Magic-Angle Line Narrowing in Optical Spectroscopy

Spin decoupling and line narrowing are observed for the first time in an optical transition, ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}\leftrightarrow {}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ of ${\mathrm{Pr}}^{3+}$ in La${\mathrm{F}}_3$ at 2°K, with use of optical free-induction decay. The ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei, when irradiated by an appropriate rf field, undergo forced precession about an effective field at the magic angle in the rotating frame. The fluctuating ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$-${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ dipolar interaction is thereby quenched and the optical linewidth drops from ∼10 to ∼2 kHz, as predicted in a theory of spin diffusion.