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 reveal strong energy dependence of the VUV photodissociation branching ratios to N(4S3/2)+N(2DJ) and N(4S3/2)+N(2PJ) product channels, the primary dissociation pathways in the 108,000-116,000 cm-1 energy region. This produces N(2DJ) and N(2PJ) 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 non-adiabatic and spin-orbit couplings. The dynamics follow the sequential evolution from the optically excited but bound 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 states. However, for both isotopomers, at about 110,000 cm-1 there is an abnormally low dissociation rate that enables comparable participation of triplet and quintet electronic states. This leads to the first peak in the branching ratio. At higher energies, trapping of the population in the bound triplet state occurs. This favors dissociation to the lower energy N(4S3/2)+N(2DJ) channel and results in the observed second switch in branching ratios.

Recombination of N Atoms in a Manifold of Electronic States Simulated by Time-Reversed Nonadiabatic Photodissociation Dynamics of N2

“… 27 − 29 The states of given symmetry and multiplicity are mixed by strong nonadiabatic terms. 30 The considerable spatial and state selectivity of the spin–orbit coupling; see, for example refs ( 2 and 31 ), is displayed for N 2 in ref ( 32 ) and here in Scheme 1 .…”

“… 3 , 24 , 27 , 36 − 38 We here consider three optically accessible bound singlet 1 Σ u + and six dissociative states: one 3 Σ u – , four 3 Π u and one 5 Π u electronic states ( Figure 1 A). This set of states was defined based on our recent computational simulation 32 of the experimentally measured branching fractions into N( 4 S 3/2 ) + N( 2 D J ) and N( 4 S 3/2 ) + N( 2 P J ) product channels shown with the blue and orange arrows in Figure 1 A, respectively. The triplet 3 Π u states have been known to be the primary channels for the photodissociation of the N 2 molecule.…”

“… 27 , 36 , 37 A special case is at an excitation energy of 110,144 cm –1 where the dissociation of the triplet 3 Π u states is particularly slow and the contribution of the 1 3 Σ u – and 2 5 Π u electronic states is found to be dominant. 32 …”

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

“…7(a), the population of the triplets 3 P u is very low and so even other low populated states become important. In a previous study, 73 we proposed that the reason of this low population is the notably long lifetime of the excited singlet state.…”

“…[69][70][71][72] We recently computed a realistic energy dependence in the dissociation branching arising from excitation of different single vibrational levels of 1 S + u electronic states of N 2 . 65,73 These computations started from the molecule in the ground electronic state and excited it by a polarized laser pulse of a very long duration and of the required mean energy. In the present work we make a step further and present the results of the branching into the channels N( 4 S) + N( 2 D) (Channel 1) and N( 4 S) + N( 2 P) (Channel 2) accessed through the excited 1 P u states of N 2 .…”