Far-infrared properties of lattice resonant modes. VI. Hydrostatic pressure effects

The frequency shifts produced by hydrostatic pressure have been measured for four resonant-mode systems NaCl:Cu⁺, KBr:Li⁺, KI:Ag⁺, and CsI:Tl⁺. The experimental results have been successfully characterized with an anharmonic-oscillator model. Shifts have also been measured for the tunneling and excited-state modes of KCl:Li⁺. The tunneling mode [1.15 cm^-1 for (⁶Li)⁺ at zero strain] increases to 30 cm^-1 at 1.1% strain. An extra low-frequency transition provides direct evidence that the tunneling levels in the lowest manifold are not equally spaced. We find that the double-harmonic-oscillator model which has been used previously to describe the tunneling levels does not adequately describe the pressure dependence of the five observed absorption lines in KCl:Li⁺. Analysis of the data indicates that the off-center Li⁺ ion goes on-center with application of a 0.5% strain. At 0.65% strain the KCl:Li⁺ spectrum is virtually identical to the KBr:Li⁺ spectrum at zero strain.