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  • 1.
    Alagia, M.
    et al.
    ISMN-CNR, Sez. Roma1, P.le A. Moro 5, I-00185 Roma, Italy and TASC-CNR, Area Science Park, Basovizza, I-34012 Trieste, Italy.
    Lavollée, M.
    LIXAM-CNRS, F-91898 Orsay-Cedex, France.
    Richter, R.
    Sincrotrone Trieste, Area Science Park, I-34012 Basovizza, Trieste, Italy.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Carravetta, V.
    Institute of Chemical Physical Processes (CNR), Via Moruzzi 1, I-56124 Pisa, Italy.
    Stranges, D.
    Dipartimento di Chimica and INSTM, Universitá La Sapienza, P.le A. Moro 5, I-00185 Roma, Italy and ISMN-CNR, Sez. Roma1, P.le A. Moro 5, I-00185 Roma, Italy.
    Brunetti, B.
    ISMN-CNR, Sez. Roma1, P.le A. Moro 5, I-00185 Roma, Italy.
    Stranges, S.
    Dipartimento di Chimica and INSTM, Universitá La Sapienza, P.le A. Moro 5, I-00185 Roma, Italy; ISMN-CNR, Sez. Roma1, P.le A. Moro 5, I-00185 Roma, Italy; and TASC-CNR, Area Science Park, Basovizza, I-34012 Trieste, Italy.
    Probing the potential energy surface by highresolution x-ray absorption spectroscopy: The umbrella motion of the coreexcited CH3 free radical2007Ingår i: Physical Review. A, ISSN 1050-2947, Vol. 76, nr 2, s. 124305-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A detailed study of the umbrellalike vibration in inner-shell spectroscopy is presented. The high-resolution x-ray absorption spectrum for the lowest lying core excitation of the CH3 free radical was recorded. High quality potential energy surfaces (PES) for the initial and final states of the transition were calculated as a function of the symmetrical stretching and the umbrella deformation coordinates. The strong anharmonicity along the umbrella coordinate in the double-well region of the PES of the core excited state has a strong effect on the bending vibrational progressions. The excellent agreement between the experiment and theory allows an accurate spectroscopic characterization of the vibrational structure of the electronic transition, and the estimation of the umbrella inversion time of 149  fs.

  • 2.
    Alagia, Michele
    et al.
    ISMN-CNR, Rom.
    Baldacchini, Chiara
    Università di Roma "La Sapienza" .
    Betti, Maria Grazia
    Università di Roma "La Sapienza" .
    Bussolotti, Fabio
    Università di Modena e Reggio Emilia.
    Carravetta, Vincenzo
    IPCF-CNR, Pisa.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Mariani, Carlo
    Università di Roma "La Sapienza" .
    Stranges, Stefano
    Università di Roma "La Sapienza" .
    Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory2005Ingår i: Journal of chemical physics Online, ISSN 1089-7690, Vol. 122, nr 12, s. 124305-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288  eV photon energy, due to absorption to * virtual orbitals, and broader structures at higher energy, involving * virtual orbitals. The sharp absorption structures to the * empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of * symmetry, from the six chemically shifted C 1s core orbitals. ©2005 American Institute of Physics

  • 3.
    Cappelli, Chiara
    et al.
    Istituto per i Processi Chimico-Fisici del CNR, Area della Ricerca, Pisa, Italy.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Rizzo, Antonio
    Istituto per i Processi Chimico-Fisici del CNR, Area della Ricerca, Pisa, Italy.
    Coriani, Sonia
    Dipartimento di Scienze Chimiche, Università degli Studi di Trieste, Trieste, Italy.
    The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl22004Ingår i: Journal of Computational Methods in Sciences and Engineering, ISSN 1472-7978, Vol. 4, nr 3, s. 365-380Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An ab initio investigation of the molecular properties rationalizing the electric-field-gradient induced birefringence (Buckingham effect) for Cl_2 is presented. The quadrupole moment is determined using hierarchies of basis sets and wavefunction models. The electric dipole polarizability, the dipole - dipole - quadrupole and dipole - dipole - magnetic dipole hyperpolarizabilities are determined exploiting a Coupled Cluster Singles and Doubles (CCSD) response approach. The properties are zero-point vibrationally averaged, and the contribution of excited ro-vibrational states accounted for. To this end, the interatomic ^1Σ_g^+ ground state potential has been computed at CCSD plus perturbative triples - CCSD(T) - level employing a large augmented correlation consistent basis set. The effect of relativity is estimated at the Dirac-Hartree-Fock level. Our best value for the quadrupole moment of Cl_2 is (2.327 ± 0.010) au and it is in excellent agreement with experiment which, after revision and dependent on the procedure employed for correcting the original estimate of (2.24 ± 0.04) au of Graham et al., [Mol. Phys., 93, 49, (1998)], ranges from (2.31 ± 0.04) au to (2.36 ± 0.04) au.

  • 4.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Birefringences and X-ray absorption from relativistic and nonrelativistic quantum chemical methods2005Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Application of quantum chemical methods for the prediction and interpretation of optical and X-ray experiments have been performed. X-ray absorption and X-ray photo-electron spectroscopy of pentacene in the gas phase has been studied using the static exchange approximation. A generalizat ion of the static exchange approximation, to the relativistic four-component realm is presented and applied to the H2S molecule. This generalization allows for a description of the effects the strong spin-orbit coupling in the core region of atoms. Two types of birefringences, Faraday effect and electric field gradient induced birefringence, a re calculated for small molecules in the gas phase, and the effects of relativity are considered for these nonlinear optical properties.

    Delarbeten
    1. The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl2
    Öppna denna publikation i ny flik eller fönster >>The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl2
    2004 (Engelska)Ingår i: Journal of Computational Methods in Sciences and Engineering, ISSN 1472-7978, Vol. 4, nr 3, s. 365-380Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    An ab initio investigation of the molecular properties rationalizing the electric-field-gradient induced birefringence (Buckingham effect) for Cl_2 is presented. The quadrupole moment is determined using hierarchies of basis sets and wavefunction models. The electric dipole polarizability, the dipole - dipole - quadrupole and dipole - dipole - magnetic dipole hyperpolarizabilities are determined exploiting a Coupled Cluster Singles and Doubles (CCSD) response approach. The properties are zero-point vibrationally averaged, and the contribution of excited ro-vibrational states accounted for. To this end, the interatomic ^1Σ_g^+ ground state potential has been computed at CCSD plus perturbative triples - CCSD(T) - level employing a large augmented correlation consistent basis set. The effect of relativity is estimated at the Dirac-Hartree-Fock level. Our best value for the quadrupole moment of Cl_2 is (2.327 ± 0.010) au and it is in excellent agreement with experiment which, after revision and dependent on the procedure employed for correcting the original estimate of (2.24 ± 0.04) au of Graham et al., [Mol. Phys., 93, 49, (1998)], ranges from (2.31 ± 0.04) au to (2.36 ± 0.04) au.

    Nyckelord
    molecular quadrupole moment, birefringence, electric field gradient, electric dipole (hyper)polarizabilities, coupled cluster theory, relativistic effects, molecular vibrations
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12696 (URN)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2013-11-25
    2. Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens
    Öppna denna publikation i ny flik eller fönster >>Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens
    2005 (Engelska)Ingår i: The Journal of Chemical Physics, ISSN 1050-2947, Vol. 122, s. 074321-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    Theeffects of relativity on the magnetic-field induced circular birefringence, orFaraday effect, in He, Ne, Ar, Xe, Rn, F2, Cl2,Br2, and I2 have been determined at the four-component Hartree–Focklevel of theory. A measure of the birefringence is givenby the Verdet constant, which is a third-order molecular propertyand thus relates to quadratic response functions. A fully analyticalnonlinear polarization propagator approach is employed. The results are gaugeinvariant as a consequence of the spatial symmetries in themolecular systems. The calculations include electronic as well as vibrationalcontributions to the property. Comparison with experiment is made forHe, Ne, Ar, Xe, and Cl2, and, apart from neon,the theoretical values of the Verdet constant are within 10%of the experimental ones. The inclusion of nonrelativistically spin-forbidden excitationsin the propagator parametrization has significant effects on the dispersionin general, but such effects are in the general caselargely explained by the use of a resonant-divergent propagator theory.In the present work we do, however, observe noticeable relativisticcorrections to the Verdet constant in the off-resonant regions forsystems with light elements (F2 and Cl2), and nonrelativistic resultsfor the Verdet constant of Br2 are in error by25% in the low-frequency region. ©2005 American Institute of Physics.

    Nyckelord
    HF calculations, birefringence, helium, neon, argon, xenon, radon, fluorine, chlorine, bromine, iodine, Faraday effect, polarisation, relativistic corrections, vibrational states, excited states, resonant states, magnetic fields
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12695 (URN)10.1063/1.1849167 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2013-11-25
    3. Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory
    Öppna denna publikation i ny flik eller fönster >>Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory
    Visa övriga...
    2005 (Engelska)Ingår i: Journal of chemical physics Online, ISSN 1089-7690, Vol. 122, nr 12, s. 124305-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288  eV photon energy, due to absorption to * virtual orbitals, and broader structures at higher energy, involving * virtual orbitals. The sharp absorption structures to the * empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of * symmetry, from the six chemically shifted C 1s core orbitals. ©2005 American Institute of Physics

    Nyckelord
    organic compounds, photoexcitation, photoelectron spectra, SCF calculations, ab initio calculations, chemical shift, molecular electronic states, molecule-photon collisions
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12685 (URN)10.1063/1.1864852 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2013-11-25
    4. Relativistic four-component static-exchange approximation for core-excitation processes in molecules
    Öppna denna publikation i ny flik eller fönster >>Relativistic four-component static-exchange approximation for core-excitation processes in molecules
    2006 (Engelska)Ingår i: Physical review A, ISSN 1050-2947, Vol. 73, nr 2, s. 022501-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    A generalization of the static-exchange approximation for core-electron spectroscopies to the relativistic four-component realm is presented. The initial state is a Kramers restricted Hartree-Fock state and the final state is formed as the configuration-interaction single excited state, based on the average of configurations for (n–1) electrons in n near-degenerate core orbitals for the reference ionic state. It is demonstrated that the static-exchange Hamiltonian can be made real by considering a set of time-reversal symmetric electron excitation operators. The static-exchange Hamiltonian is constructed at a cost that parallels a single Fock matrix construction in a quaternion framework that fully exploits time-reversal and spatial symmetries for the D2h point group and subgroups. The K- and L-edge absorption spectra of H2S are used to illustrate the methodology. The calculations adopt the Dirac-Coulomb Hamiltonian, but the theory is open ended toward improvements in the electron-electron interaction operator. It is demonstrated that relativistic effects are substantial for the L-edge spectrum of sulfur, and substantial deviations from the statistical 2:1 spin-orbit splitting of the intensity distribution are found. The average ratio in the mixed region is 1.54 at the present level of theory.

    Nyckelord
    hydrogen compounds, relativistic corrections, configuration interactions, HF calculations, excited states, spin-orbit interactions
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12688 (URN)10.1103/PhysRevA.73.022501 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2013-11-25
    5. X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach
    Öppna denna publikation i ny flik eller fönster >>X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach
    2006 (Engelska)Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, nr 4, s. 042722-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    The one-photon absorption cross sections of molecular systems have been determined in the high-energy region from the imaginary part of the electric dipole polarizability tensor. In contrast to commonly adopted state-specific methodologies, the complex polarization propagator approach does not require explicit consideration of the excited states and it is open-ended towards multiphoton absorption. It is shown that the electronic relaxation in the core-hole state is well accounted for in the present approach with use of standard density-functional based electronic structure methods. Sample calculations are presented of the K-edge x-ray absorption spectra for H2O, CO, C4H4N, and C6H6.

    Nyckelord
    water, carbon compounds, organic compounds, X-ray absorption spectra, molecular moments, polarisability, electric moments, excited states, multiphoton spectra, density functional theory
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12691 (URN)10.1103/PhysRevA.74.042722 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2017-12-14
  • 5.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Time-dependent molecular properties in the optical and x-ray regions2007Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Time-dependent molecular properties are important for the experimental characterization of molecular materials. We show how these properties can be calculated, for optical and x-ray frequencies, using novel quantum chemical methods. For xray absorption there are important relativistic effects appearing, due to the high velocity electrons near the atomic nuclei. These effects are treated rigorously within the four-component static exchange approximation. We also show how electron correlation can be taken into account in the calculation of x-ray absorption spectra, in time-dependent density functional theory based on the complex polarization propagator approach. The methods developed have been applied to systems of experimental interest|molecules in the gas phase and adsorbed on metal surfaces. The effects of molecular vibrations have been take into account both within and beyond the harmonic approximation.

    Delarbeten
    1. Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory
    Öppna denna publikation i ny flik eller fönster >>Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory
    Visa övriga...
    2005 (Engelska)Ingår i: Journal of chemical physics Online, ISSN 1089-7690, Vol. 122, nr 12, s. 124305-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288  eV photon energy, due to absorption to * virtual orbitals, and broader structures at higher energy, involving * virtual orbitals. The sharp absorption structures to the * empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of * symmetry, from the six chemically shifted C 1s core orbitals. ©2005 American Institute of Physics

    Nyckelord
    organic compounds, photoexcitation, photoelectron spectra, SCF calculations, ab initio calculations, chemical shift, molecular electronic states, molecule-photon collisions
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12685 (URN)10.1063/1.1864852 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2013-11-25
    2. Probing the potential energy surface by highresolution x-ray absorption spectroscopy: The umbrella motion of the coreexcited CH3 free radical
    Öppna denna publikation i ny flik eller fönster >>Probing the potential energy surface by highresolution x-ray absorption spectroscopy: The umbrella motion of the coreexcited CH3 free radical
    Visa övriga...
    2007 (Engelska)Ingår i: Physical Review. A, ISSN 1050-2947, Vol. 76, nr 2, s. 124305-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    A detailed study of the umbrellalike vibration in inner-shell spectroscopy is presented. The high-resolution x-ray absorption spectrum for the lowest lying core excitation of the CH3 free radical was recorded. High quality potential energy surfaces (PES) for the initial and final states of the transition were calculated as a function of the symmetrical stretching and the umbrella deformation coordinates. The strong anharmonicity along the umbrella coordinate in the double-well region of the PES of the core excited state has a strong effect on the bending vibrational progressions. The excellent agreement between the experiment and theory allows an accurate spectroscopic characterization of the vibrational structure of the electronic transition, and the estimation of the umbrella inversion time of 149  fs.

    Nyckelord
    free radicals, molecular configurations, optical rotation, organic compounds, potential energy surfaces, vibrational states, X-ray absorption spectra
    Nationell ämneskategori
    Fysik
    Identifikatorer
    urn:nbn:se:liu:diva-12686 (URN)10.1103/PhysRevA.76.022509 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2009-04-23Bibliografiskt granskad
    3. The umbrella motion of core-excited CH3 and CD3 methyl radicals
    Öppna denna publikation i ny flik eller fönster >>The umbrella motion of core-excited CH3 and CD3 methyl radicals
    Visa övriga...
    2008 (Engelska)Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 128, nr 4, s. 044302-1-044302-11Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    An accurate experimental and theoretical study of the lowest core excitationof CH3 and CD3 methyl radicals is presented. The complexvibrational structure of the lowest band of the x-ray absorptionspectrum (XAS) is due to the large variation of themolecular geometry, which is planar in the ground state andpyramidal in the core-excited state. The XAS spectra of thetwo radicals were recorded at high resolution and assigned bytheoretical simulations of the spectra, taking into account the couplingof symmetrical stretching and symmetrical bending (umbrellalike) deformations of theradicals. An excellent agreement between experimental and theoretical spectral profilesallowed us to accurately characterize the vibrational structure of theelectronic transition. The similarities, as well as the differences, ofthe peculiar vibrational progression observed for the two radicals areexplained by the strong anharmonicity along the umbrella coordinate andby the isotopic variation, leading to a different probing ofthe double-well potential energy surface of the core excited stateduring the nuclear motion.

    Ort, förlag, år, upplaga, sidor
    College Park, MD United States: American Institute of Physics (AIP), 2008
    Nyckelord
    excited states, free radicals, ground states, molecular configurations, organic compounds, potential energy surfaces, vibrational states, X-ray scattering
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12687 (URN)10.1063/1.2822246 (DOI)000252821200025 ()
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2017-12-14Bibliografiskt granskad
    4. Relativistic four-component static-exchange approximation for core-excitation processes in molecules
    Öppna denna publikation i ny flik eller fönster >>Relativistic four-component static-exchange approximation for core-excitation processes in molecules
    2006 (Engelska)Ingår i: Physical review A, ISSN 1050-2947, Vol. 73, nr 2, s. 022501-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    A generalization of the static-exchange approximation for core-electron spectroscopies to the relativistic four-component realm is presented. The initial state is a Kramers restricted Hartree-Fock state and the final state is formed as the configuration-interaction single excited state, based on the average of configurations for (n–1) electrons in n near-degenerate core orbitals for the reference ionic state. It is demonstrated that the static-exchange Hamiltonian can be made real by considering a set of time-reversal symmetric electron excitation operators. The static-exchange Hamiltonian is constructed at a cost that parallels a single Fock matrix construction in a quaternion framework that fully exploits time-reversal and spatial symmetries for the D2h point group and subgroups. The K- and L-edge absorption spectra of H2S are used to illustrate the methodology. The calculations adopt the Dirac-Coulomb Hamiltonian, but the theory is open ended toward improvements in the electron-electron interaction operator. It is demonstrated that relativistic effects are substantial for the L-edge spectrum of sulfur, and substantial deviations from the statistical 2:1 spin-orbit splitting of the intensity distribution are found. The average ratio in the mixed region is 1.54 at the present level of theory.

    Nyckelord
    hydrogen compounds, relativistic corrections, configuration interactions, HF calculations, excited states, spin-orbit interactions
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12688 (URN)10.1103/PhysRevA.73.022501 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2013-11-25
    5. Resonant LII;III x-ray raman scattering from HCl
    Öppna denna publikation i ny flik eller fönster >>Resonant LII;III x-ray raman scattering from HCl
    Visa övriga...
    2006 (Engelska)Ingår i: Physical Review A, ISSN 1050-2947, Vol. 74, nr 6, s. 062512-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    We have studied the spectral features of Cl LII,III resonant x-ray Raman scattering of HCl molecules in gas phase both experimentally and theoretically. The theory, formulated in the intermediate-coupling scheme, takes into account the spin-orbital and molecular-field splittings in the Cl 2p shells, as well as the Coulomb interaction of the core hole with unoccupied molecular orbitals. Experiment and theory display nondispersive dissociative peaks formed by decay transitions in both molecular and dissociative regions. The molecular and atomic peaks collapse in a single narrow resonance because the dissociative potentials of core-excited and final states are parallel to each other along the whole pathway of the nuclear wave packet.

    Nyckelord
    hydrogen compounds, X-ray spectra, Raman spectra, dissociation energies, spin-orbit interactions, excited states
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12689 (URN)10.1103/PhysRevA.74.062512 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2009-06-05
    6. Characterization of the chemisorption of methylsilane on a Au(1,1,1) surface from the silicon K- and L-edge spectra: a theoretical study using the four-component static exchange approximation
    Öppna denna publikation i ny flik eller fönster >>Characterization of the chemisorption of methylsilane on a Au(1,1,1) surface from the silicon K- and L-edge spectra: a theoretical study using the four-component static exchange approximation
    2005 (Engelska)Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, s. 13846-13850Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectra (NEXAFS) of methylsilane, isolated and chemisorbed to a Au(1,1,1) surface, are determined in the fully relativistic four-component static exchange approximation¯both the K- and the L-edge of silicon are addressed in this investigation. In the fully chemisorbed structure, three H(Si) atoms have been cleaved off when Si binds in the hollow site of Au forming three Si−Au bonds of normal length. As due to the tri-coordinated chemisorption, the onsets of the K- and L-edge NEXAFS absorption bands occur some 2.0 and 2.5 eV lower in energy, respectively. The spin−orbit splittings in the silicon 2p-shell are not significantly changed due to adsorption. A partly chemisorbed methylsilane with only one H(Si) bond cleaved was also studied, and it is shown that the polarization dependence in the surface spectra contains details that can be used experimentally to identify the surface coordination of silicon. The red-shifts in the XPS silicon 1s (2p) spectra upon surface binding are 0.95 (0.65) and 1.15 (0.83) eV for the mono- and tricoordinated system, respectively.

    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12690 (URN)10.1021/jp0717084 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2017-12-14
    7. X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach
    Öppna denna publikation i ny flik eller fönster >>X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach
    2006 (Engelska)Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, nr 4, s. 042722-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    The one-photon absorption cross sections of molecular systems have been determined in the high-energy region from the imaginary part of the electric dipole polarizability tensor. In contrast to commonly adopted state-specific methodologies, the complex polarization propagator approach does not require explicit consideration of the excited states and it is open-ended towards multiphoton absorption. It is shown that the electronic relaxation in the core-hole state is well accounted for in the present approach with use of standard density-functional based electronic structure methods. Sample calculations are presented of the K-edge x-ray absorption spectra for H2O, CO, C4H4N, and C6H6.

    Nyckelord
    water, carbon compounds, organic compounds, X-ray absorption spectra, molecular moments, polarisability, electric moments, excited states, multiphoton spectra, density functional theory
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12691 (URN)10.1103/PhysRevA.74.042722 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2017-12-14
    8. Polarization propagator for x-ray spectra
    Öppna denna publikation i ny flik eller fönster >>Polarization propagator for x-ray spectra
    2006 (Engelska)Ingår i: Physical Review Letters, ISSN 1050-2947, Vol. 97, nr 14, s. 143001-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    A polarization propagator for x-ray spectra is outlined and implemented in density functional theory. It rests on a formulation of a resonant-convergent first-order polarization propagator approach which makes it possible to directly calculate the x-ray absorption cross section at a particular frequency without explicitly addressing the excited states. The quality of the predicted x-ray spectrum relates only to the type of density functional applied without any separate treatment of dynamical relaxation effects.

    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12692 (URN)10.1103/PhysRevLett.97.143001 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2009-05-11
    9. Self-interaction-corrected time-dependent density functional theory calculations of x-ray absorption spectra
    Öppna denna publikation i ny flik eller fönster >>Self-interaction-corrected time-dependent density functional theory calculations of x-ray absorption spectra
    Visa övriga...
    2007 (Engelska)Ingår i: Physical Review A, ISSN 1050-2947, Vol. 76, nr 2, s. 022506-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    We outline an approach within time-dependent density functional theory that predicts x-ray spectra on an absolute scale. The approach rests on a recent formulation of the resonant-convergent first-order polarization propagator [P. Norman et al., J. Chem. Phys. 123, 194103 (2005)] and corrects for the self-interaction energy of the core orbital. This polarization propagator approach makes it possible to directly calculate the x-ray absorption cross section at a particular frequency without explicitly addressing the excited-state spectrum. The self-interaction correction for the employed density functional accounts for an energy shift of the spectrum, and fully correlated absolute-scale x-ray spectra are thereby obtained based solely on optimization of the electronic ground state. The procedure is benchmarked against experimental spectra of a set of small organic molecules at the carbon, nitrogen, and oxygen K edges.

    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12693 (URN)10.1103/PhysRevA.76.022506 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2009-05-15
    10. Near-edge X-ray absorption and natural circular dichroism spectra of L-alanine: a theoretical study based on the complex polarization propagator approach
    Öppna denna publikation i ny flik eller fönster >>Near-edge X-ray absorption and natural circular dichroism spectra of L-alanine: a theoretical study based on the complex polarization propagator approach
    2007 (Engelska)Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, nr 16, s. 165104-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    The complex polarization propagator method [J. Chem. Phys. 123, 194103 (2005)] has been employed in conjunction with density functional theory and gauge-including atomic orbitals in order to determine the near-edge x-ray absorption and natural circular dichroism spectra of L-alanine in its neutral and zwitterionic forms. Results are presented for the K-edges of carbon, nitrogen, and oxygen. In contrast to traditional methods, the proposed approach enables a direct determination ofspectra at an arbitrary frequency instead of focusing on the rotatory strengths for individual electronic transitions. The propagator includes a complete set ofand allows for full core-hole relaxation. The theoretical spectrum at the nitrogen K-edge of the zwitterion compares well with the experimental spectrum. the nonredundant electron-transfer operators

    Nyckelord
    X-ray absorption near edge structure, absorption spectra, dichroism, polarization spectroscopy, density functionals, atomic orbitals
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12724 (URN)10.1063/1.2800024 (DOI)
    Tillgänglig från: 2007-11-16 Skapad: 2007-11-16 Senast uppdaterad: 2017-12-14
    11. Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens
    Öppna denna publikation i ny flik eller fönster >>Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens
    2005 (Engelska)Ingår i: The Journal of Chemical Physics, ISSN 1050-2947, Vol. 122, s. 074321-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    Theeffects of relativity on the magnetic-field induced circular birefringence, orFaraday effect, in He, Ne, Ar, Xe, Rn, F2, Cl2,Br2, and I2 have been determined at the four-component Hartree–Focklevel of theory. A measure of the birefringence is givenby the Verdet constant, which is a third-order molecular propertyand thus relates to quadratic response functions. A fully analyticalnonlinear polarization propagator approach is employed. The results are gaugeinvariant as a consequence of the spatial symmetries in themolecular systems. The calculations include electronic as well as vibrationalcontributions to the property. Comparison with experiment is made forHe, Ne, Ar, Xe, and Cl2, and, apart from neon,the theoretical values of the Verdet constant are within 10%of the experimental ones. The inclusion of nonrelativistically spin-forbidden excitationsin the propagator parametrization has significant effects on the dispersionin general, but such effects are in the general caselargely explained by the use of a resonant-divergent propagator theory.In the present work we do, however, observe noticeable relativisticcorrections to the Verdet constant in the off-resonant regions forsystems with light elements (F2 and Cl2), and nonrelativistic resultsfor the Verdet constant of Br2 are in error by25% in the low-frequency region. ©2005 American Institute of Physics.

    Nyckelord
    HF calculations, birefringence, helium, neon, argon, xenon, radon, fluorine, chlorine, bromine, iodine, Faraday effect, polarisation, relativistic corrections, vibrational states, excited states, resonant states, magnetic fields
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12695 (URN)10.1063/1.1849167 (DOI)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2013-11-25
    12. The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl2
    Öppna denna publikation i ny flik eller fönster >>The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl2
    2004 (Engelska)Ingår i: Journal of Computational Methods in Sciences and Engineering, ISSN 1472-7978, Vol. 4, nr 3, s. 365-380Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    An ab initio investigation of the molecular properties rationalizing the electric-field-gradient induced birefringence (Buckingham effect) for Cl_2 is presented. The quadrupole moment is determined using hierarchies of basis sets and wavefunction models. The electric dipole polarizability, the dipole - dipole - quadrupole and dipole - dipole - magnetic dipole hyperpolarizabilities are determined exploiting a Coupled Cluster Singles and Doubles (CCSD) response approach. The properties are zero-point vibrationally averaged, and the contribution of excited ro-vibrational states accounted for. To this end, the interatomic ^1Σ_g^+ ground state potential has been computed at CCSD plus perturbative triples - CCSD(T) - level employing a large augmented correlation consistent basis set. The effect of relativity is estimated at the Dirac-Hartree-Fock level. Our best value for the quadrupole moment of Cl_2 is (2.327 ± 0.010) au and it is in excellent agreement with experiment which, after revision and dependent on the procedure employed for correcting the original estimate of (2.24 ± 0.04) au of Graham et al., [Mol. Phys., 93, 49, (1998)], ranges from (2.31 ± 0.04) au to (2.36 ± 0.04) au.

    Nyckelord
    molecular quadrupole moment, birefringence, electric field gradient, electric dipole (hyper)polarizabilities, coupled cluster theory, relativistic effects, molecular vibrations
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-12696 (URN)
    Tillgänglig från: 2007-10-30 Skapad: 2007-10-30 Senast uppdaterad: 2013-11-25
    13. On the evaluation of quadratic response functions at the four-component Hartree-Fock level: Nonlinear polarization and two-photon absorption in bromo- and iodobenzen
    Öppna denna publikation i ny flik eller fönster >>On the evaluation of quadratic response functions at the four-component Hartree-Fock level: Nonlinear polarization and two-photon absorption in bromo- and iodobenzen
    2006 (Engelska)Ingår i: The Journal of Chemical Physics, ISSN 0021-9606, Vol. 124, nr 21, s. 214311-Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    The nonlinear polarization and two-photon absorption parameters have been determined for dibromo- and di-iodobenzene in their meta- and ortho-conformations and with relativistic effects accounted for to a varying degree. By exclusion of small component integrals in the calculations of the first-order hyperpolarizability, results within 1% of fully relativistic four-component Hartree-Fock values are obtained at a cost of 8.7 times the corresponding nonrelativistic calculations. It is shown that the nonlinear absorption in bromobenzene (and even more so in iodobenzene) is broad banded due to spin-orbit interactions among the excited states, and nonrelativistic and scalar relativistic calculations are not to be used in this case.

    Nyckelord
    organic compounds, HF calculations, two-photon processes, relativistic corrections, polarisability, spin-orbit interactions, excited states
    Nationell ämneskategori
    Naturvetenskap
    Identifikatorer
    urn:nbn:se:liu:diva-13999 (URN)10.1063/1.2204604 (DOI)
    Tillgänglig från: 2006-10-05 Skapad: 2006-10-05 Senast uppdaterad: 2009-05-18
  • 6.
    Ekström, Ulf
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Carravetta, V.
    Institute of Chemical Physical Processes, CNR, Pisa, Italy.
    Alagia, M.
    Istituto per lo Studio dei Materiali Nanostrutturati, CNR, Roma, Italy and TASC-CNR, Area Science Park, Trieste, Italy.
    Lavollée, M.
    CNRS, Université Paris-Sud, Orsay-Cedex, France.
    Richter, R.
    Sincrotrone Trieste, Trieste, Italy.
    Bolcato, C.
    Dipartimento di Scienze Farmaceutiche, Universitá di Trieste, Trieste, Italy.
    Stranges, S.
    Dipartimento di Chimica and INSTM, Universitá La Sapienza, Roma, Italy.
    The umbrella motion of core-excited CH3 and CD3 methyl radicals2008Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 128, nr 4, s. 044302-1-044302-11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An accurate experimental and theoretical study of the lowest core excitationof CH3 and CD3 methyl radicals is presented. The complexvibrational structure of the lowest band of the x-ray absorptionspectrum (XAS) is due to the large variation of themolecular geometry, which is planar in the ground state andpyramidal in the core-excited state. The XAS spectra of thetwo radicals were recorded at high resolution and assigned bytheoretical simulations of the spectra, taking into account the couplingof symmetrical stretching and symmetrical bending (umbrellalike) deformations of theradicals. An excellent agreement between experimental and theoretical spectral profilesallowed us to accurately characterize the vibrational structure of theelectronic transition. The similarities, as well as the differences, ofthe peculiar vibrational progression observed for the two radicals areexplained by the strong anharmonicity along the umbrella coordinate andby the isotopic variation, leading to a different probing ofthe double-well potential energy surface of the core excited stateduring the nuclear motion.

  • 7.
    Ekström, Ulf
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach2006Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, nr 4, s. 042722-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The one-photon absorption cross sections of molecular systems have been determined in the high-energy region from the imaginary part of the electric dipole polarizability tensor. In contrast to commonly adopted state-specific methodologies, the complex polarization propagator approach does not require explicit consideration of the excited states and it is open-ended towards multiphoton absorption. It is shown that the electronic relaxation in the core-hole state is well accounted for in the present approach with use of standard density-functional based electronic structure methods. Sample calculations are presented of the K-edge x-ray absorption spectra for H2O, CO, C4H4N, and C6H6.

  • 8.
    Ekström, Ulf
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Carravetta, Vincenzo
    Istituto per i Processi Chimico-Fisici, Area della Ricerca del C.N.R., Pisa, Italy.
    Relativistic four-component static-exchange approximation for core-excitation processes in molecules2006Ingår i: Physical review A, ISSN 1050-2947, Vol. 73, nr 2, s. 022501-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A generalization of the static-exchange approximation for core-electron spectroscopies to the relativistic four-component realm is presented. The initial state is a Kramers restricted Hartree-Fock state and the final state is formed as the configuration-interaction single excited state, based on the average of configurations for (n–1) electrons in n near-degenerate core orbitals for the reference ionic state. It is demonstrated that the static-exchange Hamiltonian can be made real by considering a set of time-reversal symmetric electron excitation operators. The static-exchange Hamiltonian is constructed at a cost that parallels a single Fock matrix construction in a quaternion framework that fully exploits time-reversal and spatial symmetries for the D2h point group and subgroups. The K- and L-edge absorption spectra of H2S are used to illustrate the methodology. The calculations adopt the Dirac-Coulomb Hamiltonian, but the theory is open ended toward improvements in the electron-electron interaction operator. It is demonstrated that relativistic effects are substantial for the L-edge spectrum of sulfur, and substantial deviations from the statistical 2:1 spin-orbit splitting of the intensity distribution are found. The average ratio in the mixed region is 1.54 at the present level of theory.

  • 9.
    Ekström, Ulf
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Carravetta, Vincenzo
    Istituto per i Processi Chimico-Fisici, Area della Ricerca del C.N.R., Pisa, Italy.
    Ågren, Hans
    Laboratory of Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden.
    Polarization propagator for x-ray spectra2006Ingår i: Physical Review Letters, ISSN 1050-2947, Vol. 97, nr 14, s. 143001-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A polarization propagator for x-ray spectra is outlined and implemented in density functional theory. It rests on a formulation of a resonant-convergent first-order polarization propagator approach which makes it possible to directly calculate the x-ray absorption cross section at a particular frequency without explicitly addressing the excited states. The quality of the predicted x-ray spectrum relates only to the type of density functional applied without any separate treatment of dynamical relaxation effects.

  • 10.
    Ekström, Ulf
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Ottosson, Henrik
    Department of Biochemistry and Organic Chemistry, Uppsala university, Sweden.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Characterization of the chemisorption of methylsilane on a Au(1,1,1) surface from the silicon K- and L-edge spectra: a theoretical study using the four-component static exchange approximation2005Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, s. 13846-13850Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectra (NEXAFS) of methylsilane, isolated and chemisorbed to a Au(1,1,1) surface, are determined in the fully relativistic four-component static exchange approximation¯both the K- and the L-edge of silicon are addressed in this investigation. In the fully chemisorbed structure, three H(Si) atoms have been cleaved off when Si binds in the hollow site of Au forming three Si−Au bonds of normal length. As due to the tri-coordinated chemisorption, the onsets of the K- and L-edge NEXAFS absorption bands occur some 2.0 and 2.5 eV lower in energy, respectively. The spin−orbit splittings in the silicon 2p-shell are not significantly changed due to adsorption. A partly chemisorbed methylsilane with only one H(Si) bond cleaved was also studied, and it is shown that the polarization dependence in the surface spectra contains details that can be used experimentally to identify the surface coordination of silicon. The red-shifts in the XPS silicon 1s (2p) spectra upon surface binding are 0.95 (0.65) and 1.15 (0.83) eV for the mono- and tricoordinated system, respectively.

  • 11.
    Ekström, Ulf
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Rizzo, Antonio
    Istituto per i Processi Chimico-Fisici, Consiglio Nazionale delle Ricerche, Area della Ricerca, Pisa, Italy.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens2005Ingår i: The Journal of Chemical Physics, ISSN 1050-2947, Vol. 122, s. 074321-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Theeffects of relativity on the magnetic-field induced circular birefringence, orFaraday effect, in He, Ne, Ar, Xe, Rn, F2, Cl2,Br2, and I2 have been determined at the four-component Hartree–Focklevel of theory. A measure of the birefringence is givenby the Verdet constant, which is a third-order molecular propertyand thus relates to quadratic response functions. A fully analyticalnonlinear polarization propagator approach is employed. The results are gaugeinvariant as a consequence of the spatial symmetries in themolecular systems. The calculations include electronic as well as vibrationalcontributions to the property. Comparison with experiment is made forHe, Ne, Ar, Xe, and Cl2, and, apart from neon,the theoretical values of the Verdet constant are within 10%of the experimental ones. The inclusion of nonrelativistically spin-forbidden excitationsin the propagator parametrization has significant effects on the dispersionin general, but such effects are in the general caselargely explained by the use of a resonant-divergent propagator theory.In the present work we do, however, observe noticeable relativisticcorrections to the Verdet constant in the off-resonant regions forsystems with light elements (F2 and Cl2), and nonrelativistic resultsfor the Verdet constant of Br2 are in error by25% in the low-frequency region. ©2005 American Institute of Physics.

  • 12.
    Henriksson, Johan
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    On the evaluation of quadratic response functions at the four-component Hartree-Fock level: Nonlinear polarization and two-photon absorption in bromo- and iodobenzen2006Ingår i: The Journal of Chemical Physics, ISSN 0021-9606, Vol. 124, nr 21, s. 214311-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The nonlinear polarization and two-photon absorption parameters have been determined for dibromo- and di-iodobenzene in their meta- and ortho-conformations and with relativistic effects accounted for to a varying degree. By exclusion of small component integrals in the calculations of the first-order hyperpolarizability, results within 1% of fully relativistic four-component Hartree-Fock values are obtained at a cost of 8.7 times the corresponding nonrelativistic calculations. It is shown that the nonlinear absorption in bromobenzene (and even more so in iodobenzene) is broad banded due to spin-orbit interactions among the excited states, and nonrelativistic and scalar relativistic calculations are not to be used in this case.

  • 13.
    Jiemchooroj, Auayporn
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Teoretisk Fysik. Linköpings universitet, Tekniska högskolan.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Near-edge X-ray absorption and natural circular dichroism spectra of L-alanine: a theoretical study based on the complex polarization propagator approach2007Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, nr 16, s. 165104-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The complex polarization propagator method [J. Chem. Phys. 123, 194103 (2005)] has been employed in conjunction with density functional theory and gauge-including atomic orbitals in order to determine the near-edge x-ray absorption and natural circular dichroism spectra of L-alanine in its neutral and zwitterionic forms. Results are presented for the K-edges of carbon, nitrogen, and oxygen. In contrast to traditional methods, the proposed approach enables a direct determination ofspectra at an arbitrary frequency instead of focusing on the rotatory strengths for individual electronic transitions. The propagator includes a complete set ofand allows for full core-hole relaxation. The theoretical spectrum at the nitrogen K-edge of the zwitterion compares well with the experimental spectrum. the nonredundant electron-transfer operators

  • 14.
    Såthe, C.
    et al.
    Department of Physics, Uppsala University, Uppsala, Sweden.
    Guimarães, F. F.
    Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden .
    Rubensson, J.-E.
    Department of Physics, Uppsala University, Uppsala, Sweden.
    Nordgren, J.
    Department of Physics, Uppsala University, Uppsala, Sweden.
    Agui, A.
    Synchrotron Radiation Research Unit, Japan Atomic Energy Agency, 1-1-1 Kouto, Sayo-cho, Sayogun, Hyogo, Japan .
    Guo, J.
    Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Gel'mukhanov, F.
    Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden .
    Ågren, H.
    Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden .
    Resonant LII;III x-ray raman scattering from HCl2006Ingår i: Physical Review A, ISSN 1050-2947, Vol. 74, nr 6, s. 062512-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have studied the spectral features of Cl LII,III resonant x-ray Raman scattering of HCl molecules in gas phase both experimentally and theoretically. The theory, formulated in the intermediate-coupling scheme, takes into account the spin-orbital and molecular-field splittings in the Cl 2p shells, as well as the Coulomb interaction of the core hole with unoccupied molecular orbitals. Experiment and theory display nondispersive dissociative peaks formed by decay transitions in both molecular and dissociative regions. The molecular and atomic peaks collapse in a single narrow resonance because the dissociative potentials of core-excited and final states are parallel to each other along the whole pathway of the nuclear wave packet.

  • 15.
    Tu, Guangde
    et al.
    Laboratory of Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden.
    Rinkevicius, Zilvinas
    Laboratory of Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden.
    Vahtras, Olav
    Laboratory of Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden.
    Ågren, Hans
    Laboratory of Theoretical Chemistry, Royal Institute of Technology, Stockholm, Sweden.
    Ekström, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Norman, Patrick
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Beräkningsfysik. Linköpings universitet, Tekniska högskolan.
    Carravetta, Vincenzo
    nstitute of Chemical-Physical Processes, C.N.R., Pisa, Italy.
    Self-interaction-corrected time-dependent density functional theory calculations of x-ray absorption spectra2007Ingår i: Physical Review A, ISSN 1050-2947, Vol. 76, nr 2, s. 022506-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We outline an approach within time-dependent density functional theory that predicts x-ray spectra on an absolute scale. The approach rests on a recent formulation of the resonant-convergent first-order polarization propagator [P. Norman et al., J. Chem. Phys. 123, 194103 (2005)] and corrects for the self-interaction energy of the core orbital. This polarization propagator approach makes it possible to directly calculate the x-ray absorption cross section at a particular frequency without explicitly addressing the excited-state spectrum. The self-interaction correction for the employed density functional accounts for an energy shift of the spectrum, and fully correlated absolute-scale x-ray spectra are thereby obtained based solely on optimization of the electronic ground state. The procedure is benchmarked against experimental spectra of a set of small organic molecules at the carbon, nitrogen, and oxygen K edges.

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