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Birefringences and X-ray absorption from relativistic and nonrelativistic quantum chemical methods
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
2005 (English)Licentiate thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2005. , 40 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1211
Series
LIU-TEK-LIC, 66
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-31976Local ID: 17818ISBN: 91-8545-772-8 (print)OAI: oai:DiVA.org:liu-31976DiVA: diva2:252799
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-25
List of papers
1. The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl2
Open this publication in new window or tab >>The molecular electric quadrupole moment and electric field gradient induced birefringence (Buckingham effect) of Cl2
2004 (English)In: Journal of Computational Methods in Sciences and Engineering, ISSN 1472-7978, Vol. 4, no 3, 365-380 p.Article in journal (Refereed) 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.

Keyword
molecular quadrupole moment, birefringence, electric field gradient, electric dipole (hyper)polarizabilities, coupled cluster theory, relativistic effects, molecular vibrations
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12696 (URN)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2013-11-25
2. Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens
Open this publication in new window or tab >>Four-component Hartree–Fock calculations of magnetic-field induced circular birefringence—Faraday effect—in noble gases and dihalogens
2005 (English)In: The Journal of Chemical Physics, ISSN 1050-2947, Vol. 122, 074321- p.Article in journal (Refereed) 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.

Keyword
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
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12695 (URN)10.1063/1.1849167 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2013-11-25
3. Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory
Open this publication in new window or tab >>Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: Experiment and theory
Show others...
2005 (English)In: Journal of chemical physics Online, ISSN 1089-7690, Vol. 122, no 12, 124305- p.Article in journal (Refereed) 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

Keyword
organic compounds, photoexcitation, photoelectron spectra, SCF calculations, ab initio calculations, chemical shift, molecular electronic states, molecule-photon collisions
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-12685 (URN)10.1063/1.1864852 (DOI)
Available from: 2007-10-30 Created: 2007-10-30 Last updated: 2013-11-25
4. Relativistic four-component static-exchange approximation for core-excitation processes in molecules
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, 022501- p.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.

Keyword
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
5. X-ray absorption spectra from the resonantconvergent first-order polarization propagator approach
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, 042722- p.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.

Keyword
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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