Theoretical (a) and experimental (b) linear absorption spectra for dilute HDO in H2O for membrane hydration levels X = 2 (red), 4 (orange), 6 (yellow), 8 (green), 16 (blue), and bulk water (purple). The theoretical spectra were calculated from Eq. 2.2 with vibrational lifetimes given in Table 2, and the experimental spectra were adapted from Fig. 2 of Ref. 33.