$c'_{n+1}{}^1\Sigma^+_u$ states are indistinguishable at the plotted scale. The ionisation potential energy occurs at 15.4eV in this figure.[]{data-label="fig:N2potentials"}](all_potentials){width="0.9\hsize"}

Predissociation occurs through spin-orbit coupling of ${}^1\Pi_u$ states with a number of states with ${}^3\Pi_u$ symmetry, including an unbound level, and one state of ${}^3\Sigma_u^+$ symmetry. These are shown in Fig. \[fig:N2potentials\] and lead to dissociation into N atoms in the ${}^4S$, ${}^2D$, or ${}^2P$ states. The spin-selection rules forbid the appearance triplet states in optical spectra although many have been observed in the laboratory regardless, due to their coupling with ${}^1\Pi_u$ levels [@sprengers_etal2005b; @lewis_etal2008a]. The majority of excited vibrational states predissociate after an absorption event with near certainty, that is, they have a predissociation fraction of $\mathord{\sim}1$. [For those states that decay substantially by emission, a reduction of the CSE-calculated photoabsorption cross section, $\sigma^\text{abs}$, has been made by their less-than-unity predissociation fractions, $\eta^\text{pre}$, leading to a photodissociation cross section $\sigma^\text{pd}$, so that $$\sigma^\text{pd} = \eta^\text{pre} \sigma^\text{abs}.$$ ]{} This provides an example of the utility of the physically-based CSE model. That is, the $b\,{}^1\Pi_u(v=1)$ level of is known to predissociate significantly less rapidly than for [@sprengers_etal2004b; @sprengers_etal2005; @wu2011] because of mass-change-induced shifts of the interacting ${}^1\Pi_u$ and ${}^3\Pi_u$ levels. These shifts are 