Prof Helen Fielding

Research Overview

Organic photochemistry

The typical vibrational period of a covalent bond in an organic molecule is tens of femtoseconds (fs), i.e. fractions of a millionth of a millionth of a second. If we wish to observe bonds rearranging during photochemical reactions, we need to be able to take pictures of the molecule at intervals of a few tens of billionths of a millionth of a second, which is possible using femtosecond lasers. In our group, we employ femtosecond time-resolved photoelectron spectroscopy to follow photochemical reactions: a femtosecond ultraviolet laser pulse (pump) initiates the reaction, a susequent femtosecond ultraviolet laser pulse (probe) ionises the molecule and we record the resulting photoelectron spectrum as a function of pump-probe delay.

A recent highlight is an investigation of the femtosecond dynamics of electronically and vibrationally excited benzene, at the onset of “channel 3”. Femtosecond time-resolved photoelectron spectroscopy was exploited to track the evolution of the singlet and triplet components of the excited state population which, together with quantum chemistry and quantum dynamics calculations carried out by Graham Worth and coworkers (Birmingham), revealed that ultrafast intersystem crossing competes with internal conversion. The significance of this is that it challenges the generally accepted view that intersystem crossing is slow compared to internal conversion in simple hydrocarbons

In a project funded by the EU-funded Marie-Curie FASTQUAST ITN, we are currently focussing on organic molecules containing optically dark, dissociative πσ * states, which are a common feature of organic molecules containing a heteroatom and have been implicated in the intrinsic photostability of various biologically significant molecules including the nucleic acids and DNA bases. In collaboration with Jon Marangos and coworkers (Imperial), we are employing UV-pump-VUV-probe photoelectron spectroscopy to follow the femtosecond dynamics of these molecules.

Biological chromophores

We have developed a versatile new instrument, combining electrospray ionisation mass-spectrometry and photoelectron spectroscopy, for investigating the spectroscopy and dynamics of anions in the gas-phase. The capability of our instrument has been demonstrated by recording photodetachment spectra of deprotonated phenol and indole anions. Electronic structure calculations have been employed to support our interpretation of the measured adiabatic photodetachment energies. We are currently focussing on anions of biological interest, including deprotonated fluorescent protein chromophores. In a collaboration with Jan Verlet (Durham) we have also employed femtosecond time-resolved photoelectron spectroscopy to follow the excited state dynamics of a deprotonated fluorescent protein chromophore anion.

Photodynamic therapy

Photodynamic therapy uses light-activated drugs to treat diseases ranging from cancer to age-related macular degeneration and bacterial infections. The photosensitiser, dose, light flux and drug-light time interval are adjustable parameters that are usually optimised for a specific treatment, in the clinic. In an MRC funded project, we are collaborating with Sandy MacRobert and colleagues in the UCL Medical Laser Centre to develop a more detailed understanding of the fundamental photophysics, photochemistry and photobiology of photodynamic therapy at the molecular level.

We are currently focussing on porphyrin-based photosensitisers and hypericin, employing a combination of fluorescence lifetime and singlet oxygen measurements, photoelectron spectroscopy and electronic structure calculations. We are also collaborating with Nik Kaltsoyannis on some of the electronic structure calculations and Alethea Tabor for our work involving peptides.

Surface photochemistry

In collaboration with Wendy Brown, we have built an ultrahigh vacuum surface science apparatus equipped with a quadrupole mass spectrometer and hemispherical electron analyser, as well as the usual surface science diagnostics, for investigating the dynamics of femtosecond laser-induced photochemistry. A recent success has been a series of experiments to investigate the femtosecond laser-induced desorption of NO/Pd(111), CO/Pd(111) and NO+CO/Pd(111). Our future interests lie in the investigation of organic photochemistry on surfaces. 

Current research group

Jadranka Butorac (PhD 2007-11)

Roman Spesyvtsev (PhD 2009-12)

Ciarán Mooney (PhD 2009-13)

Oliver Kirkby (PhD 2010-13)

Past group members

Dr Vasilios Stavros (PhD 1999; PDRA 1999-2000) PDRA Berkeley, California, USA; Royal Society research fellow and university lecturer, Warwick

Dr James Ramswell (PhD 1999) Scientist, Defence Industry, UK

Dr Qiang Hong (PDRA 1995-7) Industry, USA

Dr Jie Lei (PDRA 1997-8) Industry, USA

Dr Jan Verlet (PhD 2003) PDRA Berkeley, California, USA; EPSRC research fellow and university lecturer, Durham

Dr Robert Smith (PhD 2003) Marie-Curie fellow, Lund, Sweden; Lawyer, London

Dr Elizabeth Boléat (PhD 2005) Medical doctor, London

Dr Robert Carley (PhD 2005; PDRA 2005-7) PDRA Imperial College, London; PDRA, Berlin, Germany

Dr Russell Minns (PhD 2004; PDRA 2007-9); PDRA Virginia, USA; Ramsay Memorial Trust research fellow, UCL; Royal Society research fellow, Southampton

Dr Eva Heesel (PDRA 2004-7) Optics Industry, Switzerland

Dr Rakhee Patel (PhD 2007) Chemistry Admin, UCL; Science administration, Paris

Dr Adam McKay (PDRA 2006-8) RCUK research fellow and lecturer, UCL; Finance

Dr Nicholas Jones (PhD 2008) Theoretical Chemistry MSc, Oxford

Dr Dorian Parker (PhD 2009) PDRA University of Hawaii

Dr Adam Kirrander (PDRA 2005-9) Marie-Curie fellow, Paris

Dr Maria Sanz (Academic visitor 2008-9) Universidad de Valladolid, Spain

Dr Emma Wilson (PDRA 2006-9) Scientist, UK

Dr Abigail Nunn (PhD 2009) PDRA Eastman dental institute

Douglas Lazenby (MPhil 2011) Maths teacher