in clouds and discs.

The model of photoabsorption and photodissociation {#sec:CSE model}
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The photodissociation of N$_2$ first requires excitation into bound states that then rapidly predissociate. Thus, the photoabsorption spectrum contains well separated electronic-rovibrational lines and must be treated at high resolution ($\mathord{\sim}\!10^{-3}$Å) but over a comparatively large wavelength range (912–1000Å). Actually, absorbs strongly at considerably shorter wavelengths but the present application is limited to longwards of the atomic H ionisation threshold at 912Å. Here, a broadband and detailed spectrum is calculated by means of a quantum mechanical model which solves a coupled Schrödinger equation (CSE) for the vibrational wavefunction of the molecule. Properties of the electronic wavefunction are described by potential-energy curves and state coupling parameters optimised with respect to a large database of laboratory line positions, oscillator strengths, and predissociation rates. These empirical parts of the CSE model formulation are independent of molecular mass, and so are unchanged by isotopic substitution of one or both nuclei. Then, the explicit calculation of vibrational wavefunctions models the quantum mixing of electronic states and its dependence on the nuclear masses. A more detailed description of the method, the important electronic states and potential-energy curves used in the model is given 