The Application of Ultraviolet/Visible Spectroscopy to the Study of Surface Astrochemistry
James W. Stubbing
University of Sussex
Tara L. Salter (University of Sussex), Skandar Taj (Heriot-Watt University), Martin R. S. McCoustra (Heriot-Watt University), Wendy A. Brown (University of Sussex)
Interstellar clouds are known to consist of gas and dust grains. Understanding the processes which occur on the surface of these grains is of interest as they are in part responsible for the rich chemistry observed in the interstellar medium (ISM). A new ultraviolet/visible (UV/vis) spectrometer assembly is being designed and commissioned that will allow us to measure the UV absorption characteristics of surfaces and adsorbates on those surfaces. Chiefly, by working in reflection-absorption mode with a variety of geometries, Kubelka-Munk and Fresnel analysis of the resulting spectra will allow relevant optical properties to be determined for graphite, amorphous carbon and silicate surfaces, and for thin films (in the nm range) of astronomically relevant adsorbates on these surfaces. Adsorbates of interest include complex organic molecules (COMs) and polycyclic aromatic hydrocarbons (PAHs) for which there is evidence of their presence in space. The determination of these parameters will allow spectra of adsorbed ices of varying compositions to be predicted, this has been demonstrated previously in the infrared (IR) region. Systems which will be examined initially will include benzene and toluene. Benzene is the simplest aromatic hydrocarbon and is considered to be a building block of larger PAHs. It will be the next step of our investigation of benzene, for which infrared (IR) spectroscopy and temperature programmed desorption (TPD) data has already been collected. Although toluene has not been detected in the ISM, feasible production routes are known and hence it offers a good comparison to the benzene system as a simple substituted benzene.