Sampling Raman advantages

Vibrational spectroscopy is a vitally important technique in inorganic chemistry, used both to identify new and known compounds, and to check the purity of samples. Infrared (IR) spectrometers are widely available and are relatively cheap, so IR spectroscopy is normally one of the first techniques to be used when studying a new sample. Raman spectrometers have also become more routinely used in recent years, and have the advantage that they can focus on tiny specimens. 

In terms of axial resolution, for transparent samples, Raman microscopy is governed by the focal depth of the objective, whereas IR microscopy is governed by the thickness of the sample (in simple transmission mode). For a 100 x objective, this is typically 1-2 pm and the axial resolution can be improved by taking advantage of confocal imaging conditions. An important consideration, however, is that whereas in the visible region the majority of cellular... 

One of the well known advantages of resonance Raman spectroscopy is that samples dissolved in water can be studied since water is transparent in the visible region. Furthennore, many molecules of biophysical interest assume their native state in water. For this reason, resonance Raman spectroscopy has been particularly strongly embraced in the biophysical connnunity. 

The Raman spectrum of an oxide sample after adsorption may be considered to consist of the spectrum of the adsorbed species superimposed on the spectrum due to the oxide adsorbent. In general, the Raman spectra of oxide adsorbents are sufficiently weak or sufficiently simple that they allow the detection of Raman lines due to the adsorbed species. This is one major advantage of Raman scattering over infrared absorption spectroscopy. The infrared spectra of most oxide adsorbents show strong absorptions which may obscure those arising from the adsorbates (Figs. 13,14). 

Taking into consideration that antenna xanthophylls not only possess original absorption but also resonance Raman spectra, and the fact that the Raman signal is virtually free from vibrational spectroscopy artifacts (water, sample condition, etc.), it seemed of obvious advantage to apply the described combination of spectroscopies for the identification of these pigments. 

The great advantage of these methods is the possibility of creating three-dimensional distributions, also referred to as tomographs, which is not possible with FTIR spectroscopy and not always possible with Raman spectroscopy, if, for instance the sample is opaque. 

Raman microscopy provides a spatial resolution slightly better than IR, and no sample preparation is necessary in many cases. It has advantages with special types of substances (e.g., systems containing conjugated double bonds, oriented systems, amorphous and crystalline carbon, oxides). SNOM techniques (with spatial resolution below 1 pm) have been more popular with Raman than with IR, so far, but as yet are not routinely practiced. 

An interesting and powerful new development in Raman spectroscopy of catalysts is the use of a UV laser to excite the sample. This has two major advantages. First, the scattering cross section, which varies with the fourth power of the frequency, is substantially increased. Second, the Raman peaks shift out of the visible region of the spectrum where fluorescence occurs. The reader is referred to Li and Stair for applications of UV Raman spectroscopy on catalysts [40]. 

It is important to appreciate that Raman shifts are, in theory, independent of the wavelength of the incident beam, and only depend on the nature of the sample, although other factors (such as the absorbance of the sample) might make some frequencies more useful than others in certain circumstances. For many materials, the Raman and infrared spectra can often contain the same information, but there are a significant number of cases, in which infrared inactive vibrational modes are important, where the Raman spectrum contains complementary information. One big advantage of Raman spectroscopy is that water is not Raman active, and is, therefore, transparent in Raman spectra (unlike in infrared spectroscopy, where water absorption often dominates the spectrum). This means that aqueous samples can be investigated by Raman spectroscopy. 

Raman spectroscopy, because of its versatility and wide applicability, has been used for a wide range of art historical and conservation science (Edwards 2000) and archaeological applications (Smith and Clark 2004). Fourier transform Raman spectroscopy (FTRS) in particular has the advantage of being a reflective method which allows direct, nondestructive analysis. It can also be used through a microscope to allow the characterization of small samples. 

It is more difficult to perform ultrafast spectroscopy on neat H20 (than it is on H0D/D20 or HOD/H20) since the neat fluid is so absorptive in the OH stretch region. One innovative and very informative technique, developed by Dlott, involves IR pumping and Raman probing. This technique has a number of advantages over traditional IR pump-probe experiments The scattered light is Stokes-shifted, which is less attenuated by the sample, and one can simultaneously monitor the populations of all Raman-active vibrations of the system at the same time. These experimental have been brought to bear on the spectral diffusion problem in neat water [18, 19, 75 77],... 

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