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(4S3/2)+N(2D J ) and N(4S3/2)+N(2P J ) product channels, the primary dissociation pathways in the 108,000–116,000 cm−1 energy region. This produces N(2D J ) and N(2P 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 Σ 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 3Π u states. However, for both isotopomers, at about 110,000 cm−1 there is an abnormally low dissociation rate into the N(4S3/2)+N(2P J ) channel that enables comparable participation of triplet 3 Σ u − and quintet 5Π u electronic states. This leads to the first peak in the branching ratio. At higher energies, trapping of the population in the 33Π u bound triplet state occurs. This favors dissociation to the lower-energy N(4S3/2)+N(2D J ) 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

“…They are accessed by spin–orbit coupling from the singlet states. − The states of given symmetry and multiplicity are mixed by strong nonadiabatic terms . The considerable spatial and state selectivity of the spin–orbit coupling; see, for example refs and , is displayed for N 2 in ref and here in Scheme .…”

“…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 Each triplet 3 Π u state is degenerate because of its spin multiplicity and its 2-fold spatial degeneracy due to its cylindrical symmetry about the molecular axis. Of the three degenerate spin states only two, m S = +1 and m S = −1, can be relevant here because the spin−orbit coupling from the singlet 1 Σ u + states does not access the state of zero magnetic quantum number.…”

“…The Supporting Information provides access to movies showing the time evolution of the density of the vibrational state in two different exit channels that are also shown for two time points in Figure 2. Experiments and also our simulations forward in time 32 demonstrate that the branching fraction into the product channels varies nonmonotonically with the total energy. Figure 3 proves that this is equally the result when we integrate backward.…”

“…The VUV photodissociation of N 2 is indirect and proceeds through the triplet and quintet electronic states. ,,,− 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 A). This set of states was defined based on our recent computational simulation 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 A, respectively. The triplet 3 Π u states have been known to be the primary channels for the photodissociation of the N 2 molecule. ,, 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 …”

“…In the energy range from 108,000 to 116,000 cm –1 both dissociation channels I and II are open. The first peak into the channel II is at 110,144 cm –1 and it is related to a very long predissociation lifetime of the singlet state , which allows the triplet 1 3 Σ u – and quintet 2 5 Π u states to contribute to the photodissociation. According to selection rules, 1 Σ u + and 3 Σ u – are coupled by the LSZ spin–orbit term; see Scheme .…”