Watching electrons move inside an atom
In the field of ultrafast optics, we like to view chemistry as the study of dynamics and transitions of electrons, and the resulting effects on atoms and molecules. This explains the need to chase ever faster time resolution: because electrons move on the time scale of attoseconds, we need attosecond resolution to resolve their motion. While the ability to trace the evolution of electrons in wave packets with both attosecond precision and excellent spatial resolution is still a dream, our experiment has taken a significant step towards realizing this goal.
When atoms and molecules are subjected to high intensity laser pulses, their outermost electrons may be ripped away by the immensely strong field strength. In a COLTRIMS (Cold Target Recoil Ion Momentum Spectrometer) apparatus, these liberated electrons are then subjected to a static electric field and mapped onto a time and position sensitive detector. Using this approach, Meckel et al.  developed a “momentum microscope” technique that permits measuring the shape of the electron’s original orbital in momentum space. They presented a static picture, simultaneously imaging the highest occupied molecular orbital and the nuclei.
At the Joint Attosecond Science Laboratory, we have strived to extend the capabilities of the momentum microscope to capture femtosecond snapshots of time-dependent orbitals. To this end, we first ionize argon atoms using an ultrashort laser pulse. This results in a coherent electron hole wave packet in the 3p-1 orbital of the argon cation, which oscillates between states with m = 0 and |m| =1 (where m is the magnetic quantum number). This is spatially represented by the distributions shown in Figure 1, which resemble a peanut and donut respectively. The goal of our experiment is to directly imaging the variations in the electron wave packet using the momentum microscope.
However, a major difficulty in performing these measurements is that, by necessity, we end up detecting two electrons. Only one of these holds the interesting wave packet information, and it is per se indistinguishable from the other. This is where our innovative technique comes in. We have recently developed a streaking technique where a relatively weak mid-infrared pulse is used to modify the motion of free electrons but will effectively not act on a bound atom or molecule . So, we use a third pulse overlapped but polarized perpendicularly to the probe pulse to separate the electrons spatially, allowing us to differentiate between the two. Figure 2 shows a visual scheme of the laser pulses used in our experiment.
The results are quite striking: we can clearly observe how the hole moves within the ion on the femtosecond time scale. Figure 3 shows the resulting delay dependent yields and electron densities in momentum space. The ionization yield maximum occurs when the m = 0 (peanut shaped) state is populated by two electrons, and there is negligible contribution from the |m| = 1 (dount shaped) state. The opposite is found at the yield minimum, when the |m| = 1 state is fully populated. There is a clear oscillation shown between these two distributions (as if looking from above), repeating with a period of approximately 23 femtoseconds.
Movie 1. Two-dimensional movie of a bound electron wave packet in the argon cation. (Top) The measured time-dependent yield of Ar2+ is modulated by the evolution of the wave packet with a frequency of 23 fs. For the part of the yield curve highlighted in red, the momentum-space orbital image is shown below. (bottom) Measured variation in the valence electron density. The revivals of the ring (spot) shape at the yield minima (maxima) indicates localization of the electron hole in the m=0 (|m|=1) states.
In the paper, we further show that spatial features of the orbital can be reconstructed. Our technique promises exciting applications in imaging electron dynamics in molecules. Future avenues for research will include the imaging of charge migration and charge transfer following ionization, or probing the electron dynamics preceding and accompanying chemical reactions.
Link to the paper: https://www.nature.com/articles/s41467-019-09036-w
 M. Meckel, et al., “Laser-induced electron tunneling and diffraction”, Science 320, 1478 (2008).