alignment of the $b\,{}^1\Pi_u(v=3)\leftarrow X\,{}^1\Sigma_g^+(v''=0)$ absorption band of and the line-centre of the shielding $4p\leftarrow 1s$ transition of atomic hydrogen. The relevant , , and H features are plotted in Fig. \[fig:cross sections\] near 972Å. For a larger H column, like that in Fig. \[fig:cross sections\], with heavily saturated and broadened H lines, the misalignment of H and transitions near 972Å loses its significance.

Also shown in Fig. \[fig:shielding functions\] is the shielding of photodissociation by . This rivals self-shielding because of the much greater abundance of . This is particularly true at higher temperatures where the wavelength offset between isotopologues is compensated for by their broader rotational distributions and Doppler widths. A discussion of the relative importance of different species in mixed shielding media is given in Sec. \[sec:photochemical models\] with respect to an example protoplanetary disc.

[ Shielding functions calculated according to Eq. (\[eq:def shielding function\]) describe the reduction in photodissociation rate due to the attenuation of unidirectional radiation. A related quantity, $\theta_\text{iso}(N)$, describes this reduction as a function of column density for an extended medium illuminated isotropically at its surface. The resultant non-perpendicular rays penetrate less deeply and $\theta_\text{iso}(N)$ is smaller than the shielding function in the 