Experimental and theoretical investigations of the 4ƒ^2→ 4ƒ5d ground-state and excited-state absorption spectra of Pr^3+ in LiYF_4

Near-UV excited-state absorption spectra obtained with a pulsed pump–probe technique were recorded in ${\mathrm{LiYF}}_4:{\mathrm{Pr}}^{3+}$ at 77 and at 8 K and revealed a detailed structure of the $4f^2{ }^1{D}_2\to 4f5d$ and $4f^2\hspace{0.5em}{(}^3{P}_0{+}^1{I}_6)\to 4f5d$ optical transitions. These experimental results, together with those previously obtained at room temperature, are compared with the numerical predictions of a full calculation of the $4f5d$ sublevels and of the electric dipole transitions toward these sublevels from different states of the $4f^2$ configuration. The agreement is good, provided that the four adjustable parameters of the theory are taken as $B_{20}(5d)=7290 {\mathrm{cm}}^{-1},$ $B_{40}(5d)=-14 900 {\mathrm{cm}}^{-1},$ $F_0=60 557 {\mathrm{cm}}^{-1},$ and $〈r{〉}_{\mathit{fd}}=0.245$ Å. The level structure of the $4f5d$ configuration of ${\mathrm{LiYF}}_4:{\mathrm{Pr}}^{3+}$ is shown to be dominated, as expected, by the crystal field coupling of the $5d$ electron but also, to a noticeable extent, by the spin–orbit coupling of the $4f$ electron and the Coulomb repulsion of both electrons. The former of these two interactions is somewhat more efficient than the latter in the low-energy part of the $4f5d$ configuration, i.e., in the spectral region where the largest amount of experimental data is currently available.