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Surface Science Lab

The experiments in the surface science lab are concerned with investigating the adsorption, desorption and formation of small molecules, other than H2, on simulated cosmic dust grains under "interstellar" conditions.

Experiment to study the formation of simple interstellar molecules on cosmic dust grain analogues

The aim of the experiments that are currently underway in the surface science lab is to use infrared spectroscopy and temperature programmed desorption to study the chemistry of heterogeneous processes that occur on the surface of analogues of dust grains in space. Many different molecules, including CH3OH, NH3 and H2O, have been observed in interstellar space. However, the quantities of molecules observed are far greater than can be explained by gas phase reactions alone. Heterogeneous processes, whereby species such as CO, N and O undergo sequential hydrogenation, are therefore thought to be fundamental to the formation of these molecules. These heterogeneous reaction mechanisms have not been studied to date and are therefore not well understood.

The apparatus that will be used for these experiments consists of a UHV chamber (shown left) equipped with a closed cycle helium refrigerator. This will allow the experiments to be performed in conditions which mimic those in interstellar space i.e. low pressure and temperatures of the order of 10-20 K. An atom source is coupled to the UHV chamber to allow the dosing of H, or O, atoms onto a surface which is a suitable analogue of a cosmic dust grain.

Reflection absorption infrared spectroscopy (RAIRS) is the main technique that is used to study these reactions. RAIRS is an infrared technique that can be used to probe the species present on a surface. This technique allows us to identify the products, and intermediates, of the reactions. This is achieved by observing the vibrational frequencies of intermolecular bonds of molecules adsorbed on the surface. Temperature programmed desorption (TPD) is also used as a probe of the species desorbing from the surface.

Current experiments are concerned with investigating the adsorption, desorption and formation of methanol, ethanol, carbon monoxide and carbon dioxide on the surface of highly oriented pyrolytic graphite (HOPG). We are also looking at the adsorption and desorption of mixed overlayers of water and ethanol on the HOPG surface. If we are to understand the formation of these species on the surface, it is first necessary to look at the adsorption and desorption of these molecules so that a fingerprint of the species in the infrared or TPD can be obtained.

Other experiments that the surface science group are currently involved in include investigating the UV processing of model interstellar ices consisting of water and benzene. These experiments are performed at the central laser facility at the Rutherford Appleton Laboratory in collaboration with Prof Martin McCoustra (Heriot-Watt) and Dr Helen Fraser (Strathclyde). The figure shows a cartoon of the water/benzene layers that are irradiated with UV light.

Daren Burke and Jon Edridge are currently working in the surface science lab under the supervision of Dr Wendy Brown.

The final report for a recent EPSRC grant for this work can be found here.

This page last modified 26 October, 2007 by John Edridge

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