Timescales for adiabatic photodissociation dynamics from the (A)over-tilde state of ammonia

Photodissociation dynamics after excitation of the (A) over tilde state nu(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)], which reported the appearance timescales for ground state NH2((X) over tilde)+H photoproducts, born from non-adiabatic passage through an (X) over tilde/(A) over tilde 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 NH2((A) over tilde)+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 NH2((A) over tilde)+H products, where nascent dissociative flux can become temporarily trapped/impeded around the upper cone of the CI on the (A) over tilde 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" NH2((X) over tilde). 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 (A) over tilde state PES with a time-constant of 113 +/- 46 fs. Subsequently, transient population encircling the CI then progresses to finally form NH2((A) over tilde)+H photoproducts from the CI region of the (A) over tilde state PES with a slower time-constant of 415 +/- 25 fs. Non-adiabatic dissociation into ro-vibrationally "hot" NH2((X) over tilde) radicals together with H-atoms is also evidenced to occur via a qualitatively similar process. (C) 2013 AIP Publishing LLC.