Date of this Version
Zandi, Omid, "Design and Construction of a High-Current Femtosecond Gas-Phase Electron Diffraction Setup" PhD Dissertation, Department of Physics and Astronomy, University of Nebraska-Lincoln, (2017).
We designed and constructed a state-of-the-art high current ultrafast gas electron diffraction experimental setup, which resolved two main challenges that constraint temporal resolution in previous setups. These aforementioned bottlenecks were: the space charge effect due to the Coulomb expansion, and the velocity mismatch between the sub-relativistic electrons (probe) and the exciting laser pulse (pump). In our setup, the problem of space charge effect was ameliorated by compressing 90 keV photo-emitted electron pulses using a radio-frequency electric field. The compression allowed us to increase the beam current by almost two orders of magnitude higher than previously reported. We developed a laser-activated streak camera with a streak velocity of 1.89 mrad/ps to evaluate the compression by measuring the electron pulse duration in situ with a resolution of 100 fs. Electron pulses composed of half a million electrons with a duration of 350 fs were obtained. The velocity mismatch problem, on the other hand, was resolved by employing the technique of laser intensity front tilting. We also constructed a setup to measure the duration of the tilted front laser pulses by an interferometric technique. The timing between the pump and the probe was determined either by photo-ionization induced lensing of the electrons in the gas for normal front laser pulses, or by a transient space charge/surface polarization creation in a copper foil that deflected the electron pulses. The change in the timing between the laser and the electrons was measured by the streak camera with a resolution of 70 fs RMS.
Advisor: Martin Centurion