Interfacial optical absorptance of air-water interfaces probed by a variable-angle spectral reflectance method

The photomolecular effect has been hypothesized to enhance water evaporation fluxes at visible wavelengths. While bulk water is transparent at these wavelengths, an interfacial optical absorption effect has been proposed to underlie the photomolecular effect. A maximum interfacial absorptance of 0.84% was reported from laser-induced evaporation experiments and the interfacial absorptance is sensitive to wavelength, polarization, and angle of incidence. The present study employs a spectrophotometric setup to directly probe the presence and magnitude of surface absorptance through the comparison of polarized reflectance for substrates with and without a layer of water on top. The reflected signal ratio is then compared with a theoretical model of the reflectance ratio to detect potential interfacial absorption or attenuation. Lightly doped silicon was used as a high-index dielectric substrate since high-quality, large-area wafers are commercially available. Platinum deposited on the silicon wafer was used as a metal substrate given its high stability and relatively high reflectivity with a moderate frequency dependence in the visible region. The pseudo-optical properties of both substrates were measured using ellipsometry. The experimental results are compared to a multilayer reflectance model to determine the theoretical sensitivity of the reflectance ratio. The measurements demonstrate close agreement with the classical model that assumes zero surface absorptance. The results suggest that under the experimental conditions used in this work, interfacial absorptance is well below 0.5%, indicating that the optical signature of the photomolecular effect is either significantly weaker than previously reported or highly dependent on specific environmental conditions not present in this experiment. The classical model is then extended to include surface absorption, which is modeled as a spectral response function utilizing Lorentzian-distributed Feibelman parameters. Although this model predicts a distinct 1–2% reduction in reflectance centered at resonant frequencies, such spectral features or localized attenuations were not observed in the experimental data.