Raman radiation

In practical application, Raman sensors exclusively use frequency-stabilised laser sources to compensate for the low intensity of the Raman radiation. For Raman sensors, prevalently compact high-intensity external cavity laser diodes are used, operated in CW (continuous wave) mode. These diode lasers combine high intensity with the spectral stability required for Raman applications and are commercially available at various wavelengths. 

The infra-red spectrum of Mo(CO)3[P(OCH3)3]3 (VIII) shows three absorption bands at 1993,1919, and 1890 cm-1 in the region in which CO stretching frequencies usually appear. But Cr(CO)3(CNCH)3 (IX) has two absorption bands in the C—O stretch region at 1942 and 1860 cm-1. Octahedral ML3(CO)3 complexes can exist in either the mer or fac isomeric forms (Figure 9.2). Assign the structures of the above two molecules. How many bands would you expect to see in the vibrational Raman spectra of these two molecules, and for which of these bands would the scattered Raman radiation be polarized ... 

Besides being always Raman active, the totally symmetric vibrational modes can also be readily identified in the spectrum. As shown in Fig. 7.3.2, the scattered Raman radiation can be resolved into two intensity components, /l and 7. The ratio of these two intensities is called the depolarization ratio p ... 

G. R. Abell and C. E. Gillespie, Remote Sensing and Analyzing of Gaseous Materials Using Raman Radiation, US Patent 3625613 19680628 (1971). 

If the Raman radiation is observed through a polarizer with its electric vector perpendicular to the plane of scattering ( c in Fig. 2.4-6) then the equation 2.4-13 changes ... 

Vibrations of totally symmetric species (defined by the first row of the character tables) emit Raman lines which are polarized, the depolarization ratio pk can assume, according to Eqs. 2.4-11. .. 13 values of 0 < p < 6/7. All other Raman-active vibrations are emitting lines which are depolarized, they have a depolarization ratio of 6/7. The value 6/7 is appropriate for an arrangement, where the Raman radiation s investigated without an analyzer. If an analyzer is used 3/4 has to be taken instead. Cubic and icosahedral point groups are a special case the depolarization ratio for totally symmetric vibrations is 0. 

Changes of each component of the scattering tensor of the unit cell can be directly observed. The arrangement of the crystal with regard to the spectrometer is described by a nomenclature recommended by Porto et al. (1966). In this system, a(bc)d stands for irradiation in a direction by radiation polarized in b direction, Raman radiation polarized in c direction is observed in d direction. Here, a, b, c, d indicate any crystal axes. 

Figure 2.7-11 Raman spectra of the thiourea single crystal at 300 K, excited by using the radiation of a HeNe laser, 50 mW at 633 nm, dah)a indicates irradiation in c direction, with radiation polarized parallel to the a axis, Raman radiation is observed in a direction with an analyzer oriented parallel to the h axis.

Figure 3.3-7 Fiber bundles for Raman spectroscopy. The diameter of the fiber for the exciting radiation is a the same, b smaller than that of the fibers transporting Raman radiation.

If coherent radiation with a very high intensity is applied continuously or as pulse, non-linear effects can be observed which produce coherent Raman radiation. This is due to the quadratic and cubic terms of Eq. 2.4-14, which describe the dipole moment of a molecule induced by an electric field. Non-linear Raman spectroscopy and its application are described in separate chapters (Secs. 3.6 and 6.1), since this technique is quite different from that of the classical Raman effect and it differs considerably in its scope. 

In dispersive spectrometers, the Rayleigh radiation may produce stray radiation in the entire spectrum, the intensity of which may be higher than that of the Raman lines. Interferometers transform the Poisson distribution of the light quanta of the Rayleigh radiation into white noise, which overlays the entire Raman spectrum. Therefore, all types of spectrometers must have means to reduce the radiant power of the exciting radiation accompanying the Raman radiation. 

Figure 3.5-2 Definition of the light fluxes inside and outside a sample of thickness d for the application of the theory of Kubelka and Munk (1931) and Schrader and Bergmann (1967) Exciting radiation /(>, unshifted radiation leaving the back /p, and the front surface Jp, Raman radiation leaving the back Ip, and the front surface Jp, ip, jp, ip, and jp are the respective light fluxes inside the sample.

It is particularly important to know the intensity of the Raman radiation relative to the unshifted radiation scattered in the same direction for forward scattering (0° arrangement) ... 

In conventional Raman spectroscopy Raman spectra are recorded from colorless samples with exciting radiation in the visible range of the spectrum. In this case, the absorption of the exciting and Raman radiation by the sample is at its minimum, and the linear decadic absorption coefficient (a = e c) is of the order of 10 -. ..10cm. ... 

Figure 3.5-4 Raman radiation at a the front Jr, and b the back Ir, c reflectance p, and d transmittance T, of samples with a thickness between 0 and 2 cm and coefficients of unshifted scattering r = 0, 10, 100, 1000 cm representing liquids, coarse, medium, and fine powders, respectively.

This is a useful prerequisite for the optimization of sample arrangements the low intensity of the Raman radiation can be considerably enhanced by utilizing multiple reflections of the exciting and the emerging Raman radiation at the sample and an external spherical mirror. 

Fig. 3.5-4 b shows the intensity of the Raman radiation of a forward-scattering arrangement. At an optimum thickness , the Raman radiation has a maximum, which increases as the elastic scattering coefficient decreases. The exciting radiation which emerges from the sample (Fig. 3.5-4 d) has a lower intensity if the elastic scattering coefficient r is higher. 

Conventional Raman spectroscopy employs nonabsorbing samples and grating spectrometers. The liquid sample is irradiated with the exciting radiation, usually a laser beam. In a direction perpendicular to the laser radiation, an image of this part of the sample is produced by the entrance optics, irradiating the grating with the Raman radiation through the entrance slit (Fig. 3.5-5 a). 

For nonabsorbing crystal powders, the 0° multiple reflection arrangement shown in Fig. 3.5-8 g has proven to be superior to other arrangements, because it combines a high intensity of the Raman radiation with the maximum ratio of Ram an/exciting radiation, Ir/Ip- All of the Raman spectra of crystal powders reproduced in the Raman/ Infrared Atlas (Schrader, 1989) have been recorded with an arrangement according to this principle. 

The exciting radiation irradiates a conical volume of the sample. The Raman radiation emerging from this volume irradiates optimally the beam splitter of the interferometer through the circular Jacquinot aperture. 

A circular spot is more aberration-tolerant than a line of the same area and thus can be demagnified further (Hirschfeld, 1977). In other words, the Raman radiation of a smaller spot on the sample can be collected within a larger solid angle. This means a further optimization of the arrangement. 

Conventional Raman spectroscopy utilizes rectangular or cylindrical cuvettes. A given spectrometer collects maximum intensity of Raman radiation of a sample, if the sample is placed in the focal region of a laser beam and if a maximum amount of the Raman radiation emerging from this sample is collected by a sample optics of the spectrometer within a maximum solid angle (Schrader, 1980). As mentioned in Sec. 3.1, the optical conductance of the entrance optics should have the same value as that of the interferometer or monochromator. Inspection of conventional sample arrangements shows that these conditions were often not fulfilled optimally ... 

The effective solid angle of the collected Raman radiation of a sample in a rectangular cell is only about 40 % of the solid angle of a suitable high-aperture optical system (Fig. 3.5-6 a, b). 

Fig. 3.5-8 c shows a sample in an NMR tube. The angle of the tube axis relative to the axis of observation is 45°. When the Raman radiation is observed through the spherical bottom of the tube, some of the advantages of the spherical cell are exploited. All arrangements of Figures 3.5-8a...c can be utilized in a 180° (back-scattering) or a 90° arrangement. 

Fig. 3.5-8 i finally exhibits another universal sample head. It utilizes an optical fiber (Christie and McCreery, 1990 Schrader et al. 1988) to transport the exciting radiation to the sample. A bundle of fibers, usually with a larger diameter (Fig. 3.3-7), is arranged around the central fiber in order to transport Raman radiation from the sample to the spectrometer. The connection to the spectrometer is at its optimum if the optical conductance of the fiber bundle is as high as that of the spectrometer. The end of the fiber bundle is covered with a shield (NMR tube) to prevent sticking and pyrolysis of particles at the end of the laser fiber. It can be used to cool the head if the laser power... 

For a given light flux of a laser source, the flux of the Raman radiation is inversely proportional to the diameter of the focus of the laser beam at the sample. This means that an optimized Raman sample is a micro sample (Schrader and Meier, 1966 Schrader, 1980). The minimum focal diameter of the laser beam is of the order of the wavelength... 

Figure 3.5-10 Scanning micro arrangements for Raman spectroscopy a normal sample arrangement b arrangement using a microscope and fiber optics c scanning of surfaces with liber optics and half-spheric mirror, which reflects the part of the exciting and Raman radiation back to the sample which is not directly collected by the fiber bundle (Schrader, 1990).

If the spatial resolving power has to be high, then the Raman radiation must be observed through microscope objectives (Fig. 3.5-10 b). Unfortunately, these objectives have a somewhat lower optical conductance than the regular sample arrangement (Schrader, 1990). As a result, the observed Raman spectrum is also considerably weaker. A microscope may be connected to the spectrometer by a mirror system or by optical fibers, as shown in Fig. 3.5-10 b. Optical fibers are e.specially useful for NIR FT Raman spectroscopy, because the transmission of the fibers may be at its maximum exactly in the range of a Raman spectrum excited by a Nd YAG laser (Fig. 3.3-5). 

Figure 3.5-12 Principle of a confocal microscope a LF fiber transporting the laser radiation, D dichroitic mirror, 0 objective, S sample, the Raman radiation produced in the illuminated spot of the sample is focused upon the diaphragm A, only the radiation from the spot is focused at the fiber SF, which transports the Raman radiation to the spectrometer b focal range in the illuminated sample, Ax spatial, Az depth resolution.

Fig. 3.5-10 c shows another sample arrangement which makes use of a fiber-optical connection from the laser to the sample and back to the spectrometer. It is specially designed for the scanning of surface layers, e.g., of precious prints or paintings. The half spheric concave mirror reflects the portion of exciting radiation and Raman radiation back to the sample which has been. scattered by the sample and is not collected by the optical fiber. Thus the mirror as a component of a multiple reflection system enhances the observed intensity of the Raman lines by a factor of 2 to 8, depending on the properties of the sample. 

This section demonstrates how to calculate the flux (radiant power) of Raman radiation which is produced by irradiating a sample with laser radiation, analyzed by a. spectrometer and recorded by a radiation detector (see Eq. 3.1-1). 

This calculation, however, does not yet take into account the absorption of exciting and Raman radiation by the sample. For colorless samples, it is very low in the visible range of the spectrum, but in the NIR region it cannot be ignored. 

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