Adsorption of astrochemically relevant molecules on interstellar dust grain analogue surfaces
In the last few decades, astronomers have found that interstellar clouds are chemical factories, in which atoms are processed into more complex chemical species in reactions energized by starlight and fast particles. Many of the identified molecules are formed in networks of ion-molecule and neutral-neutral reactions and, at the low temperatures in the interstellar medium (-15 K), can accrete on the surface of interstellar dust grains to form ices. However, it is widely speculated that some of these interstellar ices can only form sufficiently rapidly if the more abundant atoms, and carbon monoxide, are hydrogenated on the surface of dust grains. Hence, there is an urgent need for data concerning gas-grain interactions, especially with regard to whether addition reactions can take place on dust grains. This thesis presents detailed information on the adsorption and desorption of water, methanol and ammonia on suitable dust grain analogue surfaces, using a combination of temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). All three adsorbates show evidence for molecular adsorption in a physisorbed state. In addition, various degrees of hydrogen bonding are observed in the multilayer. Crystallisation is also observed to take place during the desorption of the ices in both RAIRS and TPD studies. Detailed analysis of the TPD spectra has been performed for all adsorption systems to give desorption energies, desorption orders and pre-exponential factors. The information obtained from these experiments has been incorporated into simple simulations under astrochemically relevant conditions (i.e. low heating rates and appropriate ice thicknesses). These simulations can be directly incorporated into astronomical models. This in turn helps to lead to a greater understanding of star formation, and hence of the Universe in which we live.