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The nature of inhomogeneous broadening in rare earth systems plays an important role in the dynamical optical -properties of ions in crystals. For this reason much of the work on the nature of the inhomogeneous broadening in rare earth doped systems has concentrated on the investigation of ion-ion interactions such as spectral transfer and up conversion processes. Variations in homogeneous optical dephasing times across the inhomogeneous optical line have also been reported. These studies concentrate on the variations of collective effects of ion- ion interactions at different positions in the inhomogeneous optical line in relatively heavily doped rare earth systems. We report variations in optically detected NQR (ODNQR) linewidths and shifts of Pr3+ hyperfine transitions in the optical ground state as a function of position in the inhomogeneous optical line and for satellite transitions of the 1D2-3H4 transition in lightly doped (0.5 at.%) Pr3+:LaF3. A systematic increase in the broadening of the Pr hyperfine transitions is observed as the laser is tuned toward the wings of the inhomogeneous optical transition without a shift of the NQR center frequency. The principle contribution to the 200KHz (FWHM) inhomogeneous width of the Pr hyperfine levels is explained by magnetic dipole-dipole interactions between the Pr nuclei and the neighboring fluorine spins. This contribution remains constant across the Stark broadened optical transition. Thus we show the existence of an additional broadening mechanism.

Photon echo measurements made at 2 OK on the 3H4 - ID2 transition in pr3+:LaF3 show that magnetic dipolar couplings between the Pr and F nuclei account for the 56 kHz homogeneous linewidth of this transition. The homogeneous broadening arises from the enhanced 141 Pr nuclear moment (I=5/2) interacting with the local field fluctuations of the 19 F nuclear moments undergoing mutual spin flip transitions. Such resonant fluctuations should, in the absence of a fluorine spin diffusion barrier, produce a homogeneous linewidth of about 200 kHz which is, in fact, roughly what is observed for the inhomogeneous broadening of the Pr3+ hyperfine levels and is considerably broader than that obtained by the photon echo measurements. Shelby et al 2 proposed a simple model analogous to the spin diffusion barriers responsible for narrowing the homogeneous lines in certain electron paramagnetic resonance transitions. In such systems, the field produced by the electron magnetic dipole moment (orders of magnitude larger than the enhanced nuclear moment associated with the ground state of Pr3+ in LaF3) de-tunes the nearest neighbors from each other, prohibiting mutual spin flips among them. Thus, the Fields produced by the neighboring spins are static and their interaction with the paramagnetic ion contributes to the inhomogeneous linewidth and not to homogeneous broadening.