Time-resolved studies of excited state molecular dynamics

Abstract

The interaction of light and matter is one that is universal in nature and provides problems to be solved by physicists and chemists alike. This thesis presents a collection of experiments dedicated to understanding the effect of ultraviolet (UV) and vacuum ultraviolet (VUV) radiation on simple model analogues of biologically and environmentally relevant molecules. The energy deposited into the molecular system through this high-energy radiation is typically redistributed by various nonradiative relaxation processes which take place on ultrafast (femtosecond) timescales. What these processes are and whether they can be related to the molecule’s structure and function will be explored in this text. In order to do this, the highly-differential timeresolved photoelectron imaging (TRPEI) approach was employed in conjunction with theoretical ab initio quantum chemistry calculations. A specific feature that is considered throughout this work is the use of short-wavelength exciting (pump) and ionising (probe) pulses. Initially, acetylacetone was studied using the TRPEI approach in conjunction with 267 nm pump and 160 nm VUV probe pulses. The femtosecond VUV laser pulses were produced using four-wave difference-frequency mixing in an argon-filled gas cell. These high-energy probe pulses provide a significantly extended view along the reaction-coordinate of interest, through a deep projection into the ionisation continuum. This lab-based approach was able to provide quantitative links between elements of earlier reports on relaxation dynamics in acetylacetone, which sampled smaller subsections of the reaction-coordinate. Four dynamical processes occurring on distinct timescales ranging from <10 fs to over 300 ps were identified, including one signature not previously reported. This work highlights the need for such shortwavelength VUV probes in photoionisation-based investigations of photochemical dynamics. Secondly, the non-adiabatic relaxation dynamics of nitrobenzene and three of its dimethyl-derivatives were investigated using TRPEI and ab initio calculations to gain insight into the influence exerted by the nitro-group orientation on the dynamics. Multiphoton ionisation involving two and/or three photons with wavelengths centred at 400 nm achieved a high-energy probe and revealed near-identical dynamical signatures for all four systems, despite the varying effects of steric hindrance on the nitro-group. These could be assigned to dynamical processes occurring on three timescales: sub 30 fs, in the range of 160-190 fs and finally in the range of 90-160 ps, depending on the molecule. Finally, VUV pulses were again employed, this time as the pump in the study of formamide, N,N-dimethylformamide and N,Ndimethylacetamide, which are motifs ubiquitous in nature. Dynamical signatures indicative of rapid relaxation processes were observed on timescales of 10-35 fs and 70-75 fs in all three systems. In addition to the TRPEI results, extensive quantum chemistry calculations revealed different Rydberg-to-valence evolution behaviour in formamide and the two larger amide systems.