Ultrafast Charge and Proton Transfer in Doubly Ionized Ammonia Dimers

We investigate the ultrafast energy and charge transfer processes between ammonia molecules following ionization reactions initiated by electron impact. Exploring ionization-induced processes in molecular clusters provides us with a detailed insight into the dynamics using experiments in the energy domain. We ionize the ammonia dimer with 200 eV electrons and apply the fragment ions coincident momentum spectroscopy and nonadiabatic molecular dynamics simulations. We identify two mechanisms leading to the doubly charged ammonia dimer. In the first one, a single molecule is ionized. This initiates an ultrafast proton transfer process, leading to the formation of the NH₂⁺ + NH₄⁺ pair. Alternatively, a dimer with a delocalized charge is formed dominantly via the intermolecular Coulombic decay, forming the NH₃⁺·NH₃⁺ dication. This dication further dissociates into two NH₃⁺ cations. The ab initio calculations have reproduced the measured kinetic energy release of the ion pairs and revealed the dynamical processes following the double ionization.