  We report studies of the Yb$^{3+}$ site distribution in Er$^{3+}$ co-doped
upconverting alkaline earth fluoride nanoparticles using high-resolution
absorption measurements. Similar to bulk crystals, Yb$^{3+}$ single ion cubic
(O$_h$), and preferentially formed clusters are the dominant centres for
moderate dopant concentrations. At higher concentrations, and for larger
particle sizes, a minority C$_{4v}$(F$^-$) centre is also observed.
Furthermore, the behaviour of Yb$^{3+}$ cluster centres, whose primary
absorption band is located around 10205 cm$^{-1}$ (in near resonance with 980
nm laser diodes) was studied by tuning the metal cation in the host lattice
from Ca$^{2+}{\rightarrow}$Sr$^{2+}{\rightarrow}$Ba$^{2+}$. For co-doped
materials, with heterogeneous cluster centres, Er$^{3+}$ upconversion
fluorescence was observed upon selective excitation of Yb$^{3+}$, resulting in
strong visible emission from $^4$H$_{11/2}$, $^4$S$_{3/2}$ and $^4$F$_{9/2}$
multiplets. By monitoring the Er$^{3+}$ $^4$S$_{3/2}{\rightarrow}^4$I$_{15/2}$
(540 nm) fluorescence, whilst scanning a laser through the wavelength dependent
Yb3+ absorption, we obtain an excitation spectrum for these heterogeneous
cluster centres. It is demonstrated that the group of Yb$^{3+}$ ions residing
near an Er$^{3+}$ ion represent a small subset of the overall Yb$^{3+}$
ensemble and therefore a linear absorption measurement is not necessarily a
good indication of the optimum laser pump wavelength.
