Browsing by Author "Chen, S. A."

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Core-level anionic photofragmentation of gaseous CCl(4) and solid-state analogs
(2009-09-01)
Lu, K. T.
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Chen, J. M.
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Lee, J. M.
;
Haw, S. C.
;
Chou, T. L.
The dissociation dynamics of anionic and excited neutral fragments of gaseous CCl(4) and CCl(4) adsorbed on Si(100) similar to 90 K following Cl 2p core-level excitations were investigated on combining measurements of photon-induced anionic dissociation, x-ray absorption, and uv-visible dispersed fluorescence. The transitions of core electrons to high Rydberg states or doubly excited states near Cl 2p ionization thresholds of gaseous CCl(4) remarkably enhance the production of excited neutral fragments (C* and CCl*); this enhancement is attributed to the contribution from the shake-modified resonant Auger decay and/or postcollision interaction (PCI). The Cl-anion is significantly reinforced in the vicinity of the Cl 2p(1/2,3/2) ionization threshold of gaseous CCl(4), originating from PCI-mediated photoelectron recapture. The Cl 2p -> 7a(1)* excitation for CCl(4)/Si(100) at similar to 90 K enhances the Cl(-) desorption yield at a submonolayer level. This resonant enhancement of Cl-yield at the 7a(1)* resonance in the Cl 2p edge at a submonolayer level occurs through the formation of high-lying molecular-ion states of CCl(4) adsorbed on a Si surface.
Core-level anionic photofragmentation of gaseous CCl4 and solid-state analogs
(2009-09-01)
Lu, K. T.
;
Chen, J. M.
;
Lee, J. M.
;
Haw, S. C.
;
Chou, T. L.
The dissociation dynamics of anionic and excited neutral fragments of gaseous CCl4 and CCl4 adsorbed on Si(100) similar to 90 K following Cl 2p core-level excitations were investigated on combining measurements of photon-induced anionic dissociation, x-ray absorption, and uv-visible dispersed fluorescence. The transitions of core electrons to high Rydberg states or doubly excited states near Cl 2p ionization thresholds of gaseous CCl4 remarkably enhance the production of excited neutral fragments (C* and CCl*); this enhancement is attributed to the contribution from the shake-modified resonant Auger decay and/or postcollision interaction (PCI). The Cl-anion is significantly reinforced in the vicinity of the Cl 2p(1/2,3/2) ionization threshold of gaseous CCl4, originating from PCI-mediated photoelectron recapture. The Cl 2p -> 7a(1)* excitation for CCl4/Si(100) at similar to 90 K enhances the Cl- desorption yield at a submonolayer level. This resonant enhancement of Cl-yield at the 7a(1)* resonance in the Cl 2p edge at a submonolayer level occurs through the formation of high-lying molecular-ion states of CCl4 adsorbed on a Si surface.
Dissociation dynamics of anionic and excited neutral fragments of gaseous SiCl(4) following Cl 2p and Si 2p core-level excitations
(2008-10-08)
Chen, J. M.
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Lu, K. T.
;
Lee, J. M.
;
Chou, T. L.
;
Chen, H. C.
The state-selective dissociation dynamics for anionic and excited neutral fragments of gaseous SiCl(4) following Cl 2p and Si 2p core-level excitations were characterized by combining measurements of the photon-induced anionic dissociation, x-ray absorption and UV/visible dispersed fluorescence. The transitions of core electrons to high Rydberg states) doubly excited states in the vicinity of both Si 2p and Cl 2p ionization thresholds of gaseous SiCl4 lead to a remarkably enhanced production of anionic, Si-and Cl(-), fragments and excited neutral atomic, Si*, fragments. This enhancement via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of the contributions from the Auger decay of doubly excited states, shake-modified resonant Auger decay, or/and post-collision interaction. These complementary results provide insight into the state-selective anionic and excited neutral fragmentation of gaseous molecules via core-level excitation.
Dissociation dynamics of anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations
(2008-10-08)
Chen, J. M.
;
Lu, K. T.
;
Lee, J. M.
;
Chou, T. L.
;
Chen, H. C.
The state-selective dissociation dynamics for anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations were characterized by combining measurements of the photon-induced anionic dissociation, x-ray absorption and UV/visible dispersed fluorescence. The transitions of core electrons to high Rydberg states) doubly excited states in the vicinity of both Si 2p and Cl 2p ionization thresholds of gaseous SiCl4 lead to a remarkably enhanced production of anionic, Si-and Cl-, fragments and excited neutral atomic, Si*, fragments. This enhancement via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of the contributions from the Auger decay of doubly excited states, shake-modified resonant Auger decay, or/and post-collision interaction. These complementary results provide insight into the state-selective anionic and excited neutral fragmentation of gaseous molecules via core-level excitation.
Intra- and intersite electronic excitations in multiferroic TbMnO(3) probed by resonant inelastic x-ray scattering
(2010-09-27)
Chen, J. M.
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Lee, J. M.
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Huang, S. W.
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Lu, K. T.
;
Jeng, H. T.
Mn 3d valence states and elementary electronic excitations in single-crystalline TbMnO(3) were probed with resonant inelastic x-ray scattering (RIXS) (or resonant x-ray emission spectroscopy, RXES), polarized x-ray absorption spectra and ab initio electron-structure calculations. Polarized Mn K-edge x-ray absorption spectra of TbMnO(3) crystals exhibit a significant anisotropy along three crystallographic directions, particularly for the white-line region. The 1s3p-RXES spectra obtained at the Mn K edge reveal that the quadrupolar Raman regime is restricted predominantly to below the pre-edge peak whereas the fluorescence regime starts from the first pre-edge peak of 1s -> 3d transitions, indicating a relatively delocalized character of unoccupied Mn 3d states. The additional K beta emission profile at energy loss similar to 62 eV is attributed to the off-site dipole Mn 1s-Mn' 3d and/or Mn 1s-Tb 5d transitions, originating from hybridization between Mn 4p states and neighboring Mn' 3d/Tb 5d states. The off-site dipole transition makes a considerable contribution to the pre-edge region of Mn K-edge spectrum. Three prominent RIXS features at similar to 2.9, 7, and 11 eV were observed. Based on GGA+U calculations, the 7 eV region corresponds to transitions from O 2p states to unoccupied minority Mn 3d states, whereas the 11 eV band is ascribed to transitions from the O 2p band to the empty Tb 5d band. The broad 2.9 eV band is attributed to the coexistence of on-site Mn 3d-Mn 3d and off-site Mn 3d-Mn' 3d transitions.
Intra- and intersite electronic excitations in multiferroic TbMnO3 probed by resonant inelastic x-ray scattering
(2010-09-27)
Chen, J. M.
;
Lee, J. M.
;
Huang, S. W.
;
Lu, K. T.
;
Jeng, H. T.
Mn 3d valence states and elementary electronic excitations in single-crystalline TbMnO3 were probed with resonant inelastic x-ray scattering (RIXS) (or resonant x-ray emission spectroscopy, RXES), polarized x-ray absorption spectra and ab initio electron-structure calculations. Polarized Mn K-edge x-ray absorption spectra of TbMnO3 crystals exhibit a significant anisotropy along three crystallographic directions, particularly for the white-line region. The 1s3p-RXES spectra obtained at the Mn K edge reveal that the quadrupolar Raman regime is restricted predominantly to below the pre-edge peak whereas the fluorescence regime starts from the first pre-edge peak of 1s -> 3d transitions, indicating a relatively delocalized character of unoccupied Mn 3d states. The additional K beta emission profile at energy loss similar to 62 eV is attributed to the off-site dipole Mn 1s-Mn' 3d and/or Mn 1s-Tb 5d transitions, originating from hybridization between Mn 4p states and neighboring Mn' 3d/Tb 5d states. The off-site dipole transition makes a considerable contribution to the pre-edge region of Mn K-edge spectrum. Three prominent RIXS features at similar to 2.9, 7, and 11 eV were observed. Based on GGA+U calculations, the 7 eV region corresponds to transitions from O 2p states to unoccupied minority Mn 3d states, whereas the 11 eV band is ascribed to transitions from the O 2p band to the empty Tb 5d band. The broad 2.9 eV band is attributed to the coexistence of on-site Mn 3d-Mn 3d and off-site Mn 3d-Mn' 3d transitions.
Modifications of MnO6 Octahedra in DyMnO3 under High Pressure
(2010-01-01)
Chou, T. L.
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Lee, J. M.
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Chen, S. A.
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Ishii, H.
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Hiraoka, N.
Local geometry and electronic states of octahedrally coordinated Mn ions in orthorhombic DyMnO3 have been studied by partial-fluorescence-yield spin-selective x-ray absorption spectroscopy and in-situ Raman scattering. As evidenced in absorption near-edge structure and Raman spectra, the external pressure alleviates the MnO6 octahedral distortion and Jahn-Teller effect with enhanced octahedral tilt.
Pressure dependence of the electronic structure and spin state in Fe(1.01)Se superconductors probed by x-ray absorption and x-ray emission spectroscopy
(2011-09-09)
Chen, J. M.
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Haw, S. C.
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Lee, J. M.
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Chou, T. L.
;
Chen, S. A.
Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe(1.01)Se superconductors up to similar to 66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of similar to 7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe1.01Se decreases progressively with pressure up to similar to 10 GPa. A new prepeak at energy of similar to 7113.7 eV develops for pressure above similar to 13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe(1.01)Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe(1.01)Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe(1.01)Se. A discontinuous variation of the integrated absolute difference values of the K beta emission line was observed, originating from a phase transition of Fe(1.01)Se for a pressure >12 GPa. Fe(1.01)Se shows a small net magnetic moment of Fe(2 broken vertical bar) at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line K beta' was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe(1.01)Se superconductors. The experimental results provide insight into the spin state of Fe(1.01)Se superconductors under pressure.
Pressure dependence of the electronic structure and spin state in Fe1.01Se superconductors probed by x-ray absorption and x-ray emission spectroscopy
(2011-09-09)
Chen, J. M.
;
Haw, S. C.
;
Lee, J. M.
;
Chou, T. L.
;
Chen, S. A.
Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe1.01Se superconductors up to similar to 66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of similar to 7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe1.01Se decreases progressively with pressure up to similar to 10 GPa. A new prepeak at energy of similar to 7113.7 eV develops for pressure above similar to 13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe1.01Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe1.01Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe1.01Se. A discontinuous variation of the integrated absolute difference values of the K beta emission line was observed, originating from a phase transition of Fe1.01Se for a pressure >12 GPa. Fe1.01Se shows a small net magnetic moment of Fe-2 broken vertical bar at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line K beta' was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe1.01Se superconductors. The experimental results provide insight into the spin state of Fe1.01Se superconductors under pressure.
Pressure-dependent electronic structures in multiferroic DyMnO(3): A combined lifetime-broadening-suppressed x-ray absorption spectroscopy and ab initio electronic structure study
(2010-10-21)
Chen, J. M.
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Lee, J. M.
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Chou, T. L.
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Chen, S. A.
;
Huang, S. W.
Variations in the electronic structure and structural distortion in multiferroic DyMnO(3) were probed by synchrotron x-ray diffraction, lifetime-broadening-suppressed x-ray absorption spectroscopy (XAS), and ab initio electronic structure calculations. The refined x-ray diffraction data enabled an observation of a diminished local Jahn-Teller distortion of Mn sites within MnO(6) octahedra in DyMnO(3) on applying the hydrostatic pressure. The intensity of the white line in Mn K-edge x-ray absorption spectra of DyMnO(3) progressively increased with the increasing pressure. With the increasing hydrostatic pressure, the absorption threshold of an Mn K-edge spectra of DyMnO(3) shifted toward a greater energy, whereas the pre-edge line slightly shifted to a smaller energy. We provide the spectral evidence for the pressure-induced bandwidth broadening for manganites. The intensity enhancement of the white line in Mn K-edge spectra is attributed to a diminished Jahn-Teller distortion of MnO(6) octahedra in compressed DyMnO(3). A comparison of the pressure-dependent XAS spectra with the ab initio electronic structure calculations and full calculations of multiple scattering using the code FDMNES shows the satisfactory agreement between experimental and calculated Mn K-edge spectra. (C) 2010 American Institute of Physics. [doi:10.1063/1.3490400]
Pressure-dependent electronic structures in multiferroic DyMnO3: A combined lifetime-broadening-suppressed x-ray absorption spectroscopy and ab initio electronic structure study
(2010-10-21)
Chen, J. M.
;
Lee, J. M.
;
Chou, T. L.
;
Chen, S. A.
;
Huang, S. W.
Variations in the electronic structure and structural distortion in multiferroic DyMnO3 were probed by synchrotron x-ray diffraction, lifetime-broadening-suppressed x-ray absorption spectroscopy (XAS), and ab initio electronic structure calculations. The refined x-ray diffraction data enabled an observation of a diminished local Jahn-Teller distortion of Mn sites within MnO6 octahedra in DyMnO3 on applying the hydrostatic pressure. The intensity of the white line in Mn K-edge x-ray absorption spectra of DyMnO3 progressively increased with the increasing pressure. With the increasing hydrostatic pressure, the absorption threshold of an Mn K-edge spectra of DyMnO3 shifted toward a greater energy, whereas the pre-edge line slightly shifted to a smaller energy. We provide the spectral evidence for the pressure-induced bandwidth broadening for manganites. The intensity enhancement of the white line in Mn K-edge spectra is attributed to a diminished Jahn-Teller distortion of MnO6 octahedra in compressed DyMnO3. A comparison of the pressure-dependent XAS spectra with the ab initio electronic structure calculations and full calculations of multiple scattering using the code FDMNES shows the satisfactory agreement between experimental and calculated Mn K-edge spectra. (C) 2010 American Institute of Physics. [doi:10.1063/1.3490400]
Pressure-induced structural distortion of TbMnO(3): A combined x-ray diffraction and x-ray absorption spectroscopy study
(2009-04-01)
Chen, J. M.
;
Chou, T. L.
;
Lee, J. M.
;
Chen, S. A.
;
Chan, T. S.
The variation of electronic states and structural distortion in highly compressed multiferroic TbMnO(3) was probed by x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) using synchrotron radiation. Refined XRD data enabled observation of a reduced local Jahn-Teller (JT) distortion of Mn sites within MnO(6) octahedra in TbMnO(3) with increasing hydrostatic pressure. A progressively increasing intensity of the white line in Mn K-edge x-ray absorption spectra of TbMnO(3) was detected with increasing pressure. The absorption threshold of Mn K-edge spectra of TbMnO(3) is shifted toward higher energy, whereas the pre-edge peak is slightly shifted to lower energy with increasing hydrostatic pressure. We provide spectral evidence for pressure-induced bandwidth broadening for mangnites. The enhanced intensity of the white line and the shifted absorption threshold of Mn K-edge spectra are explained in terms of a reduced JT distortion of MnO(6) octahedra in compressed TbMnO(3). Comparison of XAS data with full-multiple-scattering calculations using code FDMNES shows satisfactory agreement between experimental and calculated Mn K-edge spectra.
Pressure-induced structural distortion of TbMnO3: A combined x-ray diffraction and x-ray absorption spectroscopy study
(2009-04-01)
Chen, J. M.
;
Chou, T. L.
;
Lee, J. M.
;
Chen, S. A.
;
Chan, T. S.
The variation of electronic states and structural distortion in highly compressed multiferroic TbMnO3 was probed by x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) using synchrotron radiation. Refined XRD data enabled observation of a reduced local Jahn-Teller (JT) distortion of Mn sites within MnO6 octahedra in TbMnO3 with increasing hydrostatic pressure. A progressively increasing intensity of the white line in Mn K-edge x-ray absorption spectra of TbMnO3 was detected with increasing pressure. The absorption threshold of Mn K-edge spectra of TbMnO3 is shifted toward higher energy, whereas the pre-edge peak is slightly shifted to lower energy with increasing hydrostatic pressure. We provide spectral evidence for pressure-induced bandwidth broadening for mangnites. The enhanced intensity of the white line and the shifted absorption threshold of Mn K-edge spectra are explained in terms of a reduced JT distortion of MnO6 octahedra in compressed TbMnO3. Comparison of XAS data with full-multiple-scattering calculations using code FDMNES shows satisfactory agreement between experimental and calculated Mn K-edge spectra.