Experimental Techniques of Linear Laser Raman Spectroscopy

Optical fibre with small diameter (Fig. 3.4). Light from other points than the focal point are not transmitted by the aperture. A lateral resolution of a few microns can be obtained. The axial resolution depends on the focal length of lens Li. The light from layers above or below the focal plane has a larger diameter and therefore only a small fraction passes the aperture. 

2 Experimental Techniques of Linear Laser Raman Spectroscopy 

In earlier days of Raman spectroscopy the photographic plate was the only detector used to record the Raman spectra. The introduction of sensitive photomultipliers and, in particular, the development of image intensifiers and optical multichannel analyzers with cooled photocathodes (Vol. 1, Sect. 4.5) have greatly enhanced the detection sensitivity. Image intensifiers and instrumentation such as optical multi- 

The third experimental component that has contributed to the further improvement of the quality of Raman spectra is the introduction of digital computers to control the experimental procedure, to calibrate the Raman spectra, and to analyze the data. This has greatly reduced the time spent for preparing the data for the interpretation of the results [317]. 

Because of the increased sensitivity of an intracavity arrangement, even weak vibrational overtone bands with Ap 1 can be recorded with rotational resolution. For illustration. Fig. 3.7 shows the rotationally resolved Q-branch of the D2 molecule for the transitions (p = 2 p = 0) [318]. The photon counting rate for the overtone transitions was about 5000 times smaller than those for the fundamental 

---

In the first part of this chapter the experimental techniques of linear and nonlinear Raman spectroscopy of gases are reviewed. The nonlinear techniques (Stimulated Raman Gain Spectroscopy, Inverse Raman Spectroscopy, Coherent Anti-Stokes Raman Spectroscopy, Photo-Acoustic Raman Spectroscopy, and Ionization-Detected Stimulated Raman Spectroscopy) have the capability of very high resolution, limited by the linewidths of the lasers used and pressure broadening effects. 

It may be concluded, from the analysis of the Raman results, that the information provided by Raman spectroscopy is, in essence, similar to that of infrared spectroscopy. The exploitation of the data, namely, the frequencies and intensities due to the molecular vibrations, is of a certain benefit in giving some insight as to the conformations of carbohydrates, and their interactions with the environment. As laser-Raman spectroscopy is applicable to solids, as well as to aqueous solutions, the linear relationship between Raman intensities and mass concentrations, and the specificity and high quality of the spectra experimentally obtained, make this technique particularly promising in investigations of the chemistry and biochemistry of carbohydrates. 

The application of lasers in optical experimental techniques has led to a rapid development of research into the properties of elementary excitations in solids. In addition to the conventional methods of linear crystal optics, Raman scattering of light (RSL) has become one of the principal research methods, as have its various modifications, such as coherent active Raman spectroscopy and others. 

A larger increase of sensitivity in linear Raman spectroscopy of liquids has been achieved with optical-fiber Raman spectroscopy. This technique uses a capillary optical fiber with the refractive index nu filled with a liquid with refractive index He > m. If the incident laser beam is focused into the fiber, the laser light as well as the Raman light is trapped in the core due to internal reflection and therefore travels inside the capillary. With sufficiently long capillaries (1—30 m) and low losses, very high spontaneous Raman intensities can be achieved, which may exceed those of conventional techniques by factors of 10 [8.31]. Figure 8.7 shows schematically the experimental ar-... 

---