In situ high pressure-temperature Raman spectroscopy technique with laser-heated diamond anvil cells

We describe an in situ high pressure-temperature Raman technique for studying materials in laser-heated diamond anvil cells using a Nd:YLF laser (1053 nm) as the heating source and an ion laser as the Raman exciting source. Here we introduce the method of laser heating transparent samples using a metallic foil (Pt,Re, or W) as the laser absorber (internal heating furnace) in a diamond cell. The YLF laser is used to effectively laser-heat one side of a metal foil 5-15 mum thick with a small hole of 10-20 mum in diameter at the center. The foil, in turn, heats a transparent sample while the Raman signals excited by an Ar+ or Kr+ laser are measured. Temperature of the laser-heated foil is measured by means of spectroradiometry whereas the average temperature of the heated sample is independently determined from the intensity ratios of the anti-Stokes/Stokes excitation pairs. The intrinsic temperature-dependent asymmetry of the Raman spectra arises from the principle of the detailed balance and is independent of sample properties other than the temperatures. The average determined by the signal-to-noise ratio of anti-Stokes/Stokes excitation pairs gives the sample temperature with the statistical accuracy of the Raman spectra. Transparent samples such as CO2 have been heated up to 1600 K and 65 GPa and Raman spectra have been measured with temperature uncertainty of 50-100 K. In situ Raman spectroscopy by laser heating represents a powerful technique to characterize high pressure-temperature properties of materials including molecular systems present in planetary interiors. (C) 2004 American Institute of Physics.