Time-Resolved Photoelectron Imaging of Model Biological Chromophores

This is a PhD research project in Physics. Time-Resolved Photoelectron Imaging (TRPEI) will be employed to study the relaxation dynamics of excited states in model biological chromophores. Such systems are of great interest since they play a significant role in the process of photoprotection through efficient dissipation of excess energy following absorption of ultraviolet (UV) radiation. Typical examples of relevant species include the DNA bases, the melanin pigments and phenylpropanoids that act as natural sunscreens in plants. Molecules will be prepared in the gas-phase and excited using UV pump pulses, provided by a femtosecond laser system. Subsequently, the excited stated are then probed using photoionization by a second UV laser pulse. The delay time between the pump and probe pulses is then varied, allowing for the real time observations of the dynamic evolution of the system. The project will also involve developing novel probe sources exploiting the vacuum ultraviolet (VUV) region of the electromagnetic spectrum. This will provide a more expanded view along the various critical reaction co-ordinates that mediate the UV relaxation process, thereby providing a greater level of mechanistic insight. Work will also be undertaken to begin exploring the effects of inter- and intra-state photoionization detection sensitivity within the TRPEI approach. The aim here is to learn more about important photoproduct branching yields and also to correlate various physical molecular properties that are easy to evaluate computationally (eg polarizability volume) with photoionization cross sections (which are notoriously difficult to calculate accurately). In addition to the experimental TRPEI measurement, ab initio computational studies using commercial software packages such as Gaussian will provide information to support and predict experimental observations.