Resonant Photoemission Process Research Articles

The local metallicity of gadolinium doped compound semiconductors

The local metallicities of Hf0.97Gd0.03O2, Ga0.97Gd0.03N, Eu0.97Gd0.04O and EuO films were studied through a comparison of the findings from constant initial state spectroscopy using synchrotron light. Resonant enhancements, corresponding to the 4d → 4f transitions of Eu and Gd, were observed in some of the valence band photoemission features. The resonant photoemission intensity enhancements for the Gd 4f photoemission features are far stronger for the more insulating host systems than for the metallic system Eu0.96Gd0.04O. The evidence seems to suggest a correlation between the effective screening in the films and the resonant photoemission process.

Co 2p–3d resonant photoemission spectroscopy of CoSb 3

High-resolution synchrotron radiation photoemission spectroscopy (HRPES) has been performed in the Co 2p core excitation region on thermoelectric materials of CoSb 3 with the skutterudite crystal structure. At the Co 2p 3/2–3d peak, a photoionization cross-section of the Co 3p states becomes extremely larger than that at the Co 3p–3d peak. Co 2p3d3d, 2p3p3d, 2p3s3d, 2p3p3p and 2p3s3p Auger structures and plasmon satellites are recognized. It has been confirmed that the resonant photoemission process is less important in CoSb 3. The hν-dependence of the spectra suggests that the Co 3d electrons in CoSb 3 can be treated as the one-electron model rather than the many-body particle model, which is consistent with the covalency of CoSb 3.

Divalent admixtures to the trivalent ground-state configurations of Pr and Nd in transition-metal compounds

Resonant photoemission spectra at the $3\stackrel{\ensuremath{\rightarrow}}{d}4f$ absorption threshold are presented for some Pr and Nd transition-metal compounds. It is shown that the shape of spectra varies strongly with the composition of the compounds. Hybridization of the $4f$ electrons with valence-band states can lead to giant effects resulting in energy splittings of $4f$ emission by several electron volts. The phenomena are described quantitatively in the framework of a single-impurity Anderson model. Large splittings of the $4f$ emission are obtained if the energy position of the bare $4f$ state coincides with maxima of the valence-band density of states. For the ${4f}^{n}$ ground states, admixtures of ${4f}^{n+1}$ states of the order of some percent are obtained. Contributions of valence states to the resonant photoemission process are discussed.

Angular-dependent resonant-photoemission processes at the2pthresholds in nickel metal

Angle-resolved valence-band resonant photoemission of nickel metal has been measured close to the 2p core-level thresholds with synchrotron radiation. The well-known 6-eV correlation satellite has an intensity enhancement of about two orders of magnitude at resonance. The angular dependence of the photoemission intensity has been studied as function of photon energy and provides unambiguous evidence for interference effects all the way up to the resonance maximum. The observation of different angular asymmetries, {\beta}, for the valence band and the satellite is discussed in connection to the origin of the resonant photoemission process and the character of the satellite.

Investigation of resonant photoemission in Gd with x-ray linear dichroism

The constructive summing of direct and indirect channels above the absorption threshold of a core level can cause a massive increase in the emission cross section, leading to a phenomenon called “resonant photoemission.” Using novel magnetic linear dichroism in angular distribution photoelectron spectroscopy experiments and theoretical simulations, we have probed the nature of the resonant photoemission process in Gd metal. It now appears that temporal matching as well as energy matching is a requirement for true resonant photoemission.

Evidence for the Photoemission Nature of Gd 4f Resonant Photoemission

ABSTRACTThe constructive interference between direct and indirect channels above the absorption threshold of a core level leads to a massive increase in the emission cross section leading to a phenomenon called “resonant photoemission”. Using novel magnetic linear dichroism in angular distribution photoelectron spectroscopy experiment we have tried to understand the nature of the resonant photoemission process in Gd metal. The presence of dichroism in Gd 4f photoemission intensity at a photon energy corresponding to resonant photoemission clearly demonstrates the photoemission-like nature of the resonant photoemission process.

Angle-resolved resonant photoemission at theMIIIabsorption threshold of Co and Fe metal

We report on an angle-resolved resonant photoemission study of Co and Fe metal at the ${M}_{\mathrm{III}}$ absorption threshold. The intensity of the various spectral components was measured as a function of the electron emission angle relative to the E vector of the incident light. On ${M}_{\mathrm{III}}$ resonance, the main valence-band photoemission (PE) intensity of Co and Fe metal, located between ${E}_{F}$ and 3 eV, follows a ${\mathrm{cos}}^{2}\ensuremath{\theta}$ behavior, as is expected for a photoemission process. However, the intensity of the enhanced feature located at $\ensuremath{\approx}5$ eV is found to be almost independent of the emission angle, as expected for an incoherent Auger process. This clearly shows that the enhancement of the 5 eV PE signal is mainly caused by an incoherent Auger decay and not by a resonant PE process.

Spectator states in resonant photoemission of Eu and Gd metal

We report on a resonant photoemission study of Gd and Eu metal at the x-ray absorption threshold. The 4f photoemission signal exhibits a strong enhancement at resonance which is due to a true resonant photoemission process. For a photon energy corresponding to a satellite structure in the absorption spectra, a final state assigned to a spin-flip phenomenon in the excitation process is observed. Resonant photoemission spectra of the 4d states of Gd and Eu metal at the thresholds are also presented. No enhancement of the 4d emission is found. However, the appearance of a large emission at higher binding energy is observed. It is proposed that this feature originates from a more probable Auger transition, which reduces the probability of a resonant photoemission of the 4d states.

Resonant photoemission study of the electronic structure of K 0.3MoO 3

We report resonant photoemission measurements performed on both stoichiometric and defective (20 1 ) surfaces of the quasi-one dimensional oxide conductor K 0.3MoO 3. We investigate the resonant enhancement of the photoemission intensity in the O 2 p band and of defect-related emission 2 eV below the Fermi level. The strong Fano-like resonance profile of the defect emission allows us to identify the defects as oxygen vacancies. The destructive nature of resonant photoemission for this system is revealed, as the core hole excitations that are necessary for the resonant photoemission process also lead to photon stimulated desorption.

Resonant photoemission study of CeO2.

The 4f electronic state and the decay process of the photoexcited 4${\mathit{d}}^{9}$4${\mathit{f}}^{\mathit{n}+1}$ states in ${\mathrm{CeO}}_{2}$ are investigated by means of a resonant photoemission technique in the Ce 4d\ensuremath{\rightarrow}4f photoabsorption region. Resonant enhancement of the valence-band emission is clearly observed in the giant-absorption region. This confirms the existence of the 4f electron in the ground state of ${\mathrm{CeO}}_{2}$. The 4f-derived emission exhibits a single-peak distribution rather than a double-peak structure such as observed in other Ce compounds. The Ce 5p emission band, which spreads over about 10 eV and consists of at least three or more fine structures, also shows the resonant enhancement in the giant-absorption region. In the prethreshold region, the Ce 5p fine structures as well as the 4f emission band show enhancements, but these constant-initial-state spectra are different from each other. The obtained results are discussed in terms of the mixed valence in the ground state of ${\mathrm{CeO}}_{2}$ and of possible intermediate configurations in the resonant photoemission process.

Resonant photoemission in highly localized versus weakly localized solids

We report on comparative study of angle-resolved photoelectron spectra of Ni metal and CeGd at theMIII andNIV,V soft-x-ray absorption threshold, respectively. OnMIII resonance, the valence-band photoemission (PE) intensity of Ni follows a cos2-behavior, while the intensity of the 6-eV satellite is independent of the angle of emission relative to the E-vector of the incoming beam. On the other hand, the 4f Ce PE signal behaves ∝ cos2 onNIV,V resonance. This clearly shows that the resonant enhancement of the 6-eV PE satellite of Ni is mainly caused by an incoherent Auger decay and cannot be described as a resonant PE process.

Magnetic circularly polarized 2p resonant photoemission of nickel.

We report the observation of two resonance channels in the 2p resonant photoemission of ferromagnetic nickel using circularly polarized soft x rays. By identifying the local 3${\mathit{d}}^{8}$ and 3${\mathit{d}}^{7}$ configurations in the valence-band photoemission spectra, we are able to assign structures in 2p core-level-absorption and magnetic-circular-dichroism spectra to many-body 2${\mathit{p}}^{5}$3${\mathit{d}}^{10}$ and 2${\mathit{p}}^{5}$3${\mathit{d}}^{9}$ configurations as well as to a critical point in the band structure. The Ni data provide a critical test for theoretical models of x-ray absorption and magnetic-circular-dichroism spectroscopy of transition-metal magnetic materials. This work also demonstrates the unique possibilities provided by circularly polarized light for separating the Auger process from the resonant-photoemission process and unraveling the electronic structure.

Partial density of Mn 3dstates and exchange-splitting changes inZn1−xMnxY(Y=S,Se,Te)

Mn 3d states in ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Y (Y=S,Se,Te) have been investigated using resonant synchrotron-radiation photoemission at the Mn 3p-3d threshold. We observe chemical trends due to anion substitution for the main Mn 3d photoemission feature at 3.5, 3.6, and 3.8 eV below the valence-band maximum as well as for the d(\ensuremath{\uparrow})-d(\ensuremath{\downarrow}) exchange splitting for ${\mathrm{Zn}}_{0.81}$${\mathrm{Mn}}_{0.19}$S, ${\mathrm{Zn}}_{0.81}$${\mathrm{Mn}}_{0.19}$Se, and ${\mathrm{Zn}}_{0.68}$${\mathrm{Mn}}_{0.32}$Te, respectively. The size of relaxation effects associated with the resonant-photoemission process is derived. Valence-band offsets of 0.3 eV are found between the semimagnetic Zn compounds and their parent binary alloys.

Optical response of an embedded atom: Resonant photoemission from uranium.

We have studied the effect of solid-state hybridization on the resonant photoemission process by means of relativistic time-dependent local-density calculations for a single uranium atom embedded in a spherical drop of jellium. The computed total absorption is in good agreement with experiment. We find a dramatic suppression of 5f photoemission relative to that of the free atom.