Site-selective laser spectroscopy of Pr3+ C4v symmetry centers in hydrogenated CaF2 : Pr3+ and SrF2 : Pr3+ crystals

Abstract

Laser-selective excitation and optical-absorption techniques have been used to establish comprehensive sets of energy levels for both the ${\mathit{F}}^{\mathrm{\ensuremath{-}}}$ 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 ${\mathit{C}}_{4\mathit{v}}$ ${\mathit{F}}^{\mathrm{\ensuremath{-}}}$ and ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ 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 ${\mathit{F}}^{\mathrm{\ensuremath{-}}}$, ${\mathit{T}}^{\mathrm{\ensuremath{-}}}$, and ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ varieties of the ${\mathit{C}}_{4\mathit{v}}$ centers in ${\mathrm{SrF}}_{2}$ and for the ${\mathit{C}}_{4\mathit{v}}$ ${\mathit{F}}^{\mathrm{\ensuremath{-}}}$ center in ${\mathrm{CaF}}_{2}$ are all consistent with excitation of specific subsets of nearby ${\mathit{C}}_{4\mathit{v}}$ centers.