A velocity map imaging study of the photodissociation of the (A)over-tilde state of ammonia

作者：Rodriguez Javier D; Gonzalez Marta G; Rubio Lago Luis; Banares Luis^*

来源：Physical Chemistry Chemical Physics, 2014, 16(2): 406-413.

DOI：10.1039/c3cp53523a

NH3((A) over tilde) 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 ((A) over tilde, v(2)' = n) <- ((x) over tilde, v = 0) vibronic transitions (denoted as 0(0)(0) and 2(0)(n) 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) over tilde) rovibrational populations for the 0(0)(0), 2(0)(1) and 2(0)(2) transitions, which are in good agreement with previous measurements. For the 2(0)(3) and 2(0)(4) transitions, some features of the KED have been assigned to rovibrational states of NH2((A) over tilde) fragments produced adiabatically. In particular, the sharp feature distinctively observed at very low kinetic energies in the H-atom KED for the 2(0)(4) transition has been undoubtedly assigned to H atoms produced in correlation with rotationally excited NH2 fragments in the (A) over tilde electronic state. For these two transitions, the analysis of the KEDs has allowed the determination of the NH2((X) over tilde, (A) over tilde) rovibrational populations and precise electronic branching ratios, Phi* = [NH2((A) over tilde)]/([NH2((X) over tilde)] + [NH2((A) over tilde)]). 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.

• 出版日期2014