A velocity map imaging study of the photodissociation of the à state of ammonia

Abstract

NH₃(Ã) photodissociation dynamics has been studied using a combination of velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) of the H-atom product. H⁺ ion images have been recorded after excitation to the first five NH₃ (Ã, ν₂′ = n) ← (, ν = 0) vibronic transitions (denoted as 0⁰₀ and 2ⁿ₀ with n = 1–4). The measured high-resolution H-atom kinetic energy distributions (KED) show a dense set of sharp structures related to rovibrational states of the NH₂ co-fragment. A careful simulation of the KEDs in terms of the known internal energies of the NH₂ fragment has allowed the extraction of the non-adiabatic NH₂() rovibrational populations for the 0⁰₀, 2¹₀ and 2²₀ transitions, which are in good agreement with previous measurements. For the 2³₀ and 2⁴₀ transitions, some features of the KED have been assigned to rovibrational states of NH₂(Ã) fragments produced adiabatically. In particular, the sharp feature distinctively observed at very low kinetic energies in the H-atom KED for the 2⁴₀ transition has been undoubtedly assigned to H atoms produced in correlation with rotationally excited NH₂ fragments in the à electronic state. For these two transitions, the analysis of the KEDs has allowed the determination of the NH₂(, Ã) rovibrational populations and precise electronic branching ratios, Φ* = [NH₂(Ã)]/([NH₂()] + [NH₂(Ã)]). A speed-dependent anisotropy analysis of the H-atom images has been made for all transitions, which provides a picture of the partitioning of the available energy among the NH₂ co-product internal modes – including the electronic branching ratios – in terms of a roaming-like mechanism.