High-Order Harmonic Spectroscopy of Molecular Structure and Dynamics

Date The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. The recent developments of high repetition rate and high average power ultrafast laser open the door for probing molecular structure and dynamics with intense laser fields. In this regime, the electric field of the laser is comparable to the binding field of the electron in the outer valence shell of atoms or molecules. Thus the potential is strongly modified by the laser electric field as compared to field-free atomic or molecular potential, and the probability for ionization is dramatically enhanced. High harmonic generation (HHG), as an ionization-initiated strong field process, can be explained well by the semiclassical recollision model. In this work, we demonstrate HHG as a new spectroscopic method for determining the molecular structure of simple linear molecules (such as N 2 and CO 2), and for monitoring the real-time molecular dynamics of polyatomic molecules (such as N 2 O 4). First, we use an extreme-ultraviolet interferometry to measure the phase of high-order harmonic generation from transiently aligned CO 2 molecules. We unambiguously observe a reversal in phase of the high order harmonic emission for higher harmonic orders with a sufficient degree of alignment. This results from molecular-scale quantum interferences between the molecular electronic wave function and the re-colliding electron as it recombines with the molecule, and it is consistent with a plane wave model. We also perform similar experiments with N 2 molecules and observe behaviors that can not be predicted by the plane-wave model. Second, we perform an accurate polarimetry measurement of high harmonic emission from aligned molecules. Surprisingly, we find that harmonic emission from N 2 molecules can be strongly elliptically polarized even when driven by linearly polarized iv laser fields. We extract the phase difference between the parallel and perpendicular components of the high harmonic field, which strongly depends on the harmonic order. This nontrivial phase indicates a breakdown of plane wave approximation. This work also shows that it is possible to engineer the polarization properties of harmonic emission by carefully preparing a molecular medium. Finally, we show that high harmonic emission can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude and relative slow vibrations in the N 2 O 4 molecule, we show that …