Tunneling states of CN−in various alkali halides: Study of resolved tunneling structures and "paraelastic sidebands" in the vibrational absorption of CN−

Abstract

Tunneling structures were observed in the vibrational absorption of C${\mathrm{N}}^{-}$ in KCl, KBr, KI, and RbCl using high-resolution ir spectroscopy. Under uniaxial stress applied in various directions two prominent effects were observed: (a) integrated absorption changes due to paraelastic alignment of the C${\mathrm{N}}^{-}$ molecules and (b) drastic spectral changes due to the modification of the tunneling states under stress. In contrast to earlier work the structures could, in most cases, be fully resolved. In connection with (b), sidebands emerging from the vibrational absorption corresponding to transitions between different paraelastic states were observed, lending themselves to an accurate determination of paraelastic spittings. The results conclusively show that the equilibrium orientations of the molecule is along $〈 111 〉$ axes in all investigated systems. Experimental evidence for a considerable stress dependence of certain tunneling matrix elements and deviations of the equilibrium orientations from $〈 111 〉$ orientations under high stress lead to the introduction of a modified "soft" $〈 111 〉$ tunneling model in which the effects of external stress on the defect-lattice interaction potential are not assumed to be negligible.