Ultrafast single molecule videography with sub-Ångström resolution

现代量子材料、化学反应和生物化学过程的功能在活细胞中由其分子成分的超快动力学在微观上决定。因此，直接研究单个分子的结构和运动是所有自然科学的跨学科目标。稳态扫描隧道显微镜（STM）使研究人员能够以原子分辨率进行成像，光谱和操作。相关实验对分子的平衡电子态有了独特的认识。然而，直接在时域中解决动态问题需要将激励限制在短时间窗口内。最近，我们已经联手联合收割机结合我们的专业知识，在亚分子STM成像和亚周期光波电子学。在第一个单分子太赫兹驱动扫描隧道显微镜（THz-STM）实验中，我们将亚分子空间分辨率和亚周期时间分辨率相结合，利用THz-STM点光谱技术观察并五苯单分子在瞬态充电激发后的垂直振荡，旨在系统地研究单分子在空间和时间上的重要超快动力学行为。为此，我们将通过在四个不同的方向上扩展我们最近开发的超快THz-STM方法来引入THz-STM的新质量：（i）我们将拍摄一系列具有不同泵浦-探测时间延迟的完整图像，从而记录各种超快分子过程的第一个分子实空间慢动作电影。我们希望通过在空间和时间上的即时录像来建立单个分子内部超快动态响应的最直接和最全面的图像。(ii)除了迄今为止我们用来触发振动的电子充电之外，我们还期望有更直接的方法来激发分子的动力学，即通过利用原子尖锐STM尖端的太赫兹近场来施加超快原子局部力瞬态，这可能会准备相干结构波包。(iii)我们打算开发一种新的概念，单次激发超快作用光谱，将能够遵循单分子开关，并揭示其单向量子概率和动力学，是难以捉摸的传统的泵探测方法。(iv)最后，我们打算将组合的激发刺激从太赫兹脉冲可见光或紫外线激发。这样，我们可以以亚空间和飞秒时间分辨率跟踪单个分子中激子的重要结构和动力学。我们希望这些关键步骤能够开启对单分子动力学的观察、控制和理解的新质量，这些动力学对化学、生物化学和物理学产生深远的影响，例如视觉生物学和超紧凑分子电路。

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.