78
3 Vibrational Up-Pumping: Predicting Impact Sensitivity of Some …
Fig. 3.5 Potential energy surfaces (PES) associated with the N
−
3 anion. PES are shown for a elongation of a single N…N covalent bond, and the three symmetry independent normal modes:
b δ θ NNN , c δR S , and d δR A ; r 1 = (r eqm + α/10), r 2 = (r eqm − α/10), where r eqm is the equilibrium
bond distance. In each case the potential energy surface for S 0 (black), S 1 (red), S 2 (blue), T 1 (pink)
and T 2 (green) are given. All energies are normalized to the S 0 equilibrium energies. Figure from
Ref. [1], https://doi.org/10.1021/acs.jpcc.8b05285. Copyright 2018 American Chemical Society
of N
−
3 . For the isolated anion, these include two degenerate bending modes (δθ NNN ), a
single symmetric stretch (δR S ), and an asymmetric stretching mode (δR A ). Variations
in the electronic structure of N
−
3 were studied as a function of these normal modes,
Fig. 3.5b–d.
At the lowest frequency, δθ NNN is most responsive to mechanical perturbation. As the anion bending angle θ NNN deviates from 180
◦ , the energy of
the ground state species increases until an apparent plateau is achieved at ca.
110
◦ , Fig. 3.5b. At this plateau, an overall increase in the internal energy,
U, of 3.9 eV is observed. In contrast, as θ NNN decreases, the energy of the
S 1 state decreases, reaching its minimum energy at approximately 140
◦ , with
S1 E(θ N N N = 180
◦
) −
S1 E(θ N N N = 140
◦
) ≈ 1.1 eV. At this angle, the energy separation between the S 0 and S 1 state decreases from ca. 5.3 eV to only ca. 3.0 eV. This