Theoretical And Experimental Study Of Femtosecond Electron Diffractometer

Microscopic dynamic processes of material structure,which determine the inherent properties of substances in nature,take place on molecular,atomic and electron scales.Understanding the underlying mechanisms of the various fundamental processes and monitoring the dynamic behavior of matter at atomic scale have always been the goal of femtosecond chemistry,femtosecond physics,femtosecond biology and materials science.Ultrafast electron diffraction,combining the pump-probe and electron diffraction technique,has become an excellent tool with sufficient temporal precision to directly deliver insights into ultrafast phenomena at the atomic level.Central to this method is the ultrashort electron pulses generated from a metal photocathode.However,up to now,owing to the electron sources' energy dispersion and inherent coulomb repulsion,the state-of-the-art temporal resolution is still insufficient to resolve many microscopic basic processes.In this thesis,we first optimize and establish a sub-picosecond ultrafast electron diffraction system with cryogenic capability,and further theoretically design a 100 fs ultra-compact femtosecond electron diffractometer.Finally,the graphite-diamond ultrafast structural phase transition is studied from real space and reciprocal space.The main contents of this paper are as follows:1)Established a femtosecond electron diffraction system,including pump-probe optical path with third harmonic generation,design of cooling-type magnetic lens,introduction of vibration isolated cryocooler and faraday cup,etc.Ultrafast electron diffraction static experiments of ultra-thin materials were carried out,and the electron diffraction pattern with high signal-to-noise ratio was obtained.The spatio-temporal overlap of the ultrafast electron probe and the femtosecond pump laser was completed.2)An ultra-compact femtosecond electron diffractometer suitable for ultra-fast structural dynamics study of both single-layer materials and thick biological samples was theoretically designed.By optimizing the structure of the high voltage electrode,a temporal resolution of 100 fs and energy range covering 10 ke V to 125 ke V have been achieved.It can work in either stroboscopic or single-shot mode,achieving a wide range of energy adjustment range and 100 fs temporal resolution.3)The graphite-diamond phase transition process in real space was studied at atomic scale using the Argonne chromatic aberration-correction transmission electron microscope.The theoretically predicted metastable phase was first observed from the high-temperature and high-pressure graphite sample,which is located between graphite and diamond,and this metastable phase is further converted into hexagonal and cubic diamond by boat-and chair-buckling,respectively.These foundings provide important experimental evidence for revealing the graphite-diamond transformation mechanism.4)Using the state-of-the-art ultrafast electron diffraction system(SLAC Me V UED),the light-induced graphene-diamond phase transition was studied on the femtosecond time scale for the first time.By analyzing the time-dependent difference pair distribution function,new carbon-carbon distances of ~1.94 ? and ~3.14 ? were observed within ~100 fs in the twisted bialyer graphene with AA and AB' stacking,which means that a transient two-dimensional diamond structure is formed,and this photo-induced phase transition was not found in the single-crystal graphene sheets with pure AB stacking,indicating that the moiré pattern in the twisted bialyer graphene plays an important role in the formation of sp3 bonds.This new discovery based on the twsited bilayer graphene provides key dynamic experimental evidence for the unsolved direct graphite-diamond transformation mechanism.In addition,this work provides a possible method to synthesize two-dimensional diamond at ambient temperature and pressure.