Realizing the capability to track electron motion is one of the fundamental pursuits in modern science. Ultrashort flashes of light have in principle the desired resolution in time to track electrons in action, however, the resolution in space is limited. This has hindered researchers to look into what's going with electrons in a single microscopic entity (e.g. atoms and molecules). Electron motion was thus possible to be captured (reconstructed) only from indirect measurements. Scanning tunneling microscope (STM), on the other hand, can look into electrons and its distribution in atoms and molecules. However, here the resolution in time is only a few millisecond. Space-time resolution required to capture electron motion directly (without reconstruction) is sub-Angstrom and few hundreds of attoseconds. The figure below shows the relevant length and time scales of various processes in atoms and molecules.
A quantum microscope realized by integrating ultrashort laser pulses with an STM can achieve the desired space-time resolution to directly capture (visualize) electron motion. We have recently demonstrated the capability to simultaneously achieve such space-time resolution, as well as coherently control electron tunneling on time scales as short as a ~ 200 attoseconds (Science 367 (6476), 411-415 (2020)).
Utilizing our quantum microscope (Quantoscope) we have recently captured (directly) electron motion in molecules (Nature Photonics 16,196–202 (2022)). Electrons set in motion in molecules deposited on top of a metallic surface by ultrashort pulses (duration < 6 fs) were tracked locally (sub-Angstrom) in a low temperature UHV STM. Real-space and real-time imaging of valence electron motion in molecules has been realized! Efforts are now underway to track electron motion in a single molecule as well.
Any ultrafast experiment is too complex to decipher the coupling of electronic and vibrational motion in a molecule. Integration of a local spectroscopic tool to look into time evolution of various vibrational modes (Raman peaks) in conjunction with the recently demonstrated technique to track electron motion would solve this long standing problem in ultrafast science. We have recently demonstrated tip-enhanced Raman spectroscopy with a single molecule sensitivity in a low temperature UHV STM by using (exciting) laser pulses of duration ~ 500 fs. More reccently, coherences between various phonon modes and their dynamics in a single molecule were tracked for the first time by realizing fs broadband coherent anti-Stokes Raman spectroscopy in an STM. Atomic motion as fast as ~ 70 fs were tracked in a single molecule with sub-Angstrom scale (spatial), 30 fs (temporal) and meV-scale (energy) resolutions at the same time (arXiv:2210.02561).