Abstract
Strong-field physics is currently experiencing a shift towards the use of mid-IR driving wavelengths. This is because they permit conducting experiments unambiguously in the quasistatic regime and enable exploiting the effects related to ponderomotive scaling of electron recollisions. Initial measurements taken in the mid-IR immediately led to a deeper understanding of photoionization and allowed a discrimination among different theoretical models. Ponderomotive scaling of rescattering has enabled new avenues towards time-resolved probing of molecular structure. Essential for this paradigm shift was the convergence of two experimental tools: (1) intense mid-IR sources that can create high-energy photons and electrons while operating within the quasistatic regime and (2) detection systems that can detect the generated high-energy particles and image the entire momentum space of the interaction in full coincidence. Here, we present a unique combination of these two essential ingredients, namely, a 160-kHz mid-IR source and a reaction microscope detection system, to present an experimental methodology that provides an unprecedented three-dimensional view of strong-field interactions. The system is capable of generating and detecting electron energies that span a 6 order of magnitude dynamic range. We demonstrate the versatility of the system by investigating electron recollisions, the core process that drives strong-field phenomena, at both low (meV) and high (hundreds of eV) energies. The low-energy region is used to investigate recently discovered low-energy structures, while the high-energy electrons are used to probe atomic structure via laser-induced electron diffraction. Moreover, we present, for the first time, the correlated momentum distribution of electrons from nonsequential double ionization driven by mid-IR pulses.
4 More- Received 2 April 2015
DOI:https://doi.org/10.1103/PhysRevX.5.021034
This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Synopsis
Mid-Infrared Lasers Probe Atomic Structure
Published 29 June 2015
Researchers have imaged the structure and the response of atoms and molecules with powerful mid-infrared electric fields.
See more in Physics
Popular Summary
Strong-field physics is concerned with the motion of electrons inside atoms, molecules, or solids in the presence of intense radiation. These dynamics govern chemical reactions, modulate the operation of sensors and devices, and trigger biological change. It is therefore necessary to observe strong-field effects in a comprehensive way without missing crucial details or the most rapid time scales. However, this task is challenging because electrons move in three dimensions and effects occur on the attosecond temporal scale. Operating in the so-called quasistatic or tunneling regime with intense midinfrared light sources, we present a methodology that, for the first time, fulfills all these requirements and is able to observe the full three-dimensional interactions of electrons cleanly.
Our methodology is based on the combination of a high repetition rate (160 kHz), intense source that was built in our laboratory and is able to generate electrons with energies ranging from 1 meV up to 1 keV. The intense, ultrashort laser pulses create high field strengths in which the electric fields are comparable to the fields that bind matter together. The electron that is created by the laser field collides with its parent ion in either an elastic or inelastic manner. We couple this source with a reaction microscope detection system that can image the entire three-dimensional interaction. We highlight the versatility of the system using both extremes of the generated electron spectrum to investigate strong-field ionization and electron recollisions in xenon (both and ions). We are able to measure the entire electron recollision and tunneling momentum distribution ranging from low-energy Rydberg contributions and to high-energy multiple recollisions.
Our results pave the way for future strong-field physics experiments at long laser wavelengths. We show that many future studies, such as imaging molecular dynamics with few-femtosecond resolutions, will require a drastic change in thinking compared with current experimental techniques.