Takashi Fujikawa

Chiba University, Tiba, Chiba, Japan

Are you Takashi Fujikawa?

Claim your profile

Publications (234)331.7 Total impact

  • Takashi Fujikawa · Kaori Niki
    [Show abstract] [Hide abstract]
    ABSTRACT: Keldysh Green's function approach is extensively used in order to derive practical formulas to analyze pump–probe ultrafast photoemission and X-ray absorption spectra. Here the pump pulse is strong enough whereas the probe X-ray pulse can be treated by use of a perturbation theory. We expand full Green's function in terms of renormalized Green's function without the interaction between electrons and probe pulse. The present theoretical formulas allow us to handle the intrinsic and extrinsic losses, and furthermore resonant effects in X-ray Absorption Fine Structures (XAFS). To understand the radiation field screening in XPS spectra, we have to use more sophisticated theoretical approach. In the ultrafast XPS and XAFS analyses the intrinsic and extrinsic loss effects can interfere as well. In the XAFS studies careful analyses are necessary to handle extrinsic losses in terms of damped photoelectron propagation. The nonequilibrium dynamics after the pump pulse irradiation is well described by use of the time-dependent Dyson orbitals. Well above the edge threshold, ultrafast photoelectron diffraction and extended X-ray absorption fine structure (EXAFS) provide us with transient structural change after the laser pump excitations. In addition to these slow processes, the rapid oscillation in time plays an important role related to pump electronic excitations. Near threshold detailed information could be obtained for the combined electronic and structural dynamics. In particular high-energy photoemission and EXAFS are not so influenced by the details of excited states by pump pulse. Random-Phase Approximation (RPA)-boson approach is introduced to derive some practical formulas for time-dependent intrinsic amplitudes.
    No preview · Article · Nov 2015 · Journal of Electron Spectroscopy and Related Phenomena
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report on the measurement of deep inner-shell 2p X-ray photoelectron diffraction (XPD) patterns from laser-aligned I2 molecules using X-ray free-electron laser (XFEL) pulses. The XPD patterns of the I2 molecules, aligned parallel to the polarization vector of the XFEL, were well matched with our theoretical calculations. Further, we propose a criterion for applying our molecular-structure-determination methodology to the experimental XPD data. In turn, we have demonstrated that this approach is a significant step toward the time-resolved imaging of molecular structures.
    Full-text · Article · Sep 2015 · Scientific Reports
  • Takashi Fujikawa · Kaori Niki · Hiroto Sakuma
    [Show abstract] [Hide abstract]
    ABSTRACT: In this short review we discuss recent progress in photoemission theory for organic molecules and their crystals. We discuss some important features in Keldysh Green's function theory for the photoemission. We briefly discuss many-body aspects in photoemission from core and extended levels. In particular phonon effects are investigated in more detail since organic solids are typically soft where electron-phonon interaction is important. Debye-Waller factor suppresses the interference effects of photoelectron waves which makes ARPES analyses useless, particularly in high energy region.
    No preview · Article · Jul 2015 · Journal of Electron Spectroscopy and Related Phenomena
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The photoluminescence of Gd-doped GaN multi-quantum wells (MQWs) is presented and discussed considering the formation of a Gd 3+ :Nitrogen-vacancy (N-vacancy) complex. A lower energy photolumi-nescence peak was observed for the Gd-doped GaN MQW sample with respect to the main peak assigned to a neutral donor bound exciton (D 0 X) of the undoped GaN MQW sample. The X-ray absorption near edge structure spectrum observed at Gd L III-edge indicates a nitrogen vacancy adjacent to the Gd substituting the Ga ion in Gd-doped GaN MQW sample. Local stresses around the Gd dopants in Gd-doped GaN matrix generated due to the larger diameter of the Gd 3+ ion with respect to the Ga 3+ ion can be relieved by the creation of vacancies. The lower formation energy of N-vacancies in GaN matrix introduce them as a preferred candidate to relieve the generated stresses. A Gd 3+ :N-vacancy complex consisting of a Gd 3+ ion and the created nitrogen vacancy adjacent to the Gd 3+ dopant is likely to form in GaN:Gd matrix. The lower photoluminescence peak energy observed in the Gd-doped GaN MQW sample is assigned to the recombination of an exciton captured at the Gd 3+ :N-vacancy complex forming a small polaron-like state. A model is presented considering the small exciton-polaron population in defect sites captured around the Gd 3+ ions in the Gd-doped GaN.
    Full-text · Article · Apr 2015 · Journal of Alloys and Compounds
  • Takashi Fujikawa · Kaori Niki
    [Show abstract] [Hide abstract]
    ABSTRACT: Some features in photoemission theory from organic solids are reviewed starting from many-body scattering theory. This theoretical approach is direct and transparent, however, the extension to discuss temperature effects is rather difficult. In order to discuss phonon and many-body effects we build a theoretical frame based on Keldysh Green’s functions. Phonon effects such as Debye–Waller factor, electron–phonon interaction and recoil effects are extensively discussed. For the practical calculations multiple scattering formulas work so well. Different features obtained from UPS and XPS analyses are discussed for the excitations from extended levels.
    No preview · Article · Jan 2015 · Springer Series in Materials Science
  • Source
    Takashi Fujikawa · Hiroto Sakuma · Kaori Niki · Didier Sébilleau
    [Show abstract] [Hide abstract]
    ABSTRACT: Some important phonon effects observed in X-ray absorption and X-ray photoemission spectra are discussed on the basis of nonequilibrium Green's function theory. This theoretical framework allows us to incorporate phonon effects, such as Debye-Waller (DW) factors, Franck-Condon (FC) factors and electron-phonon interactions in a natural way. In the case of core level excitations, we can take into account the core-hole effects in lesser Green's function g< and photoelectron propagation in greater Green's function g>. For the core-hole propagation we derive some formulas to describe the thermally displaced core functions: we have p components even for deep core s orbital due to the thermal motion. We should notice that the thermal fluctuation is quite small but it is already in the order of the spread of the core functions. Applying Mermin's theorem, we can calculate the thermal average of the hole propagator g<: Here an important ingredient is the Debye-Waller factor used in X-ray and neutron diffraction. For the pre-edge structures, the intensity associated with forbidden electric dipole transition is sensitive to the temperature compared with allowed electric quadrupole transition. We also discuss the FC and their interference, which have negligible contribution to pre-edge intensity and energy shift. The quasi-particle energy is also influenced by the core displacement which can be responsible for the peak shift of the pre-edges. We also discuss the changes of the photoelectron angular distributions caused by the thermal atomic vibration.
    Preview · Article · Dec 2014 · Journal of Electron Spectroscopy and Related Phenomena
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nickel oxide is a p-type semiconductor with wide band gaps of 3–4 eV. The defects of O or Ni atoms are essential to explain some specific features of the semiconductors. The information of the local structure around Ni is important to clarify the conduction mechanism. We investigate the Ni K-edge X-ray absorption fine structure spectra of the nickel oxide thin films in order to obtain the structural information around Ni. For the X-ray absorption near edge structure (XANES) analyses, we apply a full multiple scattering theory. The experimental results suggest that the crystallinity of the thin films depends on the annealing temperature. The distances of Ni–O and Ni–Ni are 1.984 and 2.747 Å for the sample annealed at 100 °C and 2.084 and 2.947 Å for the sample annealed at 300 °C, respectively. Our multiple scattering calculations can satisfactorily explain the experimental result. Copyright © 2014 John Wiley & Sons, Ltd.
    No preview · Article · Oct 2014 · Surface and Interface Analysis
  • K. Niki · N. Yamamura · Y. Ohori · M. Kazama · T. Fujikawa · L. Kövér
    [Show abstract] [Hide abstract]
    ABSTRACT: We have studied the surface and bulk plasmon losses associated with Li 1s photoemission. Here, full multiple scatterings of photoelectrons are taken into account before and after the plasmon losses within the quantum Landau formula that can describe overall features of the photoemission bands. We compared the depth profiles of the Li 1s single-loss spectra with full multiple scatterings and without elastic scatterings. They are strongly influenced by the elastic scatterings. The model accounting for the full multiple scatterings gives a rapidly decaying function of the depth because of the defocusing effects. Copyright © 2014 John Wiley & Sons, Ltd.
    No preview · Article · Oct 2014 · Surface and Interface Analysis
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have measured O 1s photoelectron angular distributions in coincidence with the O+-CO+ fragment ion pairs of CO2 molecules at the photoelectron energies of 90, 120 and 150 eV. The observed molecular frame photoelectron angular distributions are left-right asymmetric as a consequence of an enhancement in bond-breaking probability for the CO bond involving the O atom with the 1s hole, compared with the other CO bond. Comparison of the experimental data with the calculations due to our semi-empirical model enables us to deduce the value of 1.4 for this enhancement in the relative bond breaking probability.
    No preview · Article · Jul 2014 · Chemical Physics Letters
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have measured C 1s photoelectron angular distributions (PADs) in coincidence with the CO+–O+ fragment ion pairs of CO2 molecules at the photoelectron energies of 85, 120 and 150 eV. The observed left–right asymmetric PADs have been well reproduced by our theoretical model taking into account the two degenerate zero-point bending vibrations. This leads to a conclusive result on the interpretation of such PADs; although it has been believed so far that they are the molecular frame, in fact they are the recoil frame.
    No preview · Article · Mar 2014 · Journal of Physics B Atomic Molecular and Optical Physics
  • [Show abstract] [Hide abstract]
    ABSTRACT: We study the angular and energy dependence of surface and bulk plasmon losses accompanying deep core excitations in simple metals. Here full multiple scatterings of photoelectrons are taken into account before and after the plasmon losses within the quantum Landau formula, which can describe overall features of the photoemission bands. For example, multiple plasmon loss features can be calculated by use of the formula. Two simple metals, Al and Na, are studied here. The depth profiles of the plasmon losses are strongly influenced by the elastic scatterings. The model assuming single elastic scatterings overestimates the losses from deep emitters due to the forward focusing effects, whereas the model accounting for full multiple scatterings gives a much rapidly decaying function of the depth due to the defocusing effects and rich structures due to the photoelectron diffraction. The single elastic scattering approximation gives a poor result both for the depth profiles and for the loss spectra. The present multiple scattering calculations successfully explain the azimuthal dependence of the loss spectra, which reflect the local geometry around the emitters.
    No preview · Article · Dec 2013 · Physical Review B
  • [Show abstract] [Hide abstract]
    ABSTRACT: The spin-polarized electronic structures across the interface between single-layer graphene and a Ni(111) thin film are explored by employing depth-resolved X-ray absorption and magnetic circular dichroism spectroscopy with atomic layer resolution. The depth-resolved Ni L2,3-edge analysis clarifies that the Ni atomic layers adjacent to the interface show a transition of the spin orientation to the perpendicular one in contrast to the in-plane one in the bulk region. The C K-edge analysis reveals the intensification of the spin–orbit interactions induced by the π–d hybridization at the interface as well as out-of-plane spin polarization in the π band region of graphene. The present study indicates the importance of the interface design at the atomic layer level for graphene-based spintronics.
    No preview · Article · Sep 2013 · Journal of Materials Chemistry
  • Source

    Full-text · Conference Paper · Jul 2013
  • Misato Kazama · Takashi Fujikawa · Naoki Kishimoto · Akira Yagishita
    [Show abstract] [Hide abstract]
    ABSTRACT: We provide a molecular structure determination method, based on multiple-scattering x-ray photoelectron diffraction (XPD) calculations. This method is applied to our XPD data on several molecules having different equilibrium geometries. Then it is confirmed that, by our method, bond lengths and bond angles can be determined with a resolution of less than 0.1 Å and 10∘, respectively. Differently from any other scenario of ultrafast structure determination, we measure the two- or three-dimensional XPD of aligned or oriented molecules in the energy range from 100 to 200 eV with a 4π detection velocity map imaging spectrometer. Thanks to the intense and ultrashort pulse properties of x-ray free-electron lasers, our approach exhibits the most probable method for obtaining ultrafast real-time structural information on small to medium-sized molecules consisting of light elements, i.e., a “molecular movie.”
    No preview · Article · Jun 2013 · Physical Review A
  • Ming-Hui Shang · Takashi Fujikawa · Nubuo Ueno
    [Show abstract] [Hide abstract]
    ABSTRACT: Recoil effects in valence band X-ray photoelectron spectroscopy (XPS) are studied for both abbtrifluorostyrene and styrene molecular crystal systems. The gradual changes of XPS spectra excited by several photon energies are theoretically investigated within the tight-binding approximation and harmonic approximation of lattice vibrations, and have been explained in terms of not only atomic mass but also atomic orbital (AO) population. The recoil effect of valence band photoemission strongly depends on the population and partial photoionization cross section (PICS) of AOs as well as the mass of composite ion. In abb-trifluorostyrene F 2p dominant bands show the recoil shift close to free F atom recoil shift, and C 2s dominant bands show that to free C atom recoil shift, whereas the mixed bands of C and F give rise to the peak asymmetries due to their different recoil shifts. For these systems hydrogen contribution is negligibly small which is contrast to our previous results for the crystals composed of small organic molecules. We also discuss some potential uses of the recoil shifts for these systems.
    No preview · Article · Feb 2013 · Analytical Chemistry
  • Akihiro Koide · Takashi Fujikawa · Nobuyuki Ichikuni
    [Show abstract] [Hide abstract]
    ABSTRACT: We briefly review the basic theory and recent applications in the X-ray absorption fine structure (XAFS) spectroscopy. First we discuss a dressed one-electron XAFS formula starting from many-body scattering theory, where important many-body effects, intrinsic and extrinsic losses, and optical potential are naturally introduced. Next multiple scattering renormalization, and spherical wave effects are discussed. Phonon effects such as Debye-Waller factors are also discussed. Some interesting XAFS applications, in particular, ultrafast XAFS and XAFS applications to nano-particles are discussed in some detail.
    No preview · Article · Jan 2013 · Journal of Electron Spectroscopy and Related Phenomena
  • Source
    Kei Takahashi · Takashi Fujikawa

    Full-text · Article · Jan 2013 · Shikizai Kyokaishi
  • [Show abstract] [Hide abstract]
    ABSTRACT: 1s photoelectron angular distributions from fixed-in-space CO2, NO2, BF3 and CH3F molecules have been calculated by X-ray photoelectron diffraction (XPD) theory with muffin-tin-type molecular potential. For all the molecules, the calculated results show good agreements with those by density functional theory in the energy region ≳100 eV. Furthermore, for all the molecules experimental data on the angular distributions in such energy region are well reproduced by the XPD theory. These intensive studies lead to a rather general rule that the XPD theory is an adequate tool to describe high-energy photoelectron angular distributions for any single oriented molecules.
    No preview · Article · Nov 2012 · Journal of Electron Spectroscopy and Related Phenomena
  • Takashi Fujikawa · Katsumi Kaneko
    [Show abstract] [Hide abstract]
    ABSTRACT: We theoretically discuss X-ray absorption intensity in high-energy region far from the deepest core threshold to explain the morphology-dependent mass attenuation coefficient of some carbon systems, carbon nanotubes (CNTs), highly oriented pyrolytic graphite (HOPG) and fullerenes (C60). The present theoretical approach is based on the many-body X-ray absorption theory including the intrinsic losses (shake-up losses). In the high-energy region the absorption coefficient has correction term dependent on the solid state effects given in terms of the polarization part of the screened Coulomb interaction Wp. We also discuss the tail of the valence band X-ray absorption intensity. In the carbon systems C 2s contribution has some influence on the attenuation coefficient even in the high energy region at 20 keV.
    No preview · Article · Nov 2012 · Journal of Electron Spectroscopy and Related Phenomena
  • [Show abstract] [Hide abstract]
    ABSTRACT: We calculate the inelastic mean free path (IMFP) for small metal particles based on the plasmon model for free electron gas confined in small sphere with radius a. The IMFP decreases with the radius a smaller than about 100 °A, and the rate of the decrease depends on the bulk plasmon energy ω p. We furthermore discuss the coordination numbers obtained from the EXAFS curve fitting method for small Ag particles. We find the importance of the size dependent IMFP for EXAFS analyses in order to obtain reliable coordination numbers in nanoparticles.
    No preview · Article · Oct 2012 · e-Journal of Surface Science and Nanotechnology

Publication Stats

2k Citations
331.70 Total Impact Points


  • 1995-2015
    • Chiba University
      • • Graduate School of Advanced Integration Science
      • • Graduate School of Science
      • • Department of Image and Materials Science
      • • Faculty of Science
      • • Department of Chemistry
      Tiba, Chiba, Japan
  • 2012
    • High Energy Accelerator Research Organization
      • Photon Factory
      Tsukuba, Ibaraki, Japan
  • 2009
    • The University of Electro-Communications
      Edo, Tokyo, Japan
  • 2007
    • Hokkaido University
      • Catalysis Research Center
      Sapporo, Hokkaidō, Japan
  • 1980-2006
    • Yokohama National University
      Yokohama, Kanagawa, Japan
  • 1993
    • Hirosaki University
      Khirosaki, Aomori, Japan
  • 1990-1993
    • Lund University
      • Department of Theoretical Physics
      Lund, Skåne, Sweden
  • 1974-1993
    • The University of Tokyo
      • Department of Chemistry
      白山, Tōkyō, Japan