Vacuum-ultraviolet excitation studies of $5d^{1}4f^{n-1\mathrm{}}$ to $4f^n$ and $4f^n$ to $4f^n$ transitions of ${\mathrm{Nd}}^{3+}$-, ${\mathrm{Er}}^{3+}$-, and ${\mathrm{Tm}}^{3+}$-doped trifluorides

We have measured the vacuum-uv excitation spectra of interconfiguration ($5d^{1}4f^{n-1\mathrm{}}$ to $4f^n$) and intraconfiguration ($4f^n$ to $4f^n$) transitions of Nd-, Er-, and Tm-doped La${\mathrm{F}}_3$, Y${\mathrm{F}}_3$, Lu${\mathrm{F}}_3$, and LiY${\mathrm{F}}_4$. In Nddoped samples the observed vacuum-uv excitation spectra of interconfiguration transitions differ completely from those of intraconfiguration transitions. The difference is also striking in the Er- and Tm-doped samples. The excitation spectra of interconfiguration transitions of Nd-doped samples can be explained by the crystalfield splitting of a $5d$ state, the corresponding excitation spectra for Er- and Tm-doped samples being more complicated. The vacuum-uv excitation spectra of interconfiguration transitions of rare-earth doped trifluorides show a strong excitation peak around 1200 Å which overlaps the host-lattice absorption, and exhibit concentration-quenching characteristics different from those of interconfiguration transitions. The location of this excitation peak suggests that it is associated with a process involving energy transfer from the host lattice to the rare-earth ion dopants. The large Stokes shift (∼6 eV) observed for the emission resulting from this excitation suggests also that the transfer process is accompanied by a strong lattice relaxation similar to that observed for self-trapped holes in alkaline-earth fluorides. Attempts to observe the recombination radiation of self-trapped excitons in pure Y${\mathrm{F}}_3$ and LiY${\mathrm{F}}_4$ hosts were unsuccessful. Similar attempts on pure La${\mathrm{F}}_3$ and Lu${\mathrm{F}}_3$ hosts are complicated by the presence of rare-earth impurities.