Vibrational absorption of tunneling molecular defects in crystals. II. Tunneling molecules under applied stress (KCl:CN−)

Abstract

The general considerations about the vibrational absorption of tunneling molecules, developed in Paper I of this work, are applied and tested, using ${\mathrm{CN}}^{-}$ defects in KCl. In previous work on this system the vibrational absorption of ${\mathrm{CN}}^{-}$ was reported to be strongly broadened by an unresolved tunneling splitting, and to show pure zero-moment changes under applied stress. The tunneling model, however, predicts additionally characteristic anisotropic changes of the spectral shape under applied field or stress for any absorption, broadened by orientational tunneling. A thorough experimental reinvestigation of this system at low temperatures yields results which agree with the predictions from the tunneling model. For low ${\mathrm{CN}}^{-}$ concentration, the first- and second-harmonic vibrational absorption (at 5 and 2.5 μm) consists of a resolved double structure, caused by the reorientational tunneling of the molecule in the crystal (tunneling splitting $\Delta =1.2\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$). The application of uniaxial stress of different symmetry is found to produce two basic effects: Second-moment absorption changes (due to quantum-mechanical mixing of the tunneling states by the stress), and zero-moment changes (due to classical elastic dipole alignment). The observed pronounced anisotropy of these effects yield both independently a (cigar-shaped) elastic dipole model. These results disagree with the (pancake-shaped) elastic dipole, derived from the earlier elastooptical work, which has been the basis of the so far generally accepted defect model. Consequences of this symmetry change for the interpretation of several previous investigations on KCl:C${\mathrm{N}}^{-}$ will be discussed.