Optical Absorption Spectra of Coupled ${\mathrm{Nd}}^{3+}$ Ions in Nd${\mathrm{Cl}}_3$ and Nd${\mathrm{Br}}_3$

The absorption spectra of single-crystal samples of pure Nd${\mathrm{Cl}}_3$ and Nd${\mathrm{Br}}_3$ have been obtained at low temperatures (4.2 to 1.2°K) under high resolution in the region from 2700 Å to 1 μ for Nd${\mathrm{Cl}}_3$ and in the region 3000 to 7000 Å for Nd${\mathrm{Br}}_3$. Aside from small shifts in the Stark multiplets of the order of 20 ${\mathrm{cm}}^{-1\mathrm{}}$, the principal difference from the spectrum of 1% Nd${\mathrm{Cl}}_3$/La${\mathrm{Cl}}_3$ is that each line is accompanied by two symmetrically placed satellites of comparable intensity, one on either side of the main line. Zeeman studies were made of the lines in fields up to 36 kOe, applied both parallel and perpendicular to the axis which joins the nearest-neighbor ${\mathrm{Nd}}^{3+}$ within the sample. The source of these additional lines is interactions coupling neighboring ${\mathrm{Nd}}^{3+}$ ions. Only a small part of this coupling is attributable to known, calculable interactions between the ions—electric dipole, magnetic dipole, and electric quadrupole. A satisfactory interpretation of the spectra can be obtained by assuming an additional coupling between ${\mathrm{Nd}}^{3+}$ neighbors $i$ and $j$ of the form ${\mathcal{H}}_{\mathrm{ij}}={\mathcal{J}}_{\mathrm{ij}}{S}_{\mathrm{zi}}{S}_{\mathrm{zj}}$ where $S_z$ is the $z$ component of fictitious spin ½. This coupling is assumed to be due to electron exchange. The sign and magnitude of the nearest-neighbor coupling has been obtained for the ground and five excited states in Nd${\mathrm{Cl}}_3$ and for the ground and five excited states in Nd${\mathrm{Br}}_3$. The sign and magnitude of the next-nearest-neighbor coupling has been obtained for the ground and two excited states in Nd${\mathrm{Br}}_3$.