Sampling 180° backscattering geometry

Raman spectroscopy is a very convenient technique for the identification of crystalline or molecular phases, for obtaining structural information on noncrystalline solids, for identifying molecular species in aqueous solutions, and for characterizing solid—liquid interfaces. Backscattering geometries, especially with microfocus instruments, allow films, coatings, and surfaces to be easily measured. Ambient atmospheres can be used and no special sample preparation is needed. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Because instrument volume and experiment time must both be minimized for a planetary Mossbauer spectrometer, it is desirable in backscatter geometry to illuminate as much of the sample as possible with source radiation. However, this... 

Cosine smearing. Because instrument volume and experiment time must both be minimized for a planetary Mossbauer spectrometer, it is desirable in backscatter geometry to illuminate as much of the sample as possible with source radiation. However, this requirement at some point compromises the quality of the Mossbauer spectrum because of an effect known as cosine smearing [327, 348, 349] (see also Sects. 3.1.8 and 3.3). The effect on the Mossbauer spectrum is to increase the linewidth of Mossbauer peaks (which lowers the resolution) and shift their centers outward (affects the values of Mossbauer parameters). Therefore, the diameter of the source y-ray beam incident on the sample, which is determined by a... 

Samples were characterized by X-ray diffraction, magnetic susceptibility and chemical analysis with some results summarized in Table 1. The electrical resistivity measurements were made down to 80 K using a four-probe method. Raman scattering experiments used the excitation line A = 514.5 nm of an Ar+ laser in a quasi-backscattering geometry. The laser power of 5 mW was focused to a 0.1 mm diameter spot on the (010) surface. The averaged laser power density amounts to 6 105 W/m2 which is much less compared to earlier Raman studies in manganites [12-15],... 

The experimental setup for matrix Raman spectroscopy is essentially the same as that for matrix IR spectroscopy. The major difference lies in optical geometry. Namely, backscattering geometry must be employed in Raman spectroscopy since the matrix gas and sample vapor are deposted on a cold metal (Cu, Al) surface. Figure 3-27 shows the optical arrangement... 

The Raman spectra were recorded in the backscattering geometry on a Labram I (Jobin-Yvon, Horiba Group, France) microspectrometer in conjunction with a confocal microscope. To avoid any thermal photochemical effect, we have used a minimum intensity laser power on sample of 370 pW with the 514.5 nm incident line from an Ar-Kr laser from Spectra Physics. Detection was achieved with an air cooled CCD detector and a 1800 grooves/mm, giving a spectral resolution of 4 cm-1. An acquisition time of 120 s was used for each spectrum. The confocal aperture was adjusted to 200 pm and a 50 X objective of 0.75 numerical aperture was used. 

The specific intensity is an important quantity because it depends mainly on sample D) and laser (Pd) variables and not on spectrometer parameters such as collection angle, quantum efficiently, and the like L indicates what the spectrometer has to work with while collecting and detecting scattered light. If we consider the example of a clear sample and 180° backscattered geometry with = 0.1 cm (as in Fig. 2.5), then L can be calculated for a variety of samples. Table 2.4 lists several specific intensities for samples of... 

As noted earlier, the 180° backscattered geometry is experimentally convenient and has become quite common in commercial instruments. Most fiber-optic probes and Raman microscopes also use 180° backscattered geometry, so similar principles apply in all three sampling categories. 

Fig. 6. Simple liquid N2 temperature cryostat for RR studies of frozen protein solutions. The protein solution (typically 5-15 /xl of 1-10 mM concentration) is placed in a copper cup on the end of a cold finger to give a flat surface. The glass or quartz shroud is clamped over the cold finger, and the sample is frozen by pouring liquid N2 in the horizontal Dewar flask. Once the sample is frozen, the Dewar flask is turned to a vertical position and evacuated to lO -lO" torr. The Dewar flask, filled with liquid N2, is transferred to the Raman sample compartment, and the scattered light is collected via 135° backscattering geometry directly from the surface of a frozen protein solution. ...

Figure 1. (a) Micro-Raman spectra of Si-matrix with buried and unburied layers in it, taken in backscattering geometry, (b) Raman spectra of samples with buried and unburied layers, taken in near perpendicular geometry. The configurations are shown in the insets of the figures. 

The interference enhancement technique again uses a conventional backscattering geometry but requires that the sample to be studied be deposited as the top layer of a... 

The basic experiment in HREELS in the backscattering geometry is straightforward [37], A monochromatized electron beam of 1-10 eV is directed toward the surface and the energy distribution of the reflected electrons is measured in an electron analyzer with a resolution of up to 7 meV. The spectrum consists of the elastic peak and peaks due to energy losses to the sample surface by the excitation of molecular vibrations. If plotted as wave numbers, these vibrations are very similar to those observed in IR techniques. The resolution achievable in this technique is, however, considerably less than in IR, which becomes clear if one considers that 1 meV = 8.066 cm , so the spectral resolution in HREELS is of the order of 100 cm (in IR the resolution is typically around 4 cm" or better). Detection of crystallinity or other high-resolution details as is possible in IR is therefore currently not achievable in HREELS. 

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