Hyper-Raman spectroscopy of Earth related materials

Raman and infrared spectroscopy proved extremely successful in obtaining structural information and thermodynamic data on samples under high pressure conditions in a diamond anvil cell [1,2]. With substantial advances in CCD detector technology and the possibility to focus visible laser light down to several microns, Raman spectroscopy can nowadays be regarded one of the standard techniques for diamond anvil cell investigations. Nevertheless, Raman scattering suffers from often strong fluorescence and the strong Raman signal of the diamonds. Infrared spectroscopy is limited by the sample size and the diffraction limit of mid- or far-infrared radiation. With increasing pressure, diamonds also show strong infrared activity, which can interfere with the signal from the sample. Detectors in the mid- and far-infrared are inherently noisy, often leading to low signal-to-noise ratios for infrared measurements. With new techniques and instrumentation available, such as low noise CCD cameras and stable diode-pumped solid state laser systems, more demanding techniques become feasible as well. Especially hyper-Raman scattering, a nonlinear optical variant of infrared spectroscopy, can be used on a more routine basis for the first time. Pioneering work in the 70s and 80s have explored some of the capabilities of Hyper-Raman spectroscopy [3]. Unlike infrared spectroscopy, Hyper-Raman is not limited by the diffraction limit of mid- or far-infrared radiation, typically restricting the lower frequency limit to several hundred wave numbers. The major advantages of hyper-Raman are essentially background free spectra and the use of wavelengths in the near-infrared and visible, making possible micro focusing and taking advantage of high efficiencies, low noise, and smooth wavelength dependencies of CCD detectors. Hyper-Raman does not suffer from saturation caused by strong absorption in the infrared and is therefore less sensitive to surface effects. For centrosymmetric materials conventional Raman and hyper-Raman are complimentary. In many cases the combined information of both techniques can reveal all the vibrational information of a material. This information can be used to calculate thermodynamic properties, to identify mineral phases ('finger-printing'), or to investigate the dynamics related to phase transitions ('soft-modes'). First results on planetary materials will be presented, including MgO and stichovite. Corundum as another possible high pressure transmitting material is characterized as well. Further measurements are underway, including MgSiO₃ and CaSiO₃ perovskite. [1] A. M. Hofmeister, in: Infrared Spectroscopy in Geochemistry, Exploration Geochemistry, and Remote Sensing, Vol. 33 (ed. P. K. King, M. S. Ramsey, and G. A. Swayze), Mineralogical Society of Canada (2004) [2] P. F. McMillan, R. J. Hemley, and P. Gillet, in : Mineral Spectroscopy: A Tribute to Roger G. Burns, Vol. 5 (ed. D. Dyar, C. McCammon, and M. W. Schaefer), The Geochemical Society Special Publication (1996). [3] H. Vogt, in: Topics in Applied Physics, Vol. 50, Light scattering in solids II (ed. M. Cardonna and G. Guentherodt), Springer-Verlag, Heidelberg, New York (1982).