Imaging Valence Electron Dynamics during Surface Diffusion

Abstract

The motion of valence electrons dictates the course of several physical transformations and chemical reactions of great importance in biology and for technological applications. Direct observation of bond breaking and making is an invaluable tool for understanding the intricate mechanism of the underlying processes. To capture these instances with atomic-scale spatiotemporal resolutions, time-resolved X-ray diffraction is a promising method, which harnesses the unique properties of ultrashort X-ray pulses from novel light sources. Valence electrons diffract relatively very weakly in comparison to core electrons. Since the latter compose the bulk of the measured diffraction signal, imaging only the relevant parts of the valence electron density via time-resolved X-ray diffraction is a conundrum. In this work, we demonstrate the potential of a method to overcome this challenge using smoothed low-pass Fourier filtering to post-process time-resolved X-ray diffraction data. The present method selectively highlights the instances of bond transformations during surface diffusion of a copper dimer on the MgO­(001) surface. Further, it is robust with respect to uncertainties in the phase retrieval of diffraction patterns. Our work offers a promising avenue to make four-dimensional movies of surface chemistry and of reactions relevant to heterogeneous catalysis.

Probing the Effect of Molecular Structure Saddling on Ultrafast Charge Migration via Time-Resolved X-ray Diffraction

“…The availability of ultrashort, intense x-ray pulses from various free-electron laser sources around the globe [21,22] has opened a new array of possibilities to extend x-ray diffraction from static to time domain. TRXD is an emerging method to probe ultrafast processes in nature with atomic-scale spatial and temporal resolutions and has triggered significant theoretical [23][24][25][26][27][28][29][30][31][32][33][34] and experimental [35][36][37][38][39] research on TRXD from different molecules in recent years. Additionally, we will analyse time-dependent electronic flux densities to understand the mechanistic details of the charge migration associated with the induced dynamics.…”

Ultrafast Imaging of Molecular Dynamics Using Ultrafast Low-Frequency Lasers, X-ray Free Electron Lasers, and Electron Pulses

“…As a variant of time-resolved surface X-ray diffraction (TRXD), time-resolved surface X-ray diffraction at grazing incidence develops significantly with the advent of XFEL. It has been experimentally demonstrated that TRXD is capable of selectively mapping the motion of valence electrons in an adsorbate–surface system . Using a smooth limited- Q Fourier filter, which can be understood as the convolution of the detected signal with a finite box function, and a Gaussian-smoothed cutoff apodization function, the valence electrons’ motion of Cu 2 /MgO­(100) are reconstructed.…”