Evolution of the normalized m/z = 4 ion current (black dots), equivalent to the normalized D₂ flux at the x = l surface, during (left panel) and after (middle and right panels) VUV irradiation of a 2.6 μm a-C:D analog at 130 K. Theoretical models fitting the experimental data are presented (blue solid lines), along with the derived diffusion coefficient D and the 1σ associated error. The model shown in the left panel is described by Eq. (6). The middle and right panels show the models described by Eqs. (11) and (12), respectively. The free parameter p of the latter is an indicator of how close the model is to the one described by Eq. (11). The χ² values associated with the fits of the experimental data during and after irradiation cannot be directly compared since they are not normalized.