The molecular structure of nanoscopic interfacial water layers is changed by irradiation with 670 nm laser light, a wavelength to which bulk water is practically transparent. Whereas the phenomenon was confirmed in several experimental studies, its intrinsic cause was not clear. We use X-ray spectroscopy to analyze nanoscopic interfacial water layers on nanocrystalline diamond films and demonstrate that the structural change is due to collective hydrogen bond excitation in the water layers. K-edge X-ray fluorescence yield reveals a change in H2O⋯H2O orbital hybridization upon light exposure. The corresponding electronic picture is indicative of a density drop in the condensed-phase layer. The associated volume expansion is relevant to systems involving nanoconfined liquids and may inspire innovative design principles both in nanotechnology and nanomedicine. Potential applications range from anticancer strategies to the design principles of nano steam engines.