The Chemistry of Euro banknotes
Europium(III) oxide, alongside other chemicals, is phosphorescent and is used in the anti-counterfeiting details in Euro banknotes.
Most of the trivalent rare earth elements are luminescent. This means that they can be excited by shining a light of a particular wavelength at them. When the ions relax again, they emit light – of a different wavelength. That is luminescence.
Euro notes luminesce in the red, green and blue (excited by 254 nm). The red light is clearly linked to europium and most likely to a Eu3+-β-diketone complex.