Multichannel Raman spectrometry system for weakly scattering materials at simultaneous high pressures and high temperatures

Abstract

A Raman system is described which is suitable for rapid determination of Raman spectra of silicates and other weak Raman scatterers under simultaneous high-pressure and high- temperature conditions. Samples are pressurized with an argon pressure medium and Sm:YAG pressure standard in an Inconel 718 or rhenium Merrill–Bassett diamond–anvil cell that is externally heated in a vacuum oven equipped with quartz windows on either side. Raman spectra are excited and collected through the front window using ≊160° scattering geometry. A triple spectrometer in conjunction with a holographic notch filter, disperses the Raman scattered radiation onto a liquid-nitrogen-cooled CCD detector. Simultaneously measured Sm:YAG fluorescence spectra are collected through the rear window and routed via an optical fiber to a second multichannel spectrometer system equipped with a diode-array detector. The high sensitivity of the detectors and the simultaneous measurement technique enable detection of and compensation of pressure drifts occurring at high temperature. The performance of the system is demonstrated by measurement of the positions and linewidths of the high-frequency Raman modes of polycrystalline forsterite (α-Mg2SiO4) at temperatures up to 930 K at pressures of ∼8 GPa and up to 842 K at pressures in the range 12–14 GPa.