Leading light

Helen Fielding talks to Cath O’Driscoll about what it takes, scientifically and financially, to be in control of simple chemical systems.

Helen Fielding certainly has expensive tastes. Most of her grant applications rarely come in at much under ?250 000 - so it’s not surprising that they don’t always succeed first time round, she concedes. ’The lasers are hugely expensive - of my last grant, ?200 000 was for a low power femtosecond laser system, and this is a very competitive price. Their replacement value is ?300 000-400 000’, not to mention the cost of all of the optical and electronic paraphernalia that goes alongside them.

When Chemistry World met up with her last year, most of Fielding’s possessions were securely stashed away in crates, ahead of her then impending move from King’s College, London, UK, to nearby University College London, (UCL). Fielding had in fact applied for the post before anything was known about the closure of King’s chemistry department (Chem. Br., May 2003, p7), and was clearly looking forward to the move: ’we have two separate laser rooms here but at UCL I have one exceedingly well-equipped large lab housing three laser tables, with the potential for carrying out three separate experiments’, and UCL’s recent five star RAE rating for chemistry might even improve her success rate with the EPSRC, she adds optimistically.

Fielding’s experiments are mainly to do with ’coherent control’ - the use of lasers to control the electronic and molecular dynamics of simple atoms and molecules. The coherent part is by virtue of the nature of laser light - ’Light Amplification by Stimulated Emission of Radiation’, which means that all the photons are in phase with one another. At a very fundamental level, Fielding exploits this coherence of laser light to manipulate different chemical pathways.

Most of the atoms and molecules that Fielding has been interested in probing are in highly excited or so-called Rydberg states, where one very distant electron ’orbits’ around the remaining positively charged atomic or molecular ion. ’It’s only when the distant electron comes back that it "knows" there are other electrons and atoms within the core, and that it is in a large atom or molecule. So, because for much of the time it "thinks" it’s in a hydrogen atom, one can develop an accurate model of the system - these systems are so attractive because they are tractable.’

Novel and complex
The work on Rydberg molecules is particularly novel, and Fielding is one of only a handful of researchers worldwide exploring Rydberg wavepacket dynamics in molecules. ’I think it’s because it lies on the boundary of chemistry and physics. Chemists tend to be interested in molecules and it is usually the physicists who are interested in Rydberg states and they tend to stick with atoms. Our interest in molecules is because of their greater complexity. Rydberg molecules exhibit many of the dynamical complications present in large molecular systems and so they represent a step towards real molecules.’

Much of Fielding’s work lies at the boundary of classical and quantum mechanics. ’At school we are taught the picture of small electrons orbiting around the nucleus of an atom, then at university we are told to think of a wave function and probability distributions and to consider the electron as being delocalised in an orbital’, she says. The wavepackets that interest Fielding try to reconcile these two ideas: ’By exciting many wave functions coherently we generate a wavepacket which has a more localised probability distribution and which at short times moves like a classical electron. It still has wave character, and as it evolves it begins to spread and produce interference patterns within itself. Once this happens, phase becomes important and quantum mechanics becomes more appropriate than classical mechanics’.

Rotating molecules
In 2000, Fielding and her group made the first observation of a wavepacket in a molecule. Unlike the situation in atoms, the core of a Rydberg molecule rotates, which manifests itself in the wavepacket dynamics as an interference pattern. ’These types of experiments are very difficult technically, involving many lasers and timing issues, so it was very exciting when we recorded the first spectra.’ The work on Rydberg molecules was duly recognised when Fielding was awarded the RSC’s prestigious Marlow medal in 2001.

It was this insight into the phase of the electronic and rotational motions in Rydberg molecules that eventually led Fielding to explore the possibility of coherent control. By carefully selecting the optical phase of the laser light, Fielding and coworkers aim to exploit the intrinsic phase of the Rydberg molecule and so manipulate the dynamics of simple chemical systems. Early explorations are already bearing fruit. In a recent paper1, Fielding’s work is attracting attention with a report on controlling the decay pathway of highly excited NO molecules.

In this simple molecular Rydberg system, there are two distinct decay pathways, Fielding explains: above the ionisation and dissociation limits there is a probability that each time the electron collides with the core it will escape leaving NO+ and e-; or alternatively, the electronic energy may be transferred to vibrational energy and the molecule can dissociate into neutral N and neutral O. ’By using our knowledge of the wavelength of the laser light and its phase we can choose whether most of the Rydberg molecules go down the ionisation route or the predissociation route. We are using the optical phase to manipulate the interferences between the different excitation pathways so that one decay route is the result of constructive interference [the waves are in phase] and the other is the result of destructive interference. We have shown that we can manipulate the ionisation/predissociation ratio to favour one process or the other.’

This work is especially significant, Fielding says, because it is ’one of the first examples of intuitive molecular control, where we know exactly the relative phases of light that we’re putting in, and we know why we’re doing it’. She points out that other researchers are doing some very elegant work controlling much larger chemical systems, albeit less rationally. For example, Gustav Gerber’s group at the University of W?rzburg in Germany has recently devised an ingenious way of making iron pentacarbonyl (Fe(CO)5) produce one of five different decay products by using a learning algorithm so that the ’light’ learns for itself what pulse shape is required for each selected product.

’That’s a really cool experiment. However, unfortunately you don’t learn anything about the underlying physics. It’s a beautiful example of being able to control chemistry, but we don’t yet know why the shape of the light pulse that was generated using the learning algorithm did what it did.’ Rather, what Fielding is doing now is ’going back to the simple systems to understand how optical phase links with electronic and molecular phase, in the hope that one day we will be able to take a system like the Fe(CO)5 one and predict what pulse shape is needed for a particular product - without using the learning algorithm’.

Photochemical manipulation
For chemists, the real power of coherent control will be for manipulating photochemical reactions. Controlling bond breaking and bond making is mainly down to molecular vibrations between the atoms that hold molecules together. ’But a lot of light-controlled reactions, like photoswitches and photosynthesis, happen in excited electronic states, so you do need to understand the link between the optical phase of light and the electronic phase - and this is important in big molecules as well as simple systems.’

Along with former King’s colleague and renowned quantum chemist Mike Robb (now at Imperial College London), Fielding has recently submitted a grant application to employ feedback-controlled shaped laser pulses to control the dynamics of ’chemically interesting systems’. With their combined strengths in experiment and theory, the pair hope to be able to understand how laser light and molecules interact and to develop a systematic laser driven control of photochemistry. Important applications include the photobiology of vision and photosynthesis, and molecular photoelectronics.

Control of the reaction pathways of photochemically excited molecules involves electronic, vibrational and rotational interactions. In another recent paper2, Fielding and coworkers reported being able to control the rotational quantum state of a molecular Rydberg wavepacket. Fielding elaborates: ’rather than letting the core be a superposition of many rotational states, we’ve used lasers to select one specific component of the superposition, ie we can determine how much rotational energy the molecule has, which in this particular experiment amounts to zero or non-zero rotational energy’.

The significance of this, she continues, is that ’we’ve been able to design a sequence of light pulses with a well understood phase difference between them in order to control a Rydberg molecule and its rotational state; and of course the rotational state of a molecule can determine the way the molecule falls apart or reacts’.

Life in the fast lane
Fielding’s career as a lecturer at King’s began in 1994, after only 18 months postdoctoral experience - with Ben van Linden van den Heuvell and coworkers at the University of Amsterdam, The Netherlands, where she got her first taste of using ultrafast lasers for dynamic studies. Her former PhD supervisor Tim Softley, at the University of Oxford, UK (Fielding had originally begun her PhD with him at Cambridge, where she also obtained a first degree in natural sciences), told her about the vacancy.

Fielding admits ’I thought I’d be crazy to apply for this straight after a postdoc but Tim said, "well if nothing else it’s good interview experience"’. Eight years on, in 2002, she was appointed professor of chemical physics, having also won the RSC’s Harrison prize for her work with femtosecond spectroscopy in 1996.

At UCL, Fielding hopes to extend her coherent control work to other larger molecules such as retinol, which is important in vision. She also plans to try some experiments with learning algorithms on simple, tractable, systems like H2. The aim is to determine the shape of the optical pulse and carry out complementary theory to understand why it does what it does. In the meantime, she’s waiting to hear whether her latest - ?750 000 - grant application has been approved.

Acknowledgements

Cath O’Driscoll