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High-Order Harmonic Spectroscopy of Molecular Structure and Dynamics

TitleHigh-Order Harmonic Spectroscopy of Molecular Structure and Dynamics
Publication TypeThesis
Year of Publication2009
AuthorsZhou, X

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 N2 and CO2), and for monitoring the real-time molecular dynamics of polyatomic molecules (such as N2O4).

First, we use an extreme-ultraviolet interferometry to measure the phase of highorder harmonic generation from transiently aligned CO2 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 N2 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 N2 molecules can be strongly elliptically polarized even when driven by linearly polarized 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 N2O4 molecule, we show that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration, whereas the first excited state is populated at the inner turning point of the vibration motion. This state switching due to the coupled electronic and nuclear motions is manifested as bright bursts of high harmonic light that are emitted mostly at the outer turning point of the vibration, due to the different symmetries of the ground state and the first excited state of the cation.