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Ekström, Ulf
Publications (10 of 15) Show all publications
Ekström, U., Carravetta, V., Alagia, M., Lavollée, M., Richter, R., Bolcato, C. & Stranges, S. (2008). The umbrella motion of core-excited CH3 and CD3 methyl radicals. Journal of Chemical Physics, 128(4), 044302-1-044302-11
Open this publication in new window or tab >>The umbrella motion of core-excited CH3 and CD3 methyl radicals
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2008 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 128, no 4, p. 044302-1-044302-11Article in journal (Refereed) 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.

Place, publisher, year, edition, pages
College Park, MD United States: American Institute of Physics (AIP), 2008
Keywords
excited states, free radicals, ground states, molecular configurations, organic compounds, potential energy surfaces, vibrational states, X-ray scattering
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12687 (URN)10.1063/1.2822246 (DOI)000252821200025 ()
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2017-12-14Bibliographically approved
Jiemchooroj, A., Ekström, U. & Norman, P. (2007). Near-edge X-ray absorption and natural circular dichroism spectra of L-alanine: a theoretical study based on the complex polarization propagator approach. Journal of Chemical Physics, 127(16), 165104-
Open this publication in new window or tab >>Near-edge X-ray absorption and natural circular dichroism spectra of L-alanine: a theoretical study based on the complex polarization propagator approach
2007 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 127, no 16, p. 165104-Article in journal (Refereed) 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

Keywords
X-ray absorption near edge structure, absorption spectra, dichroism, polarization spectroscopy, density functionals, atomic orbitals
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12724 (URN)10.1063/1.2800024 (DOI)
Available from: 2007-11-16 Created: 2007-11-16 Last updated: 2017-12-14
Alagia, M., Lavollée, M., Richter, R., Ekström, U., Carravetta, V., Stranges, D., . . . Stranges, S. (2007). Probing the potential energy surface by highresolution x-ray absorption spectroscopy: The umbrella motion of the coreexcited CH3 free radical. Physical Review. A, 76(2), 124305-
Open this publication in new window or tab >>Probing the potential energy surface by highresolution x-ray absorption spectroscopy: The umbrella motion of the coreexcited CH3 free radical
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2007 (English)In: Physical Review. A, ISSN 1050-2947, Vol. 76, no 2, p. 124305-Article in journal (Refereed) 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.

Keywords
free radicals, molecular configurations, optical rotation, organic compounds, potential energy surfaces, vibrational states, X-ray absorption spectra
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-12686 (URN)10.1103/PhysRevA.76.022509 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-04-23Bibliographically approved
Tu, G., Rinkevicius, Z., Vahtras, O., Ågren, H., Ekström, U., Norman, P. & Carravetta, V. (2007). Self-interaction-corrected time-dependent density functional theory calculations of x-ray absorption spectra. Physical Review A, 76(2), 022506-
Open this publication in new window or tab >>Self-interaction-corrected time-dependent density functional theory calculations of x-ray absorption spectra
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2007 (English)In: Physical Review A, ISSN 1050-2947, Vol. 76, no 2, p. 022506-Article in journal (Refereed) 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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12693 (URN)10.1103/PhysRevA.76.022506 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-05-15
Ekström, U. (2007). Time-dependent molecular properties in the optical and x-ray regions. (Doctoral dissertation). : Institutionen för fysik, kemi och biologi
Open this publication in new window or tab >>Time-dependent molecular properties in the optical and x-ray regions
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2007
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1131
Keywords
Quantum chemistry, molecular physics, x-ray spectroscopy, relativity
National Category
Other Physics Topics
Identifiers
urn:nbn:se:liu:diva-10125 (URN)978-91-85895-88-5 (ISBN)
Public defence
2007-10-23, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-04-26
Henriksson, J., Ekström, U. & Norman, P. (2006). On the evaluation of quadratic response functions at the four-component Hartree-Fock level: Nonlinear polarization and two-photon absorption in bromo- and iodobenzen. The Journal of Chemical Physics, 124(21), 214311-
Open this publication in new window or tab >>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 (English)In: The Journal of Chemical Physics, ISSN 0021-9606, Vol. 124, no 21, p. 214311-Article in journal (Refereed) 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.

Keywords
organic compounds, HF calculations, two-photon processes, relativistic corrections, polarisability, spin-orbit interactions, excited states
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13999 (URN)10.1063/1.2204604 (DOI)
Available from: 2006-10-05 Created: 2006-10-05 Last updated: 2009-05-18
Ekström, U., Norman, P., Carravetta, V. & Ågren, H. (2006). Polarization propagator for x-ray spectra. Physical Review Letters, 97(14), 143001-
Open this publication in new window or tab >>Polarization propagator for x-ray spectra
2006 (English)In: Physical Review Letters, ISSN 1050-2947, Vol. 97, no 14, p. 143001-Article in journal (Refereed) 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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12692 (URN)10.1103/PhysRevLett.97.143001 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-05-11
Ekström, U., Norman, P. & Carravetta, V. (2006). Relativistic four-component static-exchange approximation for core-excitation processes in molecules. Physical review A, 73(2), 022501-
Open this publication in new window or tab >>Relativistic four-component static-exchange approximation for core-excitation processes in molecules
2006 (English)In: Physical review A, ISSN 1050-2947, Vol. 73, no 2, p. 022501-Article in journal (Refereed) 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.

Keywords
hydrogen compounds, relativistic corrections, configuration interactions, HF calculations, excited states, spin-orbit interactions
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12688 (URN)10.1103/PhysRevA.73.022501 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2013-11-25
Såthe, C., Guimarães, F. F., Rubensson, J.-E., Nordgren, J., Agui, A., Guo, J., . . . Ågren, H. (2006). Resonant LII;III x-ray raman scattering from HCl. Physical Review A, 74(6), 062512-
Open this publication in new window or tab >>Resonant LII;III x-ray raman scattering from HCl
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2006 (English)In: Physical Review A, ISSN 1050-2947, Vol. 74, no 6, p. 062512-Article in journal (Refereed) 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.

Keywords
hydrogen compounds, X-ray spectra, Raman spectra, dissociation energies, spin-orbit interactions, excited states
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12689 (URN)10.1103/PhysRevA.74.062512 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2009-06-05
Ekström, U. & Norman, P. (2006). X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach. Physical Review A. Atomic, Molecular, and Optical Physics, 74(4), 042722-
Open this publication in new window or tab >>X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach
2006 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, no 4, p. 042722-Article in journal (Refereed) 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.

Keywords
water, carbon compounds, organic compounds, X-ray absorption spectra, molecular moments, polarisability, electric moments, excited states, multiphoton spectra, density functional theory
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12691 (URN)10.1103/PhysRevA.74.042722 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2017-12-14
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