Hydrogen bonding between water and methanol studied by temperature-programmed time-of-flight secondary ion mass spectrometry

The interactions between condensed molecules at cryogenic temperatures (15-200 K) have been investigated on the basis of secondary ion mass spectrometry. It is demonstrated that the protonated molecular ions, emitted via the proton transfer reactions, provide us unique information about the reorganization of hydrogen-bonded molecules. From the CH3OH molecules adsorbed on the D2O-ice surface, the D+(CH3OH) ions are sputtered predominantly in the temperature range between 100 and 150 K since most of the CH3OH molecules are bound to the D2O layer via hydrogen bonds. A rapid and almost complete H/D exchange, yielding the D+(CH3OD) species, occurs above 150 K due to the enhanced mobility of the surface D2O molecules. Up to the desorption temperature of 180 K, a considerable amount of methanol exists on the surface without mixing with the heavy-water layer due to hydrophobicity of the methyl group. On the methanol-ice surface, the adsorbed D2O molecules form hydrogen bonds preferentially with the CH3OH molecules and tend to be incorporated in the thin-layer bulk of methanol above 120 K. (C) 2003 American Institute of Physics.