Site-selective laser spectroscopy of ${\mathrm{Pr}}^{3+}$${\mathit{C}}_{4\mathit{v}}$ symmetry centers in hydrogenated ${\mathrm{CaF}}_2$:${\mathrm{Pr}}^{3+}$ and ${\mathrm{SrF}}_2$:${\mathrm{Pr}}^{3+}$ crystals

Laser-selective excitation and optical-absorption techniques have been used to establish comprehensive sets of energy levels for both the ${\mathit{F}}^{\mathrm{-}}$ and hydrogenic varieties of the tetragonal symmetry (${\mathit{C}}_{4\mathit{v}}$) centers in ${\mathrm{SrF}}_2$ and ${\mathrm{CaF}}_2$. Polarization of the various spectral lines was used to determine the symmetry labels of many of the energy levels. Reversals of the ordering of some levels between ${\mathrm{SrF}}_2$ and ${\mathrm{CaF}}_2$ are found and, in particular, the two lowest levels (${\mathit{D}}_1$ and ${\mathit{D}}_2$) of the ${}_{}{}^1{}_{}{}^{}$ ${\mathit{D}}_2$ multiplet, which are the originating levels for many of the observed emission transitions, have reversed-symmetry label assignments in ${\mathrm{SrF}}_2$ and ${\mathrm{CaF}}_2$. Differences were found between the ground-state Zeeman splittings for the ${\mathit{C}}_{4\mathit{v}}$ ${\mathit{F}}^{\mathrm{-}}$ and ${\mathit{D}}^{\mathrm{-}}$ centers for both ${\mathrm{SrF}}_2$ and ${\mathrm{CaF}}_2$. Fluorescence lifetimes of the ${\mathit{C}}_{4\mathit{v}}$ hydrogenic centers show a strong dependence on the particular charge-compensating ion present through nonradiative decay processes involving the respective local mode phonons. The upconversion spectra observed for ${\mathit{F}}^{\mathrm{-}}$, ${\mathit{T}}^{\mathrm{-}}$, and ${\mathit{D}}^{\mathrm{-}}$ varieties of the ${\mathit{C}}_{4\mathit{v}}$ centers in ${\mathrm{SrF}}_2$ and for the ${\mathit{C}}_{4\mathit{v}}$ ${\mathit{F}}^{\mathrm{-}}$ center in ${\mathrm{CaF}}_2$ are all consistent with excitation of specific subsets of nearby ${\mathit{C}}_{4\mathit{v}}$ centers.