Resonant Photoemission Spectroscopy Research Articles

Charge Migration Mechanism in DNA Strands Elucidated by Electronic Structure Examinations

In order to establish the charge migration mechanism, we examined the electronic structure of bases in DNA strands by means of resonant photoemission spectroscopy near the Fermi level. Because the N atoms are only included in bases in DNA strands, the electronic orbital features of the bases can be specified selectively at the resonant excitation from the N core level to unoccupied states. On the obtained resonant photoemission spectra for both poly(dG) · poly(dC) and poly(dA) · poly(dT) DNAs, we observed a kinetic energy shift of N-KLL Auger electrons and an intensity enhancement of valence electrons. These results clearly show the localized unoccupied electronic states of the bases. Therefore, it is concluded that the charge hopping model is pertinent for the charge migration mechanism in DNA strands, when electrons pass through the unoccupied states.

Ultrafast Electron Transfer from Oligo(p-phenylene-ethynylene)thiol to Gold

The electron transfer dynamics of oligo(p-phenylene-ethynylene) (OPE) SAM on Au(111) was studied by resonant photoemission spectroscopy. The ultrafast electron transfer from OPE molecules to Au substrate was clearly observed. The time scale for this charge transfer is much less than 6 fs, the core-hole lifetime for C 1s. This strongly suggests that there is an intense interfacial electronic coupling between OPE molecules and the Au substrate.

Intramolecular vibronic dynamics in molecular solids:C60

Vibronic coupling in solid ${\mathrm{C}}_{60}$ has been investigated with a combination of resonant photoemission spectroscopy (RPES) and resonant inelastic x-ray scattering (RIXS). Excitation as a function of energy within the lowest unoccupied molecular orbital resonance yielded strong oscillations in intensity and dispersion in RPES, and a strong inelastic component in RIXS. Reconciling these two observations establishes that vibronic coupling in this core hole excitation leads to predominantly inelastic scattering and localization of the excited vibrations on the molecule on a femtosecond time scale. The coupling extends throughout the widths of the frontier valence bands.

Gas-phase-dependent properties ofSnO2(110), (100), and (101) single-crystal surfaces: Structure, composition, and electronic properties

The dependence of the surface structure, composition, and electronic properties of three low index $\mathrm{Sn}{\mathrm{O}}_{2}$ surfaces on the annealing temperature in vacuum has been investigated experimentally by low energy ${\mathrm{He}}^{+}$ ion scattering spectroscopy (LEIS), low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle resolved valence band photoemission (ARUPS) using synchrotron radiation. Transitions from stoichiometric to reduced surface phases have been observed at $440--520\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, $610--660\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and $560--660\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for the $\mathrm{Sn}{\mathrm{O}}_{2}$ (110), (100), and (101) surfaces, respectively. Density functional theory has been employed to assess the oxidation state and stability of different surface structures and compositions at various oxygen chemical potentials. The reduction of the $\mathrm{Sn}{\mathrm{O}}_{2}$ surfaces is facilitated by the dual valency of Sn, and for all three surfaces a transition from Sn(IV) to Sn(II) is observed. For the (100) and (101) surfaces, theory supports the experimental observations that the phase transitions are accomplished by removal of bridging oxygen atoms from a stoichiometric $\mathrm{Sn}{\mathrm{O}}_{2}$ surface, leaving a SnO surface layer with a $1\ifmmode\times\else\texttimes\fi{}1$ periodicity. For the (110) surface the lowest energy surface under reducing conditions was predicted for a model with a SnO surface layer with all bridging oxygen and every second row of in-plane oxygen atoms removed. Ab initio atomistic thermodynamic calculations predict the phase transition conditions for the (101) surface, but there are significant differences with the experimentally observed transition temperatures for the (110) and (100) surfaces. This discrepancy between experiment and thermodynamic equilibrium calculations is likely because of a dominant role of kinetic processes in the experiment. The reduction of surface Sn atoms from a Sn(IV) to a Sn(II) valence state results in filling of the $\mathrm{Sn}\text{\ensuremath{-}}5s$ states and, consequently, the formation of Sn derived surface states for all three investigated surfaces. The dispersion of the surface states for the reduced (101) surface was determined and found to be in good agreement with the DFT results. For the (110) surface, the $4\ifmmode\times\else\texttimes\fi{}1$ reconstruction that forms after sputter and annealing cycles was also investigated. For this surface, states that span almost the entire band gap were observed. Resonant photoemission spectroscopy identified all the surface states on the reduced $\mathrm{Sn}{\mathrm{O}}_{2}$ surfaces as Sn derived.

Temperature-dependent Eu3d−4fx-ray absorption and resonant photoemission study of the valence transition inEuNi2(Si0.2Ge0.8)2

We study the mixed valence transition ($T$$_{v}$ $\sim$80 K) in EuNi$_{2}$(Si$_{0.2}$Ge$_{0.8}$)$_{2}$ using Eu 3$d-4f$ X-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The Eu$^{2+}$ and Eu$^{3+}$ main peaks show a giant resonance and the spectral features match very well with atomic multiplet calculations. The spectra show dramatic temperature ($T$)-dependent changes over large energies ($\sim$10 eV) in RESPES and XAS. The observed non-integral mean valencies of $\sim$2.35 $\pm$ 0.03 ($T$ = 120 K) and $\sim$2.70 $\pm$ 0.03 ($T$ = 40 K) indicate homogeneous mixed valence above and below $T$$_{v}$. The redistribution between Eu$^{2+}$$4f^7$+$[spd]^0$ and Eu$^{3+}$$4f^6$+$[spd]^1$ states is attributed to a hybridization change coupled to a Kondo-like volume collapse.

Correlations in the electronic structure of half-metallic ferromagneticCrO2films: An x-ray absorption and resonant photoemission spectroscopy study

The electronic structure of high-quality ${\mathrm{CrO}}_{2}(100)$ films was investigated by means of x-ray absorption and resonant photoemission spectroscopy at the $2p\text{\ensuremath{-}}3d$ excitation threshold. The obtained binding energy of the occupied Cr $3d$ states is in agreement with the results predicted within the local spin-density approximation using the dynamical mean-field theory [L. Craco et al., Phys. Rev. Lett. 90, 237203 (2003)]. The reported data support a model of ${\mathrm{CrO}}_{2}$ as a half-metallic ferromagnet with strong electron-correlation effects.

Orbital-specific dynamic charge transfer from Fe(II)-tetraphenylporphyrin molecules to molybdenum disulfide substrates

Orbital-specific femtosecond charge transfer dynamics between Fe(II)-tetraphenylporphyrin molecules and semimetallic molybdenum disulfide substrates is investigated using core-level resonant photoemission spectroscopy. The electronic coupling to the substrate and the efficiency of charge transport across the interface is found to be different for the individual molecular electronic subsystems. In particular, electrons excited at the phenyl substituents are transferred within $3--6\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, while hopping from the porphyrin ring is slower than $30\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$.

Angle-resolved and resonant photoemission spectroscopy on heavy-fermion superconductorsCe2CoIn8andCe2RhIn8

We have carried out high-resolution angle-resolved and resonant photoemission spectroscopy (RPES) on heavy-fermion superconductors ${\mathrm{Ce}}_{2}\mathrm{Co}{\mathrm{In}}_{8}$ and ${\mathrm{Ce}}_{2}\mathrm{Rh}{\mathrm{In}}_{8}$ to study the electronic band structure and the nature of the Ce $4f$ electrons. We have experimentally determined the valence-band structure and compared them with the full-potential linear augmented plane-wave band calculations. We found that both compounds have quasi-two-dimensional cylindrical Fermi surfaces centered at the $M(A)$ point in the Brillouin zone, which may be an essential parameter for the development of the superconductivity. Comparison with the band calculations based on the itinerant and localized models suggests that the Ce $4f$ electrons are essentially localized in both compounds at a measured temperature of $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. RPES results have confirmed the localized character of the Ce $4f$ electrons in both compounds, with a relatively stronger localized nature in ${\mathrm{Ce}}_{2}\mathrm{Rh}{\mathrm{In}}_{8}$ than in ${\mathrm{Ce}}_{2}\mathrm{Co}{\mathrm{In}}_{8}$. This difference in the strength of localized character well explains the difference in the magnetic properties between the two compounds.

Synchrotron-radiation photoemission and O Kα emission study on (LnO)CuS (Ln = Ce, Pr, Nd)

Ln 4f partial densities of states (DOSs) in layered oxysulfide semiconductors (LnO)CuS (Ln = Ce, Pr, Nd) have been deduced by means of the Ln 4d–4f resonant photoemission spectroscopy. The Ce 4f DOS contributes just below the valence-band maximum (VBM) of (CeO)CuS, and on going from Ln = Ce to Pr, and to Nd, the 4f DOS shifts toward the deeper binding-energy side, leading no 4f contribution near VBM for Ln = Pr and Nd. The O 2p partial DOSs derived from O Kα emission spectra are almost unchanged with the Ln elements.

Site-specific electronic structure of an oligo-ethylenedioxythiophene derivative probed by resonant photoemission

A combination of conventional and resonant photoemission spectroscopy, x-ray absorption spectroscopy and ground-state quantum-chemical calculations has been used to study the valence electronic structure of a phenyl-capped 3,4-ethylenedioxythiophene oligomer, in polycrystalline thin films. The photon energy-dependent intensities of specific resonant decay channels are interpreted in terms of the spatial overlap of the excitation site and the ground-state molecular orbital involved in the decay. By making use of chemical shifts, excitations on different atomic sites are distinguished. It is demonstrated that site-specific information on the electronic structure of relatively large and complex organic systems may be obtained experimentally from non-radiative resonant decay spectra. In addition, these spectra provide relevant insight into the interpretation of near-edge x-ray absorption fine structure spectra.

Resonantly enhanced production of excited fragments of gaseous molecules following core-level excitation

State-selective dissociation dynamics for the excited fragments of gaseous Si(CH 3) 2Cl 2 following Cl 2p and Si 2p core-level excitations have been investigated by resonant photoemission spectroscopy and dispersed UV/optical fluorescence spectroscopy. The main features in the gaseous Si(CH 3) 2Cl 2 fluorescence spectrum are identified as the emission from excited Si*, Si +*, CH* and H*. The core-to-Rydberg excitations at both Si 2p and Cl 2p edges lead to a noteworthy production of not only the excited atomic fragments, neutral and ionic (Si*, Si +*) but also the excited diatomic fragments (CH*). In particular, the excited neutral atomic fragments Si* are significantly reinforced. The experimental results provide deeper insight into the state-selective dissociation dynamics for the excited fragments of molecules via core-level excitation.

Surface metallic nature caused by an in-gap state of reduced NiO: a photoemission study

The electronic state of oxygen-reduced NiO(1 0 0) surface has been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. We focus on the behavior of an in-gap state induced by oxygen-defects, which is observed at ∼ 0.5 eV below the Fermi level ( E F ). The in-gap state shows slight energy dispersion; however, with a closer look around E F , it seems to create metallic Fermi cut-off both in h ν -dependent normal-emission spectra and in off-normal emission ones. The rest of the peaks due to bulk bands are identical to those of stoichiometric bulk NiO; therefore, the observed feature of the in-gap state reveals the fact that the surface of reduced NiO is a metal while the vast underlying bulk is still an insulator.

Spectral evidence for inherent “dead layer” formation at La 1− ySr yFeO 3/La 1− xSr xMnO 3 heterointerface

We have investigated the electronic structure of a La 0.6Sr 0.4FeO 3 (LSFO)/La 0.6Sr 0.4MnO 3 (LSMO) heterointerface using Mn 2p-3d resonant photoemission spectroscopy and X-ray absorption spectroscopy. We have found that the spectral intensity of e g↑ states near the Fermi level is gradually reduced with increasing LSFO overlayer thickness. The spectral behavior is quite similar to that of hole-doped LSMO films, indicating hole-doping in LSMO layers. We have also found that Fe 2p X-ray absorption spectrum of a LSMO/LSFO/LSMO trilayer shows the spectral evidence for electron doping in the corresponding LSFO layer. These results indicate the occurrence of charge transfer at interfaces between LSMO and LSFO layers.

Mn 3p–3d resonant photoemission spectroscopy of ferromagnetic semiconductor Ge 1− xMn xTe

In order to investigate the Mn 3d electronic structure in IV–VI ferromagnetic semiconductor Ge 1− x Mn x Te, we have carried out the Mn 3p–3d resonant photoemission spectroscopy (RPES) and Mn 2p–3d soft X-ray emission spectroscopy (SXES). The Mn 3d partial density of states derived from the RPES spectra and the excitation-energy-dependent SXES spectra indicate that the Mn 3d states of Ge 1− x Mn x Te are nearly localized with the divalent character.

Electronic structures of YNi 2 − xCo xB 2C superconductors studied by photoemission and photoabsorption spectroscopy

The electronic structures of five polycrystalline YNi 2 − x Co x B 2C ( x = 0, 0.05, 0.1, 0.15, and 0.2) borocarbide superconductors were studied by photoemission and photoabsorption spectroscopy. The resonant photoemission spectroscopy around the Ni 2p 3/2 absorption edge is used to study the 6-eV valence-band satellite of Ni. The Ni and Co K-edge X-ray absorption near edge spectra for these intermetallic compounds are compared with those of Ni and Co powder and foil, respectively.

Titanium dioxide surface stoichiometry and ordering studied by resonant photoemission spectroscopy

The electronic structure of titanium dioxide surfaces having undergone different preparations leading to different stoichiometries and crystallinities has been studied using resonant photoemission spectroscopy. Valence band photoemission spectra through the Ti 3p–3d/4s absorption edge between 45 and 55 eV were measured and allowed a characterization of defects present at the surface as well as of the quality of the surface organization. Indeed, from the comparison of the resonance results obtained for each kind of surface with the LEED patterns on the one hand and the corresponding Ti 2p core level lines on the other hand, it was evidenced that the high binding energy part of the valence band seems rather related to the ordering of the surface than to the amount of defects.

Sub-liquidus co-crystallization in the Ln 2O 3–BaO–CoO system: growth of large LnBaCo 2O 5+x (Ln=Eu, Gd, Tb, Dy) single crystals

An original approach to flux growth has been developed to provide for the first time large and high-quality single crystals of oxygen deficient LnBaCo 2O 5+ δ (Ln=Eu, Gd, Tb, Dy, Tb 0.9Dy 0.1). The data from DTA studies of both the BaO–CoO binary eutectic and temperature of crystal nucleation as well as analysis of more than 100 runs of sub-liquidus co-crystallization allow one to infer the tentative ( T–x) phase diagram of a quasi-binary cut in the Ln 2O 3–BaO–CoO system with the estimated primary crystallization field for the LnBaCo 2O 5+ δ (Ln-112) phase. As a result, in successful growth runs several crystals of a parallelepiped shape and up to 120 mm 3 in volume were grown. The dependence of the crystal shape on the rare earth ion is discussed. For most flux melts close to the ternary eutectic compositions of the corresponding quasi-binary cuts, co-crystallization of the layered cobaltite phase and a new one of LnBaCo 4O 7 (Ln-114) was found in a wide range of temperatures. Data on the twinned crystal structure of the optimally oxidized ( δ = 0.5 ) orthorhombic Ln-112 crystals and crystal characterization by chemical composition, magnetic susceptibility and resonance photoemission spectroscopy are discussed.

Electronic structure changes across the valence transition inEuNi2(Si0.2Ge0.8)2

We study the electronic structure of $\mathrm{Eu}{\mathrm{Ni}}_{2}{({\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8})}_{2}$, which exhibits a temperature dependent mixed valence transition, using $4d\text{\ensuremath{-}}4f$ resonant photoemission spectroscopy (RESPES), x-ray absorption spectroscopy (XAS) and temperature-dependent ultraviolet photoemission spectroscopy (UPS). The RESPES studies identify the divalent and trivalent Eu $4f$ character density of states (DOS) which participate in the valence transition. Using the photoionization cross section variation as a function of photon energy, we discuss the Eu, Ni, and $\mathrm{Ge}\ensuremath{-}\mathrm{Si}$ partial DOS in the valence band. The bulk divalent Eu $4f$ character states are centered at a binding energy of about $0.75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, significantly away from the Fermi level. While the surface divalent feature is negligibly affected, the spectra obtained using He $II\ensuremath{\alpha}$ UPS exhibit temperature dependent bulk Eu $4f$ character states. The bulk divalent spectral weight is transferred to the high energy trivalent states, across the valence transition temperature, ${T}_{v}\ensuremath{\sim}80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The He $I\ensuremath{\alpha}$ UPS also exhibit spectral intensity changes across ${T}_{v}$. The non-$f$ character conduction band states at and near the Fermi level exhibit spectral weight changes up to $350\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ with a small energy $(\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ temperature dependent pseudogaplike feature. The results suggest an increase in effective hybridization strength between the conduction and $4f$ electrons in the low temperature nearly trivalent phase. While the $4f$ character changes across ${T}_{v}$ are qualitatively consistent with change in valence configurations, the temperature dependent spectral changes in the non-$f$ character DOS indicate direct participation in the valence transition in $\mathrm{Eu}{\mathrm{Ni}}_{2}{({\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8})}_{2}$.

Electronic structure of protonic conductor SrCeO 3 by soft-X-ray spectroscopy

The electronic structure of protonic conductor SrCe 1− x Y x O 3− δ ( x=0, 0.05) has been studied by resonant-photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) in the soft-X-ray region. The RPES spectra exhibit that the O 2p state strongly hybridizes with Ce 4f state in the valence band. The valence band consists of the mixed-valent states of the 4f 0 (Ce 4+) and 4f 1 L (Ce 3+) configurations. The XAS spectra in the band gap energy region show a hole state at the top of the valence band and an acceptor-induced level just above the Fermi level ( E F). The energy position of the hole state accords with that of the Ce 3+ state. The energy separation between the hole state and E F is in good agreement with the activation energy.

Electronic structure of bases in DNA duplexes characterized by resonant photoemission spectroscopy near the Fermi level.

The electronic structure of bases in DNA duplexes was investigated by resonant photoemission spectroscopy near the Fermi level, in order to specify charge migration mechanisms. We observed a kinetic energy shift of N-KLL Auger electrons and an intensity enhancement of valence electrons on the resonant photoemission spectra for both poly(dG).poly(dC) and poly(dA).poly(dT) DNAs. These directly show the localized unoccupied states of the bases. We conclude that the charge hopping model is pertinent for electric conduction in a DNA duplex, when electrons pass through the unoccupied states.