UBC Theses and Dissertations
Development and application of a momentum dispersive multichannel electron momentum spectrometer Lermer, Noah
The design, evaluation, and application of a momentum dispersive multichannel spectrometer for electron momentum spectroscopy (EMS) is reported. The spectrometer employs a microchannel plate/resistive anode position sensitive detector and a channel electron multiplier, situated on the exit circle of a cylindrical mirror electron energy analyzer, to simultaneously measure (e,2e) coincidence events over a ± 30° range of azimuthal angle. A novel coincidence detection system based on the ‘pile-up’ of pulses from the detectors has been developed to provide prompt detection of coincidence events. This spectrometer provides an improvement of one to two orders of magnitude in sensitivity over typical single channel instruments. The performance of the new spectrometer has been characterized through measurements of the binding energy spectra and experimental momentum profiles (XMPs) of the valence electrons of Ne, Ar, Kr, Xe, CH 4and SiH 4. These measurements show significantly higher statistical precision than any previously reported EMS work. Consistent with earlier studies, the present multichannel XMPs exhibit very good agreement with theoretical momentum profiles calculated using high quality wavefunctions. The momentum profiles of the helium atom for the transitions to the ground (n=1) and the excited (n=2, n=3) He⁺ final ion states have been obtained with considerably greater precision than previously reported. The experimental momentum profiles ofH2 and D2 for transitions to the ground and excited (2pσ[sub u],2sσ-sub g]) ion states have also been measured to high precision. While the XMPs for the transitions to the ground ion states of each system are found to be in good agreement with theory, the XMPs for the transitions to the excited ion states show significant deviations from theoretical profiles calculated with very accurate correlated wavefunctions. It is suggested that these discrepancies arise from contributions of higher order collision processes neglected in the plane wave impulse description of the (e,2e) scattering event normally used in the theoretical interpretation of EMS experiments. While these additional processes have been discussed with regard to other photon, electron and proton impact studies of two-electron transitions (i.e. ionization plus excitation, double ionization), they have not been previously considered in the context of EMS studies.