Angle Resolved Research Articles

Atomistic Understanding of the Formation, Structure, and Decomposition of an Fe4C Iron Carbide Phase on a Copper Substrate

Having an atomistic understanding of the formation and structural characteristics of bulk and surface iron carbide phases plays a crucial role in comprehending the mechanisms of phase transformation, microstructure formation, and surface reconstructions in the development of advanced materials with improved magnetic, mechanical, and catalytic properties. This study uses synchrotron (SR) and angle-resolved (AR) XPS, STM, LEED, and theoretical calculations (DFT) with the aim to provide a detailed experimental and theoretical discussion on the formation, stabilization, structure, and decomposition of Fe4C iron carbide. FCC(100) Fe4C iron carbide multilayer films were prepared on Cu(100) by evaporating iron in the presence of an ethylene atmosphere. Angle-resolved XPS measurements reveal that surface and interior carbon can be distinguished due to their appearance at different binding energies and reveal that the surface consists of Fe2C while the bulk has Fe4C stoichiometry. LEED and STM measurements show that the surface exhibits the p4g(2 × 2) clock reconstruction. Theory simulations show that the bulk Fe4C iron carbide has a constrained crystal lattice with alternating Fe and Fe4C2 layers, which grow in a pseudomorphic way on the copper. The carbide phase is stable up to 700 K. Above this temperature, iron diffuses into the copper while carbon remains on the surface, where it forms graphite.

Impact of molecular distribution on unoccupied image potential states of PbPc/HOPG surfaces

Two-photon photoemission (2PPE) spectroscopy revealed modified image potential states (IPSs) of sub-monolayer lead phthalocyanine (PbPc) films on graphite (HOPG). Angle-resolved (AR) 2PPE for room temperature films showed unique energy/momentum maps in which the inside of the parabolic dispersion curve of IPS on HOPG area is filled up to a specific energy. For cooled films, AR maps showed two parabolic curves originating from IPSs on HOPG areas and PbPc islands. The changes are explained with simple models of electron scattering by molecular islands. IPS feature is an indicator of nano-scale superstructures, which are not easily assessed by other experiments.

Angle-resolved RABBITT: theory and numerics

Angle-resolved (AR) RABBITT measurements offer a high information content measurement scheme, due to the presence of multiple, interfering, ionization channels combined with a phase-sensitive observable in the form of angle and time-resolved photoelectron interferograms. In order to explore the characteristics and potentials of AR-RABBITT, a perturbative 2-photon model is developed; based on this model, example AR-RABBITT results are computed for model and real systems, for a range of RABBITT schemes. These results indicate some of the phenomena to be expected in AR-RABBITT measurements, and suggest various applications of the technique in photoionization metrology.

Characterization of the interface of binary mixed DOPC:DOPS liposomes in water: The impact of charge condensation.

Solutions of liposomes composed of binary mixtures of anionic dioleoylphosphatidylserine (DOPS) and zwitterionic dioleoylphosphatidylcholine (DOPC) are investigated with label-free angle-resolved (AR) second harmonic scattering (SHS) and electrophoretic mobility measurements. The membrane surface potential is extracted from the AR-SHS response. The surface potential changes from -10 to -145 mV with varying DOPS content ( from 0% to 100%) and levels off already at ∼ 10 % DOPS content. The ζ-potential shows the same trend but with a drastically lower saturation value (-44 mV). This difference is explained by the formation of a condensed layer of Na+ counterions around the outer leaflet of the liposome as predicted by charge condensation theories for polyelectrolyte systems.

Unoccupied band dispersion of Si(1 1 1):√3 × √3-Ag surface studied by time- and angle-resolved two-photon photoemission spectroscopy

Band dispersion and transient population of unoccupied electronic states on Si(111):√3×√3-Ag surface have been studied by time-resolved (TR) and angle-resolved (AR) two-photon photoemission (2PPE) spectroscopy. The band dispersions originating from unoccupied electronic states have been identified from the comparison between AR-2PPE spectra and angle-resolved one-photon photoemission spectra with synchrotron radiation. A lifetime of unoccupied surface state has been determined from the TR-2PPE spectra.

Internal Energy of Product CO2 in CO Oxidation on Pd Surfaces

Rotational and vibrational energies of desorbing products in chemical reactions provide information on dynamical processes of bond formation and bond rupture and also on structures of transition states. Energies of product CO2 in CO oxidation on metal surfaces can be determined by analysis of infrared chemiluminescence spectra from CO2. However, angle-resolved (AR) measurements of these energies in thermal surface reactions have long been lacking due to difficulty of experiments, although AR measurements is required to obtain detailed structural information. In this review, recent progress in the AR measurements in CO oxidation on Pd surfaces is described. It was found that both internal and vibrational energies changed depending on polar- and azimuthal angles, surface structures, and rotational/vibrational modes. From these phenomena, new insights into transition states and energy partition dynamics can be obtained.

Angle-Resolved Measurements of Rotational and Vibrational Energies of Product CO2 in Catalytic CO Oxidation on Pd Surfaces

Measurements of rotational and vibrational energies of desorbing products in chemical reactions provide useful information on dynamical processes of bond formation and bond rupture and also on structures of transition states. In order to obtain structural information, dependence of these energies on the desorption angle of products should be studied. However, such angle-resolved (AR) measurements of internal energies in thermal surface reactions have long time been lacking due to difficulty of experiments. Recently, measurements of internal energies of AR product CO2 in CO oxidation on Pd surfaces have become possible by detecting extremely weak infrared chemiluminescence from desorbing CO2. Rotational and vibrational energies change depending on polar and azimuth angles and also surface structures, providing new insights into structures of transition states and energy partition dynamics. In this review, recent progress in the above AR measurements is described. [DOI: 10.1380/ejssnt.2008.180]

Observation of HfO 2 thin films by deep UV spectroscopic ellipsometry

Deep UV spectroscopic ellipsometry (SE) is used for structure change observations in thin hafnia (HfO 2) layers deposited by p-MOCVD on silicon substrate. The absorption edge E g and most of the critical point transitions in HfO 2 are above 6 eV, which makes the extension to Deep UV SE (5–9 eV) very suitable. The phase mixture changes as function of thickness and deposition process temperature, deduced from SE correspond well to XRD and Angle Resolved (AR)-XPS spectroscopy observations. From the absorption spectra at 4.5 eV, defects such as oxygen vacancies are detected, whereas from XPS spectra the estimation of the O/Hf ratio follows the same trend. Deep UV SE reveals differences in the dielectric function with orthorhombic/monoclinic phase mixtures essentially with peaks at 7.5 and 8.5 eV. Quantum confinement originated from the grain size of the films and the excitonic origin of the 6 eV feature are discussed.

Photoelectron–Auger electron coincidence study for condensed matter

Advances in materials science have produced a wide array of new solid-state systems with tunable properties and previously unattainable combinations of phenomena that hold the promise of entirely new approaches to technological applications. Invariably, these new materials are increasingly complex and include a large number of constituents in a variety of chemical states. Entirely new theoretical and experimental approaches are needed to gain the insights necessary for intelligent engineering of these materials. In the past 20 years, a steadily increasing number of electron–electron coincidence experiments on atoms and molecules have demonstrated the capability of investigating complicated systems with sensitivity and specificity well beyond the limits imposed by conventional electron spectroscopies. Over the past decade or so, Auger–photoelectron coincidence spectroscopy (APECS) has emerged as a powerful technique for obtaining detailed information about complex materials systems. Moreover, the recent advent of angle-resolved (AR)-APECS has introduced a new level of discrimination in studying the distribution of electrons photoemitted from complex systems. In this review, we describe the basic ideas behind APECS and discuss a study of the SiO 2 system as an example of the unique information this technique can provide. We then introduce the concept of AR-APECS, explain its novel state and angular momentum selectivity that can be used to disentangle information about complex systems that is hidden to conventional spectroscopies. Examples of AR-APECS measurements from Cu, Ge, and graphite that exemplify the capabilities of this technique are presented.

Angle-resolved PED and AED calculations for different structures of the diamond C(111) surface

Angle-resolved (AR) photoelectron diffraction (PED) spectra for electrons excited from the C 1s core state and angle-resolved KVV Auger electron diffraction (AED) spectra are calculated for the Pandey and the Tsai stucture models of diamond C(111) which extend previous investigations of the ideal structure. It is shown how to decide on the structure model by comparing PE spectra for different directions and by comparing PED and AED spectra. Calculations have been performed by evaluating the scattering path operator for a finite cluster in a curved-wave approximation. The different matrix elements for the photoelectron excitation and for the Auger process, respectively, are included. It is shown that the PED intensities are very sensitive to the surface reconstruction for polar angles in the range of 80°. In the AED intensities, polar scans in the plane perpendicular to the chain direction can be considered.

Surface termination of GaAs(001) by Sb dimers

Angle-integrated and angle-resolved (AR) photoelectron spectroscopy (PES) experiments were performed on GaAs(001) 2 × 4-Sb surfaces. The results of angle-integrated PES show that only Sb atoms lie on the GaAs surface and that they bond to Ga atoms. ARPES experiments revealed a surface state whose dispersion is very similar to the As dimer dangling bond surface state of As-terminated GaAs(001). These results suggested that Sb dimers terminate GaAs surfaces and bond to Ga atoms of the second top atomic layer.

Angle-resolved X-ray photoelectron spectroscopy contribution to elucidation of the mechanism of cathodic deposition of AsSb alloys

AsSb alloys were deposited electrochemically onto flat, smooth Ni-plated Cu sheets from aqueous citric acid solutions and analysed by X-ray photoelectron spectroscopy (XPS) and angle-resolved (AR) XPS. Samples prepared and manipulated in air showed marked surface oxidation of both alloy elements, present as As 2O 3 and Sb 2O 3. Oxide-free samples were obtained when electrochemical synthesis and transfer to the ultrahigh vacuum XPS chamber were carried out in an inert atmosphere. ARXPS analysis of oxide-free alloy samples obtained at large negative potential showed an increase in intensity of the As(2p 3 2 ) peak at grazing take off angles. This result, showing As surface enrichment, supports a previously proposed kinetic model of AsSb alloy electrodeposition.