Site-selective spectroscopy of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Nd</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>centers in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CaF</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>:<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Nd</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">SrF</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>:<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Nd</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>

Laser-selective-excitation and optical-absorption techniques have been used to establish comprehensive sets of energy levels of the ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ and hydrogenic varieties of the tetragonal symmetry (${\mathit{C}}_{4\mathit{v}}$) centers in ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$, of three multihydrogenic centers observed in both ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ and of the M, N, and M' ${\mathrm{Nd}}^{3+}$ cluster centers present in ${\mathrm{CaF}}_{2}$. Polarization of the various spectral lines of the ${\mathit{C}}_{4\mathit{v}}$ symmetry centers was used to determine the symmetry labels of all energy levels of these centers up to the $^{4}$${\mathit{F}}_{3/2}$ multiplet, for which crystal-field fits are reported. A noteworthy feature of the ${\mathrm{Nd}}^{3+}$ centers is their strong upconversion fluorescence giving a visual perception of ``white'' emission under selective laser excitation to energy levels of the $^{4}$${\mathit{G}}_{5/2}$${,}^{2}$${\mathit{G}}_{7/2}$ multiplets near 17 350 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the yellow. Excitation with a single frequency in this region enables observation of fluorescence from many multiplets up to the $^{4}$${\mathit{D}}_{3/2}$ multiplet near 28 200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Fluorescence lifetimes for emission from the $^{4}$${\mathit{F}}_{3/2}$ multiplet are reported for several centers. The ${\mathrm{Nd}}^{3+}$ cluster centers show small splittings in their sharper transitions, which is attributed to ${\mathrm{Nd}}^{3+}$-${\mathrm{Nd}}^{3+}$ coupling effects.

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