Spin-allowed and spin-forbidden4fn↔4fn−15dtransitions for heavy lanthanides in fluoride hosts

Luminescence spectra of lanthanides in the vacuum ultraviolet spectral region have revealed interesting features for ${4f}^n\leftrightarrow {4f}^{n-1}5d$ $\mathrm{}(f-d)\mathrm{}$ transitions. For ${\mathrm{Tb}}^{3+},$ ${\mathrm{Dy}}^{3+},$ ${\mathrm{Ho}}^{3+},$ ${\mathrm{Er}}^{3+},$ and ${\mathrm{Tm}}^{3+}$ in ${\mathrm{LiYF}}_4,$ weak $f-d$ bands are present in the excitation spectra at lower energy than the usually observed strong $f-d$ bands. The weakness of the bands is explained by the spin-forbidden character of transitions from the ${4f}^n$ ground state to these ${4f}^{n-1}5d$ states. For ${\mathrm{Er}}^{3+}$ and ${\mathrm{Tm}}^{3+},$ upon excitation in the higher energetic ${4f}^{n-1}5d$ states two kinds of $f-d$ emission are observed: fast spin-allowed emission with a decay time in the order of ns, and slow spin-forbidden emission with a decay time in the order of μs. These two types of $f-d$ emission are also found for ${\mathrm{Er}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ in other fluoride hosts.