Aqueous Solution Chemistry of Ammonium Cation in the Auger Time Window

Daniel Hollas,Marvin N Pohl,Emad F Aziz,Bernd Winter,Robert Seidel,Petr Slavíček

doi:10.1038/s41598-017-00756-x

Abstract

We report on chemical reactions triggered by core-level ionization of ammonium ({{rm{NH}}}_{4}^{+}) cation in aqueous solution. Based on a combination of photoemission experiments from a liquid microjet and high-level ab initio simulations, we identified simultaneous single and double proton transfer occurring on a very short timescale spanned by the Auger-decay lifetime. Molecular dynamics simulations indicate that the proton transfer to a neighboring water molecule leads to essentially complete formation of H3O+ (aq) and core-ionized ammonia {({{rm{NH}}}_{3}^{+})}^{ast }(aq) within the ~7 fs lifetime of the nitrogen 1s core hole. A second proton transfer leads to a transient structure with the proton shared between the remaining NH2 moiety and another water molecule in the hydration shell. These ultrafast proton transfers are stimulated by very strong hydrogen bonds between the ammonium cation and water. Experimentally, the proton transfer dynamics is identified from an emerging signal at the high-kinetic energy side of the Auger-electron spectrum in analogy to observations made for other hydrogen-bonded aqueous solutions. The present study represents the most pronounced charge separation observed upon core ionization in liquids so far.

Highlights

Ionization of molecules or ions by high-energy radiation leads to the formation of excited species which relax either via radiative (X-ray fluorescence) or non-radiative (Auger-type autoionization) decay channels
The core hole is refilled by a valence electron, but instead of ejecting a local Auger electron, an electron is emitted from a water molecule in the first hydration shell
In order to discuss the results of our combined theoretical and experimental studies on the proton-transfer mediated charge separation processes in aqueous NH+4, we first present computation-based evidence for this process

Summary

Introduction

Ionization of molecules or ions by high-energy radiation leads to the formation of excited species which relax either via radiative (X-ray fluorescence) or non-radiative (Auger-type autoionization) decay channels. The core hole is refilled by a valence electron, but instead of ejecting a local Auger electron, an electron is emitted from a water molecule in the first hydration shell This so-called intermolecular Coulombic decay (ICD) creates two positive charges shared between two molecular partners, e.g. AHq+1 H2O+. The charge separation, leading to the transient core-excited species, is supported by the proton motion which has been identified in previous studies[20,21,22,23,24,25] Theoretical analysis of this process in liquid water has shown that Zundel-type transients have an increased probability to decay via ICD14, creating 1h1h states. This is because the lighter and faster moving proton forms the Zundel-type structures more efficiently compared to the heavier deuteron within the core-hole lifetime (approximately 4 fs for O 1s25 and 6.4 fs for N 1s26)

Methods

Results

Conclusion