Lifetime and quantum-efficiency measurements were used to determine the transition rates between several adjacent groups of Stark levels lying above $R$ of La${\mathrm{Cl}}_3$: ${\mathrm{Nd}}^{3+}$. By choosing transitions for which the ion pair interaction could be excluded as a relaxation mechanism, the rate of energy transfer to the lattice by emission of several phonons was determined. The 4.2°K rates were found to be due to spontaneous emission, and for $C\to B$, $S\to R$, and $D\to C$ are (2.23±0.38)×${10}^3$ ${\sec }^{-1\mathrm{}}$, (4.59±0.64)×${10}^4$ ${\sec }^{-1\mathrm{}}$, and (6.3±3.1)×${10}^3$ ${\sec }^{-1\mathrm{}}$, respectively. At higher temperatures increased rates due to emission stimulated by thermally populated phonon states were measured. Fitting the temperature dependences permitted estimates of the magnitude of the phonons involved, and determined their frequencies to be in the range of the optical branches of the La${\mathrm{Cl}}_3$ phonon spectrum. A technique for measuring quantum efficiencies is presented which does not require the use of a spectrometer which has been calibrated for transmission.

• Received 19 December 1966

DOI:https://doi.org/10.1103/PhysRev.157.252