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) ← ([X with combining tilde], 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([X with combining tilde]) 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([X with combining tilde], Ã) rovibrational populations and precise electronic branching ratios, Φ* = [NH2(Ã)]/([NH2([X with combining tilde])] + [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.