Linear Spin-Wave Theory of Librational Motion in the α Phase of N2 and CO

A linear spin-wave theory is used to calculate the frequencies of librational modes in the α phase of nitrogen and of carbon monoxide. Results are presented both for quadrupole-quadrupole interactions and for the potential of Kohin. From a comparison with the Raman scattering measurements of Anderson, Sun, and Donkersloot, it is concluded that the Kohin potential does not give a satisfactory representation of the anisotropic interactions in either crystal. The present spin-wave theory is compared with the recent excitonlike theories of Jacobi and Schnepp and of Raich. A numerical error in the classical harmonic calculations of Goodings and Henkelman is rectified.