  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.

Atomic and Molecular Clusters