Illustration of the layered model of Section 3.1, used to determine effective photon exposure in mixed ices, is shown in panel a. In this case, the ice consists of two components, CO and ethanol. The effective flux is considered to be the average of the flux incident on each layer. Panels b-c show the result of applying this model to our samples by comparing the ratio of the effective values to the zeroth-order values for the flux ${\phi }_0^{eff}$ and fluence ϕ_avg.