Conformer-Sensitive Nuclear Dynamics of the Ammonia Dimer Cation Probed by Femtosecond Time-Resolved Coulomb...

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• The existence and nature of hydrogen-bonded structures in neutral ammonia dimer (NH3)2 in the gas phase remain a long-standing debate.
• Distinguishing between hydrogen-bonded and non-hydrogen-bonded configurations is crucial for understanding intermolecular interactions in weakly bound clusters.

Purpose of the Study:

• To differentiate between hydrogen-bonded and non-hydrogen-bonded neutral ammonia dimers in the gas phase.
• To investigate the structural dynamics of ammonia dimer cations after photoionization using Coulomb explosion imaging.

Main Methods:

• Photoionization of neutral ammonia dimers followed by a pump-probe experiment.
• Analysis of distinct Coulomb explosion channels: direct dissociation (NH3+ + NH3+) and indirect dissociation with proton migration (NH2+ + NH4+).
• Quantum chemical calculations and molecular dynamics simulations to interpret experimental observations.

Main Results:

• Neutral ammonia dimers with and without hydrogen bonds yield distinct Coulomb explosion patterns in their ionic counterparts.
• Non-hydrogen-bonded dimer cations primarily undergo direct dissociation.
• Hydrogen-bonded dimer cations exhibit ultrafast proton migration (~48 fs) leading to indirect dissociation, with no potential energy barrier.

Conclusions:

• The study successfully distinguishes hydrogen-bonded from non-hydrogen-bonded neutral ammonia dimers in the gas phase.
• Coulomb explosion provides a sensitive method to probe weakly bonded and fluxional structures in molecular clusters.
• The observed ultrafast proton migration in hydrogen-bonded ammonia dimer cations offers insights into dynamics of proton transfer in such systems.