A New Network in Surface Science Applications in Laboratory Astrophysics
In dense regions of the interstellar medium (ISM), so-called molecular clouds, astronomical observations have revealed a rich chemistry with the detection of over 100 different chemical species ranging in size from simple diatomic molecules to polyatomic molecules comprising 10 or more atoms. This rich chemical soup, which comprises significantly less than 1% of the mass of the observable universe, has a surprisingly complex and crucial role to play in controlling the formation of stars, their associated planetary systems and seeding the Universe with the potential for life. It is now widely recognised by astronomers and molecular astrophysicists that understanding the chemical processes which produce this chemical soup is the gateway to understanding key aspects of these very fundamental processes in our evolving Universe. Surprisingly, crucial to understanding the chemistry of such clouds is a recent realisation that physical and chemical processes occurring at the surfaces of interstellar grains are vitally important.
In attempting to understand the gas-grain interaction, there are several key problems that the chemical physics and surface science communities are well equipped to address; molecule formation, molecule accretion, icy mantle formation, ice desorption and ice processing by heat, light and cosmic rays. In balancing the molecular inventory of the interstellar medium, it is crucial to know the routes by which molecules are formed. There is little doubt, for example, that molecular hydrogen (H2) is formed from atomic hydrogen predominantly on the surfaces of grains and much effort is already being expended by the members of the UCL Centre for Cosmic Chemistry and Physics in understanding the microscopic detail of this process. However, it is becoming increasingly clear that other simple molecules including water (H2O), carbon dioxide (CO2) and others are formed from atomic and radical precursors on grains. At the low temperatures concerned (typically below 10 K), these molecules accumulate on the grains, as molecules such as carbon monoxide (CO) accrete from the gas phase, to generate the icy mantles observed in the infrared. New experiments being developed within the UCL CCCP and elsewhere in the UK are seeking to understand both the simple adsorption of gaseous molecules on grain surfaces and, more importantly, the key atomic and free radical reactions leading to the formation of the icy grain mantles.
In star formation, the central role of molecules in radiatively cooling a collapsing core is well understood. It is also understood that the principle reservoir for such molecules is the icy mantles that accumulate on the grain surfaces. However, what is clearly less well understood is the process by which molecules are released from the icy mantles to return to the gas phase as the grains warm during cloud collapse, thermal desorption. At Heriot-Watt, experiments are underway that cast new light on the interplay of ice morphology, adsorbate trapping and desorption.
To a limited extent, attempts have been made to study icy grain mantles using traditional matrix isolation techniques in high vacuum. However, such measurements have primarily focussed on identifying characteristic vibrational spectra for the ices (H2O, NH3, CH4, CH3OH, CO, CO2, etc.) and ice mixtures that form on the grains, as an aid to the interpretation of observations made from ground- and space-based infrared telescopes. Studies of the UV photolysis or high energy electron or proton irradiation of such ice mixtures have become increasing important in recent years as an explanation has been sought for the formation of complex organic molecules. The work of the groups at NASA Ames Laboratory and Leiden Observatory are of particular significance in this respect. Unfortunately, these measurements cannot hope to explicitly investigate the gas-grain interaction, as the high vacuum environment in which they are conducted effectively precludes such measurements. More recent versions of these experiments at the OU utilise UHV techniques however and pave the way towards a better understanding of energetic processing of icy mantles and the formation of pre-biotic molecules.
From much of the work already going on in the UK, it is clear that surface science provides the means of investigating the gas-grain interaction in a well-defined and controlled manner. Indeed, the environment within an UHV chamber equipped with cryogenic sample cooling provides a realistic model of the milieu within a dense molecular cloud in terms of both the pressure and temperature. Such is the potential for surface science methodologies and techniques to contribute to our understanding of the gas-grain interaction that three major review articles in a special issue of the journal Surface Science were recently dedicated to this topic. The Network was established with EPSRC funding and with this background in mind to encourage collaborations between chemical physicists, surface scientists, molecular astrophysicists and astronomers.
The specific objectives of the Network are as follows:.
- To stimulate future research which is directly relevant to astronomical systems.
- To provide a forum for the exchange of ideas and expertise.
- To encourage collaborations between research groups, thus allowing access to a wider range of experimental and theoretical techniques.
- To promote the Network, and the research of the constituent members, both to other members of the network and to the wider scientific community and to the general public.
- To promote the inclusion of additional network members whose research is relevant to the topics of interest, or who are moving into suitable areas.