The phosphor BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺, whose excitation and emission spectra are shown below, is of interest as a combined blue and green phosphor in plasma-display panels. The Eu²⁺ ions absorb at 336 nm and emit in the blue with a maximum of 450 nm. There is also energy transfer from Eu²⁺ to Mn²⁺ that leads to the green emission at 512 nm.

(a) What are the electronic transitions responsible for emission on Eu²⁺ and Mn2+?

(b) What is the Stokes shift for Eu²⁺?

(c) What is the most likely mechanism of energy transfer from Eu²⁺ to Mn²⁺, Förster (FRET) or Dexter?

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intensity (arb. units) 200 Eu+ excitation 300 Eu+ emission Mn+ 400 500 wavelength, (nm) 600