Interfacial optical absorptance of air-water interfaces

The photomolecular effect has been hypothesized to enhance evaporation of water at visible wavelengths. This study develops a measurement technique to investigate its presence and magnitude at the liquid-vapor interface of water. The experiment detects surface absorptance by comparing polarized reflectance for substrates with and without a water layer. The reflectance ratio is used as an indicator of interfacial absorptance or attenuation. Lightly doped silicon and platinum were used as dielectric and metal substrates, and their pseudo-optical properties were measured using ellipsometry. Experimental results were compared to a multilayer reflectance model to determine theoretical sensitivity and guide angle selection. Measurements showed close agreement with a classical model assuming zero surface absorptance. The model was then extended to include surface absorptance using a spectral response based on Lorentzian-distributed Feibelman parameters derived from a recent estimate of 0.84% interfacial absorption. Although this model predicted a 1-2% reduction in reflectance near resonant frequencies, no such spectral features were observed experimentally. These results suggest that under ambient conditions, interfacial absorption is well below 1%, indicating that the optical signature of the photomolecular effect is either much weaker than previously reported or strongly dependent on conditions not present in this experiment.