Wide Photon Energy Research Articles

Resonant photoemission in titanium metal

The photoionization cross section of the titanium 3d band, including the contributions from direct photoemission and autoionization, has been measured using synchrotron radiation on a wide photon energy range (21 – 110 eV). Particularly, we have concentrated on the resonant behavior of the valence band near the 3p and 3s threshold in which the influence of autoionization appears as drastic fluctuations of a resonant profile. In addition, we have examined subthreshold Auger emission that should already occur below the threshold energy.

Magneto-optic properties and exchange interaction of the macroscopic ferrimagnet Co1−x−yTbx(EuS)y

Polar Kerr-rotation spectra and hysteresis loops have been measured in Co1−x−yTbx(EuS)y, with 0≤x≤0.21 and 0≤y≤0.21, and compared to the macroscopic ferrimagnet Co1−y(EuS)y and to amorphous Co1−xTbx. The measurements were taken at room temperature and at 7 K over a wide photon energy range of 1.1–4.6 eV in fields up to 2.77 T. The addition of Tb retains the phase-separated nature yielding a material consisting of a Co–Tb matrix containing crystalline EuS precipitate particles with about 2 nm diameter. Up to 8% Tb is substituted for Eu in the EuS phase. The Kerr rotations are rather small reaching −0.25° at room temperature and increasing to −0.85° at 7 K, i.e., no optical enhancement is observed as in Co(EuS). This is most likely due to the much smaller size of the EuS crystallites. An ‘‘s’’-shaped feature in the Kerr-rotation spectra can be assigned to an Eu2+ 4f→5d transition while a contribution from the Tb3+-4f states shows up as a decrease with increasing photon energies. At 7 K, the Tb and Eu magnetic moments are found to be parallel while the Co moment is believed to be antiferromagnetically exchange coupled to both. At room temperature, the Co moment is dominating the Tb moment which is antiferromagnetically coupled as in Co1−xTbx.

Spin-polarized photoelectrons excited by circularly polarized radiation from a nonmagnetic solid.

We report a spin analysis of core-level photoelectrons excited by circularly polarized x rays from a nonmagnetic solid. In a combined experimental and theoretical study, we show that the spin-orbit-split W 4 f 7/2 and 4 f 5/2 photoemission lines from W~110! exhibit high spin polarizations of opposite sign that vary with energy and emission direction. These results suggest the study of the magnetic structure of nonmagnetic/ferromagnetic interfaces formed on high-atomic-number substrates by spin-polarized photoelectron diffraction. Photoelectrons from unpolarized atoms can be highly spin polarized when ejected by circularly polarized ~CP! light, due to the Fano effect: 1~a! although the dipole operator does not act upon the electron spin explicitly, the photon angular momentum is partially transferred to the photoelectron spin by the spin-orbit ~SO! interaction. Shortly after its prediction in 1969, this effect was observed in the valence levels of alkali-metal 1~b! and rare-gas atoms 2 in gaseous and solid-state phases, and today it is well known as the basic mechanism behind the GaAs source for spin-polarized electrons. 3 In particular, photoelectrons excited by CP light from p, d, and f shells can acquire substantial spin polarization over a wide photon energy range. 4 Up to now, the spin analysis of photoelectrons from unpolarized targets has mostly served fundamental interests of ~i! understanding photoemission ~PE! dynamics through quantum mechanically ‘‘complete’’ experiments 5,6 and ~ii! characterizing the symmetry of valence bands in nonmagnetic solids. 7

Study of correlation states of acetylene by synchrotron photoelectron spectroscopy

The inner valence photoelectron spectra of acetylene (C2H2) and isotopically labeled acetylene (1,213C–C2H2) are obtained using high resolution synchrotron photoelectron spectroscopy. Four distinct correlation (satellite) peaks, consistent with previous x-ray photoelectron spectroscopy measurements, are resolved. The photon energy dependence of the intensity ratios of these satellites to the 2σ−1g main peak is observed over a wide photon energy range (32–72 eV). Three of these satellites (26.6, 28.0, and 29.8 eV binding energy) exhibited constant photon energy dependence while the fourth satellite (31.2 eV binding energy) showed enhancement of intensity towards the threshold. The photon energy dependence of correlation (satellite) peak 4 can be explained in either of two ways: (1) Peak 4 is a dynamic correlation peak associated with the 2σ−1g ionization process or (2) peak 4 is an intrinsic correlation peak associated with the 3σ−1g ionization process. A multireference singles and doubles configuration interaction (MRSDCI-ANO) calculation of the theoretical photoelectron inner valence spectrum using average natural orbitals indicates that the latter explanation (2) is more likely. Semiquantitative agreement (in terms of the peak positions and intensities) is also obtained between the experimental photoelectron spectrum and the MRSDCI(ANO) calculation.

Proton photoproduction from 12C

Abstract The 12 C(γ,p) differential cross-section in the Δ-resonance region has been measured using tagged photons. Data are presented for 20 MeV wide photon energy bins at 200, 300, 400 and 500 MeV and for proton angles from 52° to 128°. The present 12 C(γ,p) measurements are compared with previous experiments carried out at Frascati, Tokyo and Bonn. Comparisons are also made with theoretical calculations developed at Gent and Valencia which provide an interpretation of the photon reaction mechanism in terms of the major contributing processes. Coincidence data with correlated neutrons, protons and pions are also presented.

Photoinduced evaporation of benzene cluster ions

Photofragmentation of benzene cluster ions, (C 6H 6) n + with n = 5–8, is studied in a wide photon energy range of 0.5-3.0 eV. Apparent evaporation energies for n = 5–8 are determined to be 0.55-0.52 eV. These are approximately 0.2 eV larger than the binding energies for one neutral monomer to the cluster ions. The average translational energies of 50–75 meV for ejected neutral monomers are obtained in the photon energy range of 1.1–2.0 eV. The remainders of nearly 0.2 eV cannot be explained only in terms of the translational energies of the photofragments. This result indicates that a part of the imparted energy is partitioned into photofragments as internal energies.

Critical analysis of a band picture description of CeRh 3 spectral function in inverse photoemission

Inverse photoemission spectra from CeRh 3 have been measured in a wide photon energy range in the isochromat mode (10.5 ⩽ hv ⩽ 33 eV in the UV inverse photoemission and hv = 1486.7 eV in the BIS regime). At the lower UV energies the weak 4f cross section allows us to probe the non-f contribution and a good agreement with LDA band calculations is obtained. However, the 4f contribution, estimated from measurements at higher photon energies do not agree with an LDA band; moreover, the 4f contribution cannot be described by a single impurity model with configuration independent parameters.

Magneto-optic spectroscopy perovskite-type La/sub 1-x/(Ba,Sr)/sub x/MnO/sub 3/

Epitaxial thin films of ferromagnetic perovskite-type La/sub 1-x/(Ba,Sr)/sub x/MnO/sub 3/ have been studied at room temperature by means of polar Kerr rotation and ellipsometric measurements over a wide photon energy range. The Kerr-rotation spectra show two prominent features, one at 2 eV and a second, larger one around 3.2 eV but the spectra do not differ much with Sr or Ba substitution. This indicates that exclusively electronic transitions involving Mn-3d and O-2p electrons are magnetooptically active. The Kerr rotation reaches a maximum value of -0.23/spl deg/ in La/sub 0.7/Sr/sub 0.3/MnO/sub 3/ at a photon energy of 3.2 eV. The structure at 2 eV is tentatively assigned to a dipole-forbidden, spin-allowed intra-Mn transition between crystal-field split 3d levels. The feature at 3.2 eV arises from a charge-transfer transition from occupied O-2p into empty Mn-3d levels. At low Sr doping (x=0.1), a narrow peak appears in the Kerr spectra at 4.6 eV. The full width at half maximum is estimated to be 0.25 eV. This is a factor of three smaller than the width of the two structures at lower energies. From the narrow width it is concluded that well localized electronic states are involved. Yet, the origin of this structure remains unclear.

Correlation of surface core levels and structural building blocks for the Si(111)-7 x 7 reconstruction through high-resolution core-level spectroscopy.

The Si(111)-7\ifmmode\times\else\texttimes\fi{}7 reconstruction is investigated by high-resolution core-level spectroscopy. A detailed line-shape analysis of spectra recorded over a wide photon energy range demonstrates the presence of four surface-derived components in the Si 2p line. These are attributed to the structural building blocks of the (7\ifmmode\times\else\texttimes\fi{}7) structure through their relative intensities, evaluated for different photon energies in order to take photoelectron-diffraction effects into account, and on the basis of temperature-dependent changes in line shape that occur in the (7\ifmmode\times\else\texttimes\fi{}7)-(1\ifmmode\times\else\texttimes\fi{}1) phase transition at around 1130 K.

The interesting photon energy dependence of a correlation peak of ethylene

Synchrotron photoelectron spectra (PES) of ethylene are obtained over a wide photon energy range (30 to 220 eV). A new correlation (satellite) peak is found at 21.4 eV binding energy. A preliminary multi-reference singles and doubles configuration interaction (MRSDCI) calcualtion, based on average natural orbitals (ANO), is reported to provide a tentative assignment for this new peak. The relative photoionization cross section of this peak shows a strong photon energy dependence and appears to be caused by dynamic correlations.

Core-level magnetic circular dichroism in 3d and 4f magnetic systems (invited) (abstract)

With the recent availability of circulary polarized synchrotron radiation over a wide photon energy range from VUV to hard X rays, the magnetic circular dichroism (MCD) in core-level photoabsorption has rapidly attracted growing interest, both experimentally and theoretically. This novel technique can provide element-specific and site-selective information about the magnetic and the electronic states in various magnetic substances because the core-level MCD process involves optical transitions in which the one-electron initial states are well localized and have well-defined angular momenta. In order to get insight into the local magnetic states in 3d and 4f magnetic systems, we have studied MCD of ferrites, Fe1−xPtx alloys, and mixed-valence CeRh3B2 at the core-absorption edges in the VUV∼soft x-ray region. The experiments were performed by utilizing directly characterized, circularly polarized undulator radiation and off-plane synchrotron radiation1 in conjunction with an ultrahigh vacuum compatible superconducting magnet of special design.2 Clear MCD signals were observed for CeRh3B2 in the prethreshold region of the Ce 4d→4f (N4,5) edges. A comparison of the experimental MCD spectrum with theoretical ones3 for uniaxial crystal fields of Δc=0 and 0.2 eV shows that the experimental spectrum qualitatively agrees with the theoretical one for Δc=0 eV. Theory predicts that the MCD pattern for Δc<0.14 eV will be essentially the same as that for Δc=0 eV. Considering a Δc value of 0.1–0.2 eV, which was determined from the linear dichroism measurement,4 the present result leads to the conclusion that Δc∼0.1 eV. With a model based on the strong hybridization between the neighboring Ce 4f0(lz=0) and Ce 5d0(lz=0) orbitals together with the value of Δc∼0.1 eV, we can successfully explain many of the extraordinary magnetic properties of CeRh3B2. We will also present the MCD data in the M2,3 core-absorption region for ferrites (Fe3O4 and CoFe2O4) and Fe1−xPtx alloys, discussing the results.

Interpretation of the photoelectron spectra of molecularly adsorbed CO

We have measured the photoemission spectra both for CO adsorbed on Ni and Pd films and for gas phase CO over a wide photon energy range. The comparison of these two sets of measurements lead to the following interpretation of the two observed electronic energy levels of molecularly adsorbed CO. The lower binding energy CO-induced level corresponds to the 1π gas phase level with some admixture of the 5σ level and the higher binding energy level corresponds to the 4σ gas phase level. The h ̷ ω-dependence of the photoionization cross section is shown to play a predominant role in determining the surface sensitivity.

Parametrization of the total photon mass attenuation coefficients in the energy range 0.1–1000 keV

It is convenient to generate mass attenuation coefficients using semi-empirical schemes. The validity of most of the existing schemes is limited to a relatively narrow energy interval (1–40 keV) and their accuracies are poor in some energy regions. In this work, a semi-empirical scheme flexible enough to give a good fit to data in a very wide photon energy range (0.1–1000 keV) was employed. Fitting coefficients for the entire range were obtained by utilizing mass attenuation data from two sources: (1) semi-empirical data of Henke et al. in the low photon energy region, and (2) theoretical values generated with the XCOM code for fitting in the high energy region. The root mean square of the fit is generally less than 0.2% except for energies below 1 keV where the available data are scattered. A computer code for generating mass attenuation coefficients based on the proposed scheme has been developed.

Nonlinear optical spectroscopy on one-dimensional excitons in silicon polymer, polysilane.

The modulus and phase of the complex third-order nonlinear optical (NLO) susceptibility \ensuremath{\chi}${\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}}^{(3)}$(-3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega}) have been measured on an all-trans silicon polymer, poly(dihexylsilane), over a wide photon energy range. Several distinct structures observed in the \ensuremath{\chi}${\mathrm{\ifmmode \tilde{}\else \~{}\fi{}}}^{(3)}$ spectra are in excellent agreement with the theoretical results of a one-dimensional (1D) exciton model. Important aspects of the 1D Wannier-like exciton structure in polysilanes are elucidated in relation to the NLO processes.

Optical Properties of Hydrogenated Amorphous Silicon, Silicon-Germanium and Silicon-Carbon Thin Films

ABSTRACTThe optical properties of solar cell grade hydrogenated amorphous silicon (a-Si:H), silicon germanium (a-SiGe:H) and silicon carbon (a-SiC:H) alloy thin films have been investigated over a wide photon energy range (0.8–4.8 eV) using a combination of subgap photoconductivity, reflection and transmission, and spectroscopie ellipsometry techniques in order to obtain accurate optical functions for solar cell modelling. Studies on films with thicknesses ranging from a few hundred Å to a few microns show that the optical spectra obtained by the different techniques agree closely over the energy ranges of overlap and display no thickness dependence from the Urbach tail energies and above. Thus, the results appear to be free of measurement and sample related artifacts. Three different methods provide a common value for the optical gap within ±0.02 eV and the result for a-Si:H is ∼0.1 eV below the mobility gap.

Complete valence-band structure of Ge determined by photoemission.

The valence-band structure along the high-symmetry directions \ensuremath{\Gamma}L, \ensuremath{\Gamma}X, and \ensuremath{\Gamma}KX was studied with synchrotron radiation for a wide photon energy range (10--106 eV). A cylindrically shaped Ge crystal with [11\ifmmode\bar\else\textasciimacron\fi{}0] axis was used, which allowed us to record the spectra in normal emission for different orientations. The data were analyzed by applying the model of direct transition into free-electron final-state bands. Due to the wide photon energy range, a complete band scheme along the three directions including critical points was obtained. The data are compared with theoretical results and other measurements. The bands along the \ensuremath{\Gamma}KX direction are reported for the first time to our knowledge. Applying the wide photon energy range, the inner potential ${\mathit{V}}_{0}$=-8.8 eV was determined self-consistently using the method suggested by Middelmann et al. [Phys. Rev. B 34, 957 (1986)].

Photoemission study of the final band in Ag(111)

For direct transitions in angle-resolved photoemission from a single crystal, the final electronic states inside the crystal can be approximately described by a free-electron-like band for energies higher than approximately=30 eV above the Fermi level. The authors report a determination of the dispersion relation for this band in Ag along the (111) direction over a wide photon energy range (40-180 eV), from which the inner potential (average electrostatic potential in the solid within the Hartree approximation) and the energy-dependent electron effective mass are deduced. The effective mass is found to increase from approximately 1.00 m at 54 eV to 1.08 m at 149 eV above the Fermi level (m is the free electron mass). This result is compared with existing many-body model calculations. The present study is also relevant to the problem of surface state resonance in this system discussed previously.

The electronic structure of 2 × 1-Au(110)

An extensive angle-resolved photoemission study using synchrotron radiation has been performed upon 2 × 1-Au(110). Because a wide photon energy range was used ( hv = 8 to 150 eV), the occupied electronic states could be mapped completely along X KΓ. This was done using the direct transition model and an experimentally determined, free-electron-like, final state. Despite the success of this model in predicting a vast majority of the spectral features, several failures of this approach were observed. These anomalies can be explained by invoking final states other than the free-electron-like state and non-direct transitions. A very sharp feature observed near B F = 1.8 eV was observed and will be discussed.

Photon energy dependence of bulk and interface phonons frequencies of a thin layer superlattice

The first order resonant Raman scattering of thin layer GaAs/AlAs superlattices has been studied over a wide incident photon energy range. An oscillatory relationship between the GaAs and AlAs LO modes is observed as a function of the laser energy, which is explained by a modulation of the optical modes by the acoustical vibrations of the superlattice.

Magneto-optical properties of metallic bilayer thin films

Metallic bilayer thin films are discussed as a novel and encouraging approach to substantially increasing the figure of merit R/sup 1/2/- theta /sub K/ in magnetooptical data storage media with a potential gain of more than a factor of two. It is demonstrated by Kerr-rotation and reflectivity measurements in a wide photon energy range that for the metallic bilayer model system Fe/sub 50/Co/sub 50/ on top of Au, the figure of merit R/sup 1/2/- theta /sub K/ is enhanced by up to 65% at a high level of reflectivity, which allows further enhancement by conventional dielectric layers. It is shown that Fresnel's formulas allow a quantitative description of the observed optical and magnetooptical response over the whole energy range. These model calculations allow general conclusions regarding optimum metal combinations with respect to the maximum figure of merit. With a proper choice of the reflector metal the magnetooptical medium can be adapted to any wavelength determined by a particular application. >