Dr Wendy Brown
Our perspective giving an overview of surface science investigations into interstellar ices has been featured on the cover of PCCP. This perspective provides an overview of the current level of understanding of the adsorption and desorption of astrophysically relevant molecules from a range of dust grain analogue surfaces.
Work in our group focuses on investigating adsorption and reactions on surfaces. Current research focuses on the following areas:
- Experimental and theoretical investigations of the adsorption, desorption and formation of molecules on the surface of cosmic dust grains.
- Experimental investigations of UV processing of interstellar ices.
- Investigations of ultrafast surface processes in collaboration with Prof. Helen Fielding.
Investigation of the adsorption, desorption and formation of molecules on the surface of cosmic dust grains
Current work is concerned with investigating the adsorption, desorption and formation of small molecules, such as methanol, water, carbon dioxide and carbonyl sulphide, on the surface of interstellar dust grains. These molecules have all been observed in interstellar regions frozen out in ices in abundances that cannot be accounted for by gas phase reactions. Astronomers have proposed that they may instead be formed by heterogeneous reactions which take place on the surface of dust grains (see figure).
Cone nebula (NGC 2264). Copyright NASA, H. Ford (JHU), G. Illingworth (USCS/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA
Our work uses the technique of surface infrared spectroscopy to probe the adsorbate species that are formed on a model dust grain surface, dosed with the relevant reactants. The experiments involve studying molecular synthesis on graphite, as well as on thin films of water, carbon and, in the future, SiO2.
Recent experiments include temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) investigations of the adsorption/desorption of model interstellar ices containing carbon dioxide, carbonyl sulphide (see figure below), and sulphur dioxide.
In all cases we have investigated pure ices, layered ices where these species are adsorbed on water and mixtures of these species in water. The latter systems are particularly relevant to the interstellar medium, as most ices usually consist of a mixture of molecules These experiments provide detailed kinetic parameters for desorption which can be incorporated directly into astronomical models.
We are just beginning a new project to look at the adsorption, desorption, stability and formation of glycolaldehyde in the interstellar medium. Glycolaldehyde has recently been discovered in star-forming regions. It is the simplest of the monosaccharide sugars and it reacts with propenal to form ribose, a central constituent of RNA. Because of this, it is thought that glycolaldehyde may have a role to play in the origins of life in our universe. Gas phase reactions are thought to be too slow to produce detectable abundances of glycolaldehyde during the star and planet formation process. It is therefore thought that surface processes, involving thermal, UV and electron processing of interstellar ices, may be responsible for the formation of this species. This project will investigate glycolaldehyde formation, desorption and destruction from several fronts: experimental, theoretical, astrochemical modelling and observational, bringing together experts from astronomy, astrochemistry, surface chemistry and computational chemistry.
To complement the experimental studies, we have also undertaken theoretical investigations of the formation of small molecules on the surface of model dust grains using DFT and QM/MM techniques. Our most recent results show that silica surfaces catalyse the formation of methanol, particularly when negatively charged defects are present. Other results also show that the formation of OCS can both take place on a coronene surface (see figure below).
This work forms part of a wider investigation of the formation of molecules in interstellar space which is currently taking place in the UCL Centre for Cosmic Chemistry and Physics. This work also forms part of the LASSIE ITN network. Dr Brown is also a founding member of the AstroSurf network. This work was featured in the 2004 and 2006 Royal Society Science exhibition via an exhibit called Stars R Us!.
In a related project we are also undertaking investigations of the UV processing of interstellar ices at the RAL Lasers for Science facility. These experiments are performed in collaboration with Prof. Martin McCoustra, Dr. Helen Fraser and Prof. Nigel Mason. Current experiments include investigations of the UV processing of PAH doped water ices. The figure below shows time-of-flight spectra recorded for various different layered configurations of water and benzene adsorbed on an underlying sapphire substrate.
ToF profiles of (A) benzene and (B) water desorption for varying layer configurations, laser energy = 1.8 mJ, λ = 250.0 nm. Thin grey lines: raw data; thick lines: single component Maxwell-Boltzmann fits.
The ultimate goal of any branch of chemistry, including surface chemistry, is to understand reactions at a fundamental level. In order to achieve this, it is necessary to both observe the reaction on an atomic length scale, and also to monitor the reaction on the same time scale as bond breaking and making - the femtosecond timescale. With the advent of scanning probe microscopy, the observation of surface reactions at the atomic length scale has become almost routine. However, observing the dynamics of surface reactions with femtosecond accuracy still remains a considerable challenge.
In collaboration with Prof. Helen Fielding, we have built a new experiment to investigate the detailed dynamics of surface processes, by using state-of-the-art femtosecond pump-probe techniques to monitor reactions in real time. We also intend to investigate the possibility, long discussed theoretically but not yet achieved experimentally, of using shaped femtosecond laser pulses to control surface photochemistry. Further details of this experiment can be found here.
- D.J. Burke and W.A. Brown, "Ice in space: surface science investigations of the thermal desorption of model interstellar ices on dust grain analogue surfaces", Phys. Chem. Chem. Phys.12 (2010) 5947-5969.
- D.A. Adriaens, T.P.M. Goumans, C.R.A. Catlow and W.A. Brown, "Computational study of carbonyl sulphide formation on model interstellar dust grains", J. Phys. Chem. C114 (2010) 1892-1900.
- E.L. Wilson and W.A. Brown, "Low pressure RAIRS studies of model catalysts", J. Phys. Chem. C114 (2010) 6879-6893.
- S.D. Green, A.S. Bolina, R. Chen, M.P. Collings, W.A. Brown & M.R.S. McCoustra, "Applying laboratory thermal desorption data in an interstellar context: sublimation of methanol thin films", Mon. Not. R. Astron. Soc. 398 (2009) 357-367.
- T.P.M. Goumans, C.R.A. Catlow and W.A. Brown, "Catalysis of addition reactions by a negatively charged silica surface site on a dust grain" J. Phys. Chem. C112 (2008) 15419-15422..
- D.J. Burke, A.J. Wolff, J.L Edridge, and W.A. Brown, "Thermally induced mixing of water dominated interstellar ices", Phys. Chem. Chem. Phys. 10 (2008) 4956-4967.