  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 process might be happening widely in nature,
potentially impacting climate and plants growth, and can be exploited for clean
water and drying technologies.
