What happens when electrons return to europium?

• A. Ultraviolet light emitted
• B. Green light (300-500 nm) emitted
• C. No light emitted
• D. Red light emitted

Answer: (B)

When europium ions light up, it’s because excited electrons drop to lower energy levels and release the extra energy as photons. For europium in the common luminescent state, the relevant transition is from the higher 4f65d1 level down to the 4f7 ground configuration. This transition is allowed and forms a broad emission band that typically lands in the blue-green part of the spectrum, so the light you see is green rather than UV or red. The exact hue can shift with the host material, but green is a characteristic outcome for this Eu2+-related relaxation. Red emission would come from Eu3+ transitions, which are different and much narrower, and ultraviolet emission would require energy gaps outside this typical 4f–5d relaxation. No light would occur only if the excited state relaxed nonradiatively, which isn’t the usual case here.