Vacuum Ultraviolet Time-Resolved Photoelectron Spectroscopy Using Velocity Map Imaging

Abstract

This thesis covers two separate but related projects. The first project covers the design and simulation of a new angle-resolved photoelectron-photoion coincidence (PEPICO) spectrometer. The spectrometer utilizes velocity map imaging (VMI) to measure the photoelectron kinetic energy and angular distributions in coincidence or covariance with the photoion mass spectrum using a Wiley-McLaren time-of-flight mass spectrometer. The instrument was also designed for 3D VMI, a technique that measures the full three dimensional photoelectron velocity distribution. Simulations were performed to measure the spectrometer performance with respect to photoelectron kinetic energy resolution, mass resolution and photoelectron time spread resolution, the latter being important for 3D VMI.

The second project covers a time-resolved velocity map imaging photoelectron spectroscopy study of nonadiabatic excited state dynamics in carbonyl sulfide (OCS). An ultrafast tunable vacuum ultraviolet light source was tuned to two different vibrational states of the singlet sigma plus excited state. A direct measurement of the excited state lifetimes was made. The photoelectron kinetic energy and angular distributions were used to determine three different ionization mechanisms: we found evidence for direct ionization, superexcited OCS* molecular autoionization and superexcited photofragment S* autoionization. In the future, definitive evidence of our assignments could be made with a coincidence measurement, thus the results from this experiment will be used as a benchmark for the new spectrometer described above.