Abstract
High-order harmonic spectroscopy allows one to extract information on fundamental quantum processes, such as the exit time in the tunneling of an electron through a barrier with attosecond time resolution and molecular structure with angstrom spatial resolution. Here, we study the spatial motion of the electron during high-order harmonic generation in an in situ pump-probe measurement using high-density liquid water droplets as a target. We show that molecules adjacent to the emitting electron-ion pair can disrupt the electron’s trajectory when positioned within the range of the maximum electronic excursion distance. This allows us to use the parent ion and the neighboring molecules as boundaries for the electronic motion to measure the maximum electronic excursion distance during the high-order harmonic generation process. Our analysis of the process is relevant for optimizing high-harmonic yields in dense media.
- Received 15 January 2016
DOI:https://doi.org/10.1103/PhysRevX.6.031029
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Coherent radiation in the extreme ultraviolet spectral domain can be applied to reveal phenomena with attosecond-level temporal resolution and angstrom-level spatial resolution. This type of radiation is therefore an excellent tool for studying not only fundamental physical processes but also chemical and biological processes with unprecedented precision. Here, we study the underlying mechanics that result in attosecond pulses: high-order harmonic generation.
High-order harmonic generation can be described by a semiclassical three-step model. First, an intense laser field tunnel ionizes a molecule, releasing an electron into the continuum. Second, the electron is accelerated by the electric field, making an excursion that can be described by a classical trajectory. Finally, the electron recombines with its parent ion by emitting a high-energy photon. While the first and the last steps are objects of recent pioneering investigations in high-order harmonic spectroscopy and attosecond science, the electronic excursion in real space has not thus far been investigated in detail. In this study, we inject a 15--diameter liquid water droplet into a vacuum; this droplet is used as a target for high-order harmonic generation in an in situ pump-probe setup that allows us to modulate the density of the target by over 6 orders of magnitude. Increasing the density (and therefore reducing the mean interparticle distance in the target) can cause a collision of the free electron with an adjacent molecule and accordingly a decrement in the extreme-ultraviolet emission. We focus on determining the relative proximity of an ion and a molecule to induce a perturbation in the electron’s trajectory. This process allows us to use the parent ion and the neighboring molecules as boundaries for the electronic motion, thereby using the high-order harmonic generating medium itself to measure the maximum electronic excursion distance in situ.
We expect that our findings will pave the way for additional experimental studies of the attosecond dynamics of atoms and molecules.