State-to-state dissociative photoionization of molecular nitrogen: the full story

Abstract

N 2 is a major constituent of Earth and planetary atmospheres. First, evidenced in 1952, the dissociative photoionization of molecular nitrogen, N 2 , plays an important role in the species abundance, out of equilibrium evolution, and chemical reactivity of diverse media including upper atmospheres (the so-called ionospheres) and plasma. Many scenarios were proposed for rationalizing the dissociative ionization mechanisms and exit channels, which are reviewed here, mainly involving the N 2 + (C 2 ∑ u + , v +) vibrational levels state-to-state dynamics on which we focus. We show, however, that previous studies are not comprehensive enough for fully shedding light on the complex undergoing processes. As a complementary global work, we used state-of-the-art quantum chemistry, time dependent and independent theoretical approaches associated to advanced experimental techniques to study the unimolecular decomposition of the N 2 + ions forming the N + + N products. In addition to the already suggested spin-orbit-induced predissociation of the cationic C 2 ∑ u + state, we documented a new mechanism based on vibronic coupling and tunneling dissociation. Besides, the quantum processes highlighted here should be also in action in the dynamics of electronically excited larger molecular systems involved in physical and chemical phenomena in plasma and in various natural environments on Earth and beyond.

Multipole-moment effects in ion–molecule reactions at low temperatures: part II – charge–quadrupole-interaction-induced suppression of the He+ + N2 reaction at collision energies below kB·10 K

“…Two recent studies have examined the predissociation of the different vibrational levels of the C state of N 2 + . 57,70 As explained in the introduction, only v ≤ 4 levels of the C state can be produced at low collision energies and, of these levels, only the v = 3 and 4 levels are located above the lowest dissociation limit of N 2 + (see inset of Fig. 1).…”

“…The radiative decay to the X 2 Σ g + ( v ′′) state is governed by Franck–Condon factors, which favour Δ v = v − v ′′ = −6 transitions. 66 The radiative lifetimes were measured to be τ rad,(C, v = 1,2) ≈ 78 ns, 65 and determined theoretically by Ayari et al 57 to be about 60 ns for the C ( v = 0–2) states. From these results, we conclude that charge transfer to the C ( v = 0–2) states exclusively leads to the formation of N 2 + and that the branching ratios for the formation of N 2 + ( η N 2 + = I (N 2 + )/[ I (N 2 + ) + I (N + )]) decrease to about 34% for the C ( v = 3) level and to about 1% for the C ( v = 4) level.…”

“…The predissociation of the C 2 Σ u + ( v = 3, 4) states most likely takes place through interactions with the 1 4 Π u state. 39,57,63,64 The radiative and predissociation lifetimes of the first seven vibrational states of the C 2 Σ u + have been measured in ref. 65.…”

“…The complex decay dynamics of N 2 + molecules excited in the energy region of the C state by charge-transfer reactions or photoexcitation has been recently discussed by Ayari et al , who have determined the radiative and predissociation lifetimes of the vibrational levels of the C state on the basis of a full set of potential energy curves obtained in high-level ab initio calculations. 57 The potential energy curves for the ground state of N 2 and of the ground and several electronically excited states of N 2 + , reproduced from ref. 57 , are presented in Fig.…”

“… 57 The potential energy curves for the ground state of N 2 and of the ground and several electronically excited states of N 2 + , reproduced from ref. 57 , are presented in Fig. 1 .…”