Start the clock: a new direct method to study collisions of electronically excited molecules

Understanding the chemistry of the atmosphere, combustion systems and technological plasmas is of great importance in the modern world. A very significant process in these gas-phase environments is the transfer of energy between molecules during collisions. The total amount of internal energy that a molecule possesses is important, but equally if not more important is what form that energy takes. It may be in the form of translational, rotational or vibrational motion of the molecule, or even in the arrangement of the electrons. A large amount of energy may be stored in a rearrangement of the electronic structure of a molecule, producing an electronically excited state. Such excited states occur in energetic environments such as combustion or plasmas, and their decay is what is responsible for the typically observed light emission. The excited states are often also created when molecules are probed in experiments using optical methods. The electronic energy may be re-radiated, but generally not before the molecule has undergone collisions with other gas-phase molecules. This may result in translational, rotational or vibrational energy transfer within the excited electronic state. Significantly, it may also involve either a reaction that removes the excited molecule altogether, or a quenching collision in which the electronic energy is lost to the collision partner.

Despite their importance, surprisingly little is known about the fundamental forces involved in collisions of electronically excited molecules. The best way to determine the detailed dynamics of the collisions of such molecules is using a technique called crossed molecular beam (CMB) scattering. In this method, defined beams of different molecules are collided in a vacuum chamber, and the details of their directions of travel and internal energies are observed in some fashion. We will extend this technique previously used for electronic ground state molecules to the collisions of electronically excited molecules, specifically NO, which is an important molecule in the atmosphere and combustion. We will build a new state-of-the-art CMB scattering apparatus, and by using a combination of laser pulses of different wavelengths will prepare NO molecules in their excited state. After these have collided with the target molecules we will probe the scattered molecules with a laser-based detection technique called velocity map ion-imaging, which is capable of accurately measuring their velocities. The laser pulse that prepares the excited NO defines a zero-time for the experiment, and so 'starts the clock' ticking. This will give us much better definition of the experiment than is usual in CMB experiments, and hence higher sensitivity to the scattering dynamics. We will use this sensitivity to study the collisions of NO with simple molecules relevant to combustion and the atmosphere, namely N2, O2 and CO. These are known to show very different quenching and reactive behaviours with excited NO, but little or nothing is known about the forces involved in these interactions.

The results from our experiments will be used to deepen our understanding of what is important is directing energy transfer in electronically excited molecules, and will help to drive the development of better theoretical models and calculations, as well as providing information of direct relevance to scientist working in the combustion and atmospheric probing of NO.

The proposed work is fundamental in nature and therefore it is expected that the societal and economic benefits deriving from the results will primarily be realised in the longer term. These specifically project-related benefits will follow from the improved understanding and capacity to model inelastic energy transfer and electronic quenching effects. These collisional processes are central to the behaviour of any gas-phase systems where energetic free radicals are present, initiated either by exothermic reactions or photolysis. The principal practical examples are the atmosphere, combustion, and technological plasmas. More accurate modelling and improved predictive capability will help to guide actions to mitigate the effects of undesirable atmospheric phenomena such as global warming or ozone destruction. They will similarly enable improved efficiency of combustion processes, with consequential economic benefits and reductions in environmental impact. The specific molecular target of the current research, NO, is one of the key species in both cases. Equally, one of the principal methods to determine trace-species concentrations and temperatures in the atmosphere or in combustion is optical probing based on various kinds of laser spectroscopy. Under the prevailing relatively high-pressure conditions, the relationship between signal strengths and species concentration is intimately dependent on collisional energy-transfer processes of the type whose mechanisms are to be investigated here. The improved understanding will enhance the accuracy and hence the value of the information derived from such optically based diagnostics.

There will, in addition, be more immediate impact from the output of highly trained personnel with technical skills in the use of modern laser, vacuum, electronic-data capture and data processing technologies. They will also have well-developed generic and transferable communication, presentation and problem-solving skills. They will be ideally suited to contribute to the growth or creation of high-technology companies, enhancing innovative capacity and consequently increasing business revenues.

We will also continue to enhance public awareness of science by disseminating the aims and results of this work to the wider public, at a suitable level, through available media including lectures, on-line video presentations and websites.