Physicists tracked electron recollision in real-time

The classical description of electron motion is justified.

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The measurement of the fastest dynamical processes in nature typically relies on observing the nonlinear response of a system to precisely timed interactions with external stimuli. This usually requires two (or more) controlled events, with time-resolved information gained by controllably varying the interpulse delays.

Using a breakthrough technique created by MPIK physicists and used to verify quantum-dynamics theory by collaborators at MPI-PKS, the movement of an electron in a strong infrared laser field is tracked in real-time. The experimental method connects the free-electron mobility caused by the subsequent near-infrared pulse to the absorption spectrum of the ionizing extreme ultraviolet pulse.

Although the electron is a quantum object, the classical description of its motion is appropriate for our experimental technique.

Strong-field physics fundamentally depends on high-harmonic generation, which converts optical or near-infrared (NIR) light into the extreme ultraviolet (XUV) regime. In the well-known three-step concept, the driving light field (1) ionizes the electron by tunnel ionization, (2) accelerates it away and back to the ionic core, where the electron (3) recollides and emits XUV light if it recombines.

In this study, physicists replaced the first step with an XUV single-photon ionization, which has a twofold advantage: First, one can choose the ionization time relative to the NIR phase. Second, the NIR laser can be tuned to low intensities where tunnel ionization is practically impossible. This allows us to study strong-field-driven electron recollision in a low-intensity limiting case.

Attosecond transient absorption spectroscopy, previously established by a team led by Christian Ott, for bound electrons, is the method used here, along with reconstructing the time-dependent dipole moment. It links the time-dependent dipole moment with the classical motion (trajectories) of the ionized electrons, in this case, by extending the approach to free electrons.

Ph.D. student Tobias Heldt said, “Our new method, applied to helium as a model system, links the absorption spectrum of the ionizing light to the electron trajectories. This allows us to study ultrafast dynamics with a single spectroscopic measurement without scanning a time delay to compose the dynamics frame by frame.”

classical trajectory
Illustration of the classical trajectory (green) with multiple recollisions and changes of the quantum wave packets (red/blue background) for a NIR (red) laser-driven electron emerging from XUV (violet) ionization of helium.

The results of the measurements indicate that, depending on the experimental settings, circular polarisation of the light wave can increase the likelihood of bringing the electron back to the ion. Despite seeming counterintuitive, theorists had anticipated this result.

This interpretation of recolliding periodic orbits is also justified by classical simulations. Whenever electron (re-)collides with the helium atom (the green line intersects the white center line), it leads to a characteristic modification and increase of the time-dependent atomic dipole (the result of the quick red-blue oscillation near the center line), which an attosecond absorption-spectroscopy experiment can pick up.

Group leader Christian Ott is optimistic about the future potential of this new approach: “In general, our technique allows us to explore laser-driven electron motion in a new lower-intensity regime, and it could further be applied on various systems, e.g., for studying the laser-driven electron dynamics within larger atoms or molecules.”

Journal Reference:

  1. Tobias Heldt, Jonathan Dubois, et al. Attosecond real-time observation of recolliding electron trajectories in helium at low laser intensities. Physical Review Letters 130, 183201 (2023). DOI: 10.1103/PhysRevLett.130.183201 

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