Visible light leaves evaporation and interfacial structure of neat water unchanged at the air–water interface

Abstract

Visible light-induced “photomolecular” enhancement of water evaporation has been proposed as a fundamentally new mode of light–matter interaction at aqueous interfaces, with far-reaching implications for climate processes and light-driven water technologies. Yet, the response of neat interfacial water to visible photons has not been directly quantified at both macroscopic and molecular scales. Here, we combine high-precision confocal displacement measurements with a surface-specific vibrational probe that detects the OH-stretch vibrations of interfacial water molecules with subnanometer depth sensitivity, allowing us to resolve even subtle blue shifts of the hydrogen-bonded continuum and changes in the amplitude and position of the free-OH peak that report on interfacial hydrogen-bond strength and molecular orientation. This approach enables us to track both evaporation kinetics and molecular structure at the air–water boundary. Continuous 450, 532, and 635 nm illumination leaves the macroscopic evaporation rate unchanged over a wide range of humidities, while the interfacial vibrational spectra show no detectable modification of water hydrogen-bonding or molecular orientation under identical environmental conditions. To directly test a nonthermal, field-driven mechanism, we further exposed the interface to femtosecond visible and near-infrared pulses spanning 515, 800, and 1030 nm; the peak intensities were increased by more than ten orders of magnitude, and no structural change was observed. Together, these measurements show that the neat air–water interface is remarkably insensitive to visible irradiation, implying that any substantial light-enhanced evaporation is most consistent with photothermal or geometric effects in complex materials rather than from a new, nonthermal and nonresonant pathway intrinsic to liquid water.