Research Guides

This group has a diverse range of research interests covering structure and reactivity, laser spectroscopy in both the gas and condensed phases and a healthy mixture of experiment and computation. Much of the experimental work is novel instrument development in which we devise new techniques to provide new insights to interesting scientific problems.  A large part of our work is interdisciplinary and we have active collaborations with several other internationally leading research groups both within the UK and overseas.

I. Structure and reactivity of small gas-phase metal clusters

Small clusters of atoms (up to 20 atoms) – particularly those of transition metal atoms – exhibit many remarkable size-dependent physical properties which are quite unlike those of either isolated atoms or the bulk metal. Indeed, in many ways these clusters represent a unique state of matter of their own. The structures of small metal clusters remain, however, largely a mystery (at least to experiment) and very little is known of the apparently complex and subtle relationship between structure - both electronic and geometrical - and reactivity.

We take several parallel approaches to studying these systems: In Oxford, we have developed the world's first velocity map imaging instrument to be fitted with a laser ablation cluster source  Following laser fragmentation, we can use conservation of momentum and energy to determine the partitioning of energy within the fragments. This, in turn, is used to determine properties such as the electronic and geometrical structure of the cluster and the nature and strength of any adsorbate binding. Understanding the evolution of these properties with cluster size and composition is key to understanding nanoparticle chemistry.

In a recent collaboration with groups in Berlin, we have used mid- and far-infrared radiation of the FELIX free electron laser in the Netherlands both to study the vibrational structure of naked (and decorated) metal clusters and to initiate cluster surface reactivity. A recent example of infrared-driven N₂O decomposition on rhodium clusters (see image) mimics the chemistry which occurs in the three-way catalytic converter.

All aspects of our cluster work are supported by calculations at the level of density functional theory (DFT). These cluster systems represent a major challenge for DFT as very often the exchange-correlation functionals available have been optimised for bulk and/or atomic properties. Ironically, however, it is the very fact that the electronic structure of small clusters differs markedly from those of the bulk endows transition metal clusters with unique and interesting physico-chemical properties including catalytic activity. 

 II. Optical cavity based spectroscopy in the condensed phase

In the last decade, cavity-based optical techniques such as cavity ring-down and cavity enhanced absorption spectroscopy have revolutionised sensitive gas-phase trace detection. Extension of the technique to the condensed phase has been comparatively slow and yet it is here that such methods could help address important technological problems. We are developing a range of fast condensed phase variants of both cavity ring-down and cavity enhanced absorption spectroscopy including some utilising evanescent fields and supercontinuum sources in order to study dynamical interfacial phenomena. Recent systems studied include the seeding and growth of nanostructured materials, DNA assisted desorption of porphyrins and redox reactions of surface immobilised cytochrome-c.

In a recent extension of this work, we collaborate with Professor Peter Hore and Dr Chris Timmel in applying these sensitive detection methods to the study of the dynamics of spin-correlated radical pairs in solution.  

Gas-phase clusters

Xe⁺ formation following photolysis of Au-Xe: A velocity map imaging study
W. Scott Hopkins et al., J. Chem. Phys., 113, 094311, 2011

Photodissociation dynamics of Li(NH₄)₃: A velocity map imaging study
W. Scott Hopkins et al., J. Phys. Chem. Lett., 2, 257, 2011

Infrared induced reactivity on the surface of isolated size-selected clusters: Dissociation of N₂O on rhodium clusters
Suzanne M. Hamilton et al.,  J. Am. Chem. Soc., 132, 1448, 2010

Velocity Map Imaging Study of the Gold – Rare Gas Complexes: Au–Ar, Au–Kr and Au–Xe
W. Scott Hopkins et al., J. Chem. Phys., 132, 214303, 2010

The structure of Rh₈⁺ in the gas-phase
D.J. Harding et al., J. Chem. Phys. 132, 011101, 2010

VUV photodissociation dynamics of diatomic gold, Au₂: A velocity map imaging study at 157 nm
W. Scott Hopkins et al., Chem. Phys. Lett., 483, 10, 2009

The chemistry of nitrogen oxides on small size-selected cobalt clusters Co_n⁺
Marie L. Anderson et al., J. Chem. Phys., 130, 064305, 2009
 

Cavity-based spectroscopy

Evanescent wave cavity-based spectroscopic techniques as probes of interfacial processes
Mathias Schnippering et al., Chem. Soc. Rev., 40, 207, 2011

Probing redox reactions of immobilized cytochrome c using evanescent wave cavity ring-down spectroscopy
Hayley V. Powell et al., Chem. Phys. Chem., 11, 2985, 2010

Kinetics of porphyrin adsorption and DNA-assisted desorption at the silica - water interface
Hayley V. Powell et al., Langmuir, 26, 4004, 2010

Cavity enhanced detection methods for probing the dynamics of spin correlated radical pairs in solution
Simon R.T. Neil et al., Mol. Phys., 108, 993, 2010

Real Time Monitoring of Polyaniline Nanoparticle Formation on Surfaces
Mathias Schnippering et al. J. Phys. Chem. C, 113, 20221, 2009