MULTICHANNEL RAMAN-SPECTROMETRY SYSTEM FOR WEAKLY SCATTERING MATERIALS AT SIMULTANEOUS HIGH-PRESSURES AND HIGH-TEMPERATURES

A Raman system is described which is suitable for rapid determination of Raman spectra of silicates and other weak Raman scatterers under si multaneous 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 almost-equal-to 160-degrees scattering geometry. A tripl e spectrometer in conjunction with a holographic notch filter, dispers es the Raman scattered radiation onto a liquid-nitrogen-cooled CCD det ector. Simultaneously measured Sm:YAG fluorescence spectra are collect ed 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 measureme nt technique enable detection of and compensation of pressure drifts o ccurring at high temperature. The performance of the system is demonst rated by measurement of the positions and linewidths of the high-frequ ency Raman modes of polycrystalline forsterite (alpha-Mg2SiO4) at temp eratures up to 930 K at pressures of approximately 8 GPa and up to 842 K at pressures in the range 12-14 GPa.