Vibrational absorption of tunneling molecular defects in crystals. I. Tunneling molecules in electric fields (KCl:OH−)

Abstract

We shall discuss the general question of how orientational tunneling of substitutional molecular crystal defects affects the vibrational absorption of the molecule and its behavior under electric fields or stress. As a model case, oriented dipoles are considered with a tunneling splitting $\Delta$ sizable enough to contribute to the low-temperature width of the vibrational absorption. It is shown that field application should cause drastic anisotropic changes in the spectral structure of this system: while the "main transition" (with strength of order 1) becomes narrowed under field application, weak satellite absorption ("paraelectric resonance sidebands") of strength $\propto {\left(\frac{\Delta }{pE}\right)}^2$ should become shifted out of the vibrational spectrum in such a way that the second-moment contribution (from tunneling) of the total absorption remains constant. For the main vibrational transitions (the only ones observed so far), the tunneling model predicts second-moment decreases under field (or stress) application, the magnitude of which is highly anisotropic in terms of dipole, field, and polarization direction. These general considerations are applied to previously uninterpreted electrooptical measurements on the O${\mathrm{H}}^{-}$ vibrational absorption in KCl. The available data for various field directions and $\parallel$ and $\perp$ polarization are found in agreement with the above analysis, and allow a determination of the tunneling parameter $\Delta$ and a dipole orientation which agrees with other experiments. Previously assumed serious contradictions between vibrational band width and tunneling splitting can be resolved within the model taking into account random background strain or fields in the crystal.