A new femtosecond electron diffractometer for structural dynamics experiments at cryogenic temperatures

[摘要] ENGLISH ABSTRACT: In this thesis, a femtosecond electron diffraction (FED) set-up that is capable of investigatingthe photo-induced switching of Cu(DCNQI)2 from being an insulator to being a conductoris presented. Movies of atomic structural changes with temporal resolution within the typicalphoto-switching transition timescales (sub-picoseconds) are obtainable with this set-upby employing a femtosecond laser. The experimental technique and the design of a crucialinstrument of the machine, the electron gun, are extensively described and characterised bothnumerically and experimentally.The interest in observing atomic structural changes of Cu(DCNQI)2 in real time is becauseof the rich variety of the radical salts available that show alloy-specific Charge Density Wave(CDW) transitions. Valuable insights about the driving mechanisms behind these structuralchanges that are responsible for a change in conductivity are obtainable, as well as the relationbetween crystal alloys and their transition characteristics. Electron diffraction patternsof crystals in their metallic phase (room temperature) are shown in this thesis, but diffractionpatterns of cryo-cooled Cu(DCNQI)2 in its insulating phase are still to be acquired.The temporal resolution of the atomic movie can be improved by recompression of electronpulses that are debunched due to Coulomb repulsion and electron energy spread within apulse. Numerical and preliminary experimental results presented in this work expose the potentialof a simple compression technique. In this way, more electrons in a single electron pulsecan be afforded which allows to perform experiments at shorter integration time or lower repetitionrate.

[发布机构] Stellenbosch University