Electron paramagnetic resonance of photoexcitedNd3+144ion pairs in LaCl3single crystals

The electron paramagnetic resonance spectrum of photoexcited nearest-neighbor ${}_{}{}^{144}{}_{}{}^{}\mathrm{Nd}_{}^{3+}{}_{}{}^{}$ ion pairs in La${\mathrm{Cl}}_3$ single crystals has been obtained at a microwave frequency, ∼24.5 GHz, and a temperature, 1.3-1.8 K. The data for four lines of these spectra are well described by the spin Hamiltonian, ${\mathcal{H}}_S=+\left|{\mu }_B\right|{\overrightarrow{\mathrm{B}}}_0·({}_{}{}^1{}_{}{}^{}\mathit{g}·{}_{}{}^1{}_{}{}^{}\overrightarrow{\mathcal{J}}+{}_{}{}^2{}_{}{}^{}\mathit{g}·{}_{}{}^2{}_{}{}^{}\overrightarrow{\mathcal{J}})+{}_{}{}^1{}_{}{}^{}\overrightarrow{\mathcal{J}}·\mathit{K}·{}_{}{}^2{}_{}{}^{}\overrightarrow{\mathcal{J}}, {}_{}{}^1{}_{}{}^{}S={}_{}{}^2{}_{}{}^{}S=\frac{1}{2}$ in which the principal values of ${}_{}{}^1{}_{}{}^{}\mathit{g}$ and ${}_{}{}^2{}_{}{}^{}\mathit{g}$ are shifted by only very small relative amounts from those previously found for isolated ${}_{}{}^{144}{}_{}{}^{}\mathrm{Nd}_{}^{3+}{}_{}{}^{}$ ions in their ground ${}_{}{}^4{}_{}{}^{}I_{\frac{9}{2}}^{}{}_{}{}^{}$ and their photoexcited ${}_{}{}^4{}_{}{}^{}I_{\frac{15}{2}}^{}{}_{}{}^{}$ states, respectively, in the La${\mathrm{Cl}}_3$ crystal. It is concluded that these lines originate with nearest-neighbor pairs in which one of the ions is in the ground state and the other is in the photoexcited state. The signs and magnitudes of the shifts of the $g$ values from those for the isolated ions are accounted for by small distortions of the crystal structure in the vicinity of the photoexcited pair. The measured principal values of $\mathit{K}$ are shown to arise predominantly from magnetic dipole-dipole and superexchange ion-ion interactions. The difference in the values found for the nondipolar interaction between nearest-neighbor pairs in the photoexcited $\frac{9}{2}$, $\frac{15}{2}$ state and in the ground