Temperature-programmed time of flight secondary ion mass spectrometry study of hydration of ammonia and formic acid adsorbed on the water–ice surface

The interactions between condensed ${\mathrm{H}}_2\mathrm{O},$ ${\mathrm{NH}}_3,$ and HCOOH molecules have been investigated on the basis of secondary ion mass spectrometry (SIMS). The protonated molecular ions are created during energetic collisions of hydrogen-bonded neutral molecules as a consequence of proton transfer reactions. The ${\mathrm{NH}}_4^{+}$ yield from the ${\mathrm{NH}}_3$ molecules adsorbed on the HCOOH surface is about 30 times as high as that from the pure ammonia surface since the ions are formed in the course of the dissociation of the ${\mathrm{NH}}_4{\mathrm{HCO}}_2$ molecule or its precursors. The hydration or reorganization of hydrogen bonding between the adsorbed molecules and the water–ice surface is discussed as a function of temperature (15–300 K). The adsorbed ${\mathrm{NH}}_3$ or HCOOH molecule forms a bound state on the water–ice surface at a temperature of 60–140 K. In the same temperature regime, the coadsorbed ${\mathrm{NH}}_3$ and HCOOH molecules react with each other to create the ${\mathrm{NH}}_4{\mathrm{HCO}}_2$ molecule prior to hydration. The hydration occurs drastically for all these molecules above 140 K as evidenced by the occurrence of rapid and almost complete H/D exchange.