| Abstract
| - In this paper, we investigated the interaction of simple carboxylic acids, formic acid (FA), acetic acid (AA),and propionic acid (PA) with thin layers of water ice in the temperature range of 110−190 K in ultrahighvacuum. The focus, however, is on the AA−ice system. Molecularly thin layers of these systems were preparedon a pre-cooled polycrystalline copper substrate. The interactions and phase changes in the system weremonitored with chemical sputtering using low-energy (≤30 eV) Ar+, which probes the topmost surface layers.At 110 K, the deposited AA exists as dimers in its amorphous solid form. At the same temperature, in thepresence of water ice, this dimeric form gets converted to chainlike oligomers. Chemical sputtering spectrashow distinct features for these two surface species. The data suggest that ion formation reflects the surfacestructure, implying a unique mechanism for its formation. Detailed studies have been made with amorphoussolid water (ASW) and crystalline water (CW) to get a complete understanding of the system. Experimentscarried out with AA−D2O ice confirmed the proton-transfer mechanism during chemical sputtering. Otherstudies were conducted with AA−CH3OH and AA−CCl4 systems. Detailed investigations performed tounderstand the effect of thickness of AA and ice overlayers suggest that the extent of water molecules requiredto effect the structural transformation in the acid is dependent on the amount of the latter. Dimeric-to-oligomerictransformation does not occur for the PA−ice system. Detection of a structural transition at the very top ofice in molecularly thin films adds additional capabilities to the low-energy ion scattering technique.
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