Theoretical distribution of the ammonia binding energy at interstellar icy grains: a new computational framework

TL;DR

This paper introduces a new computational framework to predict the distribution of ammonia binding energies on amorphous interstellar ice grains, accounting for structural variability and matching experimental data.

Contribution

It presents a novel multi-scale computational approach to model binding energy distributions on amorphous icy grains, improving upon single-value estimates.

Findings

Main peak of NH₃ BE distribution aligns with experimental data.

A second broad low-BE peak suggests a possible explanation for gaseous NH₃ in cold ISM.

Framework captures the heterogeneity of adsorption sites on amorphous ice.

Abstract

The binding energies (BE) of molecules on the interstellar grains are crucial in the chemical evolution of the interstellar medium (ISM). Both temperature programmed desorption (TPD) laboratory experiments and quantum chemistry computations have often provided, so far, only single values of the BE for each molecule. This is a severe limitation, as the ices enveloping the grain mantles are structurally amorphous, giving rise to a manifold of possible adsorption sites, each with different BEs. However, the ice amorphous nature prevents the knowledge of structural details, hindering the development of a common accepted atomistic icy model. In this work, we propose a computational framework that closely mimics the formation of the interstellar grain mantle through a water by water accretion. On that grain, an unbiased random (but well reproducible) positioning of the studied molecule is then carried out. Here we present the test case of NH$_3$, an ubiquitous species in the molecular ISM. We provide the BE distribution computed by a hierarchy approach, using the semiempirical xTB-GFN2 as low-level method to describe the whole icy cluster combined with the B97D3 DFT functional as high-level method on the local zone of the NH$_3$ interaction. The final ZPE corrected BE is computed at ONIOM(DLPNO-CCSD(T)//B97D3:xTB-GFN2) level, ensuring the best cost/accuracy ratio. The main peak of the predicted NH$_3$ BE distribution is in agreement with experimental TPD and literature computed data. A second broad peak at very low BE values is also present, never detected before. It may provide the solution to a long-standing puzzle about the presence of gaseous NH$_3$ observed also in cold ISM objects.