  In the coldest (10--20 K) regions of the interstellar medium, the icy
surfaces of interstellar grains serve as solid-state supports for chemical
reactions. Among their plausible roles, that of third body is advocated, in
which the reaction energies of surface reactions dissipate throughout the
grain, stabilizing the product. This energy dissipation process is poorly
understood at the atomic scale, although it can have a high impact on
Astrochemistry. Here, we study, by means of quantum mechanical simulations, the
formation of NH3 via successive H-additions to atomic N on water ice surfaces,
paying special attention to the third body role. We first characterize the
hydrogenation reactions and the possible competitive processes (i.e.,
H-abstractions), in which the H-additions are more favourable than the
H-abstractions. Subsequently, we study the fate of the hydrogenation reaction
energies by means of ab initio molecular dynamics simulations. Results show
that around 58--90\% of the released energy is quickly absorbed by the ice
surface, inducing a temporary increase of the ice temperature. Different energy
dissipation mechanisms are distinguished. One mechanism, more general, is based
on the coupling of the highly excited vibrational modes of the newly formed
species and the libration modes of the icy water molecules. A second mechanism,
exclusive during the NH$_3$ formation, is based on the formation of a transient
H$_3$O$^+$/NH$_2^-$ ion pair, which significantly accelerates the energy
transfer to the surface. Finally, the astrophysical implications of our
findings relative to the interstellar synthesis of NH$_3$ and its chemical
desorption into the gas are discussed.
