Timescales for adiabatic photodissociation dynamics from the |${\rm \tilde A}$|$Ã$ state of ammonia

Photodissociation dynamics after excitation of the |${\rm \tilde A}$|$Ã$ state ν′₂ = 4 (umbrella) level of ammonia are investigated using ultrafast time-resolved velocity map ion imaging (TR-VMI). These studies extend upon previous TR-VMI measurements [K. L. Wells, G. Perriam, and V. G. Stavros, J. Chem. Phys. 130, 074308 (2009)] https://doi.org/10.1063/1.3072763, which reported the appearance timescales for ground state |${\rm NH}_2 {\rm (\tilde X)} + {\rm H}$|$NH 2(X̃)+H$ photoproducts, born from non-adiabatic passage through an |${\rm \tilde X/\tilde A}$|$X̃/Ã$ state conical intersection (CI) at elongated H–NH₂ bond distances. In particular, the present work sheds new light on the formation timescales for electronically excited |${\rm NH}_2 {\rm (\tilde A)} + {\rm H}$|$NH 2(Ã)+H$ species, generated from NH₃ parent molecules that avoid the CI and dissociate adiabatically. The results reveal a step-wise dynamical picture for the production of |${\rm NH}_2 {\rm (\tilde A)} + {\rm H}$|$NH 2(Ã)+H$ products, where nascent dissociative flux can become temporarily trapped/impeded around the upper cone of the CI on the |${\rm \tilde A}$|$Ã$ state potential energy surface (PES), while on course towards the adiabatic dissociation asymptote – this behavior contrasts the concerted mechanism previously observed for non-adiabatic dissociation into H-atoms associated with ro-vibrationally “cold” |${\rm NH}_2 {\rm (\tilde X)}$|$NH 2(X̃)$⁠. Initially, non-planar NH₃ molecules (species which have the capacity to yield adiabatic photoproducts) are found to evolve out of the vertical Franck-Condon excitation region and towards the CI region of the |${\rm \tilde A}$|$Ã$ state PES with a time-constant of 113 ± 46 fs. Subsequently, transient population encircling the CI then progresses to finally form |${\rm NH}_2 {\rm (\tilde A)} + {\rm H}$|$NH 2(Ã)+H$ photoproducts from the CI region of the |$\tilde{\rm A}$|$Ã$ state PES with a slower time-constant of 415 ± 25 fs. Non-adiabatic dissociation into ro-vibrationally “hot” |${\rm NH}_2 {\rm (\tilde X)}$|$NH 2(X̃)$ radicals together with H-atoms is also evidenced to occur via a qualitatively similar process.