[Paper Review] Photomolecular Effect: Visible Light Absorption at Water-Vapor Interface

Yaodong Tu, Gang Chen|arXiv (Cornell University)|Feb 22, 2022

Solar-Powered Water Purification Methods4 citations

TL;DR

This paper proposes the photomolecular effect, a novel mechanism where visible light is directly absorbed at the water-vapor interface through the cleavage of water clusters, enabling efficient photothermal evaporation without external absorbers. Despite weak bulk absorption, the effect is demonstrated via wavelength, angle, and polarization dependence of evaporation rates, with spectral signatures in the vapor phase supporting the mechanism, suggesting widespread implications for atmospheric processes and solar evaporation technologies.

ABSTRACT

The evaporation of water is ubiquitous in nature and industrial technologies. The known mechanism for evaporation is "thermal evaporation" which highlights the energy input for evaporation is via heat. Due to the weak absorption of water to visible light, the first step to using solar energy to evaporate water is usually by converting it into thermal energy through photothermal processes via additional absorbing materials. Contrary to this conventional wisdom, we report here strong absorption of photons in the visible spectrum at the water-vapor interface by direct cleavage of water clusters via a process we call photomolecular effect. We show that this process happens at the water-vapor interface by measuring the dependence of the photomolecular evaporation rate on the wavelength, the angle of incidence, and the polarization of the incident light. The spectra signatures in the vapor phase further support the photomolecular effect. Despite the long propagation lengths of visible light in bulk water, we demonstrate that they can heat a thin layer of fog easily, suggesting that this process is ubiquitous. The photomolecular effect will have significant implications for the earth's water cycle, global warming, plant transpiration, as well as different technologies involving the evaporation of liquids from drying to power generation

Motivation & Objective

• To investigate whether visible light can directly drive water evaporation at the water-vapor interface through a previously unrecognized mechanism.
• To challenge the conventional paradigm that solar evaporation requires photothermal conversion via added absorbers due to water's weak visible light absorption.
• To explore the physical origin of enhanced evaporation under visible light illumination at the air-water interface.
• To determine whether the observed effects are due to interfacial photophysics rather than bulk optical properties or experimental artifacts.
• To assess the implications of this effect for natural processes like the water cycle and technological applications such as solar evaporation and power generation.

Proposed method

• Conducted experiments measuring evaporation rates of water under visible light illumination at varying wavelengths, angles of incidence, and polarization states.
• Used precise mass measurements of evaporating water droplets to quantify the photomolecular effect, comparing results under different light conditions.
• Analyzed the vapor phase using spectroscopic techniques to detect molecular fragments and signatures indicative of cluster cleavage.
• Performed control experiments to rule out thermal convection or conduction effects as primary drivers of observed evaporation enhancements.
• Re-evaluated data after identifying systematic errors in initial balance readings due to oblique light incidence, leading to revised experimental protocols.
• Validated findings with new experiments, confirming the photomolecular effect persists despite earlier measurement artifacts.

Experimental results

Research questions

• RQ1 Can visible light directly induce water evaporation at the water-vapor interface through a mechanism other than thermal evaporation?
• RQ2 What is the role of water cluster structure at the interface in enabling visible light absorption and dissociation?
• RQ3 How do the wavelength, angle of incidence, and polarization of light affect the evaporation rate, and what do these dependencies reveal about the underlying mechanism?
• RQ4 Are the observed spectral signatures in the vapor phase consistent with the cleavage of transient water clusters?
• RQ5 To what extent does this effect occur in natural and engineered systems, such as fog or solar evaporation devices?

Key findings

• The photomolecular effect enables strong absorption of visible light at the water-vapor interface, despite bulk water's weak absorption in this range.
• Evaporation rates increase significantly under visible light illumination, with dependence on wavelength, angle of incidence, and polarization, indicating a directional and selective interfacial process.
• Spectral signatures in the vapor phase support the cleavage of water clusters during irradiation, providing direct evidence for the proposed mechanism.
• Despite long propagation lengths of visible light in bulk water, the effect is highly localized at the interface, enabling efficient heating of thin fog layers.
• The phenomenon is robust and reproducible, with new experiments confirming the effect after correcting for initial measurement artifacts related to light incidence angle.
• The photomolecular effect has broad implications for atmospheric science, including the Earth's water cycle, global warming, and plant transpiration, as well as for solar energy harvesting technologies.