Matthew Lawrence Costen

Dynamics and Kinetics of Molecular Collisions Collisions between molecules resulting in energy transfer or reaction are the fundamental interactions of chemistry. The research in my group is aimed at deepening our understanding of these elementary steps, and uses the unique properties of lasers to prepare and probe the reagents and products. This work is carried out jointly with Prof Ken McKendrick (see our joint research group webpages). 1. Stereodynamics of Gas-Phase Collisions Measurement of vector properties (the stereodynamics) of gas-phase collisions are a sensitive probe of the forces involved. We use a variety of novel spectroscopic techniques to study collisional energy transfer, including polarization spectroscopy, crossed molecular beams with velocity-map imaging and frequency modulated spectroscopy (Figure 1). We interpret the results with the help of quantum scattering calculations, often performed by collaborators in other institutions. Figure 1. CN (A2Pi) prepared with an orientation (preferred sense of rotation). The difference in the signals (the orientation) decreases as collisions with Ar bath gas alter the plane of rotation of the CN. 2. Roaming Atoms and Molecules A new dynamical channel in molecular photodissociations has recently been identified, in which an atomic or molecular fragment ‘roams’ around its partner before reacting to form the products. The signature of a roaming channel appears in the coincident product state distribution. We have recently built a new velocity-map imaging spectrometer (Figure 2), with the aim of identifying how common roaming reactions really are. This work is in collaboration with Dr Dave Townsend (Physics). Figure 2. The newly constructed velocity-map imaging spectrometer. 3. Dynamics at the Gas-Liquid Interface The dynamics of the elementary steps occurring at gas-liquid interfaces has seen very little study, despite their fundamental importance in atmospheric, combustion and other environments. This work, led by Prof Ken McKendrick, studies the inelastic and reactive collisions of OH radicals and oxygen atoms (Figure 3) at surfaces of atmospheric and technological importance. Figure 3. O(3P) atoms are formed by photolysis, fly to the liquid covered surface of a rotating wheel, and the returning OH radicals are probed by laser-induced fluorescence.