Hydration and H/D exchange of CH3OH adsorbed on the D2O-ice surface studied by time-of-flight secondary-ion mass spectrometry (TOF-SIMS)

It is demonstrated that the reorganization of the water-methanol hydrogen bond can be investigated on the basis of the TOF-SIMS experiments with increasing temperature. From the CH3OH molecules adsorbed on the D2O-ice surface, the D+(CH3OH) ions are sputtered predominantly in the temperature range between 120 and 140 K since most of the CH3OH molecules form the hydrogen bonds with the D2O layer. A rapid and almost complete H/D exchange, yielding the D+(CH3OD) species, occurs above 140 K due to the enhanced mobility of the D2O molecules. This is supported by the fact that the CH3OH molecules buried in the thick D2O layer segregate to the topmost surface layer above 140 K. The methanol exists on the surface of the heavy-water layer up to the evaporation temperature (180 K), whereas the heavy-water tends to be incorporated in the thin-layer bulk of the methanol ice at 100 K. (C) 2004 Elsevier B.V. All rights reserved.