Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, USA.
J Chem Phys. 2022 Nov 14;157(18):180903. doi: 10.1063/5.0128109.
The spatial and energy resolutions of state-of-the-art transmission electron microscopes (TEMs) have surpassed 50 pm and 5 meV. However, with respect to the time domain, even the fastest detectors combined with the brightest sources may only be able to reach the microsecond timescale. Thus, conventional methods are incapable of resolving the myriad fundamental ultrafast (i.e., attosecond to picosecond) atomic-scale dynamics. The successful demonstration of femtosecond (fs) laser-based (LB) ultrafast electron microscopy (UEM) nearly 20 years ago provided a means to span this nearly 10-order-of-magnitude temporal gap. While nanometer-picosecond UEM studies of dynamics are now well established, ultrafast Å-scale imaging has gone largely unrealized. Further, while instrument development has rightly been an emphasis, and while new modalities and uses of pulsed-beam TEM continue to emerge, the overall chemical and materials application space has been only modestly explored to date. In this Perspective, we argue that these apparent shortfalls can be attributed to a simple lack of data and detail. We speculate that present work and continued growth of the field will ultimately lead to the realization that Å-scale fs dynamics can indeed be imaged with minimally modified UEM instrumentation and with repetition rates (f) below-and perhaps even well below-1 MHz. We further argue that the use of low f, whether for LB UEM or for chopped/bunched beams, significantly expands the accessible application space. This calls for systematically establishing modality-specific limits so that especially promising technologies can be pursued, thus, ultimately facilitating broader adoption as individual instrument capabilities expand.
最先进的透射电子显微镜（TEM）的空间和能量分辨率已经超过了 50pm 和 5meV。然而，就时间域而言，即使是最快的探测器与最亮的光源相结合，也只能达到微秒的时间尺度。因此，传统的方法无法分辨无数的基本超快（即飞秒到皮秒）原子尺度的动力学。近 20 年前成功演示的飞秒（fs）激光超快电子显微镜（UEM）提供了跨越这一近 10 个数量级时间差的手段。虽然纳米皮秒 UEM 对动力学的研究已经很成熟，但超快Å尺度成像在很大程度上仍未实现。此外，尽管仪器的发展一直是重点，尽管脉冲束 TEM 的新模态和用途继续涌现，但迄今为止，对该仪器的化学和材料应用领域的探索还相当有限。在本观点中，我们认为这些明显的不足可以归因于简单的数据和细节的缺乏。我们推测，目前的工作和该领域的持续发展最终将导致人们认识到，Å尺度的 fs 动力学确实可以用最小化修改的 UEM 仪器成像，且重复率（f）低于-甚至可能远低于-1MHz。我们进一步认为，无论是使用 LB UEM 还是使用分束/聚束光束，低 f 的使用都大大扩展了可应用的空间。这需要系统地建立特定模态的限制，以便可以追求特别有前途的技术，从而随着单个仪器能力的扩展，最终促进更广泛的采用。
