To make a `molecular movie', an `ultrafast camera' with simultaneously very high spatial and temporal resolution to match the atomic dynamics is required. The ultrafast electron diffraction (UED) technique based on femtosecond laser technology can provide a basic framework for realizing such an `ultrafast camera' although this technology has not achieved its full utility as a universal imaging and spectroscopy tool, due to limitations in generation and preservation of a high-brightness electron beam in the ultrafast regime.With moderate electron pulse intensity (10^3-10^4 electrons per pulse), UED experiments have been successfully applied to investigate photo-induced non-thermal melting processes, structural phase transitions, and transient surface charge dynamics. Based on the previous development of ultrafast electron diffractive voltammetry (UEDV), we extend the UEDV with an aim to identify the different constituents of the measured transient surface voltage (TSV) and discuss their respective roles in Coulomb refraction. From applying this methodology on Si/SiO2 interface and surfaces decorated with nano-structures, we are able to elucidate localized charge injection, dielectric relaxation, carrier diffusion, and enhancements on such processes through surface plasmon resonances, with direct resolution in the charge state and possibly correlated structural dynamics at these interfaces. These new results highlight the high sensitivity of the interfacial charge transfer to the nanoscale modification, environment, and surface plasmonics enhancement and demonstrate the diffraction-based ultrafast surface voltage probe as a unique method to resolve the nanometer scale charge carrier dynamics. The future applications of the UED and UEDV techniques lie in the direct visualization and site-selected studies such as nano-structured interfaces, a single nanoparticle or domain, which can be enabled by the development of high-brightness ultrafast electron beam system for ultrafast electron diffraction. To realize the high-brightness beam, we have developed a high-brightness ultrafast electron beam column equipped with a 100 keV Pierce photoelectron gun and an RF compressor. We are able to generate up to 5×10^6 electron per pulse, and, more importantly with the capability of micro-focusing, we have achieved a high dose delivery to the sample by three orders of magnitude (up to 2000 electron/μm2) higher than those of conventional UED electron systems. In this high intensity pulsed electron beam system, the major challenge is overcoming strong Coulomb repulsion among electrons (space-charge effect) in the pulse, because the space-charge effect causes the electron pulse expansion in transverse and longitudinal direction. To correct the space-charge effect, we have implemented the magnetic lenses for transverse focusing of the electron pulse, and radio frequency (RF) cavity for longitudinal recompression of the pulse. Such a system will provide enough flexibility to manipulate electron pulse phase space, so various experiments that require high spatial and temporal coherence and/or high-density beam optimized for microdiffraction can be achieved.
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