Vibronic transitions are drawn in figure 2 as narrow, equally spaced Lorentzian lineshapes.
Cauchy distribution, also known as the Lorentz distribution, Lorentzian function, or Cauchy–Lorentz distribution
A QPO, on the other hand, appears as a broader peak, sometimes with a Lorentzian shape.
The shape of the zero-phonon line is Lorentzian with a width determined by the excited state lifetime T10 according to the Heisenberg uncertainty principle.
R.D. Sorkin, E. Woolgar; A Causal Order for Spacetimes with C^0 Lorentzian Metrics: Proof of Compactness of the Space of Causal Curves; Classical & Quantum Gravity 13: 1971-1994 (1996); arXiv:gr-qc/9508018 (Causal Structure)
Although the only Riemannian manifold with VSI property is flat space, the Lorentzian case admits nontrivial spacetimes with this property.