Photoelectron Diffraction Effects Research Articles

Unraveling Atomic and Electronic Surface Structure and Dynamics from Angular Photoelectron Distributions

Angle-resolved photoelectron spectroscopy (ARPES) is a powerful tool in solid state sciences. Beside the direct measurement of the energy-momentum dispersion relation, the angular distribution of the photoelectron current reveals the structural environment of the emitting atoms via photoelectron diffraction effects. Moreover, in the case of molecular layers, the angular distribution of emission from molecular orbitals can be directly related to their charge density distribution via so-called orbital tomography. In the present paper we summarize our efforts undertaken over the past 12 years to add the dimension of time to these two methods via pump-probe experiments with femtosecond resolution. We give a comprehensive introduction to standard ARPES and time-resolved two photon photoemission and then focus on our efforts towards time-resolved versions of photoelectron diffraction and orbital tomography. Both, optimization of experimental parameters and data acquisition procedures, as well as new numerical tools are needed in order to realize such challenging full stop missing after experiments.

High-accuracy bulk electronic bandmapping with eliminated diffraction effects using hard X-ray photoelectron momentum microscopy

A key benefit of angle-resolved photoelectron spectroscopy (ARPES) in the X-ray range is the significant increase of the information depth, thanks to the large inelastic mean-free-path of the escaping photoelectrons. In practice hard X-ray ARPES (HARPES) faces severe challenges by low cross sections, large photon momentum transfer, and in particular strong phonon scattering and photoelectron diffraction effects. Here, we show that these challenges can be overcome by extending ultra-efficient time-of-flight momentum microscopy into the hard X-ray regime. Phonon scattering destroys the initial momentum distribution but subsequent diffraction at the lattice imprints a pronounced Kikuchi-type pattern on the background signal. Moreover, the pattern of the valence electrons is modulated by diffraction as well. For the examples of the medium-weight element materials Mo and layered TiTe2, we demonstrate how comprehensive valence-band and core-level photoemission data taken under identical conditions can be used to effectively remove photoelectron diffraction effects in HARPES band maps.

Time-Resolved Photoelectron Imaging of Molecular Coherent Excitation and Charge Migration by Ultrashort Laser Pulses

Charge migration is a fundamental and important process in the photochemistry of molecules and has been explored by time-resolved photoelectron angular distributions. A scheme based on UV pump and polarized soft X-ray probe techniques shows that photoelectron diffraction effects enable us to reconstruct electronic coherences encoding the information of the charge migration with extreme time resolutions. We discuss how probe pulse helicity influences the probing photoelectron spectra in the presence of molecular nonspherical Coulomb potentials. This phenomenon is analyzed theoretically and simulated via ab initio calculations for the molecular hydrogen ion, offering a reliable approach for measurements of charge migration and for the exploration of molecular structure in attosecond science.

Gap state imaging and spin‐orbit effects in resonant photoelectron diffraction

In core‐level photoelectron diffraction, the surface atomic structure is imaged through a local source wave. In valence photoemission, the source wave gets effectively localized at a core‐valence resonance, and local information on valence states can be obtained. Through photoelectron diffraction at the Ti 2p − 3d resonance, we have determined the charge distribution of the gap state at n‐doped TiO2 surfaces. Second, the origin of spin polarization in resonant photoemission from non‐ferromagnetic surfaces is critically reviewed. We have developed a multiple scattering computational method for resonant photoemission and have applied it to Cr(110) and Fe(100). For circular polarized light, we find that the resonant photocurrent is strongly spin polarized in antiferromagnets and disordered ferromagnets, in agreement with experiment. Contrary to previous interpretations, we show that this spin polarization is essentially unrelated to local magnetic moments, but because of an angular momentum transfer from the photon to the photoelectron via core spin‐orbit coupling and exchange autoionization decay. Copyright © 2016 John Wiley & Sons, Ltd.

A detailed look beneath the surface: Evidence of a surface reconstruction beneath a capping layer

Many physical effects are strongly depending on the composition of the interfaces between separating layers. Hence, the knowledge of the interfacial characteristics such as structure, chemical bonds, or magnetic properties of the corresponding materials is essential for an understanding and optimization of these effects. This study reports on a combined magnetic and structural analysis using X-ray photoelectron diffraction (XPD) and transverse magneto-optical Kerr effect (T-MOKE). The information depth of these methods is demonstrated by investigating the uppermost GaAs(001) layer beneath a Fe-film and the interfacial regimes of Fe/GaAs(001) beneath an MgO capping layer.Iron was prepared on a clean GaAs(001) surface and a GaAs(001)-(4×2)-reconstructed surface. Beneath the Fe-film, the (4×2)-reconstruction is not lifted, which is clearly shown by the diffraction pattern of the GaAs(4×2)-Fe surface. It is shown that Fe inter-diffusion, resulting in an amorphous interface, is almost prevented by the Ga-rich reconstruction. The magneto-optical measurements with T-MOKE clearly demonstrated the Fe-interlayer in a ferromagnetic state. We find no evidence for magnetic properties neither within the signal of the GaAs-substrate nor the MgO-film.

Probing bulk electronic structure with hard X-ray angle-resolved photoemission

Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.

Theoretical study of focusing and double slit effects in x-ray photoelectron diffraction

Double-slit and forward focusing effects in XPD spectra from H2O and GeCl2 molecules have been theoretically studied within the single scattering approximation. In low-and intermediate-energy region, the double-slit effect can be observed. In contrast, this effect wears off and only forward focusing peaks with simple structure are given in high-energy region. The double-slit effects can be observed under special conditions where the two scattering amplitudes have finite overlap in the bisecting direction.

Photoelectron diffraction effect in the highly ordered Si(5 5 7)/Pb surface

The electronic structure of vicinal Si(5 5 7) surface covered with 2 ML Pb, ordered after annealing at 640 K as found previously [C. Tegenkamp, Z. Kallassy, H.-L. Gunter, V. Zielasek, H. Pfnür, Eur. Phys. J. B 43 (2005) 557; C. Tegenkamp, Z. Kallassy, H. Pfnür, H.-L. Gunter, V. Zielasek, M. Henzler, Phys. Rev. Lett. 95 (2005) 176804], is studied with angle-resolved photoemission spectroscopy (ARPES) at a temperature of 130 K. The spectra show a superposition of Pb-induced electronic surface states and the Si(1 1 1) bulk band states. The observed splitting of a Si bulk band suggests photoelectron diffraction on the one-dimensional grid of the vicinal surface.

Investigation of surface structure related features in the multiple-scattering simulations of photoelectron diffraction of 3C–SiC(0 0 1)- [formula omitted

In this paper we have conducted a systematic study of the photoelectron diffraction (PED) effects from the 3C–SiC(0 0 1)- c ( 4 × 2 ) surface by means of multiple-scattering calculations. Two models have been proposed to describe this surface: alternately up and down dimers (AUDD) [P. Soukiassian, F. Semond, L. Douillard, A. Mayne, G. Djuradin, L. Pizzagalli, C. Joachim, Phys. Rev. Lett. 78 (1997) 907], based on the scanning tunneling microscopy data, and missing-row asymmetric-dimer (MRAD) [W. Lu, P. Krüger, J. Pollmann, Phys. Rev. Lett. 81 (1998) 2292], based on the total energy pseudopotential calculations. By calculating PED patterns from different emitters in these two models, we show that the surface structure induced features are visible even in total emission diffraction patterns. For the overall diffractogram a scatterer properties of the first layer under the surface become crucial due to the backscattering at low kinetic energies. This layer is Si-layer in the MRAD model, but C-layer in the AUDD model. While this causes clear differences between the diffraction patterns of the AUDD and the MRAD models, it is shown to diminish the differences between the PED patterns of the AUDD and the other models which consist of only one monolayer of Si on top.

Nondipole effects in photoemission angular distribution excited by high-energy X-rays

Angular distribution of emitted photoelectrons is theoretically studied for high-energy X-rays where electric quadrupole and magnetic dipole transitions play some important roles in addition to the electric dipole transitions. We apply irreducible tensor expansion of photon field to discuss the high-energy photoelectron angular distribution instead of the usual power series expansion. The present numerical calculations show that the conventional power series expansion works so well up to quite high energy ( ≈ 10 keV) for core excitation. Photoelectron diffraction effects make the nondipole contribution sensitive to the geometrical setup of surrounding scattering atoms because of the strong focusing effect in the high-energy region. Total high-energy photoemission intensities from extended levels are well described by use of the Gelius formula because the interference terms are effectively smeared out by Debye–Waller factors.

Observation and resonant x-ray optical interpretation of multi-atom resonant photoemission effects in O1semission from NiO

We present experimental and theoretical results for the variation of the O 1s intensity from a NiO(001) surface as the excitation energy is varied through the Ni 2p1/2,3/2 absorption resonances, and as the incidence angle of the radiation is varied from grazing to larger values. For grazing incidence, a strong multi-atom resonant photoemission (MARPE) effect is seen on the O 1s intensity as the Ni 2p resonances are crossed, but its magnitude decreases rapidly as the incidence angle is increased. Resonant x-ray optical (RXRO) calculations are found to predict these effects very well, although the experimental effects are found to decrease at higher incidence angles faster than those in theory. The potential influence of photoelectron diffraction effects on such measurements are also considered, including experimental data with azimuthal-angle variation and corresponding multiple-scattering-diffraction calculations, but we conclude that they do not vary beyond what is expected on the basis of the change in photoelectron kinetic energy. Varying from linear polarization to circular polarization is found to enhance these effects in NiO considerably, although the reasons are not clear. We also discuss the relationship of these measurements to other related interatomic resonance experiments and theoretical developments, and make some suggestions for future studies in this area.

Photoemission study ofCOadsorption on orderedPb∕Ni(111)surface phases

$\mathrm{Pb}∕\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}(111)$ surface phases were investigated by synchrotron radiation photoemission and low-energy electron diffraction. For room temperature deposition of Pb, two surface ordered layers, $(3\ifmmode\times\else\texttimes\fi{}3)$ and $(4\ifmmode\times\else\texttimes\fi{}4)$, were observed. The $\mathrm{Pb}\phantom{\rule{0.3em}{0ex}}5d$ and $\mathrm{Pb}\phantom{\rule{0.3em}{0ex}}4f$ core levels as well as valence band spectra indicated a weak chemical interaction between Pb and Ni, and the formation of a close-packed overlayer with Pb atoms in two different adsorption sites. Annealing of the $\mathrm{Pb}∕\mathrm{Ni}\phantom{\rule{0.3em}{0ex}}(111)$ surface led to the formation of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction, characterized by a topmost layer consisting of a substitutional alloy. The transformation to the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structure was accompanied by the appearance of a strong photoelectron diffraction effect, confirming embedding of Pb atoms in the $\mathrm{Ni}(111)$ first surface layer. $\mathrm{CO}$ adsorption results showed that lead simply blocked the $\mathrm{CO}$ adsorption sites for the unannealed surface while the surface alloy exhibited a chemical effect of weakening of the $\mathrm{CO}\text{\ensuremath{-}}\mathrm{Ni}$ bond. The $\mathrm{Pb}\phantom{\rule{0.3em}{0ex}}5d$ core-level shift indicated charge transfer from Pb to the surface, particularly for $(4\ifmmode\times\else\texttimes\fi{}4)$ and $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structures.

X-ray photoelectron spectroscopy and diffraction in the hard X-ray regime: Fundamental considerations and future possibilities

The prospects for extending X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) measurements into the hard X-ray regime of 5–15 keV excitation energies are discussed from a fundamental point of view, in some cases using prior results obtained in the 1–2 keV range as starting points of discussion, together with theoretical estimates of behavior at higher energies. Subjects treated are: the instrumentation improvements needed to optimize peak intensities; the tuning of experimental conditions to achieve bulk or surface sensitivity; the use of grazing incidence to suppress spectral backgrounds; the use of standing waves created by Bragg reflection from crystal planes or synthetic multilayers to achieve position-sensitive densities of states, compositions, and magnetizations; photoelectron diffraction and Kikuchi-band effects as element-specific local structure probes; and valence-level measurements, including the role of non-dipole effects and mechanisms leading to complete Brillouin zone averaging and density-of-states like spectra. Several distinct advantages are found for such high-energy extensions of the XPS and XPD techniques.

Real-time analysis of a surface phase transition of GaAs (0 0 1) by core-level photoelectron spectroscopy and photoelectron diffraction

The surface phase transition of GaAs (0 0 1) from the As-rich 2×4 surface to the Ga-rich 4×2 surface and the reverse transition were analyzed in real-time using time-resolved core-level photoelectron spectroscopy incorporating the photoelectron diffraction effect. From the analysis of the time dependence of core-level photoelectron intensities and spectra, we found a large anisotropy in the intensities and spectra caused by the photoelectron diffraction effect. This anisotropy in the intermediate state of the transition from the 2×4 to the 4×2 differs from that of the reverse transition, indicating that the surface phase transition pathways differ.

Multiple-scattering cluster model of photoelectron diffraction in magnetic solids

Photoelectron diffraction (PED) effects in magnetic solids are described within a one-electron non-relativistic model. The theory presented permits a straightforward calculation of spin-resolved PED patterns, whereby different spin arrangements in a magnetic solid may be considered. Both spin–orbit interaction and exchange interaction with the spin-polarized valence band have been taken into account in the description of the initial core state. The multiple scattering of the excited photoelectron wave is treated within a multiple-scattering cluster model. The theory may be applied to explain PED effects in magnetic solids in comparison to the nonmagnetic case.

Angular and temperature dependence of the magnetic circular dichroism in4dcore-level photoemission from Gd(0001)

We present experimental and theoretical results for the angular and temperature dependence of magnetic circular dichroism in Gd $4d$ core-level photoelectron emission from a Gd(0001) surface in both normal and off-normal directions and with azimuthal variation. Two theoretical approaches are used to model this data: a single electron theory with full multiple scattering of the outgoing photoelectron and a full-relativistic many-electron theory with single scattering only. Thermal effects due to atomic vibrations and the excitation of initial-state multiplets are also included. For normal emission, we find smooth free-atom-like angular variations in emission intensity, while for off-normal emission, deviations from a purely atomic model due to photoelectron diffraction effects are seen that are well predicted by photoelectron diffraction theory. We also compare dichroism measurements using two different approaches (fixed magnetization with variable light helicity and fixed light helicity with rotated sample magnetization) and find significant differences between them that are also well predicted by theory. The angular dependence of magnetic circular dichroism for a specific set of Gd $4d$ multiplet states has also been measured with high electron energy resolution (\ensuremath{\leqslant}100 meV), permitting a state-specific decomposition of the dichroism. Such state-resolved dichroism is found to be very well described by our many-electron approach. Finally, we present temperature-dependent magnetic circular dichroism (MCD) data for $4d$ emission that should permit the study of near-surface magnetic phase transitions, and discuss the relationship of such MCD measurements to magnetization. Some future prospects and applications of such core-photoemission dichroism measurements are discussed.

Optical properties in photoelectron diffraction theory

Photoelectron diffraction effects may be strongly influenced by optical properties of the solid. Due to refraction and absorption of light at a surface, the state of polarization of light may be changed and, therefore, the corresponding photoelectron diffraction intensity. In order to include optical properties of solids in photoelectron diffraction theory, a general polarization vector of light is defined taking into account both refraction and absorption of light at the vacuum–solid boundary. In a first step, the radiation field of light inside the solid is approximated macroscopically according to classical electrodynamics. Analytical expressions are derived within a real-angle representation of Fresnel equations to reveal the influence of refraction and absorption of light on the state of polarization of light. A general refractive index in dependence on the incident angle of light is introduced which determines the refractive angle inside the solid (refraction law with real angles) and the order of influence of absorption of light. The general polarization vector is applied in the calculation of dipole transition matrix elements in a multiple-scattering cluster model of photoelectron diffraction, where the process of photoabsorption (dipole transition matrix element) and multiple scattering processes of photoelectrons (scattering path operator) are considered separately. Different analytical and numerical results of photoelectron diffraction effects are presented for light of general polarization incident at general angles taking into account the optical properties of the solid. Examples are shown and discussed in detail for Cu(001) and GaAs(110) surfaces.

High-resolution core level photoelectron spectroscopy on InP(110)

We investigate the lineshape of In4d and P2p core level emission spectra recorded with high spectral resolution on cleaved InP(110) surfaces for a wide range of photon energies. A curve fitting analysis with Voigt functions is used to decompose the spectra in surface and bulk core level emission lines. We find that an intrinsic broadening of approximately 250 meV limits the linewidth of the In4d and P2p emission components of both surface and bulk. We attribute the broadening to local band bending variations induced by surface inhomogeneities such as cleavage steps and atomic defects. Surface core level shifts of 320 meV for In4d and 305 meV for P2p are found, in excellent agreement with previously published data. The intensity of the surface components with respect to those of the bulk is found to deviate from the expectation based upon the electron escape depth due to photoelectron diffraction effects.

Angular dependence of magnetic dichroism and spin polarization in angle-resolved core-level photoemission

Photoelectron diffraction theory is applied to the investigation of the dependence of magnetic dichroism and spin polarization on the emission direction of photoelectrons in angle-resolved core-level photoemission. It is shown that photoelectron diffraction effects may cause significant modulations in the angular dependence of magnetic dichroism in angle-resolved core-level photoemission. Furthermore, it is pointed out that both magnetic dichroism and spin polarization should show the same angular dependence on the electron emission direction. Results of photoelectron diffraction theory are discussed in detail for p-type core-level photoemission in comparison with those of the single-atom approach. The applied theory gives a good explanation of the experimental data of Fe 3p and Co 2p photoelectrons excited in thin Fe and Co films, respectively.

Influence of interface formation on magnetic properties: magnetic dichroism in photoemission on Ni/Fe(1 0 0)

Ultrathin Ni layers were grown onto Fe(1 0 0) and subsequently annealed. Results obtained by linear magnetic dichroism in the angular dependent (LMDAD) photoemission show a variation of the magnetic signal of the Fe 3p core levels upon annealing, suggesting an increase of the Fe magnetic moment when Ni is diluted in the interface. Strong modulations in the LMDAD signal for both Ni 3p and Fe 3p core levels as a function of either photon energy or emission angle are found, indicating photoelectron diffraction effects.