Vibronic Transitions of Hexagonal Rare-Earth Trichlorides. II. ${\mathrm{Pr}}^{3+}$ in La${\mathrm{Cl}}_3$, Pr${\mathrm{Cl}}_3$, and Gd${\mathrm{Cl}}_3$

The absorption spectra of ${\mathrm{Pr}}^{3+}$ in La${\mathrm{Cl}}_3$, Pr${\mathrm{Cl}}_3$, and Gd${\mathrm{Cl}}_3$ have been obtained under high resolution in the region 4300-4900 Å. Strong vibronic transitions are observed accompanying the transitions from ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}(\mu =\pm 2)$ to all components of the groups ${}_{}{}^3{}_{}{}^{}P_{0,1,2}^{}{}_{}{}^{}$. The correspondence between the vibronic transitions and their parent electronic transitions is made by comparing the splittings in a magnetic field parallel to the crystal $c$ axis. Assuming a model of a single ${\mathrm{Pr}}^{3+}$ ion interacting with the lattice vibrations, electric dipole selection rules are derived for the case of an applied magnetic field. Using these selection rules, possible assignments of phonon branches are given to the observed vibrational levels. The shifts in phonon frequencies as one goes from La${\mathrm{Cl}}_3$ to Gd${\mathrm{Cl}}_3$ are found to be larger for branches which are flat near k=0 than for those which are flat closer to the edge of the Brillouin zone.