Molecular Recollision Interferometry in High Harmonic Generation

There has been considerable recent interest in using high-order harmonic generation to observe molecular structure and dynamics. Besides measuring the intensity of high-order harmonics, we use 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 recolliding electron as it recombines with the molecule, and is consistent with a two-center model. Furthermore, using the combined harmonic intensity and phase information, we extract accurate information on the dispersion relation of the returning electron wave packet as a function of harmonic order. This analysis shows evidence of the effect of the molecular potential on the recolliding electron wave. Our measurements are critical for developing new approaches for molecular imaging.