Nonadiabatic quantum dynamics explores non-monotonic photodissociation branching of N2 into the N(4S) + N(2D) and N(4S) + N(2P) product channels

Abstract

Vacuum ultraviolet (VUV) photodissociation of N₂ molecules is a source of reactive N atoms in the interstellar medium. In the energy range of VUV optical excitation of N₂, the N–N triple bond cleavage leads to three types of atoms: ground-state N(⁴S) and excited-state N(²P) and N(²D). The latter is the highest reactive and it is believed to be the primary participant in reactions with hydrocarbons in Titan's atmosphere. Experimental studies have observed a non-monotonic energy dependence and non-statistical character of the photodissociation of N₂. This implies different dissociation pathways and final atomic products for different wavelength regions in the sunlight spectrum. We here apply ab initio quantum chemical and nonadiabatic quantum dynamical techniques to follow the path of an electronic state from the excitation of a particular singlet ¹Σ⁺_u and ¹Π_u vibronic level of N₂ to its dissociation into different atomic products. We simulate dynamics for two isotopomers of the nitrogen molecule, ¹⁴N₂ and ¹⁴N¹⁵N for which experimental data on the branching are available. Our computations capture the non-monotonic energy dependence of the photodissociation branching ratios in the energy range 108 000–116 000 cm⁻¹. Tracing the quantum dynamics in a bunch of electronic states enables us to identify the key components that determine the efficacy of singlet to triplet population transfer and therefore predissociation lifetimes and branching ratios for different energy regions.