Real-time dynamics of ammonia clusters excited through the à state: formation of the protonated cluster ions Original Research Article

Femtosecond pump-probe techniques combined with a reflectron time-of-flight mass spectrometer are employed to study the dynamics of ammonia clusters excited to the à state. In this paper we present data on the formation of protonated ammonia cluster ions and consider a reaction scheme for their formation through the dissociation of (NH3)n (Ã1A″2). We propose that the formation of the radical (NH3)n -,NH, species is due to a reaction involving the dissociation of (NH3)n (Ã1A″2) to NH, (à 2A,). A competing process in the dissociation of (NH3)n (Ã1A″2) to NH, (X 2B,), yields fragments that are too translationally “hot” to react to form the (NH3)n − 2 NH4 species. We also quantitatively model these reactions and report the lifetimes associated with the dissociation processes of (NH3)n, and concomitant formation of (NH3)n − 2NH4. Lifetimes of 250–900 fs are observed for the dissociation of (NH3)3_23 excited to the A state.