Raman intensity ratio

For most purposes only the Stokes-shifted Raman spectmm, which results from molecules in the ground electronic and vibrational states being excited, is measured and reported. Anti-Stokes spectra arise from molecules in vibrational excited states returning to the ground state. The relative intensities of the Stokes and anti-Stokes bands are proportional to the relative populations of the ground and excited vibrational states. These proportions are temperature-dependent and foUow a Boltzmann distribution. At room temperature, the anti-Stokes Stokes intensity ratio decreases by a factor of 10 with each 480 cm from the exciting frequency. Because of the weakness of the anti-Stokes spectmm (except at low frequency shift), the most important use of this spectmm is for optical temperature measurement (qv) using the Boltzmann distribution function. 

Large Specific Surface Area Porous materials can have a large proportion of surface atoms - their surface area within a typical sampling volume of 10 pm can reach 10 pm, which is approximately 10 larger than for a smooth surface crossing the same volume. These effects lead to clearly increased Raman intensities of surface species and also to improved intensity ratios of surface and bulk Raman bands. 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

Quantification at surfaces is more difficult, because the Raman intensities depend not only on the surface concentration but also on the orientation of the Raman scat-terers and the, usually unknown, refractive index of the surface layer. If noticeable changes of orientation and refractive index can be excluded, the Raman intensities are roughly proportional to the surface concentration, and intensity ratios with a reference substance at the surface give quite accurate concentration data. 

Present formal definitions of intensities and Raman depolarization ratios,... 

A frequency job begins by computing the energy of the input structure. It then goes on to compute the frequencies at that structure. Gaussian predicts the frequencies, intensities, and Raman depolarization ratios and scattering activities for each spectral line ... 

Raman spectroscopy of matrix-isolated molecules carries some difficulties conneeted with the possibility of local heating of the matrix under laser irradiation. Besides, because of the relatively low intensity of Raman bands, higher concentrations of the species to be studied are needed in the matrix (the ratio of matrix gas to reagent = 100-500). As a result, the effective isolation of reactive intermediates is prevented. 

This picture was found to be consistent with the comparison of Raman spectra and optical gap of a-C H films deposited by RFPECVD, with increasing self-bias [41], It was found that both, the band intensity ratio /d//g and the peak position (DQ increased upon increasing self-bias potential. At the same time, a decrease on the optical gap was observed. Within the cluster model for the electronic structure of amorphous carbon films, a decrease in the optical gap is expected for the increase of the sp -carbon clusters size. From this, one can admit that in a-C H films, the modifications mentioned earlier in the Raman spectra really correspond to an increase in the graphitic clusters size. 

Figure 3.17 presents ps-TR spectra of the olehnic C=C Raman band region (a) and the low wavenumber anti-Stokes and Stokes region (b) of Si-rra i-stilbene in chloroform solution obtained at selected time delays upto 100 ps. Inspection of Figure 3.17 (a) shows that the Raman bandwidths narrow and the band positions up-shift for the olehnic C=C stretch Raman band over the hrst 20-30 ps. Similarly, the ratios of the Raman intensity in the anti-Stokes and Stokes Raman bands in the low frequency region also vary noticeably in the hrst 20-30 ps. In order to better understand the time-dependent changes in the Raman band positions and anti-Stokes/Stokes intensity ratios, a least squares htting of Lorentzian band shapes to the spectral bands of interest was performed to determine the Raman band positions for the olehnic... 

C=C stretch 1570 cirr band and intensity ratios of the anti-Stokes/Stokes 285cnT Raman band. Results from this analysis are shown in Figure 3.18. ... 

Figure 3.18. Time dependence of the peak position of the 1570 cm Raman band of Sj trans-stilbene in chloroform solution (filled triangle). The time dependence of the anti-Stokes/Stokes intensity ratio is also shown with open circles. The best fit of the peak position change with a single-exponential function is shown with a solid curve, while the best fit of the anti-Stokes/Stokes intensity ratio is shown with a dotted curve. The obtained lifetime for both single-exponential decay functions was 12ps. (Reprinted with permission from reference [78]. Copyright (1997) American Chemical Society.)...

Because the determination of the order parameters by Raman spectroscopy is not straightforward, some works have focused on using intensity ratios to evaluate the molecular orientation (see above). Frisk et al. [56], in particular, have shown that the simple parameter R = 1 — Ixx/Izz — 1 — (axx)/(azz) can efficiently, although qualitatively, characterize orientation in polymers. 

FIGURE 1.8 Change of electrical conductivity against Raman scattering intensity ratio I260l I640 at 260 cnr1 and at 640 cm-1 [29]. 

For the carbon number larger than two n > 2), the 625/603-cm Raman intensity ratio increases with increasing n. The trans band at 625 cm... 

Intensity ratio of Raman bands f(A]g)//(E2g) and the corresponding apparent crystal diameter, L. for various carbon fibers ... 

Before melting and for some time after only the band at 625 cm of the AA [C4CiIm]+ cation was observed in the 600-630 cm i region. Gradually 603 cm i band due to the GA conformer became stronger. After about 10 min the AA/GA intensity ratio became constant. The interpretation [50] is that the rotational isomers do not interconvert momentarily at the molecular level. Most probably it involves a conversion of a larger local structure as a whole. The existence of such local structures of different rotamers has been found by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [82], Coherent anti-Stokes Raman scattering (CARS) [83],... 

Figures 28 and 29 show resonance Raman spectra of two specimens soaked at low and high iodine concentrations. In the previous section, both the 109 cm-1 and the 161 cm-1 peak are assigned to I3- and I5- mode polyiodines, respectively. According to Fig. 28 the intensity ratios are different between the two specimens the 109cm-1 peak is stronger than the 161 cm-1 peak in the heavily iodinated specimen. This corresponds to the fact observed at high iodine concentrations that the I3- mode polyiodines is formed within crystals and that the visible light absorption peak due to I3- mode complex remarkably increases its intensity as shown by Fig. 29. According to Fig. 29, the 109cm-1 peak intensity is enhanced much more by 488 nm excitation than by S14.Snm excitation.

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