Conventional Raman Spectrometer

To get a Raman spectrum, it is necessary to expose a specimen to a monochromatic source of exciting photons, and then measure the intensity of the scattered beam of light. The intensity of the Raman scattered constituent is, by far, inferior than the Rayleigh scattered part. Then, filters and diffraction gratings are utilized to suppress the Rayleigh component. Besides, an extremely sensitive detector is necessary to sense the almost imperceptibly scattered Raman photons [57], 

In order to get a Raman spectrum, a sample is located in the sample cell. Then, a laser light is focused on the sample using a lens. Usually, the sample cell is a capillary tube, normally made of Pyrex glass, where liquids and solids are sampled in [57], Or another appropriate sample holder system where is given the possibility that the light scattered during its interaction with the sample is accumulated using an additional lens and is then focused at the entry slit of the monochromator [32,62], 

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The SERS spectra can be obtained on a conventional Raman spectrometer (see Figure 2). In this particular system, 632.8 nm radiation from a Helium-Neon laser is used with an excitation power of 5 mW. Signal collection is performed at 0° with respect to the incident laser beam. This coaxial excitation/collection geometry is achieved with a small prism, which is used to direct the excitation beam to the sample while allowing... 

In a conventional Raman spectrometer (Figure 4.19), a visible laser radiation is used as the source of exciting photons these photons are typically of much higher energies than those of the fundamental vibrations of most chemical bonds or systems of bonds. 

FIGURE 4.19 Schematic representation of a conventional Raman spectrometer. 

Micro-Raman spectroscopy (pRS) involves acquiring spatially resolved Raman spectra by combining the conventional Raman spectrometer with a microscopic tool, typically an optical microscope. This chapter introduces the basic methodology of micro-Raman spectroscopy and presents an overview of its application to organic and inorganic nanostructures using specific examples from literature. 

Raman spectroscopy requires highly monochromatic light, which can be provided only by a laser source. The laser source is commonly a continuous-wave laser, not a pulsed laser. The laser source generates laser beams with the wavelengths in the visible light range or close to the range. In a Raman microscope, sample illumination and collection are accomplished in the microscope. The microscope s optical system enables us to obtain a Raman spectrum from a microscopic area this is the main difference between the micro-Raman and conventional Raman spectrometers. 

Thus, the choice of an incident radiation Aj defines k , whereas the choice of a scattering angle (usually 90° or 180") unambiguously determines and, henceforth, the Brillouin shifi related to each of the three acoustic modes with their own velocities. Typically, for an incident visible radiation, and a material characterized by an index of refraction about 1.5 and sound velocities of a few km/s, the Brillouin shift lies in the range of 10-30 GHz (0.33-1 cm ) in backscattering geometry. Obviously, the study of Brillouin lines requires a more consequent resoiution than the one of a conventional Raman spectrometer. 

Comparison of Different Raman Systems The Raman microprobe system has a very high collection efficiency compared with the conventional Raman spectrometer. However, since the laser spot is highly focused, it is very difficult to avoid... 

In principle, no special Raman instrumentation is needed to perform RRS because RR spectra can be obtained with conventional Raman spectrometers, if only the suitable excitation wavelength is applied. However, resonance Raman scattering is experimentally more difficult to implement than normal spontaneous Raman scattering. The excitation wavelength must be made to match the absorption band of the electronic chromophore of interest. The absorption band makes both the excitation intensity and Raman scattered intensity dependent on sample thickness, complicating quantitative analysis. Absorption of the excitation intensity can damage the sample due to heating and/or photochemistry. 

FT-Raman spectroscopy using FTIR instmmentation solves another problem frequently encountered in conventional Raman spectrometers, which is the lack of sufficient frequency precision required to perform spectral subtractions. This poor precision arises from back-lash in the mechanical drive and fluctuations in the laboratory environment which causes changes in alignment. 

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