Dynamics of Reactive and Inelastic Molecular Collisions

Collisions abound in almost all chemical processes of practical interest, resulting either in chemical reactions or energy transfer. We aim to understand what happens during intermolecular collisions by preparing systems as precisely as possible and then examining the detailed motions of the products immediately afterwards, mainly through novel types of laser spectroscopy. This work is carried out jointly with Dr Matt Costen (see our joint research group webpages).

1. Dynamics of Collisions at Liquid Surfaces

Despite their widespread practical importance in, e.g., heterogeneous atmospheric chemistry and the combustion of liquid fuels, collisions at the gas-liquid interface remain relatively unexplored fundamentally. We have developed a successful, laser-based method to study reactive and inelastic collisions of gas-phase radicals at liquid surfaces (Figure 1). We have also demonstrated that it can be exploited as a new type of analytical probe of complex surfaces such as those of ionic liquids.

Figure 1. Schematic reaction of gas-phase O(³P) atoms at the surface of the long-chain hydrocarbon, squalane (C₃₀H₆₂).

2. Dynamics of Inelastic Collisions of Open-Shell Radicals

Small open-shell radicals, such as OH, CN and NO, are the key species in many gas-phase environments. Their intermolecular collisions govern the dissipation of energy and are also crucial in the interpretation of spectroscopic probes of concentrations and temperatures. We have developed several methods, including polarisation spectroscopy and frequency-modulated absorption spectroscopy, to study such collisions, with particular focus on the destruction of rotational polarisation (Figure 2).

Figure 2. 2-colour polarisation spectroscopy measurements of the decay of orientation of OH(X)v = 0 in different rotational levels in collisions with Ar and He at low (300 mTorr, blue) and high (1000 mTorr, red) pressures.

3. Dynamics of High-Energy Reactive Collisions

In collaboration with Prof Tim Minton at Montana State University, we have been involved in a number of studies of the dynamics of collisions of O(³P) atoms generated from a unique high-energy source. This has been exploited to study reactions at selected liquid surfaces (see above), complemented by realistic molecular dynamics simulations (Figure 3), and in on-going work on benchmark elementary gas-phase reactions such as O(³P) + D₂.

Figure 3. Snapshot molecular dynamics simulations of the surfaces of the ionic liquids C₂mim NTf₂ (upper) and C₁₂mim NTf₂ (lower), showing the preferential accumulation of the longer alkyl chains at the surface of the liquid.

Research Group Webpage

• Head of Dynamics and Structure research grouping
• Member of Institute of Chemical Sciences Management Group
• MChem Study Director
• School of Engineering & Physical Sciences Representative of Heriot-Watt University Senate