The Rayleigh-Brillouin Spectra of Ammonium Chloride

Abstract

We have measured the spectrum of light scattered in single crystals of NH₄C1. This crystal is an interesting system to study as it undergoes a second order lambda transition at T_λ = -30. 55 °C. This transition is of the order-disorder type involving the ordering of the NH₄-tetrahedra in the cubic unit cell. We have made measurements on the spectrum of light scattered by thermally excited hypersonic sound waves (the Brillouin spectrum) and on the spectrum of light scattered by non-propagating critical fluctuations in the optical dielectric constant (the Rayleigh spectrum). Specifically, we have used a high power (80 mw) multi-mode He-Ne laser and a 12 m grating spectrograph to measure two purely longitudinal mode velocities and one purely transverse mode velocity as a function of temperature in the interval -50 to + 50 °C. We also measured the Brillouin intensities of these modes. We have measured the intensity of the elastically scattered light in the same interval. In addition, we have used a high power (15 mw) stabilized single frequency He-Ne laser, a pressure scanned flat Fabry-Perot interferometer with an instrumental width of 250 MHz, and a light detection system having a dark current of.2 counts per second to measure the natural width of the <110> longitudinal mode Brillouin component in the frequency range 8 to 24 GHz at 24°C.

This work supported by the Advanced Research Projects Agency (Project Defender) and monitored by the U. S. Army Research Office-Durham under contract DA-31-124-ARO-D-425.