A broad range of optical spectroscopy techniques are used by scientists at the Geophysical Laboratory studying high-pressure phenomena. These techniques include absorption, reflectivity, and emission spectra over a wide spectral range (240-16,000 nm), infrared absorption spectroscopy, and Raman spectroscopy -- These techniques can be used at combined high pressures and variable temperatures from cryogenic to laser heating conditions.
UV-Visible-IR Spectroscopy- For the routine mid- and near-IR measurements we use Nicolet 750 Magna FT-IR spectrometer coupled to a custom made infrared microscope. Recently, we have set up a dedicated and integrated facility (Fig. 1) for studying of optical properties in a wide spectral range (240-16,000nm). It includes the FT-IR spectrometer, UV-visible grating monochromator-spectrograph with CCD detector, UV-visible grating monochromator-spectrograph with intensified gated CCD detector (for pulsed Raman and absorption), UV-visible light sources, and a custom made Cassegrain-type mirror optics. The same facility can be used for the ruby fluorescence (optical pressure sensor) and Raman spectra, which makes it very convenient for high-pressure high-temperature applications.
We have performed measurements of the optical properties of mw to 80 GPa at room temperature (Goncharov et al., 2006) (Fig. 2). In contrast to previous predictions, we find that the spin-pairing transition results in greater absorption in the near-IR range, indicative of reduced radiative conductivity, and therefore the radiative component of thermal conductivity may be blocked. The notion of reduced thermal conductivity in the lower mantle may challenge existing theories on the tability of superplumes, which appear to require enhanced thermal conductivity in order to mitigate excessive temperature gradients. It is possible that reduced radiative component of heat transfer in low-spin phases is compensated by elevated lattice conduction (low-spin phase lattice is more stiff), both of which contribute to the overall thermal conductivity of the lower mantle. During the summer of 2006 our team has performed a set of measurements of optical properties of pv up to 130 GPa (Haugen et al., 2006). The results are currently being processed and interpreted.
Raman spectroscopy — Our laser heating apparatus combines conventional one-sided (two-sided in the latest modification) laser heating with a Raman system [Lin et al., 2004; Goncharov et al., 2005a]. A Mitutoyo near IR 20x long-working distance lens was employed for the collection of Raman spectra in the backscattering geometry and for focusing of the heating IR radiation on the sample using the same optical path. A continuous-wave 70 W YLF laser is used as the heating source. The YAG laser radiation is injected into the Raman system using a polarizing beam-splitter cube and is focused to a 15-20 m spot. The heating laser power was increased in steps (by rotating a /2 wave plate coupled to a polarization cube).
The Raman part of our confocal Raman system has previously been described in detail [Goncharov, 2000]. We use the 458 nm line of a 300 mW Ar ion laser to excite the Raman spectra. The Ar ion laser radiation is injected into the Raman system using a beamsplitter made of a Kaiser Optics laser bandpass filter. The radiation scattered/ emitted by the sample was analyzed by two dedicated single-stage grating spectrographs: Jobin Yvon HR460 for Raman measurements and Kaiser Optics Holospec– for thermal radiometry. We determined the temperature of the sample in the LHDAC by measuring thermal emission spectra over a wide spectral region. The Planck radiation function is then fitted to the measured spectra. We also estimate temperature by analyzing the relative intensities of the Stokes and Anti-Stokes Raman peaks (see, e.g., Ref. [Lin et al., 2004]). This technique is based on an intrinsic property of the studied system, so it is presumably more accurate.
The use of continuous 458 nm Raman excitation and a spatial filter allows one to suppress the thermal radiation from the sample and the coupler sufficiently to obtain high quality Raman spectra up to approximately 2000 K. At higher temperatures, when thermal radiation rises above a critical threshold and obscures the Raman signal, a pulsed Raman system with 532 nm excitation synchronized with the gated intensified CCD detector will be used [Goncharov et al, 2005b]. This system has been recently constructed at the Geophysical laboratory.