High-resolution resonant photoemission study of the 4f states in a typical Kondo system: CePd3

We exploited resonant photoemission at the Ce absorption edge to investigate the Ce 4f states in . High resolution spectra reveal, near the Fermi level, the characteristic fine structure of intermediate valence Ce compounds. The spectral lineshape is consistent with the typical “Kondo” character of CePd, but the prominent ionization peak is found at the unusually low binding energy of 1 eV. We briefly discuss the implications of these observations. Received: 13 October 1997 / Accepted: 21 January 1998     

Electronic band structure of Cu3Au: An angle-resolved photoemission study along the [111]-direction

High-resolution normal photoemission (ARPE) spectra have been recorded for Cu₃Au(111) with the use of polarized synchrotron and rare-gas resonance radiation in the photon energy range from 9 to 27 eV. It is for the first time that dispersions of the gold-like bands have been found experimentally. Using a fully relativistic layer-KKR photoemission formalism, occupied and unoccupied bands as well as one-step-model photoemission spectra have been calculated. The comparison of calculated spectra with experimental ones and the observation of direct-transition resonances upon photon energy near the Brillouin zone-center reveal a shift of the unoccupied ground-state bands by about +2.5 eV (self-energy shift). The direct-transition structures in the experimental spectra have been exploited to determine the dispersions of the occupied bands along the [111] direction (A line in k space). In order to determine the wave vector of the experimental direct transitions we used as final state that calculated unoccupied band along [111], which also exists in pure copper and gold up to about 20 eV above the Fermi energy (“unfolded” band structure), shifted by + 2.5 eV. The experimental occupied bands with Cu character are in very good agreement with theory after shifting the latter by about 0.3 eV to lower energy, whereas somewhat bigger discrepancies exist for the gold-like bands.     

Electronic surface states of Cu(1 1 0) surface

Surface states are a unique and important class of quantum states that shave an important effect on the electronic properties of Cu(1 1 0) surface. The Cu(1 1 0) surface has been studied using ultraviolet photoemission spectroscopy (PES), inverse photoemission spectroscopy (IPES), and reflection anisotropy spectroscopy (RAS), and shows a resonance in the RAS spectra at 2.1 eV due to a transition between occupied and unoccupied surface states. The unoccupied surface state involved in the RAS transition at an energy of 1.7 eV at the point of the surface Brillouin zone has been investigated using IPES and the occupied surface state is seen in PES spectra at 0.45 eV below the Fermi level. The energy difference of the surface states, 2.15 eV, is a good match to the transition energy found in the RAS experiments.     

Electronic structure of dendrimer-encapsulated Au nanocluster

We have carried out optical and X-ray photoemission studies of the dendrimer-encapsulated Au nanoclusters. The dendrimer-encapsulated Au nanoclusters are prepared by the chemical reduction of Au ions loaded within the dendrimer templates. Photoluminescence spectrum of the dendrimer-encapsulated Au nanoclusters with diameter of about 1.0 nm shows the visible luminescence centered at about 2.8 eV. In addition, we have measured the nanocluster-size dependent photoemission spectra in the valence-band region. From line shape analysis of Au 4f X-ray photoemission spectra, Au 4f core-level spectra of the dendrimer-encapsulated Au nanoclusters reflect the size dependent chemical-states. From these results, we discuss electronic structures and chemical states of the dendrimer-encapsulated Au nanoclusters.     

Inverse photoemission spectroscopy of the unoccupied electronic structure of Na/Cu(110)

The unoccupied electronic states of Na thin films on a Cu(110) substrate have been measured by inverse photoemission spectroscopy (IPES). The IPES spectrum provides the intensity of the unoccupied states, which decreases with increasing Na coverage at off-normal incidence of the electron beam. The IPES spectra at 17 and 19 eV incident electron energies show a shift towards the Fermi level with increasing Na coverage for the peak at ∼7.8 eV.     

Electronic structure of regular bacterial surface layers

We report photoemission and near-edge x-ray absorption fine structure measurements of the occupied and unoccupied valence electronic states of the regular surface layer of Bacillus sphaericus, which is widely used as the protein template for the fabrication of metallic nanostructures. The two-dimensional protein crystal shows a semiconductorlike behavior with a gap value of approximately 3.0 eV and the Fermi energy close to the bottom of the lowest unoccupied molecular orbital. We anticipate that these results will open up new possibilities for the electric addressability of biotemplated low-dimensional hybrid structures.     

Electronic structure of Sr2FeMoO6

We have used electron spectroscopies to investigate the electronic structure of the double perovskite Sr₂FeMoO₆. The valence-band photoemission spectra present a well-defined cut-off at the Fermi level, indicative of the metallic character of the material. The O 1s X-ray absorption spectrum presents three peaks, which are in good correspondence with the main structures in the unoccupied density-of-states of DF-LDA calculations. The electron energy-loss spectra present two structures which are also interpreted in terms of transitions between the bands obtained in the DF-LDA calculations.     

Electronic structure of the molecular switch tetra-<Emphasis Type="Italic">tert</Emphasis>-butyl-azobenzene adsorbed on Ag(111)

Occupied and unoccupied electronic states in tetra-tert-butyl-azobenzene (TBA) absorbed on Ag(111) have been investigated by one-photon and two-photon photoemission spectroscopy. These measurements allow the quantitative determination of energetic positions of the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as the n=1 image potential state. The assignment of the electronic states are supported by quantum chemical calculations. Experimentally a HOMO–LUMO gap of 2.85 eV is observed, whereas the gap obtained from the calculated molecular orbital energies is 0.92 eV larger. This discrepancy can be explained by image charge screening. Furthermore, two unoccupied final states located 0.18 and 0.43 eV above the vacuum level, respectively, have been identified. PACS 73.20.-r; 74.25.Jb; 79.60.-i; 79.60.Dp; 68.43.Vx     

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(1 1 1):√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.     

Observation of high-energy Pt(111) surface resonances excited by laser-generated XUV radiation

The surface electronic structure of platinum is investigated by photoemission using extreme ultraviolet light. High-order harmonics of amplified femtosecond Ti:sapphire radiation in the range h=20 to 50eV are employed. In the normal emission spectra of Pt(111), three well-distinguished maxima at fixed kinetic energies of E_kin=10,12.5 and 19eV are observed. These structures, which are far above the Fermi energy, show a strong dependence upon adsorption of CO. This observation and layer-resolved photoemission calculations indicate the surface character of the involved unoccupied platinum states. These empty states are populated by the primary photo-excited electrons, which decay into the unoccupied states due to inelastic scattering. The pronounced emission of the electrons originates mainly from these surface resonances. PACS 79.60.-i; 42.65.Ky; 71.20.-b; 79.20.Hx; 79.60.Dp     

Time-resolved two-photon photoemission of unoccupied electronic states of periodically rippled graphene on Ru(0001)

The unoccupied electronic states of epitaxially grown graphene on Ru(0001) have been explored by time- and angle-resolved two-photon photoemission. We identify a Ru derived resonance and a Ru/graphene interface state at 0.91 and 2.58?eV above the Fermi level, as well as three image-potential derived states close to the vacuum level. The most strongly bound, short-lived, and least dispersing image-potential state is suggested to have some quantum-well character with a large amplitude below the graphene hills. The two other image-potential states are attributed to a series of slightly decoupled states. Their lifetimes and dispersions are indicative of electrons moving almost freely above the valley areas of the moiré superstructure of graphene.     

苝四甲酸二酐薄膜电子结构的同步辐射共振光电子能谱研究

利用基于同步辐射的近边X射线吸收精细结构谱(NEXAFS)和共振光电子谱(RPES)研究了苝四甲酸二酐分子(PTCDA)薄膜的电子结构.碳K边NEXAFS谱中能量小于290 eV的四个峰对应于PTCDA分子不同化学环境碳原子1s电子到未占据分子轨道的共振跃迁.RPES谱中观察到共振光电子发射和共振俄歇电子发射导致的共振峰结构,以及二次谐波激发的碳1s信号.根据电子动能对入射光能量的依赖性分别对三类峰结构进行了归属.同时,发现PTCDA分子轨道共振光电子峰的强度具有光子能量依赖性.这种能量选择性共振增强效应是由于PTCDA分子轨道空间分布差异导致的.共振俄歇峰主要源于高结合能(4.1 eV)分子轨道能级电子参与的退激发过程.明确RPES实验谱图中各个峰结构的起源有助于准确利用基于RPES的芯能级空穴时钟谱技术定量估算有机分子/电极异质界面处电子从分子未占据轨道到电极导带的超快转移时间.     

First principles study of structural, elastic, electronic and optical properties of the cubic perovskite BaHfO3

First principles study of structural, elastic, electronic and optical properties of the cubic perovskite-type BaHfO₃ has been reported using the pseudo-potential plane wave method within the local density approximation. The calculated equilibrium lattice is in a reasonable agreement with the available experimental data. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear pressure dependence of the elastic stiffnesses is found. Band structures show that BaHfO₃ is a direct band gap between the occupied O 2p and unoccupied Hf d states. The variation of the gap versus pressure is well fitted to a quadratic function. Furthermore, in order to understand the optical properties of BaHfO₃, the dielectric function, absorption coefficient, optical reflectivity, refractive index, extinction coefficient, and electron energy loss are calculated for radiation up to 30 eV. We have found that O 2p states and Hf 5d states play a major role in the optical transitions as initial and final states, respectively. This is the first quantitative theoretical prediction of the elastic, electronic and optical properties of BaHfO₃ compound, and it still awaits experimental confirmation.     

FeS2多晶薄膜电子结构的实验研究

用磁控溅射方法制各纯Fe薄膜，并硫化合成FeS₂. 采用同步辐射X射线近边吸收谱与X射线光电子能谱研究了薄膜的电子结构. 结果表明，合成的FeS₂薄膜，在费米能级附近，有较强的Fe 3d态密度存在，同时，在价带谱中2—10eV处有强度较大的S 3p态密度存在；Fe的3d轨道在八面体配位场作用下分别为t_2g和e_g轨道，实验中由Fe的吸收谱计算得到两分裂能级之差为2.1eV;实验测得FeS₂价带结构中导带宽度约为2.4eV，导带上方仍存在第二能隙，其宽度约为2.8eV. 关键词： 磁控溅射 二硫化铁 X射线吸收近边结构 电子结构     

Charge-transfer at silver/phthalocyanines interfaces

Valence band photoemission spectroscopy (VB-PES) and inverse photoemission spectroscopy (IPES) were employed to determine the occupied and unoccupied density of states upon silver deposition onto layers of two phthalocyanines (H₂Pc and CuPc). The two different Pc molecules give rise to very distinct behaviour already during the initial stage of silver deposition. While in the CuPc case no shift occurs in the energy levels, the H₂Pc highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are shifting simultaneously by 0.3 eV, i.e., the HOMO shifts away from the Fermi level while LUMO shifts towards the Fermi level. As the silver quantity increases the HOMO levels of both Pcs are shifting towards the Fermi level. When the Fermi level is resolved in the VB spectra, the characteristic features of H₂Pc and CuPc are smeared out to some extent. Shifts in HOMO and LUMO energy positions as well as changes in line shapes are discussed in terms of charge-transfer and chemical reactions at the interfaces.     

The unoccupied electronic structure characterization of hydrothermally grown ThO2 single crystals

Single crystals of thorium dioxide ThO₂, grown by the hydrothermal growth technique, have been investigated by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and L₃, M₃, M₄, and M₅ X‐ray absorption near edge spectroscopy (XANES). The experimental band gap for large single crystals has been determined to be 6 eV to 7 eV, from UPS and IPES, in line with expectations. The combined UPS and IPES, place the Fermi level near the conduction band minimum, making these crystals n‐type, with extensive band tailing, suggesting an optical gap in the region of 4.8 eV for excitations from occupied to unoccupied edge states. Hybridization between the Th 6d/5f bands with O 2p is strongly implicated. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Angle-resolved photoemission study of Ag nanofilms grown on fcc transition-metal (111) substrates

A comparative study of the electronic structures of Ag nanofilms on the pseudomorphic metastable fcc Fe(111) and bulk-like fcc Co(111) substrates has been carried out to investigate their quantized electronic structures. The photoemission spectra of both Ag nanofilms exhibit the fine structures derived from the quantized sates of Ag sp valence electron. The nanofilm-thickness dependences of the binding energy of the quantized states are reproduced by the calculated results based on the phase accumulation model. From the angle-resolved photoemission measurements, the effective masses of the quantized electronic states along the direction parallel to the nanofilm surface were directly determined. We discuss the electronic hybridization effect between quantized states in Ag nanofilm and 3d-derived electronic states in transition-metal substrates.     

Surface states at the clean surfaces of cleaved Si(111) and GaAs(110)

A combination of ellipsometric data on the electronic transitions from occupied to unoccupied surface states and published photoemission data on the energy distribution of the occupied surface states has been used to construct models of the surface states densities at the cleaved Si (111) and GaAs (110) surfaces.     

Thermal behavior of the roughened Cu(110) surface

The roughened Cu(110) surface was prepared by annealing the clean surface at various temperatures ranging from 700 to ∼1000 K. A significant drop in intensity of reflection anisotropy spectroscopy (RAS) peak at 2.1 eV photon energy as a function of increasing sample temperature was found for annealing above the roughening transition at 900 K. The observed change of 2.1 eV peak in RAS spectra is because of the surface state Fermi level shift due to temperature change. The RAS result is in good agreement with an unoccupied surface state energy using inverse photoemission spectroscopy (IPES). New IPES results indicate that the unoccupied surface state intensity decreases with increasing annealing temperature. It was also found that the unoccupied surface state was shifted. IPES results provide that the contributions of the surface state to surface optical properties at 2.1 eV are relevant for the RAS technique.     

Electronic structure of semiconductor surfaces

Our present understanding of the electronic structure of semiconductor surfaces is reviewed. It is shown that photoemission and inverse photoemission are ideal techniques for probing occupied and unoccupied electronic states, respectively. All quantum numbers of an electron can be determined, i.e., energy, momentum, spin and angular symmetries. For simple systems, such as clean ordered surfaces with a small unit cell it is possible to understand the electronic structure from first-principles calculations. For complex systems, such as encountered during oxidation and dry etching one is restricted to measuring the properties determined by short-range order. Core level spectroscopy with synchrotron radiation is able to determine the oxidation state and the local bonding of surface and interface atoms.