Experimental study of the binding energy of NH3 on different types of ice and its impact on the snow line of NH3 and H2O.

Context. Nitrogen-bearing molecules (such as N₂H⁺ and NH₃) are excellent tracers of high-density and low-temperature regions, such as dense cloud cores. Notably, they could help advance the understanding of snow lines in protoplanetary discs and the chemical evolution of comets. However, much remains unknown about the chemistry of N-bearing molecules on grain surfaces, which could play an important role in their formation and evolution. Aims. In this work, we experimentally study the behaviour of NH₃ on surfaces that mimic grain surfaces under interstellar conditions in the presence of some other major components of interstellar ices (i.e. H₂O, CO₂, CO). We measure the binding energy distributions of NH₃ from different H₂O ice substrates and also investigate how it could affect the NH₃ snow line in protoplanetary discs. Methods. We performed laboratory experiments using the ultra-high vacuum (UHV) set-up VENUS (VErs des NoUvelles Syntheses). We co-deposited NH₃ along with other adsorbates (H₂O, ¹³CO, and CO₂) and performed temperature programmed desorption (TPD) and temperature programmed-during exposure desorption (TP-DED) experiments. The experiments were monitored using a quadrupole mass spectrometer and a Fourier transform reflection absorption infrared spectrometer (FT-RAIRS). We obtained the binding energy distribution of NH₃ on crystalline ice (CI) and compact amorphous solid water ice by analysing the TPD profiles of NH₃ obtained after depositions on these substrates. Results. In the co-deposition experiments, we observed a significant delay in the desorption and a decrease of the desorption rate of NH₃ when H₂O is introduced into the co-deposited mixture of NH₃–¹³CO or NH₃–CO₂, which is not the case in the absence of H₂O. Secondly, we noticed that H₂O traps roughly 5–9% of the co-deposited NH₃, which is released during the phase change of water from amorphous to crystalline. Thirdly, we obtained a distribution of binding energy values of NH₃ on both ice substrates instead of an individual value, as assumed in previous works. For CI, we obtained an energy distribution between 3780 K and 4080 K, and in the case of amorphous ice, the binding energy values were distributed between 3630 K and 5280 K; in both cases we used a pre-exponential factor of A = 1.94 × 10¹⁵ s⁻¹. Conclusions. From our experiments, we conclude that the behaviour of NH₃ is significantly influenced by the presence of water, owing to the formation of hydrogen bonds with water, in line with quantum calculations. This interaction, in turn, preserves NH₃ on the grain surfaces longer and up to higher temperatures, making it available closer to the central protostar in protoplanetary discs than previously thought. It explains well why the NH₃ freeze-out in pre-stellar cores is efficient. When present along with H₂O, CO₂ also appears to impact the behaviour of NH₃, retaining it at temperatures similar to those of water. This may impact the overall composition of comets, particularly the desorption of molecules from their surface as they approach the Sun. [ABSTRACT FROM AUTHOR]