The hyperfine structure and relaxation character of the terminal levels of the ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}-{}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ (4777 Å) transition in La${\mathrm{F}}_3$: ${\mathrm{Pr}}^{3+}$ (0.03 at.%) were studied using the photon-echo technique. Using two independent nitrogen-laser pumped dye lasers we have observed modulated photon echoes for pulse separations over a ∼10-μsec range. In this interval our modulated photon echoes decay in a simple exponential manner by a factor of ${10}^5$ and yield a homogeneous linewidth of 70 kHz [full width at half maximum (FWHM)]. Fourier transformation of the echo-modulation data yields directly the nuclear level splittings and their associated linewidths in the ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ states. In the ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}\left({}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}\right)$ state the nuclear splittings are 0.73 and 1.12 MHz (8.48 and 16.68 MHz). The measurements for the nuclear level splittings and associated linewidths in the ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ state are in excellent agreement with those obtained from the optical-rf double-resonance experiments of Erickson. We find that the calculated echo modulation is a sensitive function of the relative orientation of the principal axes associated with the ground-and excited-state Hamiltonians. By comparing our experimental and calculated echo-modulation patterns we are able to determine their relative orientations.

• Received 16 July 1979

DOI:https://doi.org/10.1103/PhysRevB.21.40