The water window

The water window is the wavelength range between the K-absorption edges of oxygen
(l = 2.34 nm, hn = 532 eV) and carbon (l = 4.38 nm, hn = 283 eV). Due to the presence of this absorption edges a natural contrast within the water window is available between water and biological material containing carbon, e.g. proteins (see picture). The relative transparency of water makes this an attractive region for investigations on organic molecules in their natural aqueous environment, for instance with soft X-ray microscopy or soft X-ray absorption spectroscopy.