Microscopy Raman

Raman is another vibrational spectroscopy, complementary to IR in that it is sensitive to the excitation of bonds that are non-polar but polarizable, while IR requires a dipole moment. The specimen is illuminated by a laser and most light is scattered elastically. Some small part is shifted down in frequency (Stokes) by a bond becoming vibrationally excited, and an even smaller part is shifted up (anti-Stokes) by vibrational de-excitation. The whole spectrum allows compounds and polymers to be identified and 

Most Raman spectrometers are dispersive, not Fourier transform, so the Raman spectrum is spread out in space. A line detector gives the Raman spectrum of a point and a 2D detector can only give the spectra from a line. Thus most Raman imaging is obtained by scanning the sample. This can be relatively slow because of the small signal, with 1 s data acquisition time at each point. Even so there are many uses of Raman imaging, in gel structure for example [313,314]. It may be more effective to use filter- 

As the CSLM (see Section 6.2.1) uses an optical laser as its illumination source, it is relatively straightforward to attach a Raman spectrometer to its output to give a confocal Raman microscope. This further limits the region being analyzed and can produce optical sections and 3D images of chemical groups [316, 317]. Dichroism in a confocal Raman microscope has been used to measure polymer orientation in films [318, 319] and fibers [320]. 

A technique that is more complex optically and gives a large Raman signal is called coher- 

Raman spectroscopy coupled to a microscope permits the analysis of very small samples nondestructively. The use of Raman microscopy allows the characterization of specific 

FTIR microscopy was discussed earlier in the chapter, and given the complementary nature of IR and Raman, it is reasonable that laboratories performing IR microscopy might well need Raman microscopy and vice versa. Two microscope systems were required and the sample had to be moved from one system to the other. The difficulty of relocating the exact spot to be sampled can be imagined. A new combination dispersive Raman and FTIR microscopy system was introduced in 2002. The system, called the LabRam-IR (JY Floriba, Edison, NJ), allows both Raman and IR spectra to be collected at exactly the same location on the sample. The resolution depends on the 

One example of the application of this technique is the investigation of different phases of fluid inclusions in a quartz crystal found in the Swiss Alps. Their Raman spectra permitted determination of CO2 as a gaseous inclusion and water as a liquid inclusion, and they showed that the mineral thought to be CaS04 was in factCaCOs [367]. 

A typical experimental arrangement for Raman microscopy is shown in Fig. 3.25. The output beam of an argon laser or a dye laser is focused by a microscope objective into the microsample. The backscattered Raman light is imaged onto the entrance slit of a double or triple monochromator, which effectively supresses scattered laser light. A CCD camera at the exit of the monochromator records the wanted spectral range of the Raman radiation [364, 368, 369], 

Since spatial resolution is diffraction limited, short wavelength lasers are optimal for analyzing small sample features. In order to achieve micron-level spatial resolution, the alignment of the Raman microscope is critical. The visual light path, the excitation laser beam path, and the Raman scatter beam path from the sample to the detector must aU be targeted precisely on the same spot. 

Raman microscopes are available from a number of instrument companies including Bruker, HORIBA Scientific, Jasco, Thermo Fisher Scientific, WITec, and Renishaw pic, among others. 

Raman microscopes are more commonly used for materials characterization than other Raman instruments. Raman microscopes are able to examine microscopic areas of materials by focusing the laser beam down to the micrometer level without much sample preparation as long as a surface of the sample is free from contamination. This technique should be referred to as Raman microspectroscopy because Raman microscopy is not mainly used for imaging purposes, similar to FUR microspectroscopy. An important difference between Raman micro-and FUR microspectroscopies is their spatial resolution. The spatial resolution of the Raman microscope is at least one order of magnitude higher than the FTIR microscope. 

Raman microspectroscopy (often called micro-Raman), like most Raman spectrometry, is of the dispersive type. It requires collecting a spectrum at each wavenumber separately, not like the FUR type that collects a spectrum in a range of wavenumbers simultaneously. Although this chapter only describes the instrumentation for Raman microscopy, its working principles and spectra are basically the same as those of conventional dispersive Raman instruments, which consist of the following elements  

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. 

The microscope in the Raman system is different from conventional microscopes used for observation of microstructure in two aspects the microscope only needs to illuminate a 

The scattered light from the microscope must be passed through special filters before reaching the spectral analyzer in order to remove elastically scattered light. The Raman light cannot 

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Figure Bl.2.12. Schematic diagram of apparatus for confocal Raman microscopy. From [3], used with penuission.

Raman microscopy is more developed than its IR counterpart. There are several reasons for this. First, the diffraction limit for focusing a visible beam is about 10 times smaller than an IR beam. Second, Raman spectroscopy can be done in a backscattering geometry, whereas IR is best done in transmission. A microscope is most easily adapted to a backscattermg geometry, but it is possible to do it in transmission. 

Raman microscopy is particularly adept at providing infonuation on heterogeneous samples, where a... 

Turrell G and Corset J (eds) 1996 Raman Microscopy Developments and Applications (New York Academic)... 

In addition to covering Raman microscopy, this book has a wealth of information on Raman instrumentation in general. Elving P J and Winefordner J D (eds) 1986 Fourier Transform Infrared Spectroscopy (New York Wiley)... 

Schrof W, Klingler J, Heckmann W and Horn D 1998 Confocal fluorescence and Raman microscopy in industrial research Colloid Polym. Sc/. 276 577-88... 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

The chemical composition of the glycerin liquid after the EEF test was measured with a Raman microscopy as shown in Fig. 54. Curve (a) is a typical Raman spectrum of glycerin without any EEF applied, and Curve (b) is the Raman spectrum of the glycerin after the positive EEF intensity... 

With regard to the confinement and enhancement ability of a metallic nano-tip, we have proposed near-field Raman microscopy using a metallic nano-tip [9]. The metallic nano-tip is able to enhance not only the illuminating light but also the Raman scattered light [9, 15, 16]. Figure 2.5 illustrates our nano-Raman microscope that mainly comprises an inverted microscope for illumination and collection of Raman... 

When detailed information is needed about local variations in composition, Raman microscopy is used. 

Raman microscopy has been used for analysis of very small samples or small heterogeneities in larger samples. Recent developments and applications of this technique have been reviewed by Turrell and Corset (1996), including a discussion of the coupling of Raman microscopy with electron, ion and x-ray microscopies, and these authors give a description of a number of prototype instruments with this facility. 

A schematic diagram of conventional Raman microscopy, due to Turrell and Corset (1966), is given in Figure 3.7 which illustrates the laser focussing, sample... 

The use of Raman microscopy in the detection and identification of pigments on manuscripts, paintings, ceramics and papyri was reviewed by Clark (1999). He concludes that it is arguable the best single technique to be applied to this area, since it combines the attributes of reproducibility and sensitivity with those of being nondestructive and immune to interference from both pigments and binders. He points... 

Turrell, G. Corset, J. (1996) Raman Microscopy Developments and Applications, Academic Press, London. 

Raman microscopy has the ability to investigate regions down to 1 pm, and, by the aid of fibre optics, remote sampling is possible. The molecular information... 

Infrared and ultraviolet probes for surface analysis are then considered.The applications of IR spectroscopy and Raman microscopy are discussed, and a brief account is also given of laser-microprobe mass spectrometry (LAMMA). 

Burgio, L., R. J. H. Clark, T. Stratoudaki, M. Doulgeridis, and D. Anglos (2000), Pigment identification in painted artworks A dual analytical approach employing LIBS (laser-induced breakdown spectroscopy) and Raman microscopy, Appl. Spectrosc. 54(4), 463-469. 

Clark, R. J. H. and P. J. Gibbs (1997), Non-destructive in situ study of ancient Egyptian Faience by Raman microscopy, /. Raman Spectrosc. 28, 99-103. 

Corset, J., P. Dhamelincourt, and J. Barbillat (1989), Raman microscopy, Chem. Britain (June), 612-616. 

G. Turrell, M. Delhaye and P. Dhamelincourt, In G. Turrell and J. Corset (Eds.), Raman Microscopy-Developments and Applications, Elsevier, Amsterdam, 1996, pp. 27-49. 

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