Photomolecular Effect Leading to Water Evaporation Exceeding Thermal Limit*

We report the discovery of photomolecular effect: cleavage of water clusters off surfaces by photons. This effect is demonstrated through surprising absorption of partially wetted hydrogel in the visible spectrum where both water and hydrogel materials’ absorption are negligible. Illumination of hydrogel under solar or visible-spectrum light-emitting-diode leads to evaporation rates exceeding the thermal evaporation limit, even in hydrogels without additional absorbers. Measurements of temperature and transmission spectrum of vapor above evaporating surfaces show clear signatures of water clusters. The photomolecular effect happens at liquid-vapor interface due to large electrical field gradients and quadrupole force on molecular clusters. This photomolecular evaporation processes might be happening widely in nature, potentially impacting climate and plants growth, and can be exploited for clean water and drying technologies.