Study of the libration of the ammonium ion in ammonium bromide in the ordered tetragonal phase by Raman spectroscopy

The frequency, intensity, spectral bandwidth, and the Raman polarizability correlation function associated with the librational motion of the ${\mathrm{NH}}_4^{+}$ ion in NH₄Br below T_λ has been studied in detail as a function of temperature by Raman spectroscopy. From the second moment data, the earlier proposed reorientational mechanism was ruled out as a cause for the spectral broadening. The correlation time and spectral linewidth versus temperature data both show an Arrhenius type of temperature dependence. This temperature dependence was attributed to the thermally induced hydrogen bond breaking process.