Royal Institute of Technology
School of Biotechnology

PhD Thesis

Electron-nuclear dynamics in nonlinear optics and
x-ray spectroscopy

Abstract

This thesis is devoted to theoretical studies of the role of nuclear vibrations on nonlinear and linear absorption, pulse propagation, and resonant scattering of light. The molecular parameters needed for the
simulations are obtained through suitable quantum chemical calculations, which are compared with available experimental data.

The first part of the thesis addresses to modeling of amplified spontaneous emission (ASE) in
organic chromophores recently studied in a series of experiments. To explain the threshold behavior of the ASE spectra we invoke the idea of competition between different ASE channels and non-radiative quenching of the lasing levels. We show that the ASE spectrum changes drastically when the pump
intensity approaches the threshold level, namely, when the ASE rate approaches the rate of vibrational relaxation or the rate of solute-solvent relaxation in the first excited state. According to our simulations
the ASE intensity experiences oscillations. Temporal self-pulsations of forward and backward propagating ASE pulses occur due to two reasons: i) the interaction of co- and counter-propagating
ASE, and ii) the competition between the amplified spontaneous emission and off-resonant absorption.

In the second part of the thesis we explore two-photon absorption taking into account nuclear vibrational degrees of freedom. The theory, applied to the N101 molecule [p-nitro-p'-diphenylamine stilbene], shows that two-step absorption is red shifted relative to one-photon absorption spectrum in agreement with the measurements. The reason for this effect is the one-photon absorption from the first excited state. Simulations show that two mechanisms are responsible for the population of this state, two-photon absorption and off-resonant one-photon absorption by the wing of the spectral line.

In the third part of the thesis we study multi-photon dynamics of photobleaching by a periodical sequence of short laser pulses. It is found that the photobleaching as well as the fluorescence follow double-exponential dynamics.

The fourth part of the thesis is devoted to the role of the nuclear dynamics in x-ray spectroscopy. Our studies show that the vibronic coupling of close lying core excited states strongly affects the resonant x-ray Raman scattering from ethylene and benzene molecules. We demonstrate that the manifestation of the non-adiabatic effects depends strongly on the detuning of photon energy from the top of photoabsorption. The electronic selection rules are shown to break down when the excitation energy is tuned in resonance with the symmetry breaking vibrational modes. Selection rules are then restored for large detuning. We obtained good agreement with experiment. Finally, our multi-mode theory is applied to simulations of the resonant Auger and x-ray absorption spectra of the ethyne molecule.

ISBN 978-91-7178-634-0

Dissertation to be publicly examined
at the Royal Institute of Technology, AlbaNova University Center,
Roslagstullsbacken 21, room FB42 on May 4, 2007, at 10:00 a.m.
(The examination will be conducted in English).

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