Rotational Degrees of Freedom of Molecules in Solids. II. The Nitrite Ion in Alkali Halides

In continuation of the work described in the preceding paper we have studied a defect with three different principal axes of inertia, namely the N${\mathrm{O}}_2^{-}$ ion. In KCl and KBr, the energy levels corresponding to free rotation, libration, and tunneling have been found. They have been analyzed in the same manner as in the preceding paper. Very different potential barriers for the three different axes of rotation have been found. Strong phonon scattering is again observed in connection with the states of free rotation and of tunneling. In NaCl the ion is frozen in even at room temperature and is only librating. In KI further evidence is found that the N${\mathrm{O}}_2^{-}$ ion does occupy an off-center position. Consequently the molecule can perform not only rotational, but also translational motion, i.e., its potential is considerably more complex than the Pauling-Devonshire potential. Hence the KI:N${\mathrm{O}}_2$ system seems to present the most general case of motion of molecules in solid solution. From the fine structure of the near infrared spectrum of KI:N${\mathrm{O}}_2$ we have found all the impurity modes detected so far by different techniques, which shows that this type of spectroscopy is a very useful tool for the study of impurity modes. We suggest further that some of these impurity modes are librational motions.