Vibrationally mediated photodissociation of ammonia: product angular distributions from adiabatic and nonadiabatic dissociation

Abstract

Velocity map ion imaging of H atoms from the photodissociation of vibrationally excited ammonia molecules measures the angular distribution of the fragments from dissociation of the excited state symmetric N–H stretch (11) and the antisymmetric N–H stretch (31). For the symmetric stretching state, the high rotational energy NH2 fragments recoil with an angular distribution whose maximum is parallel to the polarization direction of the lasers, and the low rotational energy fragments recoil with a perpendicular angular distribution. This behavior is similar to that observed for photodissociation from the origin (00) but with a larger range in the anisotropy parameter (β 2) caused by additional alignment in the two-photon excitation. The dissociation through the excited state antisymmetric N–H stretch produces primarily electronically excited products (). The initial alignment of molecules in the antisymmetric stretching state is a combination of perpendicular alignment from the vibrational excitation and parallel alignment from the electronic excitation. Fitting the more complicated angular distributions for the channel requires a higher order term (β 4) that also varies with recoil energy to describe the angular distribution.