Interfacial optical absorptance of air-water interfaces

TL;DR

This study develops a measurement method to detect interfacial optical absorptance at the water surface, finding that under ambient conditions, absorption is negligible and the photomolecular effect is likely very weak or condition-dependent.

Contribution

It introduces a novel experimental approach to quantify interfacial absorptance at water surfaces and compares results with theoretical models including the photomolecular effect.

Findings

Interfacial absorption is below 1% under ambient conditions.

No spectral features indicative of the photomolecular effect were observed.

Experimental results align with classical models assuming negligible surface absorptance.

Abstract

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.