Fission of charged nano-hydrated ammonia clusters -- microscopic insights on the nucleation processes

TL;DR

This study combines experimental and theoretical methods to investigate the molecular mechanisms of nucleation and fission in charged ammonia-water clusters, revealing water's stabilizing role and ammonium's charge transport function.

Contribution

It provides new insights into the molecular dynamics and energetics of ammonia-water clusters, highlighting water's stabilizing effect and ammonium's role in atmospheric nucleation.

Findings

Water stabilizes ammonia clusters even at low concentrations.

Ammonium acts as a charge carrier bound to ammonia molecules.

Molecular dynamics reveal fragmentation within 500 fs.

Abstract

While largely studied on the macroscopic scale, the dynamics leading to nucleation and fission processes in atmospheric aerosols are still poorly understood at the molecular level. Here, we present a joint experimental-theoretical study of a model system consisting of hydrogen-bonded ammonia and water molecules. Experimentally, the clusters were produced via adiabatic co-expansion. Double ionization ionic products were prepared using synchrotron radiation and analyzed with coincident mass- and 3D momentum spectroscopy. Calculations were carried out using ab initio molecular dynamics to understand the fragmentation within the first 500 fs. Further exploration of the potential energy surfaces were performed at a DFT level of theory to gain information on the energetics of the processes. Water was identified as an efficient nano-droplet stabilizer, and is found to have a significant effect even at low water content. On the molecular level, the stabilizing role of water can be related to an increase in the dissociation energy between ammonia molecules and the water enriched environment at the cluster surface. Furthermore, our results support the role of ammonium as a charge carrier in the solution, preferentially bound to surrounding ammonia molecules, which can influence atmospheric nucleation process.