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Professor David Bird

Tel: 01225 386586
Fax: 01225 386110
E-mail: d.bird@bath.ac.uk
Address: Department of Physics, University of Bath, Bath BA2 7AY
Office: 3W 3.6

My research involves the development and application of methods for solving two of the fundamental equations of Physics: Schrodinger's equation and Maxwell's equations.

Quantum mechanics of bond making and bond breaking at surfaces

My research in quantum mechanics focusses on the way atoms and molecules interact with surfaces. The motivation for this work comes from a long-standing quest in surface science to be able to manipulate chemical reaction pathways to allow us to control, for example, catalysis or corrosion. To achieve this control we need to understand how bonds are formed and broken as atoms or molecules move over the surface. At a fundamental theoretical level, this requires a full understanding of how the electronic states of the system evolve. This is an extraordinarily difficult problem in quantum mechanics, involving very large numbers of strongly interacting particles.

Over recent years, great progress has been made in solving this problem, largely through developments in density functional theory to tackle the many-electron aspects, and through new computational approaches to solving partial differential equations like Schrodinger's equation. I have been involved in several of these developments, for example, in references 1 to 4. However, despite much progress, we have still only got to the point where we can solve for the ground state electronic properties of surface systems. This is useful for predicting the structure of surfaces, and how the energy changes as the atoms are moved, but it leaves a huge gap. We currently have no reliable theoretical methods for treating the dynamical evolution of the electronic states. This is crucial for understanding the way in which energy is transferred from a moving atom or molecule into exciting the electrons of the surface. A number of recent experiments have demonstrated the importance of this electronic excitation in surface reactions. I am currently developing new methods to solve Schrodinger's equation, based on a time-dependent formulation of density functional theory. So far, we have been able to analyse the case where the atoms move very slowly (the nearly-adiabatic limit) and to find analytic time-dependent solutions for a simple model system (see references 5 to 7, and the figure). These have been able to provide some very useful insights into the process of energy transfer and we are now trying to extend our fully non-adiabatic approaches to include a more realistic description of the electronic structure of the molecule-surface system.

Theory and modelling of photonic crystals

It is well known that interesting phenomena occur when a wave interacts with an object whose size is similar to the wavelength of the wave. In recent years it has become possible to artificially structure materials (such as glass) on a sub-micron scale; such structures interact strongly with light waves and the resulting effects have opened up a new field of research: photonics. My aim is to understand the propagation of light in structured materials by developing methods to solve the governing equations of photonics, which are based on Maxwell's equations.

My work in this field is closely associated with the Physics department's Centre for Photonics and Photonic Materials. Experimentalists in this Centre have invented a novel type of optical fibre (called a photonic crystal fibre, PCF) which has air holes running along its length. A cross section of a PCF is shown in the figure; the dark areas represent glass and the light areas represent the air holes. Normally, it is very difficult to confine light in air because the waves have an intrinsic tendency to spread out. In PCF, the wavelength-sized structure surrounding the central core supports a photonic band gap, which means that light can be trapped in the core. The blue area in the figure shows the profile of the fundamental air-guided mode in this fibre. We have developed a very fast computer code to calculate the properties of PCFs, which has been used in a number of joint theoretical and experimental studies of particular types of PCF (see references 8 to 11). Current work is focussed on understanding the way in which light escapes from a PCF when it is bent, and on the interaction of the very intense light field with gas molecules that can be introduced into the hollow core of the fibre. The latter project provides a nice link between my two areas of expertise; Maxwell's equations govern the light field, but the quantum mechanics of the molecular states is also crucial in this field of quantum optics.

References

[1] J.A. White, D.M. Bird, M.C. Payne and I. Stich, Phys. Rev. Lett. 73, 1404 (1994)
[2] J.A. White and D.M. Bird, Phys. Rev. B50, 4954 (1994)
[3] J.A. White, D.M. Bird and M.C. Payne, Phys. Rev. B53, 1667 (1996)
[4] P.A. Gravil, D.M. Bird and J.A. White, Phys. Rev. Lett. 77, 3933 (1996)
[5] J.R. Trail, M.C. Graham, D.M. Bird, M. Persson and S. Holloway, Phys. Rev. Lett. 88, 166802 (2002)
[6] J.R. Trail, D.M. Bird, M. Persson and S. Holloway, J. Chem. Phys. 119, 4539 (2003)
[7] M.S. Mizielinski, D.M. Bird, M. Persson and S. Holloway, J. Chem. Phys. 122, 084710 (2005)
[8] J.M. Pottage D.M. Bird, T.D. Hedley, T.A. Birks, J.C. Knight, P.S. Russell and P.J. Roberts, Optics Express 11, 2854 (2003)
[9] T.A. Birks, D.M. Bird, T.D. Hedley, J.M. Pottage and P.S. Russell, Optics Express 12, 69 (2004)
[10] G.J. Pearce, T.D. Hedley and D.M. Bird, Phys. Rev. B71, 195108 (2005)
[11] G.J. Pearce, J.M. Pottage, D.M. Bird, P.J. Roberts, J.C. Knight and P.S. Russell, Optics Express, 13, 6937 (2005)

Department of Physics, University of Bath, Bath BA2 7AY
Tel: +44 (0) 1225 386154 · Fax: +44 (0) 1225 386110
e-mail: physics@bath.ac.uk

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