Watching and controlling atomic motion in a single molecule
Motion of atoms in a single molecule can now be filmed in real-space and real-time, opening routes to directly watching chemical reactions.
Can one see how atoms move inside a single molecule? By performing ultrafast spectroscopy in a scanning tunneling microscope, researchers from Max Planck Institute for Solid State Research (MPI-FKF Stuttgart) and Autonomous University of Madrid (UAM) showed that the periodic motion of the atoms (vibrations) in a single molecule can be captured and precisely controlled. The work opens the path to directly capture the snapshots of atomic motion in molecules/materials undergoing chemical/phase transformations.
The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Of particular interest is the periodic motion of atoms around their equilibrium configuration, molecular vibration, which is widely used for the characterization of molecules and materials. Upon activation with an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast time scale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. However, tracking and controlling atomic motion locally at the molecular scale is much more challenging and has remained beyond reach so far.
In a work published in Nature Communications, researchers demonstrate that the molecular vibrations triggered by broadband laser pulses in a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM).
Utilizing the extreme confinement of light in a cavity merely picometers in size (a picocavity, please see the illustrative figure), formed between the gold tip and substrate in an STM, it is possible to increase the strength of light-matter interaction by a million times. This is the key ingredient towards the efforts in imaging vibrational dynamics in a single molecule. The vibrational motions of atoms in a GNR were set by two delay-controlled ultrashort pulses incident on the picocavity, then tracked by a third pulse, which performs anti-Stokes Raman spectroscopy of the vibrational features. The researchers have finally realized a long sought-after capability to capture atomic motion in a single-molecule with very high spatial (sub-nm), temporal (~ 50 femtoseconds) and energy (meV) resolutions, all at the same time.
The work opens new possibilities to coherently prepare and manipulate vibrational wave packets in individual molecules and quantum materials, so as to drive them to a preferred pathway in light-induced transformations. It paves the way to study the intricate role of the vibrational degrees of freedom of a molecule/material undergoing a geometrical/chemical transformation or a photo-induced charge transfer process in real-space and real-time.