Hydration of ammonia and hydrogen chloride at cryogenic temperature studied by TP-TOF-SIMS

Temperature-programmed time-of-flight secondary ion mass spectroscopy is utilized in investigating the interactions between the adsorbed HCl (NH3) molecules and hydrated NH3 (HCl) molecules at the water-ice surface in the temperature range of 15-200 K. NH4Cl molecules or their precursors are generated above 120 K when NH3 is adsorbed on an acidic water-ice surface prepared by the hydration of HCl molecules. The reaction occurs at temperature higher than that on a pure HCl film due to the presence of hydrating water molecules, and unreacted NH3 and HCl molecules coexist considerably with their own hydration shells. HCl adsorbed on the basic water-ice surface formed by the hydration of NH3 molecules results in the creation of both NH4Cl and hydrated HCl molecules above 40 K. These results clearly indicate that the interactions between NH3 and H2O molecules are much weaker than those between HCl and H2O molecules, as far as the hydration and ionization of solute molecules are concerned.