THE DYNAMICS OF ELEMENTARY CHEMICAL PROCESSES
Individual elementary chemical processes, such as reaction or transfer of energy between colliding molecules, underlie the whole of chemistry. The research in my group is aimed at deepening the understanding of these fundamental steps that are the building blocks of all chemical reactions. The work is carried out jointly with Dr Matt Costen (see our joint research group website for more detailed descriptions of the experiments in our group).
Our experimental methods generally exploit the unique properties of lasers to prepare reactants with well-defined energies and velocities. The state of the products is probed spectroscopically, often also through some form of laser spectroscopy. The polarisation properties of light are used to extract additional, stereochemical information. There is a close interplay between the experiments and the theoretical prediction of potential surfaces, which describe the forces on all the constituent atoms during the course of the collision.
In our current experiments, we are probing collisions that lead to reaction or the transfer of energy, either between pairs of gas-phase molecules or when a gas-phase molecule strikes a liquid surface.
Dynamics of Collisions at a Gas-Liquid Interface
The dynamics of collisions that take place at the gas-liquid interface are relatively unexplored, largely due to the difficulty of maintaining and characterising liquid surfaces under vacuum and developing experimental methods to probe the collision products.
| We have pioneered a new experimental approach based on the use of lasers to create fragments in the gas-phase above a freshly prepared liquid surface and to monitor spectroscopically the products that escape the surface. We have exploited it to study the reactions of O(3P) atoms with liquid hydrocarbons, including the ‘benchmark’ branched-chain molecule, squalane, and a series of linear alkanes (C22-C28). We detect the nascent OH radicals as they leave the surface. By varying delay between the photolysis and probe lasers we can measure the OH translational energy distributions. Alternatively, by tuning the probe laser wavelength for a fixed photolysis-probe delay, we determine the distribution of OH internal energies. These results have revealed details of the reaction mechanisms, which must involve at least two components: one where the OH leaves the surface directly, carrying away a significant fraction of the incoming O-atom translational energy, and a second where the OH is trapped at the surface for a sufficient period to exchange energy with molecules of the liquid before escaping. The results indirectly reveal new information about the structures of the liquid surfaces, which are difficult to access directly. |
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We have recently gone on to study the scattering of OH radicals themselves with liquid hydrocarbon surfaces, comparing the results with a chemically inert perfluoropolyether surface. We once again find two components in the scattered OH, in different proportions for the two surfaces, reflecting their abilities to absorb momentum form the incoming molecules and trap them at the surface. By examining the relative yields of OH from the two liquids, we can also infer that a significant fraction react at the hydrocarbon liquid surface. This could have direct implications, for example, for the oxidation of the surfaces of aerosol particles by OH, which is an important primary process in the atmosphere.
Gas-phase Inelastic Collisions
Studies of the collisional transfer of energy are of fundamental importance in the understanding of a wide range of gas-phase environments, such as combustion systems, technological plasmas and the atmosphere.
Many experiments have studied the scalar properties of these processes, measuring rates of transfer between different quantum levels. In our current work, we aim to get further, more detailed information by exploiting the vector properties of light (direction of propagation and polarisation of the electric vector) to gain further insight into the stereochemical aspects of molecular collisions. One particular aspect is to study the relationship between the polarisation of the rotational angular momentum vectors (corresponding classically to the plane in which the molecule is rotating) before and after collisions, using a technique known as Polarisation Spectroscopy (PS).
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Polarisation Spectroscopy is a variant of the more general category of non-linear spectroscopies known as ‘Four-wave mixing’. We are using it to study the collisional evolution of rotational angular momentum in simple, important free radicals such as OH and NO. The radicals are generated by laser photolysis (if necessary) in the presence of a controlled pressure of the collision partner, which would typical be chosen for its fundamental interest or relevance in practical environments. The PS signal is excited by a combination of polarised pump and probe pulses from separately tuneable dye lasers. The (circularly or linearly) polarised pump laser creates a polarisation in the sample, which is monitored by the linearly polarised probe laser. The main aim is to study collisions that destroy the polarisation of the sample during the delay between the pump and probe lasers.
We have recently applied this approach successfully to study systematically collisions of ground-state OH radicals with Ar and He colliders. The results are being compared to quantum scattering calculations performed on accurate ab initio potential energy surfaces, and are being used to construct theoretical models of collisional energy transfer. |
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