Fractional composition and D/H ratios in the surface ice layers as functions of the total number of icy layers in model MC2B. The left panel represents the formation stage of a molecular cloud taken from Paper I, while the right panel represents the static and collapse stages at t_final−t< 8.3 × 10⁴ yr, where dust temperature is 10 K, in the same fluid parcel shown in Fig. 3. The visual extinction for calculating photochemical rates increases from 2 mag at the end of the cloud formation stage to 5 mag at the prestellar core stage. The sudden increase leads to the sharp change of the fractional composition especially of H₂CO and CH₃OH in the surface ice layers.