Ultrafast single molecule videography with sub-Ångström resolution

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Functionalities of modern quantum materials, chemical reactions, and bio-chemical processes in a living cell are microscopically determined by the ultrafast dynamics of their molecular constituents. It has, thus, been an interdisciplinary goal of all natural sciences to gain direct access to the structure and motion of individual molecules. Steady-state scanning tunnelling microscopy (STM) has allowed researchers to perform imaging, spectroscopy, and manipulation with atomic resolution. Related experiments have yielded unique insights into the equilibrium electronic states of molecules. Resolving dynamics directly in the time domain, however, requires that excitations are confined to a short time window. Recently, we have joined forces to combine our expertise in sub-molecular STM imaging and subcycle lightwave electronics. In the first terahertz-driven STM (THz-STM) experiment of a single molecule, we combined sub-molecular spatial with subcycle temporal resolution and watched the vertical oscillations of a single pentacene molecule after its excitation by transient charging in THz-STM point-spectroscopy.The aim of this project is to systematically explore important ultrafast dynamics of individual molecules directly in space and time. To this end, we will introduce a new quality of THz-STM by extending our recently developed method of ultrafast THz-STM in four different directions: (i) We will take series of full images with varying pump-probe time delays, thereby recording the first molecular real-space slow-motion movies of various ultrafast molecular processes. We expect to establish the most direct and comprehensive picture of the ultrafast dynamic response inside individual molecules by immediate videography in space and time. (ii) Apart from electronic charging that we used so far to trigger vibrations, we anticipate yet more direct ways to excite dynamics of molecules, namely by exploiting the electric THz near-field of atomically sharp STM tips to exert an ultrafast atomically localized force transient that may prepare coherent structural wave packets. (iii) We intend to develop a novel concept of single-shot ultrafast action spectroscopy that will be able to follow single-molecule switching and reveal its unidirectional quantum probability and dynamics, being elusive to conventional pump-probe approaches. (iv) Finally, we intend to extend the portfolio of excitation stimuli from THz pulses to visible or ultraviolet excitation. This way, we may follow the important structure and dynamics of excitons in individual molecules with sub-Å spatial and femtosecond temporal resolution. We expect these key steps to open a new quality of observation, control, and understanding of single-molecule dynamics that have far-reaching consequences for chemistry, biochemistry, and physics, such as photocatalysis, the biology of vision, and ultracompact molecular circuits.