Theoretical And Experimental Investigations Of Megaelectron Volt Ultrafast Electron Diffraction

Ultrafast electron di?raction (UED), which employs ultrashort electron bunchesto probe ultrafast structural evolutions, is a very powerful tool in physics, chemistry,biological and material sciences. Conventional keV UEDs are based on direct-currenthigh voltage acceleration technique, in which the relatively low kinetic energy andtherefore the relatively strong space charge (SC) e?ects limit the range of samplesand the temporal resolution. Recently, it has been proposed that MeV UED, whichis based on a photocathode radio-frequency (rf) gun, may overcome above-mentionedlimitations thus dramatically enhance UED performances. This dissertation is devotedto investigate several theoretical and technical challenges of MeV UED.We began with theoretical analysis of the dominating physics in MeV UED, in-cluding the di?raction and beam dynamics of the electron bunch, and then establisheda start-to-end particle tracking algorithm to perform quantitative investigation. Weshowed how the reciprocal space resolution and the temporal resolution of MeV UEDdepend on the relevant parameters and how to optimize them. We observed that the SCe?ects induced shifts of the peaks of di?raction rings. We found that the SC e?ectshave di?erent scalings with the electron kinetic energy in the transverse and longitu-dinal directions, which explains that in keV UED it is the longitudinal SC e?ects thatlimit the system performance while in MeV UED the transverse SC e?ects are muchmore evident. In MeV UED, there is correlation between the electron bunch lengthand the time-of-?ight from the cathode to the sample, since they both depend on therf gun phase when the electron bunch is launched from the cathode. We derived theexpression of the temporal resolution of MeV UED, and discussed how to optimize it.The temporal resolution of MeV UED can approach 100 fs level with state-of-the-arthardware performances.Based on theoretical and simulation optimizations, we built a prototype MeVUED system. We obtained accumulative di?raction patterns (DPs) in which the peaks of several main low-order rings are clearly distinguishable. With further simulationand experimental optimizations, we achieved single-shot DPs of even higher qualities.MeV UED can also be operated in a novel so-called continuously time-resolved(CTR) mode. We investigated this scheme theoretically, in simulation and with exper-iment. With a poly-crystalline sample, we obtained CTR patterns in which the streaksof several main low-order rings are clearly resolved, and are of a ps level temporalresolution. More excitingly, with a single-crystalline sample, we achieved single-shot,high quality CTR DPs, which allowed the structural evolutions within several ps to becontinuously resolved with a～200 fs resolution. CTR mode is capable of boostingthe temporal resolution to 100 fs, thus truly allows us to observe ultrafast phenomenaswith unprecedented capabilities.