Multielectron effects in strong field processes in molecules

<p>Laser technology has experienced a rapid evolution in available intensities, frequencies, and pulse durations over the last three decades. Many new laser induced phenomena in atoms have been discovered, such as multiphoton ionization, above-threshold ionization, high-order harmonic generation etc. For the interaction with atoms, usually only one electron in the outermost shell is assumed to be active (called single-active-electron approximation) while all other electrons are considered to remain frozen in their initial states. Due to the extra degrees of freedom (vibration and rotation) and the more complex structures, the interaction of molecules with intense laser pulses reveals many new features. Recent experiments have indicated that electrons from inner valence orbitals of molecules can have significant contributions to ionization and high harmonic generation. Theoretical analysis of these processes in molecules faces the challenge to extend previous theories developed for the atomic case by including the multielectron character of the molecular target.</p> <p>In this thesis we systematically investigate multielectron effects in the interaction of moleculeswith intense laser light. To this end, we apply time-dependent density-functional theory to solve the multielectron Schrödinger equation and analyze highly nonlinear processes such as high harmonic generation, laser-induced ionization and nonadiabatic electron localization. Based on the results of our numerical simulations we predict a new feature in the harmonic spectra of molecules, namely the occurrence of fractional harmonics in the form of Mollow sidebands. Such additional peaks in the spectra appear due to a field-induced resonant coupling of an inner valence orbital with the outermost orbital in a molecule. Furthermore, we show that the theoretical explanation of recent experimental data for the ellipticity of high harmonics in N2 and CO2 require the systematic consideration of all inner valence shells as well as the proper alignment distribution in the experiment. We also show that the coupling of molecular orbitals in the field can lead to an enhancement of (inner-shell) ionization, potentially leading to a population inversion in the ion, as well as nonadiabatic electron dynamics, where the electron can be trapped at one side of the molecule over several field cycles. Finally, we present the development of a new intense-field theory based on the Floquet theorem with complex Gaussian basis sets and show results of first applications for ionization of simple systems.</p>
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University of Colorado Boulder
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