High Harmonic Generation from Rotationally Excited Molecules

Lock, Robynne Marie

Graduate Thesis Or Dissertation

High Harmonic Generation from Rotationally Excited Molecules Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/hq37vn595

Abstract

High harmonic generation (HHG) is understood through a three-step model. A strong laser field ionizes an atom or molecule. The free electron propagates in the laser field and may recombine with the atom or molecule leading to the generation of extreme ultraviolet or soft x-ray light at odd harmonics of the fundamental. Since the wavelength of the recombining electron is on the order of internuclear distances in molecules, HHG acts as a probe of molecular structure and dynamics. Conversely, control of the molecules leads to control of the properties (intensity, phase, and polarization) of the harmonic emission. Rotationally exciting molecules provides field-free molecular alignment at time intervals corresponding to fractions of the rotational period of the molecule. Alignment is necessary for understanding how the harmonic emission depends on molecular structure and alignment. Additionally, HHG acts as a probe of the rotational wavepackets.

This thesis reports three experiments on HHG from rotationally excited molecules. Before we can use HHG as a probe of complex molecular dynamics or control harmonic properties through molecules, the harmonic emission from aligned, linear molecules must first be understood. To that end, the first experiment measures the intensity and phase of harmonics generated from N₂O and N₂near times of strong alignment revealing interferences during recombination. The second experiment demonstrates HHG as a sensitive probe of rotational wavepacket dynamics in CO₂ and N₂O, revealing new revival features not detected by any other probe. The final experiment focuses on understanding and controlling the polarization state of the harmonic emission. Generating elliptically polarized harmonics would be very useful for probing molecular and materials systems. We observe an elliptical dichroism in polarization-resolved measurements of the harmonic emission from aligned N₂and CO₂ molecules, revealing evidence for electron-hole dynamics between the times of ionization and recombination.

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