Effects of Uniaxial Stress on the Absorption of theFCenter in KCl and theUCenter in KBr

The stress-induced linear dichroism in the absorption of the $F$ center in KCl and the $U$ center in KBr has been measured in the range from 7 to 120°K. The moments analysis of Henry, Schnatterly, and Slichter has been used to interpret the results. For the $F$ center, a measurement of the changes in the third moment of the $F$ band allows the determination of the separate contributions to the second moment by lattice vibrations of $\Gamma _1^{}{}_{}{}^{+}$, $\Gamma _3^{}{}_{}{}^{+}$, and $\Gamma _5^{}{}_{}{}^{+}$ symmetry. Within the harmonic approximation, the temperature dependence of these separate contributions defines an average vibrational frequency for each symmetry mode broadening the band. The measured frequencies, ${\omega }_1=2.3\mathrm{}\pm 0.1\times {10}^{13}$ ${\sec }^{-1\mathrm{}}$, ${\omega }_5=1.95\mathrm{}\pm 0.1\times {10}^{13}$ ${\sec }^{-1\mathrm{}}$, and ${\omega }_3=1.4\mathrm{}\pm 0.1\times {10}^{13}$ ${\sec }^{-1\mathrm{}}$, lie in the acoustic and low-frequency optical region of the host-lattice vibrational spectrum. The stress response for the $U$ center was found to be qualitatively identical to that of the $F$ center. The changes in the first moment of the $U$ band determine the excited-state coupling to unit strains of $\Gamma _3^{}{}_{}{}^{+}$ and $\Gamma _5^{}{}_{}{}^{+}$ symmetry. From the changes in the third moment, it was found that cubic vibrations dominate the broadening of the $U$ band. A broad peak in the dichroism pattern to the high-energy side of the $U$ band is interpreted as evidence for transitions to higher excited states of the $U$ center analogous to the $K$-band transitions associated with the $F$ center.