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:${\mathrm{Cu}}^{+}$, KBr:${\mathrm{Li}}^{+}$, KI:${\mathrm{Ag}}^{+}$, and CsI:${\mathrm{Tl}}^{+}$. 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:${\mathrm{Li}}^{+}$. The tunneling mode [1.15 ${\mathrm{cm}}^{-1\mathrm{}}$ for ${\left({}_{}{}^6{}_{}{}^{}\mathrm{Li}\right)}^{+}$ at zero strain] increases to 30 ${\mathrm{cm}}^{-1\mathrm{}}$ 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:${\mathrm{Li}}^{+}$. Analysis of the data indicates that the off-center ${\mathrm{Li}}^{+}$ ion goes on-center with application of a 0.5% strain. At 0.65% strain the KCl:${\mathrm{Li}}^{+}$ spectrum is virtually identical to the KBr:${\mathrm{Li}}^{+}$ spectrum at zero strain.