Multi-atom Resonant Photoemission Research Articles

Photoelectron diffraction, multi-atom resonant photoemission, pioneered by Fadley

We discuss two important subjects pioneered by Fadley. As the first topic, we discuss the energy dependence of the intensity ratio observed in Mn 3s multiplet splitting excited from antiferromagnetic MnO, MnF2 and KMnF3. We apply the many-body scattering theory developed by Hedin in order to incorporate the extrinsic effects. We find that the extrinsic effects play an important role to explain the energy dependence of the branching ratio. Preliminary calculations show the smaller energy dependence than that observed. As the second topic we briefly discuss some features in multi-atom resonant photoemission (MARPE) based on the first principle quantum theory. Importance of the local symmetry around an emitter is stressed.

Theory of multi-atom resonant Raman scattering

We theoretically study X-ray resonant Raman scattering where the incident photon energy ωλ is tuned near deep core threshold, and the scattered one ωλ′ is tuned so that ωλ−ωλ′ should be near shallow core threshold: In contrast to ordinary resonant inelastic X-ray scattering (RIXS), they are on different atomic sites. The many-body scattering theory developed by Almbladh and Hedin has been extended to include multi-atom resonant effects. A quite similar formula to multi-atom resonant photoemission (MARPE) was obtained, which cannot be observed in the highly symmetric environment around the shallow core hole sites. In comparison with RIXS, a further non-radiative processes has to be taken into account between the two radiative interactions in the scattering amplitude. The structure factors severely control the possibility to observe MARX-Raman scatterings. They are in the order of R−2 which is in contrast to the order of R−3 for the structure factors in MARPE: R is the distance to the nearby atoms. The condition where we cannot observe MARX-Raman spectra is different from that for MARPE spectra. For the systems with spatial inversion symmetry around the shallow core hole site, we cannot expect to observe the MARX-Raman spectra. For some oxides including heavy metals like PbO and Ag2O, though they are in low symmetries around oxygens, we cannot expect to observe the MARX-Raman spectra due to the structure factors.

A medium-energy photoemission and ab-initio investigation of cubic yttria-stabilised zirconia

Experimental and theoretical investigations into the electronic properties and structure of cubic yttria-stabilized zirconia are presented. Medium-energy x-ray photoemission spectroscopy measurements have been carried out for material with a concentration of 8-9 mol. % yttria. Resonant photoemission spectra are obtained for a range of photon energies that traverse the L2 absorption edge for both zirconium and yttrium. Through correlation with results from density-functional theory (DFT) calculations, based on structural models proposed in the literature, we assign photoemission peaks appearing in the spectra to core lines and Auger transitions. An analysis of the core level features enables the identification of shifts in the core level energies due to different local chemical environments of the constituent atoms. In general, each core line feature can be decomposed into three contributions, with associated energy shifts. Their identification with results of DFT calculations carried out for proposed atomic structures, lends support to these structural models. The experimental results indicate a multi-atom resonant photoemission effect between nearest-neighbour oxygen and yttrium atoms. Near-edge x-ray absorption fine structure spectra for zirconium and yttrium are also presented, which correlate well with calculated Zr- and Y-4d electron partial density-of-states and with Auger electron peak area versus photon energy curve.

Interatomic resonant Auger effects in core-level photoemission from NO and CS2 molecules

Core-level interatomic resonant Auger in isolated molecules can take place when the photon energy is tuned to a resonant core-level excitation of an atom neighbouring the electron-emitter atom, provided that the emitter atom has smaller ionization energy of core electrons than the resonant energy. A multi-atomic nature of the Auger is a molecular analogue of multi-atom resonant photoemission (MARPE) observed in a condensed phase. This phenomenon in linear molecules, NO and CS2, has been explored by means of a photoelectron–fragment-ion–fragment-ion multi-coincidence velocity mapping technique. The N 1s (S 2p) photoionization experiments have been conducted at the O 1s → π* (C 1s → π*) resonance energy and at the off-resonance one, in which photoelectron angular distributions were obtained with respect to the recoil axis of the fragment ions produced by an Auger decay following the core-level photoemission. A resonance enhancement was observed along the photon polarization direction and this effect strongly depends on the relative geometry of the polarization vector and the recoil axis. By comparing the N 1s photoemission of NO with the S 2p photoemission of CS2, it is shown that the resonance enhancement also depends on the core-orbital characters that participate in the process. Such phenomenon should be considered to be general and the present results of the simplest system thus shed new light on photoemission studies for both isolated molecules and condensed systems consisting of different elements.

Introductory Photoemission Theory

An introductory review is presented on the basis of many-body scattering theory. Some fundamental aspects of photoemission theory are discussed in detail. A few applications are also discussed; photoelectron diffraction, depth distribution function and multi-atom resonant photoemission are also discussed briefly.

New Theoretical Aspects for X-ray Photoemission from Solids

We review some new developments in theory of photoemission from solids. In particular, multi-atom resonant photoemission (MARPE) and recoil processes in high-energy photoemission are discussed in detail. In the former and latter phenomena, radiation field screening and phonon excitation around an X-ray absorbing atom, respectively, play an important role.

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.

Single-site and multisite resonant photoemission theory studied by Keldysh Green’s functions

Single (on-site) and multisite (multiatom) resonant photoemission (MARPE) processes are systematically studied by use of nonrelativistic Keldysh Green's function theory. We apply skeleton expansion in terms of renormalized one-electron Green's functions. In this theoretical framework we discuss the importance of the radiation field screening and the dynamically polarized part ${W}_{p}$ in the screened Coulomb propagator. The radiation field screening plays a crucial role in observing the MARPE: We obtain expressions of the resonant processes (on site and multiatom) in terms of an x-ray absorption factor whose imaginary part is proportional to the x-ray absorption intensity. If we neglect ${W}_{p}$, the calculated MARPE intensity is much smaller than the observed one. We also point out the importance of the structure factor in the MARPE analyses. Typically highly symmetric atomic arrangement around a photoemitting atom provides us with no MARPE signal, but outermost oxygen atoms give rise to considerably strong MARPE because of symmetry lowering. On the other hand, low symmetric systems like rutile, perovskite, and $\ensuremath{\alpha}$-alumina $({\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ structures can give rise to finite MARPE even in the case of photoemission from inner layers of perfect crystals. The polarization dependence of the MARPE follows the same selection rule as the main photoemission processes, whereas the O $1s$ MARPE from ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ shows a rather complicated rule. These specific features of MARPE provide useful local structural information.

Search for a local effect in multiatom resonant core excitation in a surface species: Photoemission and photon-stimulated desorption fromN2on Ni(111)

We have investigated photoabsorption, N $1s$ photoelectron emission, and photon-stimulated desorption (PSD) of ${\mathrm{N}}^{+}$ ions, from ${\mathrm{N}}_{2}$ molecules perpendicularly chemisorbed on Ni(111) surfaces, in the energy range of the Ni $2{p}_{3∕2,1∕2}$ excitations. For this system, N $1s$ photoemission monitors single-core-hole production, whereas PSD of ${\mathrm{N}}^{+}$ is mainly due to excitation of multiply excited N $1s$ core-hole states. The amplitude variations of these two signals and the kinetic energy distributions (KED's) of the ${\mathrm{N}}^{+}$ ions were recorded as functions of the photon energy. In addition, we measured the amplitude variations of PE and PSD as a function of the photon incidence angle, which was varied from grazing (7\ifmmode^\circ\else\textdegree\fi{} with respect to the surface) to steeper angles (43\ifmmode^\circ\else\textdegree\fi{} and 50\ifmmode^\circ\else\textdegree\fi{} with respect to the surface). For grazing incidence, strong variations of both the photoelectron and the ${\mathrm{N}}^{+}$ signals with photon energy and angle of incidence were found in the Ni $2p$ region which are compatible with x-ray optical (dielectric) effects, one manifestation of multiatom resonant photoemission. The ${\mathrm{N}}^{+}$ KED's, which are known to depend strongly on the nature of the electronic excitation responsible, did not change across the Ni $2{p}_{3∕2}$ threshold, which excludes any type of state selectivity in the interatomic core-coupling effects observed. For N $1s$ photoemission, a first analysis of our data suggests a variation of the N $1s$ signal at the Ni $2p$ edges also for steeper angles of light incidence, of comparable magnitude to that at grazing incidence. However, more careful x-ray photoelectron spectroscopy experiments and the investigation of electronically stimulated desorption of neutral ${\mathrm{N}}_{2}$ molecules and N atoms reveal that these effects are due to a strong increase of beam damage when passing the Ni $2p$ edge; these effects could be reduced by rapidly scanning the sample under the beam. We thus conclude that for high angles of incidence most of the Ni $2p$-related changes in our N $1s$ photoemission signal are due to beam damage.

Importance of radiation field screening in XPS and XAFS spectra

In this work the importance of radiation field screening is stressed in Keldysh’s Green’s function theory to study X-ray photoelectron spectroscopy (XPS) spectra and X-ray absorption fine structure (XAFS). So far these terms have been supposed to be small, however, we have found that these effects play a crucial role to analyze resonance effects. In particular the importance of these vertex correction is important in the analyses of multiatom resonant photoemission (MARPE) spectra and the resonance effects in XAFS. We also investigate the contribution from polarized part W p of the screened Coulomb interaction W to the resonance effects. We find that W≈ v is quite a good approximation. Off the resonance these radiation field screening diagrams are not important. They have no contribution to loss processes even though they include W > and W <.

An ab initio method for computing multi-atom resonant photoemission

We present an ab initio method for computing multi-atom resonant photoemission (MARPE) in molecules and use this method to investigate the possibility of having a sizable MARPE effect. The MARPE cross section is obtained by making multi-center expansions of both the dipole matrix element – direct emission – and the hamiltonian matrix element between the resonant state and the autoionizing states – resonant emission. We show by sample calculations that the resonant contribution exhibits a strong distance dependence of the atoms participating in the MARPE process, and that it is energy dependent and element specific for the neighboring atoms. The ratio of resonant MARPE and direct photoionization cross sections is found to amount to less than one percent which probably makes the molecular MARPE effect difficult to apply in practice.

Many-body theory of core-level photoemission by Keldysh Green’s functions

The many-body theory of X-ray photoelectron diffraction (XPD) spectra is developed by using Keldysh’s non-equilibrium Green’s functions, which was basically applied by Caroli et al. We have applied skeleton diagram expansion in terms of dressed one-electron Green’s function G and the screened Coulomb interaction W. The damping photoelectron wave functions under the influence of the optical potential play a central role in the present XPD theory. On the basis of the present theoretical framework, we provide detailed discussion on Debye–Waller and Franck–Condon factors in XPD spectra at finite temperature, resonance effects in particular multiatom resonant photoemission (MARPE) effects, and also on the loss effects. The XPD spectra are simply described by a product of the Franck–Condon factor and the XPD intensity including the Debye–Waller factor. This result is in contrast to the X-ray absorption fine structure (XAFS) where the Franck–Condon factor is smeared out by a convolution product. We have discussed the conditions which give rise to prominent MARPE spectra. In particular, symmetry lowering plays an important role giving rise to prominent MARPE spectra. The angular dependence provides us with useful information on near neighbor atomic arrangement. Some model calculations clarify the basic physical processes of MARPE spectra. To discuss the loss spectra, we stress the importance of the interference effects between the intrinsic and extrinsic losses. The explicit formulas, which include both inelastic and elastic scatterings, are newly derived in the present theoretical framework, and enable us to calculate an XPD pattern associated with losses.

Search for multi-atom resonant photoemission in magnetic thin films

In order to test the presence of interatomic multi-atom resonant photoemission (MARPE) in 3 d magnetic ultrathin films, 2 p photoelectron emission of various ferromagnetic 3 d metals was studied while the excitation energy was tuned around the L 2,3 core level absorption edge of a different 3 d metal. Both elements were in direct contact either as bilayer or as alloys to maximize this possible effect. Systems under study were Fe/Co, Fe/Ni, Fe 30Ni 70, Co, Co 90Mn 10, and Ni 90Mn 10, all grown as epitaxial films on Cu(001). No measurable change in intensity of the 2 p photoemission intensities in normal emission for 45° incidence of more than 2.5–5% was found as a result of resonant excitation of neighboring atoms in any of these systems. We can thus exclude the presence of significant MARPE in these metallic systems in the present experimental geometry. This is further confirmed by the photon energy dependence of the linear magnetic dichroism in the Fe 2 p photoemission of a Fe/Co film, which is not influenced by Co L 2,3 edge resonant absorption.

Multi-atom resonances and soft X-ray emission

We present a study of the effects of interatomic multi-atom resonant photoemission monitored by soft X-ray emission. The C K α partial fluorescence yield is measured while the excitation energy is scanned through the higher lying TiL 2,3 threshold of the neighboring atoms in a multi-component system (TiNbC). No evidence for a resonant enhancement in the fluorescence signal as suggested by the MARPE effect is found.

Limits to the quantitative analysis of multiatom resonant photoemission: The case ofc(2×2)O/Ni(100)

We have investigated multiatom resonant photoemission (MARPE) on an adsorbate system $c(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{O}/\mathrm{N}\mathrm{i}(100).$ We observe a negligible MARPE enhancement $(2\ifmmode\pm\else\textpm\fi{}5%)$ of the O $1s$ photoelectron intensity when the excitation energy is tuned to the Ni ${L}_{3}$ absorption resonance. A strong nonlinearity in our photoelectron detection system is shown to produce artificial MARPE effects of up to $140%$ for uncorrected spectra. The possibly dominating influence of measurement artifacts in systems besides the studied adsorbates suggests a careful review of the intensity of the MARPE effect and its applicability in general.

Multi-atom resonant photoemission

Abstract We review the present status of multi-atom resonant photoemission (MARPE), in which the photoelectron intensity from a core level on one atom is influenced by core-level absorption resonances on neighboring atoms of different atomic number. We find that some prior experimental data has been strongly influenced by detector non-linearity and that the effects seen in new corrected data are generally smaller and of different form. The corrected data are found to be well described by an extension of resonant photoemission theory to the interatomic case, provided that interactions beyond the usual second-order Kramers–Heisenberg treatment are included. This microscopic X-ray theory is also found to simplify under certain conditions so as to yield results equivalent to a classical X-ray optical approach, with the latter providing an alternative, although less detailed and general, physical picture of these effects. The present status of experimental results for such MARPE effects is discussed, together with the potential utility of these effects as near-neighbor probes, as well as their implications for X-ray emission and X-ray scattering experiments.

Multiatomic resonant photoemission spectroscopy on CuO and NiO: Observation of antiresonant behavior

The intensity of the O 1s core photoemission peak has been measured for CuO and NiO as the photon energy is scanned across the L-3 absorption edge of the metal ion. In CuO, the O 1s peak shows a typical antiresonant behavior, with a sizable decrease of its cross section at photon energies below the Cu L-3 threshold. No detectable effect is observed for NiO. The CuO data are well reproduced by a multiatom resonant photoemission model based on resonant light scattering.

No multiatom resonances observed in x-ray fluorescence

We present a study of the effects of interatomic multiatom resonant photoemission monitored by soft x-ray emission. The partial fluorescence yield of certain transitions $\mathrm{F}K\ensuremath{\alpha}$ and $\mathrm{C}K\ensuremath{\alpha}$ is measured while the excitation energy is scanned through the higher lying thresholds $(\mathrm{La}{M}_{4.5}$ and $\mathrm{Ti}{L}_{2,3},$ respectively) of the neighboring atoms in the multicomponent systems $({\mathrm{LaF}}_{3}{,\mathrm{T}\mathrm{i}}_{x}{\mathrm{Nb}}_{1\ensuremath{-}x}\mathrm{C}).$ No evidence for a resonant enhancement in the fluorescence signal as suggested by the multiatom resonant photoemission effect is found.

Multi-atom resonant photoemission in transition metal chlorides

Recently multi-atom resonances in photoemission of transition metal oxides were observed by Kay et al. [A. Kay, E. Arenholz, S. Mun, F.J. Garcia de Abajo, C.S. Fadley, R. Denecke, Z. Hussain, M.A. Van Hove, Science 281 (1998) 679]. We present the results of a systematic study of multi-atom resonant photoemission in transition metal chlorides. We measured the intensity of the chlorine 2p photoelectron line excited around the metal 2p photoabsorption edge of the transition metal compounds MnCl 2, CrCl 2, and VCl 2 as well as of the ionic solid KCl. Our experimental results show that the intensity of Cl 2p photoelectron lines enhance by about 29% for MnCl 2, 21% for CrCl 2, 17% for VCl 2 and 11% in the case of KCl at the metal 2p photoabsorption edge. We ascribe the systematic decrease of the magnitude of this effect, going from MnCl 2 to KCl, to the extension of the excited metal 3d wavefunction and, as a consequence, decrease of the overlap between the 3d and 2p atomic orbitals.

Multiatom resonant photoemission observed via secondary processes: Auger decay and x-ray fluorescence

We present experimental evidence that the newly discovered multiatom resonant photoemission effect can be observed via secondary decay processes of the primary core hole: Auger electron emission, from ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3},$ where the O ${\mathrm{KL}}_{23}{L}_{23}$ intensity is enhanced by 90% when $h\ensuremath{\nu}$ is tuned to the Fe ${L}_{3}$ edge, and fluorescent x-ray emission, from MnO, where the O $K\ensuremath{\alpha}$ fluorescence intensity shows an enhancement of more than 100% at the Mn ${L}_{3}$ edge when properly corrected for saturation and self-absorption effects.