Molecular Beam Studies of Interactions Between Ice Surfaces and Atmospherically Relevant Molecules

Abstract: The discovery of the ozone hole over Antarctica in 1985 was one of the environmental issues that attracted most attention worldwide in the end of the last century. The cause of the ozone hole formation was proven to be coupled to the emission of chlorofluorocarbons (CFCs). The impact of the CFCs on the stratospheric chemistry was proven to be strikingly high. Eventually, research of heterogeneous chemistry showed the importance of the multi-phase reactions on stratospheric chemistry. The present thesis aims to contribute to the understanding of the heterogeneous processes relevant for stratospheric chemistry. The present thesis presents results from experimental studies of molecular beam interactions with surfaces, mainly water ice and graphite. Besides this, formation of adlayers of N2O5 and HNO3 on graphite surfaces has been studied by the use of helium scattering. Our results show that the interactions of both molecular chlorine and nitrogen oxides with the studied surfaces are highly inelastic in nature. At low incident energies (below about 0.15 eV), trapping desorption dominates completely and only at the highest energies employed, inelastic scattering is also present. Elastic scattering was not observed in any case. Furthermore, the results in this work show that the pre-exponential factor may deviate significantly from the common value of 1013 s-1 previously observed. The obtained pre-exponential factors ranged from about 1010 to 1016 s-1. This indicates that several of the studied desorption processes are complicated. The obtained activation energies were low in comparison to chemisorption energies, and were in the range of < 0.19-0.41 eV.