Imaging Valence Electron Dynamics during Surface Diffusion
Résumé
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.