Spin-allowed and spin-forbiddenfdemission fromEr3+andLiYF4

Luminescence measurements in the vacuum ultraviolet of ${4f}^{11}\to {4f}^{10}5d$ transitions on ${\mathrm{Er}}^{3+}$ in ${\mathrm{LiYF}}_4$ have shown the presence of a weak $\mathrm{fd}$ band. The weakness of the band is explained by the spin-forbidden character of transitions from the ground state to this $\mathrm{fd}$ state, which is at lower energy than the spin-allowed $\mathrm{fd}$ bands. Upon excitation in the spin-allowed $\mathrm{fd}$ bands, both fast spin-allowed $\mathrm{fd}$ emission (τ=5 ns) and slow spin-forbidden $\mathrm{fd}$ emission (τ>2 μs) are observed. These results can be understood with a configurational coordinate diagram analogous to the Jablonski diagrams that are used to explain the fluorescence and phosphorescence from organic molecules.