Where Does the Energy Go during the Interstellar NH₃ Formation on Water Ice? A Computational Study

Abstract In the coldest (10–20 K) regions of interstellar medium, icy surfaces grains serve as solid-state supports for chemical reactions. Among their plausible roles, that third body is advocated, in which reaction energies surface reactions dissipate throughout grain, stabilizing product. This energy dissipation process poorly understood at atomic scale, although it can have a high impact on astrochemistry. Here we study, by means quantum mechanical simulations, formation NH 3 via successive H-additions to N water ice surfaces, paying special attention third-body role. We first characterize hydrogenation and possible competitive processes (i.e., H-abstractions), are more favorable than H-abstractions. Subsequently, study fate ab initio molecular dynamics simulations. Results show around 58%–90% released quickly absorbed surface, inducing temporary increase temperature. Different mechanisms distinguished. One mechanism, general, based coupling highly excited vibrational modes newly formed species libration molecules. A second exclusive during formation, transient H O + /NH 2 − ion pair, significantly accelerates transfer surface. Finally, astrophysical implications our findings relative synthesis its desorption into gas discussed.