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
