Raman spectrum detection

Nitrophenyl groups covalently bonded to classy carbon and graphite surfaces have been detected and characterized by unenhanced Raman spectroscopy in combination with voltammetry and XPS [4.292]. Difference spectra from glassy carbon with and without nitrophenyl modification contained several Raman bands from the nitrophenyl group with a comparatively large signal-to-noise ratio (Fig. 4.58). Electrochemical modification of the adsorbed monolayer was observed spectrally, because this led to clear changes in the Raman spectrum. 

The Raman spectrum of an oxide sample after adsorption may be considered to consist of the spectrum of the adsorbed species superimposed on the spectrum due to the oxide adsorbent. In general, the Raman spectra of oxide adsorbents are sufficiently weak or sufficiently simple that they allow the detection of Raman lines due to the adsorbed species. This is one major advantage of Raman scattering over infrared absorption spectroscopy. The infrared spectra of most oxide adsorbents show strong absorptions which may obscure those arising from the adsorbates (Figs. 13,14). 

Fiq. 20a. The pulsed Raman spectrum of Mn-doped ZnSe single crystal using a detection interval of 200 nsec. Broad band fluorescence superimposed on a large instrumental scattered light component was observed. Recordings taken with ratemeter time constants (TC) of 1 sec and 10 sec are shown (37). 

Fiq. 20b. The pulsed Raman spectrum of Mn-doped ZnSe with a 1 /xsec detection interval. The fluorescent background was significantly reduced from that observed with a 200 nsec detection interval in Fig. 20a (37). 

Fig. 29 Raman spectrum of p-S at high pressure and room temperature [109]. The wavenumbers indicated are given for the actual pressure. No signals of other allotropes have been detected. The line at 48 cm (ca. 25 cm atp 0 GPa) may arise from lattice vibrations, while the other lines resemble the typical pattern of internal vibrations of sulfur molecules...

There are four disulfide bonds in short-chain (Type I) neurotoxins. This means that there are eight half-cystines. However, all Hydrophiinae toxins have nine halfcystines with one cysteine residue. An extra cysteine residue can be readily detected from the Raman spectrum as the sulfhydryl group shows a distinct S-H stretching vibration at 2578 cm" Some Laticaudinae toxins do not have a free cysteine residue as in the cases of L. laticaudata and L. semifasciata toxins. In long toxins (Type II) there are five disulfide bonds (Table III). 

In the time-domain detection of the vibrational coherence, the high-wavenumber limit of the spectral range is determined by the time width of the pump and probe pulses. Actually, the highest-wavenumber band identified in the time-domain fourth-order coherent Raman spectrum is the phonon band of Ti02 at 826 cm. Direct observation of a frequency-domain spectrum is free from the high-wavenum-ber limit. On the other hand, the narrow-bandwidth, picosecond light pulse will be less intense than the femtosecond pulse that is used in the time-domain method and may cause a problem in detecting weak fourth-order responses. 

Raman spectroscopy is a useful probe for detecting transannular S - S interactions in bicyclic or cage S-N molecules or ions. The strongly Raman active vibrations occur at frequencies in the range 180-300 cm-1, and for S- -S distances in the range 2.4-2.7 A. On the basis of symmetry considerations, the Raman spectrum of the mixed sulfur-selenium nitride S2Se2N4 was assigned to the 1,5- rather than the 1,3- isomer.37... 

The number of fundamental vibrational modes of a molecule is equal to the number of degrees of vibrational freedom. For a nonlinear molecule of N atoms, 3N - 6 degrees of vibrational freedom exist. Hence, 3N - 6 fundamental vibrational modes. Six degrees of freedom are subtracted from a nonlinear molecule since (1) three coordinates are required to locate the molecule in space, and (2) an additional three coordinates are required to describe the orientation of the molecule based upon the three coordinates defining the position of the molecule in space. For a linear molecule, 3N - 5 fundamental vibrational modes are possible since only two degrees of rotational freedom exist. Thus, in a total vibrational analysis of a molecule by complementary IR and Raman techniques, 31V - 6 or 3N - 5 vibrational frequencies should be observed. It must be kept in mind that the fundamental modes of vibration of a molecule are described as transitions from one vibration state (energy level) to another (n = 1 in Eq. (2), Fig. 2). Sometimes, additional vibrational frequencies are detected in an IR and/or Raman spectrum. These additional absorption bands are due to forbidden transitions that occur and are described in the section on near-IR theory. Additionally, not all vibrational bands may be observed since some fundamental vibrations may be too weak to observe or give rise to overtone and/or combination bands (discussed later in the chapter). 

Resonance Raman Spectroscopy. A review of the interpretation of resonance Raman spectra of biological molecules includes a consideration of carotenoids and retinal derivatives. Another review of resonance Raman studies of visual pigments deals extensively with retinals. Excitation profiles of the coherent anti-Stokes resonance Raman spectrum of j8-carotene have been presented. Resonance Raman spectroscopic methods have been used for the detection of very low concentrations of carotenoids in blood plasma and for the determination of carotenoid concentrations in marine phytoplankton, either in situ or in acetone extracts. ... 

The infrared (IR) spectra of 1,10-phenanthroline, its hydrate and perchlorate in the region 600-2000 cm-1 have been obtained, and the principal features of the spectra interpreted.66 Further studies on the IR spectra of 1,10-phenanthroline,67-69 substituted 1,10-phenanthrolines,70,71 and 1,7-phenanthroline67 have also been reported. The IR spectrum of 4,7-phenanthroline in the region 650-900 cm-1 has been analyzed, and the C—H out-of-plane deformation frequencies were compared with those of phenanthrene and benzo[/]quinoline.72 The IR spectra of salts of 1,10-phenanthroline have been taken, and the NH vibrations determined.28,73 Infrared spectroscopy has been used to detect water associated with 1,10-phenanthroline and some of its derivatives on extraction into nitromethane from aqueous solution.74 The Raman spectrum of 1,10-phenanthroline has been compared with its IR spectrum.75 Recently, the Raman and IR spectra of all ten isomeric phenanthrolines were measured in solution and solid states, and the spectra were fully discussed.76... 

Recently Eliasson and Matousek [26, 62] demonstrated that SORS can provide a chemical signature of the internal content of opaque plastic containers. This is demonstrated in Fig. 3.11 for aspirin tablets held inside an opaque (white) plastic pharmaceutical bottle (1.3mm thick). The conventional Raman signal is overwhelmed by the Raman component originating from the container wall and is consequently ineffective in determining the contents of the bottle. In contrast, the SORS method using a scaled subtraction of two SORS spectra measured at different spatial offsets yields a clean Raman spectrum of the tablets inside the bottle. SORS has also been used in the detection of counterfeit anti-malarial tablets by Ricci et al. [63] the chemical specificity of Raman spectroscopy readily distinguished between genuine and fake tablets and identified the content of the counterfeit tablets. 

Analogous to the principal concept of multiplex CARS microspectroscopy (cf. Sect. 6.3.5), in multiplex SRS detection a pair of a broad-bandwidth pulse, eg., white-light femtosecond pulse, and a narrow-bandwidth picosecond pulse that determine the spectral width of the SRS spectrum and its inherent spectral resolution, respectively, is used to simultaneously excite multiple Raman resonances in the sample. Due to SRS, modulations appear in the spectrum of the transmitted broad-bandwidth pulse, which are read out using a photodiode array detector. Unlike SRS imaging, it is difficult to integrate phase-sensitive lock-in detection with a multiplex detector in order to directly retrieve the Raman spectrum from these modulations. Instead, two consecutive spectra, i.e., one with the narrow-bandwidth picosecond beam present and one with that beam blocked, are recorded. Their ratio allows the computation of the linear Raman spectrum that can readily be interpreted in a quantitative manner [49]. Unlike the spectral analysis of a multiplex CARS spectrum, no retrieval of hidden phase information is required to obtain the spontaneous Raman response in multiplex SRS microspectroscopy. 

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