Temporal Resolution Characteristics Of An Ultrafast Electron Diffraction Facility

Ultrafast electron diffraction(UED)is a technology which is able to directly resolve structural changes of material on a sub-picosecond time scale.It combines has ultra-high spatial resolution and ultra-high temporal resolution.UED can be used in studies of physics,chemistry,biology,and materials.UED achieves ultra-fast temporal resolution by exploiting a pump-probe setup,in which laser pulses excite the dynamics process,and then electron pulses record the sample change after excitation.The whole dynamics process is obtained by controlling the delay between two pulse sequences.The chirped pulse amplification technology of femtosecond laser makes possible a laser single-pulse enough energy to guarantee the excitation of dynamic process under different UED experimental setup conditions.This thesis firstly introduces the components and layout of our home-made multi-mode UED facility,then discuss the realization of time-resolving and the factors influencing the temporal resolution of UED system.A method of controlling the pump laser pulse to improve the temporal resolution of UED in reflection mode is studies.Finally we present experimental studies of the temporal resolution characteristics of our UED facility,and an observation of ultrafast structural dynamics of polycrystalline graphite.The work is as following:(1)Construction of a multi-mode UED facility.Its main components are optical system,electron gun,imaging system,sample control system,and vacuum system.The facility is designed with two experimental layouts of transmission and reflection,and combined with emission of the convergent beam and the parallel beam by the electron gun,providing multiple working-modes for different studies.(2)Discussion of the realization of time-resolving UED diffraction and the factors influencing the temporal resolution of system,and a study of improving temporal resolution of UED in the reflection layout.Factors that affect the system temporal resolution include the time width of pump pulse,the time width of the electron pulse when it arrives at the sample position,and the time broadening caused by the mismatch between the two.The broadening caused by the mismatch is particularly significant in the reflection layout.When the beam spot diameter is 150 μm,the time broadening caused by the mismatch possibly reaches 43 ps in a typical experiment setup.Tilting the intensity distribution front of the laser pulse regarding to the propagation direction by optical design is able to reduce the mismatch broadening substantially,to optimize the system temporal resolution.We use an angulardispersion optical element and an imaging system to tilt the leading edge of the laser pulse intensity distribution to a specific angle with respect to the propagation direction.When the tilting angle is modulated to 60°,measured by the auto-correlation method,we obtained the time width of the tilting pulse of 106 fs in a range of-1 to 1 mm,with an increasing broadening at both ends of the range.Using modulated pulses as pump beam is able to significantly reduce the velocity and geometry mismatch with the electron beam.(3)Experimental studies of temporal resolution characteristics of the built UED system.We measured the spectrum of the excitation laser for photogenerated electron pulse,the stability of the emission intensity of electron pulse,the diameter of the electron beam spot,and the diameter of the electron beam under different electron optics conditions.The time zero of dynamics process was determined by the shadow imaging method.Finally,we used UED in transmission mode to measure the structural dynamics of a polycrystalline graphite thin film sample.The intensity decreasing curve of the(110)diffraction peak was fitted with a bi-exponential decay model,and the processes of interaction between strongly coupled optical phonons and the electron,and phonon thermalization were observed,demonstrating the sub-picosecond temporal resolution of our UED facility.