Electron and photon induced dynamics of astrochemical molecules

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Since the first molecules were identified in the interstellar medium 75 years ago,[1] our understanding of astrochemistry has increased significantly. However, there is still a long way to go before we are even close to achieving a full understanding of the complex environment within an interstellar gas cloud. Due to the impracticality of studying molecules in space, laboratory-based studies must be used to acquire the data required to develop and refine astrochemical models.[2] To model ionisation accurately, both total and partial ionisation cross sections (probabilities), as well as an understanding of the fragmentation dynamics, i.e. the fragment kinetic energy release and angular distribution, are required. Our laboratory is well placed to study both electron and photoionisation in detail.The primary objective of this project will be the experimental and theoretical study of electron-induced and photon-induced processes in a variety of astrochemical molecules that are stable under terrestrial conditions. The results of these studies, along with those carried out by other members of the group, will be used to produce an online database of ionisation cross sections, branching ratios, and fragment kinetic energy release distributions for the molecules studied.Total electron ionisation cross sections will be measured as a function of electron energy using a beam-gas instrument already operational within our laboratory. These cross sections quantify the probability of ionising the parent ions, but give no information on the fragment ions formed. Partial ionisation cross sections give the probability of forming a particular fragment ion, and can be measured in our laboratory as a function of electron energy using an electron-molecule crossed beam instrument equipped with velocity-map imaging detection. The partial ionisation cross-sections obtained in these measurements are only relative values, but can be converted to absolute values by normalising to the total ionisation cross section measured in the beam-gas instrument.Before partial ionisation cross section and fragment kinetic energy release measurements can begin, some recommissioning work is needed on our electron-molecule crossed beam instrument. This will include optimising both the electron and molecular beams, as well as installing a Pixel Imaging Mass Spectrometry (PImMS) camera. The PImMS camera is a fast imaging sensor with a 12.5 ns time resolution which, when used in a time of flight experiment, allows all fragments to be imaged during each acquisition. This will significantly reduce acquisition time, and allow correlations between the motions of different ions formed in the same process to be detected. The ion imaging detector will be fitted with a new ultrafast scintillator to replace the existing P47 phosphor. The reduced decay time of the new scintillator will improve the mass resolution of the experiment. Theoretical studies using Kim and Rudd's binary-encounter-Bethe (BEB) model,[3,4] used to predict total electron ionisation cross-sections, will also be carried out, with a particular focus initially on optimising a correction required for molecules containing third-row atoms. An attempt will also be made to extend the model to predict partial electron ionisation cross sections. Other models will also be considered, and the results compared with the BEB model. Our aim is to achieve good agreement between theory and experiment, so that we can predict ionisation cross sections of molecules which would be a challenge to study experimentally.This project falls within the EPSRC chemical reaction dynamics and mechanisms, computational and theoretical chemistry, analytical science and plasma and lasers research areas.