Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging.
Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging.
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DOI：
10.1038/nature19816
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发表时间：
2016-11-10
期刊：
影响因子：
64.8
通讯作者：
Huber R
中科院分区：
文献类型：
--
作者：
Cocker TL;Peller D;Yu P;Repp J;Huber R
观察单个分子在其固有时间尺度上的运动——现代纳米科学的中心目标之一——需要将超快时间分辨率与原子空间分辨率结合起来的测量方法。稳态实验实现了所需的空间分辨率，如使用扫描隧道显微镜对单个分子轨道进行直接成像或以亚分子分辨率获取尖端增强拉曼光谱和发光光谱所示。但是，在时域中直接跟踪单个分子的动力学面临的挑战是，单分子的激发需要限制在超短的时间窗口内。克服这一挑战的第一步是将扫描隧道显微镜与所谓的“光波电子学”相结合，它使用定制光脉冲的振荡载波直接操纵时间尺度上的电子运动，甚至比单周期光更快。在这里，我们使用这种超快太赫兹扫描隧道显微镜来访问状态选择性隧道机制，其中太赫兹电场波形的峰值通过单个分子状态瞬间打开了一个否则被禁止的隧道通道，从而在比太赫兹波的一个振荡周期短的时间窗口内从单个并五苯分子的最高占据分子轨道上移除一个电子。我们利用这一效应记录了有关轨道结构的~100 fs快照图像，并通过泵浦探针测量直接在时域和亚埃空间分辨率下揭示了太赫兹频率下的相干分子振动。我们预计，光波电子学和原子分辨率的结合将为以光时钟速率控制单个分子内的电子运动打开大门。
Watching a single molecule move on its intrinsic time scale—one of the central goals of modern nanoscience—calls for measurements that combine ultrafast temporal resolution with atomic spatial resolution. Steady-state experiments achieve the requisite spatial resolution, as illustrated by direct imaging of individual molecular orbitals using scanning tunnelling microscopy or the acquisition of tip-enhanced Raman and luminescence spectra with sub-molecular resolution. But tracking the dynamics of a single molecule directly in the time domain faces the challenge that single-molecule excitations need to be confined to an ultrashort time window. A first step towards overcoming this challenge has combined scanning tunnelling microscopy with so-called ‘lightwave electronics”, which uses the oscillating carrier wave of tailored light pulses to directly manipulate electronic motion on time scales faster even than that of a single cycle of light. Here we use such ultrafast terahertz scanning tunnelling microscopy to access a state-selective tunnelling regime, where the peak of a terahertz electric-field waveform transiently opens an otherwise forbidden tunnelling channel through a single molecular state and thereby removes a single electron from an individual pentacene molecule’s highest occupied molecular orbital within a time window shorter than one oscillation cycle of the terahertz wave. We exploit this effect to record ~100 fs snapshot images of the structure of the orbital involved, and to reveal through pump-probe measurements coherent molecular vibrations at terahertz frequencies directly in the time domain and with sub-angstrom spatial resolution. We anticipate that the combination of lightwave electronics and atomic resolution of our approach will open the door to controlling electronic motion inside individual molecules at optical clock rates.
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