On the Energy-specific Photodissociation Pathways of 14N2 and 14N15N Isotopomers to N Atoms of Different Reactivity: A Quantum Dynamical Perspective

Abstract

Photodissociation of the nitrogen molecule in the vacuum ultraviolet (VUV) is a major source of reactive nitrogen atoms in the upper atmosphere of Earth and throughout the solar system. Recent experimental studies have revealed strong energy dependence of the VUV photodissociation branching ratios to the N( ^4 S _3/2 )+N( ^2 D _J ) and N( ^4 S _3/2 )+N( ^2 P _J ) product channels, the primary dissociation pathways in the 108,000–116,000 cm ^−1 energy region. This produces N( ^2 D _J ) and N( ^2 P _J ) excited atoms that differ significantly in their chemical reactivity. The branching ratios oscillate with increase in the VUV excitation energy. We use high-level ab initio quantum chemistry to compute the potential curves of 17 electronic excited states and their nonadiabatic and spin–orbit couplings. The dynamics follow the sequential evolution from the optically excited but bound ${}^{1}{{\rm{\Sigma }}}_{u}^{+}$ singlets. Spin–orbit coupling enables transfer to the dissociative triplet and quintet states. We compute the photodissociation yields through the dense manifold of electronic states leading to both exit channels. The dynamical simulations accurately capture the branching oscillations and enable a detailed look into the photodissociation mechanism. The major contribution to the dissociation is through the two lowest ^3 Π _u states. However, for both isotopomers, at about 110,000 cm ^−1 there is an abnormally low dissociation rate into the N( ^4 S _3/2 )+N( ^2 P _J ) channel that enables comparable participation of triplet ${}^{3}{{\rm{\Sigma }}}_{u}^{-}$ and quintet ^5 Π _u electronic states. This leads to the first peak in the branching ratio. At higher energies, trapping of the population in the 3 ^3 Π _u bound triplet state occurs. This favors dissociation to the lower-energy N( ^4 S _3/2 )+N( ^2 D _J ) channel and results in the observed second switch in branching ratios.

Keywords

• Cosmochemistry
• Small molecules
• Ultraviolet spectroscopy
• Isotopic abundances
• Reaction rates
• Theoretical models