Work in my group is concerned with studying the mechanisms of simple gas phase reactions using polarized laser pump and probe (flash photolysis) techniques. We have developed various experimental methods, including the photon initiated reaction technique1, that enable us to investigate the dynamics of elementary unimolecular (photochemical)2,3 and bimolecular4,5 reactions with product quantum state resolution. Our experiments currently employ either laser induced fluorescence (LIF) or resonantly enhanced multiphoton ionization (REMPI) spectroscopy to probe the nascent reaction products of simple chemical processes. The REMPI detection scheme is usually coupled with a technique known as velocity-map ion imaging6, that provides a two-dimensional picture of isolated chemical reactions a few nanoseconds after they have taken place. An example of an image obtained in this way is shown and discussed further below. Experiments like these provide a wealth of information that allow us to elucidate the mechanisms of simple chemical processes at the molecular level (further information about which can be found on our group web page).
One consequence of using polarized light, both to initiate reactions and to interrogate the fragments generated, is that our experiments are sensitive to the direction of recoil of the products, and to the polarization of their electronic or rotational angular momenta1-6. Determining the product angular momentum polarization, equivalent to measuring the spatial distribution of angular momentum vectors, provides insight into electronic and nuclear motion during chemical reactions. We have measured such effects for a wide variety of chemical processes, ranging from the polarization of the electronic angular momentum of the atomic fragments generated via molecular photodissociation3, through to the rotational polarization of radical species produced via photolysis2 and bimolecular reaction4,5. The picture on the right is a velocity-map ion image (in false colour) from one such study, in which the atomic photofragments from the photolysis of a triatomic molecular have been probed by REMPI. The intensity as a function of distance and angle from the centre of the image provides information about the velocity (speed and angular) distribution of the fragment atoms, and their electronic angular momentum distribution. The latter information can be used to help infer which electronic states are involved during photodissociation.
A focus of some of our recent ion-imaging studies has been in the area of imaging mass spectrometry7. This work is carried out in collaboration with Claire Vallance, also in the Physical and Theoretical Chemistry sub-Department, and with Andrei Nomerotski, from Oxford Physics. The aim of this research is to develop novel types of mass spectrometry, in which velocity or spatial information is provided simultaneously with each mass peak in a time-of-flight mass spectrum.
Over the last few years we have also been investigating a new method for measuring angular momentum polarization effects in molecules8, based on Zeeman quantum beat spectroscopy. These experiments make use of weak magnetic fields to cause polarized ensembles of molecules to precess about the field direction. When these precessing molecules are observed in the laboratory using linearly polarized light, the intensity of the absorbed or emitted radiation is found to oscillate, or beat, at the Larmor frequency. The amplitude of the beat signal provides information about the degree of polarization of the sample. In addition to providing a means of determining angular momentum polarization, the use of weak magnetic fields also offers the opportunity to control the direction in which the angular momentum of an atom or molecule is pointing. This has several potentially useful applications in reaction dynamics.