evaluated explicitly by the CSE model and calculated predissociation fractions for $b\,{}^1\Pi_u(v=1)$ are shown in Fig. \[fig:b01 predissoc frac\] [for rotational levels with angular momentum quantum number, $J\leq 30$.]{}

![An example of isotopically-dependent predissociation. The predissociation fraction[, $\eta^\text{pre}$,]{} of individual rotational levels, $J$, of $b\,{}^1\Pi_u(v=1)$ calculated by the CSE model for and . []{data-label="fig:b01 predissoc frac"}](b01_predissoc_frac)

There is a strong dependence on rotational angular momentum, $J$, for both isotopologues. At the low temperatures of interstellar space only rotational levels with $J<10$ are accessible. Then, photons absorbed by and leading to excitation of the $b\,{}^1\Pi_u(v=1)$ level are nearly twice as likely to lead to a dissociation event than for .

We have calculated photoabsorption and photodissociation cross sections as @li2013 did for . A comparison is made in Fig. \[fig:cross sections\] between transmission functions derived from the two CSE-calculated photoabsorption cross sections. The principal difference is a shift of absorption bands to longer wavelengths which mostly increases with vibrational excitation, that is, with shortening wavelength. However, there are many further differences between the spectra of different N$_2$ isotopologues due to the strong coupling between excited states, such as depicted in Fig. \[fig:b01 predissoc frac\] with respect to the $b\,{}^1\Pi_u(v=1)$ predissociation fraction, 