Isotope Effects in the Predissociation of Excited States of N2+ Produced by Photoionization of 14N2 and 15N2 at Energies Between 24.2 and 25.6 eV

Abstract

Photoelectron/photoion imaging spectrometry employing dispersed VUV radiation from the SOLEIL synchrotron has been used to study the predissociation of N 2 + states located up to 1.3 eV above the ion's first dissociation limit. Branching ratios for unimolecular decay into either N 2 + or N + were obtained by measuring coincidences between threshold electrons and mass-selected product ions, using a supersonic beam of either 14 N 2 or 15 N 2 as photoionization target. The results confirm that predissociation of the state of 14 N 2 + is faster than emission to the electronic ground-state by a factor 10 or more for all vibrational levels v′ ≥ 3, while for 15 N 2 + the two decay modes have comparable probabilities for the levels v′ = 3, 4, and 5. In contrast, no significant isotope effect could be observed for the other states of N 2 + identified in the photoelectron spectrum. For both 14 N 2 + and 15 N 2 + isotopologues all vibrational levels of these other states decay to an extent of at least 95% by predissociation.

Bond-forming and electron-transfer reactivity between Ar2+ and N2

“…To interpret this exoergicity, we must consider previous studies of N 2 + dissociation. As noted above, the dissociation threshold, corresponding to the lowest energy N + + N asymptote, L1 (N + ( 3 P) + N( 4 S 0 ), Table 3), lies at B24.3 eV above the molecular ground state and corresponds to N 2 [71][72][73] At energies above the second dissociation limit, L2 (N + ( 1 D) + N( 4 S 0 ), B26.2 eV, Table 3), there is competition between dissociation to L1 and L2. 88,90,91 In studies of N 2 excitation, at the energies involved in the processes we see in our experiment (24.3-32 eV), the C 2 S u + state is the dominant state populated in photoelectron spectra and dissociation to the three lowest energy dissociation asymptotes is observed, with lifetimes of the order of nanoseconds.…”

“…26,66,67 There are two energetically accessible electronic states for the Ar + product ( 2 P and 2 S). 68 The reactant N 2 molecule, admitted as an effusive beam, will be in its ground vibronic state, X 1 68,71 Photoionisation studies have shown that N 2 + states generated with a higher internal energy than 24.3 eV have dissociation lifetimes less than the timescale of our experiment and therefore will not contribute to the N 2 + counts observed in this channel. [71][72][73][74] From the above energetic considerations, we find that there are four possible ND-SET reaction pathways that match the range of exergicities 69,70 shown in Fig.…”

“…69 The competition between fluorescence and predissociation has been studied in depth for N 2 + (C 2 S u + ). 71,81,82 Predissociation dominates over fluorescence when N 2 + is formed with more energy than the lowest energy dissociation asymptote (24.3 eV). [82][83][84] However, predissociation of the C state will not generate counts in this ND-SET channel, but instead contributes to the counts in the DSET channel, Rxn II., as discussed below.…”

“…S1, ESI †) we note that, as previously discussed, if N 2 + is formed with an energy of over 24.33 eV relative to the ground state of N 2 (equivalent to N 2 + (C 2 S u + v = 3)) and does not fluoresce, it will dissociate within the lifetime of our experiment and therefore can contribute the DSET channel. [71][72][73][74] The maximum energy that can be released from Ar 2+ ( 1 S) accepting an electron to form the ground state monocation, Ar + ( 2 P), is 31.75 eV. Therefore, the maximum exoergicity in this channel is 7.42 eV if we restrict ourselves to the p 4 states of Ar 2+ .…”

Depletion and fractionation of nitrogen in collapsing cores

“…and has attracted much interest because the energy needed to effect the transition N 2 (X 1 Σ + g ) → N + 2 (C 2 Σ + u ) nearly equals the helium ionization potential (see Hrodmarsson et al 2019, and references therein). In the UGAN network, the total rate constant is 1.2 × 10 −9 cm 3 s −1 and the branching ratio between the dissociative and non-dissociative channels is 1.94, for both N 2 isotopologs.…”