The measurements in Fig. were taken under similar conditions as those in Fig. , however, the measurements in Fig. were taken with X-rays reflected at a smaller angle of incidence, one which is below the angle of total reflection. In this case, X-rays are totally reflected from the surface at a wave vector transfer of Qz = 0.018 Å -1 . Total reflection creates an evanescent wave of X-rays whose penetration depth into the water is 10 nm, which is 280 times smaller than the penetration depth when Qz = 0.025 Å -1 (above the angle of total reflection) used for the measurements in Figs. and. Greater penetration of X-rays into the water exposes a greater volume of water to X-rays and produces a larger decay of Nd surface density for pH ≤ 3.0, as illustrated by comparing Fig. to Fig. . This suggests that the radiolysis of water by X-rays produces species that interact with DHDP to release Nd ions. The larger effects observed at lower pH in Fig. are consistent with previous reports of higher yields of radiolytic species in acidic media than in alkaline media. The XFNTR measurements shown in Fig. and Fig. were measured from the same sample for a given value of pH. Measurements in Fig. were taken first from one spot of the sample surface, then the beam was moved to another spot for the measurements in Fig. , with a time delay of about 2 minutes. Control experiments showed that the second spot was far enough from the first to be unaffected by direct X-ray exposure of the first spot. The near equivalence of the final values of surface density in Fig. to the initial values in Fig. shows that the temporal evolution in Fig. was due to X-ray exposure that affected the entire sample, not just the part of the sample within the X-ray footprint. Ozone produced by X-rays in the vapor above the sample surface might be responsible for the changes observed in Fig. , despite attempts to purge oxygen from the vapor by a helium flow. These effects were not observed at lower X-ray exposure (Fig. ).