Central peak properties of NH4Cl studied by light scattering

The central peak spectra of NH 4 Cl were precisely examied by three Fabry-Perot interferometer systems. It was found that besides an unresolvably narrow central peak which is of static origin, a broad dynamic central peak exists in the VV and HH polarizations. The polarization dependence and the orientation dependence of the q vector were examined for both the static and dynamic central peaks. It was also found that the static central peak can be eliminated by heat treatment, which enabled us to measure the dynamic central peak very close to the transition temperature and in the ordered phase. The dynamic central peak was found to narrow down to 30 MHz FWHM, as the transition temperature is approached, contrary to the assertion of Andrews et al.