instrumental techniques applied to mineralogy and geochemistry

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Raman, Conventional Infrared and Synchrotron Infrared Spectroscopy in Mineralogy and Geochemistry: Basics and Applications 65 frequency-shifted Raman scattering from the other radiation. The second monochromator increases the dispersion and separates individual Raman peaks. However, filter technology has improved with the development of effective notch and edge filters. The notch filters, in particular, are widely used. They are designed to absorb the light of the frequency of the incident laser light. Usually a filter, which collects most of the light within 200 cm -1 of the excitation frequency is regarded as sufficient. Some experiments do require measurements closer to the exciting line and in these cases, the use of monochromators would still be the preferred method for separating the Raman scattering. FT-Raman was introduced in 1980s. By that time FTIR was developed to a high level of refinement and many components were transferred from FTIR to FT-Raman with minor modifications. Many vendors offer FT-Raman attachments to otherwise conventional FTIR spectrometers, so that both techniques share the same interferometer. Detectors There are mainly two classes of IR detectors: thermal and quantum detectors. Thermal detectors detect temperature changes related to the infrared radiation coming from the sample. These detectors have slow response times, but generally have a wide wavelength range of operation. Quantum detectors rely on the interaction of incoming IR photons with the electronic wavefunction of the solid detector material. These detectors have fast response times, but only cover narrow wavelength ranges. Many IR detectors have been developed to operate optimally at liquid-N 2 or liquid-He temperatures, thus reducing the noise from the thermal IR background found even at room temperature. Typical detectors in the MIR range are TGS (triglycin sulphate) or MCT (mercury cadmium telluride). For FIR work TGS is often used despite their low sensitivity. Liquid-He-cooled bolometers with Si or Ge elements cover the FIR with a hundred-fold increase of sensitivity. The main disadvantages of these detectors are their cost and the need of handling liquid-He as coolant. For the NIR liquid-N 2 -cooled InAs or InSb detectors are available. Recent developments include two-dimensional arrays of e.g. MCT or bolometer detectors for IR imaging. In Raman spectroscopy single-channel detectors (photomultipliers and semiconductor photodiodes) are now only employed in scanning, dispersive
66 Biliana Gasharova spectrometers with special requirements such as access to the very low frequency region as well as in interferometric instruments working in the NIR region. The multichannel solid-state detectors tend now to predominate in micro-Raman systems, either for spectral analysis or Raman imaging. Most of these detectors are based on silicon technology and thus are sensitive to photon energies from the near-UV, ~280 nm, to ~1 μm in the NIR region. Other devices are based on different semiconductors such as Ge or InGaAs whose response extend further into the NIR region. Microscopy Raman and IR microscopes developed independently over the past 30 years. The development of both was motivated by the need for acquiring spectra from microspots. Raman microbeam techniques found applications in the earth sciences already in the 1970s. The spatial resolution is determined by physics based on the wavelength of the light used. For Raman in which the wavelengths of excitation and detection are done in the visible range, typically between 400 and 850 nm, the spatial resolution, which is diffraction limited is observed to be better than 1 μm. In micro-Raman spectroscopy, the incident laser beam is directed into an optical microscope and focused through the objective onto the sample. The scattered light is collected back through the same objective (~180º geometry), and sent into the spectrometer. The focus of the incident beam forms an approximate cylinder, whose dimensions are dictated by the wavelength of the laser light and the optical characteristics of the objective and the sample. The diameter of the cylinder is fixed by the diffraction limit of light in air, and is ~0.5-1 μm for an objective with numerical aperture (NA) ~0.9 and laser light in the blue-green region of the spectrum. Corresponding to this is a depth (length) of the scattering cylinder of ~3 μm. This shows that micro-Raman spectroscopy has better lateral resolution than depth spatial resolution. The depth resolution is greatly improved in recently widely employed confocal systems, but is still the less resolved direction. Exact optical conjugation onto the sample of the pinhole apertures, employed for both illumination and detection, rejects the stray light background due to the out-of-focus regions of the specimen. Thus the main contribution to the signal comes selectively from a thin layer close to the focal plane. Modern developments in IR spectroscopy include the application of the IR technique
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