Power Raman emission

The intensity or radiant power of a normal Raman band depends in a complex way on the polarizability of the molecule, the intensity of the source, and the concentration of the active group, as well as other factors. In the absence of absorption, the power of Raman emission increases with the fourth power of the frequency of the source. Howeser. advantage can seldom he taken of this rclation.ship because of the likelihood that ultraviolet irradiation will cause pholodccomposi-lion of the analvle. 

Recently, a XeCl laser (308 nm) with high spectral brightness has been used by Maeda and Takahashi [lO] to excite up to the 11th AS component at 128.3 nm in H2 gas at 10 atm. This source and therefore the AS Raman emission is tunable over the bandwidth of the XeCl laser ( V IOA). The potential for emission at much shorter wavelengths now exists with excitation of stimulated Raman scattering with an E-beam pumped Ar2 ex-cimer laser operating at 124 to 127.5 nm with 2 MW output power. In preliminary experiments, Sasaki et al. [ll] have generated X IOO kW power in the first and second order Stokes emission (at 134 and 141 nm) of H2 at 8 atm pressure. 

Raman spectroscopy, while typically used as a micro-analytical tool, can be conducted remotely. Performance of remote Raman analysis have been recently explored and reahzed for experiments on the surface of Mars (Sharma et al. 2001 Sharma et al. 2003). Raman spectroscopy is a powerful technique for mineralogical analysis, where the sharpness of spectral features of minerals allows for much less ambiguous detection, especially in the presence of mixtures. Visible, near-infrared, thermal, reflectance and in many cases emission spectroscopy of minerals all suffer from broad overlapping spectral features, which complicates interpretation of their spectra. On the other hand, Raman spectra of minerals exhibit sharp and largely non-overlapping features that are much more easily identified and assigned to various mineral species. 

Raman spectroscopy has been widely used to study the composition and molecular structure of polymers [100, 101, 102, 103, 104]. Assessment of conformation, tacticity, orientation, chain bonds and crystallinity bands are quite well established. However, some difficulties have been found when analysing Raman data since the band intensities depend upon several factors, such as laser power and sample and instrument alignment, which are not dependent on the sample chemical properties. Raman spectra may show a non-linear base line to fluorescence (or incandescence in near infrared excited Raman spectra). Fluorescence is a strong light emission, which interferes with or totally swaps the weak Raman signal. It is therefore necessary to remove the effects of these variables. Several methods and mathematical artefacts have been used in order to remove the effects of fluorescence on the spectra [105, 106, 107]. 

With any sample, recording should begin with low laser power. While observing the interferogram and the resulting spectra, fine adjustment of the sample is possible. The laser power may then be increased until the maximum intensity of the Raman spectrum is reached, while no thermal emission (as in Fig. 3.5-16 c) is visible. 

In wavenumbers relative to indicated laser. Standard emission curve (with maximum equal to 1.0 arbitrary units) is reconstructed from the coefficients and the Raman shifts (relative to 514.5 or 785 nm) raised to the indicated powers. From Reference 20. 

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