OUT experiments will be discussed with an emphasis on grazing incidence geometry, that is expected to be particularly suited for studying surfaces. The crucial question of which is the dominant mechanism that leads to ejection of pairs of electron from the surface will be addressed. It will be shown that, depending on the kinematics chosen, the correlated behaviour of the pairs of electrons detected might be singled out from independent particle one.

A novel apparatus for laser-excited time-resolved photoemission spectroscopy

A novel apparatus devoted to time-resolved photoemission experiments in the sub-picosecond regime will be presented. The system is composed of a Ti:sapphire laser source and a time of flight (TOF) electron energy analyzer mounted in a UHV experimental chamber. The laser source is characterized by a pulse duration of 150 fs at a wavelength of 790 nm (1.57 eV) and operates at a repetition rate of 1 kHz. To perform photoemission measurements, UV radiation up to 6.28 eV is produced with sequential steps of frequency conversion by employing crystals with a second order nonlinearity. Photoelectrons are collected by a TOF spectrometer designed to analyze electrons from tenths of eV up to 5 eV. It can be operated in two different angular resolution modes switching on and off an electrostatic collection optics: the high angular resolution mode (Δα = ±2.7°) and the low angular resolution mode (Δα = ±5.6°). Single photon photoemission spectra from the Ag(100) clean surface have been recorded at room temperature using the fourth harmonic light (λ = 200 nm and hν = 6.28 eV). The Fermi edge profile convoluted with a Gaussian-shaped energy transmission function of the TOF spectrometer sets an upper limit for the energy resolution which is about 65 meV (FWHM) at 2 eV of electron energy.

Correlation effects in Auger cascade studied by angle resolved Coincidence Electron Spectroscopy: the 1s->3p excitation in neon

Abstract In this work, the angular correlation of the two electrons produced in the cascade of Ne 1s −1 3p→Ne + 2s −1 2p −1 ( 1 P) 3p+e A ( E =778.4 eV)→Ne ++ 2p −2 ( 1 D, 3 P)+e C ( E =22.3, 25.7 eV) has been measured for the first time in a coincidence experiment. These measurements have been complemented by the non-coincidence angular distributions and energy spectra of both resonant Auger and second step Coster–Kronig electrons. Some results of an ab-initio calculation of the first step decay are also reported.

Study of electronic correlations in the Auger cascade decay from Ne*1s−13p

We present an experimental and theoretical study of the angular correlation between resonant Auger and subsequent Coster–Kronig electrons emitted in the cascade decay of Ne (1s−13p) resonance excited by linearly polarized light. The angular correlation has been measured for different relative orientations between the light polarization, the emission direction of the Coster–Kronig electron and the emission angle of the resonant Auger electron. All the experimental angular patterns are measured as a function of the resonant Auger emission angle and are compared with multi-configuration Dirac–Fock model numerical results, which include electron correlations in each step of the cascade. The agreement between theory and experiment is different for the different considered decay channels; for the cases in which disagreement is larger it is argued that an extended configuration space must be taken into account to reproduce the experimental data.

On the scattering mechanism of the electron impact on surfaces in specular reflection geometry: Nickel (110)

Lately several (e,2e) experiments on surfaces have been performed under specular reflection geometry. To ascertain the dominant interaction mechanism is of key relevance in order to derive, from these experiments, the correct information on the target electronic structure. This can be done performing a simple angle resolved energy loss experiment instead of a complex coincidence one. Elastically and inelastically specularly reflected electrons have been measured as a function of the primary electron kinetic energy (IV curve) on graphite and Ni. The incident beam energy was varied between 150 and 450 eV. Several IV curves, each one at fixed energy loss between 6 and 35 eV, have been collected. These results show that an inelastic scattering is always accompanied by an elastic one giving rise to two independent channels: the elastic event follow (L+D) or precede (D+L) the inelastic one. They also indicate that none of the two channels dominates over the other irrespective of the degree of localization of the excitation involved.