Hydration of the HCl and NH3 molecules adsorbed on amorphous water–ice surface

The interactions between HCl and NH3 molecules adsorbed on amorphous solid water (ASW) have been investigated by temperature-programmed time-of-flight secondary ion mass spectrometry (TP-TOF-SIMS). The NH4þ ion is sputtered predominantly from codeposited HCl and NH3 molecules on ASW due to collision-induced proton transfer reactions. The NH4þ yield is enhanced above 60 K as a consequence of the formation of the NH4Cl molecule. The NH4þ yield steeply decays above 100 K due to the hydration of the NH4Cl molecules, and the almost complete hydration occurs above 140 K. The H/D exchange is induced rapidly between D2O and NH4Cl molecules above 120 K. These behaviors strongly suggest that mobile water molecules exist on the surface above 100–120 K