Quantitative correlation of 1 + Δ𝑅∕𝑅 with the variations in the thickness of surface films (𝛿) stems from Fresnel equations, details of which are available in other sources. In brief, correlating such measurements necessitates information on the refractive index (𝑛) of the surface films which can be estimated based on their known chemical composition, gathered from the literature, and are discussed in the subsequent sections of the main text. In this work the refractive indices of all used species are 𝑛 𝐻 2 𝑂 = 1.33, , 𝑛 𝐹 𝑒 = 1.95 + 3.04i 𝑛 𝐶𝑟 2 𝑂 3 = 2.55 𝑛 𝐹 𝑒 2 𝑂 3 = 2.94 + 0.23i .