Revisiting the intricate photodissociation mechanism of ammonia along the minor NH + H₂ pathway

In this manuscript we revisit and extend the analysis of the internal conversion and intersystem crossing dynamics of ammonia originating from excitation to its first excited singlet state S₁ and leading to the major NH₂ + H and minor NH + H₂ pathways as investigated by Wang <i>et al.</i> (<i>Phys. Chem. Chem. Phys.</i> 2022, <b>24</b>, 15060). To perform extensive simulations, we use the machine-learned interatomic potentials, developed by the same authors (Y. Wang <i>et al.</i>, <i>J. Chem. Phys.</i> 2021, <b>154</b>, 094121) interfaced with the SHARC program package. Based on the analysis of 50 000 coupled trajectories, we fit a kinetic model for the major and minor reaction channels that also provides the minimal amount of time required for the dissociation to occur. The model predicts that the time constant associated with the rare pathway is two orders of magnitude larger than that of the frequent reaction, leading to an extrapolated quantum yield of 0.64% for the NH + H₂ photoproduct. This quantum yield is in agreement with available experimental measurements carried out with an excitation pulse of 193 nm, around which we excite ammonia. A comprehensive analysis of the electronic states and structures involved in the minor channel reveals that dissociation occurs in a concerted manner <i>via</i> three main mechanisms. The majority of the trajectories (98%) undergo nonradiative relaxation to the electronic ground state, from where 29% directly dissociate. Additionally, we observe reverse internal conversion (58%) as well as intersystem crossing (10%), as operative pathways responsible of the rare photodissociation reaction. These findings provide valuable insights into the dynamics of ammonia photodissociation, particularly its less-studied fragmentation pathway.