Seeing is believing. Watching how atoms move during the defining moments of chemistry has been a long held dream of chemists.  It is chemistry that drives biological processes, and similar concepts of structural transitions are relevant to functional properties of materials. This prospect seemed to be out of the realm of actual measurement given the combined requirements of extremely high time resolution requirement.  This dream was thought to be the purest form of a thought experiment; yet is has served as a very important pedagogical tool in teaching chemistry and inspiring ways to control barrier heights that offer exponential gain in directing chemical processes. The main limitation to achieve this grand objective was one of source brightness. Using a movie camera analogy, as one goes to higher and higher shutter speeds for capturing live action, it is necessary to have a brighter and brighter source or one soon runs out of enough photons or light contrast to discern the object or its motion. The source brightness with wavelengths sufficiently short in wavelength and duration to capture atomic motions in real time has been solved for both electron and x-ray sources, achieving now the fundamental space-time limit to imaging chemistry.  However, the one specific class of chemistry and structural dynamics that remains illusive to reach the same level of space-time resolution has been surface chemistry and surface structural dynamics.  This distinction is largely due to the fundamental requirement to observe a single atomic layer in action.  This gap needs to be bridged as surface chemistry is at the heart of many chemical processes, from nanoscale devices, catalysis, to the whole field of electrochemistry and advanced energy storage/battery technology.

The most direct probe to capture atomic motions at surfaces is to exploit the extremely high scattering cross section of low energy electrons (<1 KV) to gain single atomic layer sensitivity.  These sources are now available but are much more limited in electron number density or brightness.  More problematic is that the surface process requires access nonequilibrium processes, i.e., a trigger is needed as the start pulse to the phenomenon of interest. This trigger pulse must be on the required picosecond (10^-12 s) to 100 femtosecond (10^-13 s) time scale to resolve the key atomic motions in directing surface chemistry or transitions.  Given the single atomic layer involved, the very act exciting a surface makes it prone to light induced emission processes at the surface plane creating surface fields and complicated surface charge recombination processes, along with the associated surface field amplitude effects. Low energy electrons are particularly prone to surface field effects that could be interpreted as structural dynamics.  Using a known structural effect, simple lattice heating, we were able to clearly determine the presence of the surface field effects on electron trajectories and provide a quantitative means to remove this aberration from directly imaging atomic motions at surfaces or single atomic layers.

We believe we have cracked the problem in fully exploiting ultrafast low-energy electron diffraction (ULEED) to follow surface processes with full atomic resolution. This emerging field has recently showcased several outstanding results⁴ revealing nonequilibrium surface atomic structures, drastically distinguishable from their 3D bulk phase one.  In this domain, low-energy electrons  have major advantages by providing exquisite sensitivity from few atomic layers to even monolayer sensitivity in catching atomic motions with the prospect for even full reconstruction of wavefunctions by providing additional phase information. This work provides a new capability to remove aberrations from realizing this grand goal of directly following surface process and understanding the role of surfaces at the most fundamental - atomic level - possible.

1         Ischenko, A. A., Weber, P. M. & Miller, R. J. D. Capturing chemistry in action with electrons: realization of atomically resolved reaction dynamics. Chemical reviews 117, 11066-11124 (2017).

2         Park, H. & Zuo, J. Direct measurement of transient electric fields induced by ultrafast pulsed laser irradiation of silicon. Applied Physics Letters 94, 251103 (2009).

3         Schäfer, S., Liang, W. & Zewail, A. H. Structural dynamics and transient electric-field effects in ultrafast electron diffraction from surfaces. Chemical Physics Letters 493, 11-18 (2010).

4         Horstmann, J. G. et al. Coherent control of a surface structural phase transition. Nature 583, 232-236 (2020).