title: UV Induced Processes in Biomolecules, Hydrated Clusters and Stark Deflected Beams
Creators: name: Pandey, Rahul Kumar
Publisher: name: The Open University
Year: 2017

description: The nanoscale processes by which UV irradiation initiates damage in biological material have not yet been fully elucidated. This represents a barrier to innovations in radiotherapy and also limits our understanding of the molecular origins of life. Experiments on gas-phase biological building blocks can reveal detailed information via comparisons with high-level calculations, while parallel studies of biomolecular clusters offer a route to assess the effects of condensed biological environments. This thesis investigates the stabilities and relaxation pathways of isolated and clustered nucleobases in neutral electronic excited states and ionic states. Furthermore, it presents a new <i>laser thermal desorption</i> facility and reports advances in applying <i>Stark deflection</i> to narrow the range of molecular and cluster configurations in continuous supersonic beams. <br><br> The key analytical tool in this thesis is UV <i>multi-photon ionization time-of-flight mass spectrometry</i> (MPI-TOF). High-resolution measurements on the RNA base uracil and its fully-deuterated analogue provide new evidence supporting a ring-opening process in a neutral electronic excited state. Comparing experiments on uracil in supersonic beams and from the laser thermal desorption source show that the MPI channel that marks this ring-opening process is not sensitive to the vaporization method and aid comparisons with time-resolved experiments at Heriot Watt University. <br><br> Exploiting the capabilities of <i>reflectron</i> mass spectrometry to study the metastable dissociation of excited ions and cluster ions is a major theme in this thesis. Photon energy thresholds for metastable dissociation from multi-photon ionized uracil and thymine are reported for the first time. We argue that these thresholds can provide a new route to deduce the adiabatic energies of neutral electronic excited states. Further metastable dissociation experiments and detailed comparisons with <i>ab initio</i> calculations provide the most detailed study of the stable configurations of thymine-water cluster ions to date. <br><br> The variety of different structural configurations of biomolecules and clusters in neutral target beams presents a major challenge in the experimental study of radiation-induced processes. Stark deflection in electric field gradients offers a powerful method to address this challenge and has previously been used to select specific polar species from mixed pulsed supersonic beams. However, the low pulsing frequencies of such beams are unsuitable to study many collision-induced processes. This thesis reports on the development of a system at the Open University that uses Stark deflection to manipulate continuous beams with the ultimate aim of probing electron attachment processes in selected clusters. Controlled deflection of nitromethane is demonstrated in a helium beam with 94% recovery of the deflected molecules. The manipulation of nucleobases using Stark deflection is achieved for the first time, with continuous argon and krypton expansions providing the necessary rotational cooling. Isolated and clustered nucleobases show very different responses to the deflecting fields, providing a method to gain insights into the origins of specific fragment ions in MPI and electron impact ionization experiments.