Spectrometers Raman

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

Raman spectrometers are not as widespread as their IR counterparts. This is partially due to the more stringent... 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

Even while Raman spectrometers today incorporate modem teclmology, the fiindamental components remain unchanged. Connnercially, one still has an excitation source, sample illuminating optics, a scattered light collection system, a dispersive element and a detechon system. Each is now briefly discussed. 

Continuous wave (CW) lasers such as Ar and He-Ne are employed in conmionplace Raman spectrometers. However laser sources for Raman spectroscopy now extend from the edge of the vacuum UV to the near infrared. Lasers serve as an energetic source which at the same hme can be highly monochromatic, thus effectively supplying the single excitation frequency, v. The beams have a small diameter which may be... 

From a recent Raman study of thiazole on a laser Raman Spectrometer it appears that the actual data differ somewhat from those of Ref. 203. 

An FT-Raman spectrometer is often simply an FTIR spectrometer adapted to accommodate the laser source, filters to remove the laser radiation and a variety of infrared detectors. 

The use of vibrational Raman spectroscopy in qualitative analysis has increased greatly since the introduction of lasers, which have replaced mercury arcs as monochromatic sources. Although a laser Raman spectrometer is more expensive than a typical infrared spectrometer used for qualitative analysis, it does have the advantage that low- and high-wavenumber vibrations can be observed with equal ease whereas in the infrared a different, far-infrared, spectrometer may be required for observations below about 400 cm. ... 

Fig. 3. Schematic diagram of an axial transmissive siagle-stage Raman spectrometer.

Figure 1 Drawing of single-channei Raman spectrometer showing Czerny-Turner type doubie monochromator. Coiiecting optica for scattered beam are not shown.

Photomultipliers are used as detectors in the single-channel instruments. GaAs cathode tubes give a flat frequency response over the visible spectrum to 800 nm in the near IR. Contemporary Raman spectrometers use computers for instrument control, and data collection and storage, and permit versatile displays. 

Fig. 7. Block diagram of components of a laser Raman spectrometer.

A modern laser Raman spectrometer consists of four fundamental components a laser source, an optical system for focusing the laser beam on to the sample and for directing the Raman scattered light to the monochromator entrance slit, a double or triple monochromator to disperse the scattered light, and a photoelectric detection system to measure the intensity of the light passing through the monochromator exit slit (Fig. 7). 

The monochronomator in a Raman spectrometer must have excellent stray light and resolution characteristics. 

Table IV lists some commercial Raman spectrometers with their performance specifications.

In-sltu Raman experiments were performed on a Spex 1401 double monochrometer Raman spectrometer, using a Spectra-Physlcs Model 165 argon Ion laser with an exciting wavelength of 5145 A. The In-sltu Raman cell consists of a quartz tube situated In a temperature controlled heating block. The Raman spectra were collected In the 180° backscatterlng mode. 

Figure 6.2 A fourth-order coherent Raman spectrometer constructed with a Ti sapphire regenerative amplifier (Ti sapphire) and noncollinear optical parametric amplifier (NOPA).

Raman spectroscopy has enjoyed a dramatic improvement during the last few years the interference by fluorescence of impurities is virtually eliminated. Up-to-date near-infrared Raman spectrometers now meet most demands for a modern analytical instrument concerning applicability, analytical information and convenience. In spite of its potential abilities, Raman spectroscopy has until recently not been extensively used for real-life polymer/additive-related problem solving, but does hold promise. Resonance Raman spectroscopy exhibits very high selectivity. Further improvements in spectropho-tometric measurement detection limits are also closely related to advances in laser technology. Apart from Raman spectroscopy, areas in which the laser is proving indispensable include molecular and fluorescence spectroscopy. The major use of lasers in analytical atomic... 

Figure 3.7. Schematic diagram of the basic layout of the apparatus typically employed in micro-Raman spectrometers, or microprobes. (From Turrell and Corset 1996.)...

We thank Julius Chang and Jerome P. Downey for providing the iron carbide sample. This work was supported by DARPA via ONR contract. The purchase of the Raman spectrometer and environmental SEM was supported byNSF grants DMR-0116645 and BES-0216343. 

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... 

Figure 6 reproduces the Raman spectra in the region 800-1200 cm-1 reported by these authors for pure silicalite (sample 1) and for two TS-1 samples, 3 and 5, which contain 1.4 and 3.0 wt% Ti02. The spectra shown in Fig. 6a were recorded with a Fourier transfrom (FT) Raman spectrometer at an excitation wavelength of Aexc = 1064 nm (9398 cm-1), whereas those shown in Fig. 6b were excited with a UV-laser line at Aexc = 244 nm (40,984 cm-1). With each excitation wavelength, the pure silicalite gives rise to weak bands at 975 and 1085 cm -1 and a complex band centered near 800 cm-1. In the FT-Raman spectra of the dehydrated TS-1 samples (Fig. 6a), a band is clearly visible at 960 cm-1, the intensity of which increases with Ti02 content. 

A comparison between the efficiency of excitation with lasers and mercury lamps has been undertaken by Evans etal. and Brandmuller etal. Since that time, lasers have improved considerably and a later comparison would be even more in favor of laser applications. Since several commercial laser Raman spectrometers are now available 190 ), with He-Ne lasers, Ar" or Kr -ion lasers and neodymium lasers, most current investigations employ lasers as light sources, j)... 

Fig. 46. An overhead view of the electrodynamic balance and Raman spectrometer developed by Buehler (1991) for gas/microparticle chemical reaction studies.

The nse of Raman spectroscopy to monitor the reaction of chlorine gas and elemental phosphorous to produce phosphorous trichloride was first reported in 1991 and has been extensively described in patents and journals [67,89-93]. AFT-Raman spectrometer was selected to minimize fluorescence, but signal was... 

R.L. Green and C.D. Brown, Raw-material authentication using a handheld Raman spectrometer, Pharm. TechnoL, 32, 148-162 (2008). 

W. Schabel, I. Ludwig and M. Kind, Measurements of concentration profiles in polymeric solvent coatings by means of an inverse confocal micro Raman spectrometer - Initial results. Drying Technol., 22, 285-294 (2004). 

Raman spectra were obtained on a Coberg PHO Raman spectrometer equipped with a Coherent Radiation Model 52B Ar laser using 800-1200 mW of power from the 488.8-nm line a small twoprism monochromator was used to remove Imck-ground plasma lines. 

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