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PhD Thesis

Linear and Nonlinear Optical and X-ray Scattering by Molecules

Yi Luo

Abstract

The contents of this thesis is dedicated to theoretical investigations of light scattering by molecules, where special attention has been paid on molecules in gas phase and in solutions. Radiation from two modern light sources, lasers and the third generation synchrotron storage rings, are considered, thereby covering a wide wavelength region, from optical to soft x-ray wavelengths.

Accurate values for the polarizabilities and hyperpolarizabilities of small molecules in gas phase have been obtained by means of response theory. Solvent effects on molecular properties have been investigated for charge transfer organic molecules, solvated water, and molecular ions using reaction field theory. The absorption spectra and the dynamic polarizabilities and hyperpolarizabilities of long polyene chains, up to C28H30, have been studied. Geometry distortion effects, substitution effects, and the role of isomers on the ground and excited state hyperpolarizabilities have been studied in that context.

The response formalism has been extended to include vibronically induced two-photon spectra of molecules. Primary applications have been carried out for a sample of organic molecules. Two-photon absorption spectra of polyenes have been calculated, aiming at the understanding of the mechanism behind optical nonlinearity of polyenes.

Theory and applications for resonant x-ray inelastic scattering (RIXS) by molecules are presented. It is shown that the symmetries of occupied and unoccupied molecular orbitals can be assigned in terms of polarized RIXS. The polarization anisotropy and the integrated unpolaxized cross sections of RIXS have been investigated using one-step, two-step, and classical formulations. The C60 molecule, with its exceptional symmetry properties, has been used to demonstrate the particular symmetry-related aspects of RIXS. It is shown that the electronic symmetry selection rules apply especially well to the spectra caused by excitations in the band gap. RIXS spectra of chemically shifted species were found to be site selective, and small chemical shifts could be identified from RIXS spectra as a result of interference effects. We have used benzene for demonstrating the symmetry selection rules, caxbon monoxide for showing the screening effect and angular dependence, and carbon dioxide for illustrating the role of vibronic coupling.

Department of Physics and Measurement Technology
Linköping University, S-58183, Linköping, Sweden

Linköping 1996

ISBN 91-7871-739-6 ISSN 0345-7524

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