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Visualizing electronic motion

Seeing’ electron motion at both its natural length and time scales is a long-awaited dream of natural sciences, as it is the key to understand and achieve full control of electronic processes in chemical reactions, exciton dynamics in atomically thin 2D materials, cooper-pair dynamics in superconductors, etc.  Reaching such spatial or temporal resolutions is nowadays possible, but NOT simultaneously.  Indeed, on the one hand, the impressive development of attosecond science techniques allows us to generate and track electronic motion in real time (200 attoseconds to few femtoseconds) in ever larger systems, but not in real space. Consequently, the local time-evolution of the electron density can so far only be inferred indirectly, typically from reconstruction of the (often-elusive) features appearing in electron, ion, absorption or emission spectra, which is not a viable procedure for complex systems. Scanning tunneling microscopy (STM), on the other hand, can locally probe electron density in molecules and solids with the necessary atomic (sub-angstrom) resolution, but cannot provide by itself the dynamical information at the ultrafast time scale that is inherent to electronic motion.


Until date, theoretical simulations reconstructed very speculative and engrossing snapshots of electron motion in atoms and molecules. By integrating STM and attosecond technologies, we have recently broken this fundamental limitation of physical sciences and recorded the first real space-time snapshots of electronic motion in molecules, bypassing any kind of reconstruction (Garg et al., Nature Photonics 16, 196-202 (2022) and Garg et al., Science 367, 411-415 (2020)). We have shown that the coherent electronic motion generated by < 6 fs CEP-stable near-infrared pulses can be locally and non-invasively probed with both picometer and 300 attosecond resolution, thus allowing for a direct visualization of electron dynamics without the need for any additional reconstruction. In other words, our technique allows us to directly record the “motion picture” of electron dynamics by reaching the quantum space-time limit of such motion, thus going well beyond the current picometer-picosecond limit in the investigation of molecular dynamics (ACS Photonics 7, 296-320 (2020)). Importantly, we have also shown that, in this limit, full control of the generated coherent electron dynamics can be exerted in real space and real time, which can open new avenues to drive chemical reactions on surfaces and charge transfer in molecules.

~ 5 fs long carrier envelope phase stable laser pulses illuminate an STM junction comprising of tungsten nanotip and Au(111) surface.

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