Study Of Femtosecond Electron Beam Propagation In Ultrafast Electron Diffraction System

Ultrafast electron diffraction?UED?,which can directly observe atom motion on a time scale of sub-picosecond,is a novel method for studying ultrafast structural dynamics.It has the advantages of ultrashort time resolution of femtosecond laser and ultrahigh spatial resolution of electron diffraction.We can use this method to study surface dynamics,lattice structure dynamics and chemical reactions in gas phase,and so on.A UED system consists of optics system,femtosecond electron resource,ultrahigh vacuum system and imaging system.The fundamental output of the femtosecond laser provides the ultrahigh temporal resolution for UED system.The laser pulses from the femtosecond laser is divided into two arms in the pump-probe optics setup,one arm is used to excite the dynamics in the sample,the other arm,after frequency tripled is used to excite the photocathode in the electron source to generate ultrafast electron pulses.Ultrafast electron pulses provide the ultrahigh spatial resolution of sub-angstrom for UED system,by diffraction through the sample under study.The femtosecond electron source works in an ultrahigh vacuum system the pressure is maintained lower than 5×10^-8 Pa.The imaging system uses the combination of a vacuum phosphor screen and an image intensified CCD?Charge Coupled Device?,capable for amplifying weak electron signals.This thesis firstly describes each part of the multi-mode UED system and the construction.After the system was built up,we conducted experimental researches on the propagation characteristics of the femtosecond electron pulses basing on the electron optics design of the convergent beam femtosecond electron source.These works provide setting parameters for ultrafast structural research using the UED system.The works are as following:1.The construction and setup of the multi-mode UED system,include the ultrahigh vacuum system,optics system,femtosecond electron source,sample manipulator with5-dimentional adjustments.At the end we prepared the working condition for the system.2.Experimental studies of the propagation characteristics of the femtosecond electron pulses,include measuring the stability of emission from the femtosecond electron source,measuring the size of electron spot on the screen under different setting values of condenser lens?CL?and objective lens?OL?,measuring the beam shapes of electron spot in the sample area under different CL and OL values,obtaining the work parameters for parallel beam and convergent beam,which provide the instruction for analyzing and adjusting diffraction patterns.In the experimental measurements we obtained that the femtosecond electron pulses propagate in an almost parallel path when the CL is set to 14 kV and the OL is set to 7 kV,while the diameter of the beam spot at the position of sample is around 200?m.The femtosecond electron pulses propagate in a convergent path when the CL is set to14kV and the OL is set to 13.5 kV,while the minimum diameter of the beam spot at the position of sample is 20?m.Then we measured the effects of transient electric field on the direct beam when the electron pulses and the laser pulses overlapped in the sample position,determining the time zero of the system.3.Calibration of imaging parameters of the detector and the first dynamics measurement using polycrystalline aluminum.Basing on different setting values of CL and OL,we analyzed the diffraction features of single crystal phase and polycrystalline phase in the Sb₂Se₃ thin film sample.