Transmission more intense bands

Transmission geometry has been exclusively applied to the investigation of zeolites. Sendoda et al. (1975) compared transmission and reflection spectra and reported good agreement, except that the transmission mode gave "more intense bands". Forster et al. (1983) compared their transmission spectra of CoNaY to reflection spectra in the literature and claimed that the former are of at least equal quality. 

This characteristic of RAIR can be observed experimentally. Fig. 8 shows the transmission spectrum of polydimethylsiloxane (PDMS) while Fig. 9 shows the RAIR spectrum of a thin film of PDMS spin-coated onto a chromium substrate. It can be observed that the bands near 1024 and 1095 cm have similar intensities in the transmission spectra but the band at higher frequencies is clearly much more intense in the RAIR spectrum. This change in relative intensity when PDMS is deposited onto a reflecting substrate is related to optical effects and is not related to orientation effects. 

Figures 17 and 18 represent FT-IR transmission and RA spectra, respectively, of the alternating S(PS)9-Ba films at various temperatures from 0 ° to 120 °C [7], Two intense bands at 2919 and 2852 cm 1 are the antisymmetric and symmetric CH2 stretching bands of DOPC, and two bands at 2192 and 2088 cm 1 are the antisymmetric and symmetric CD2 stretching bands of St-d35, respectively. Apparently, all these bands decreases their intensities with the increase in temperature in Figure 17. At the same time, intensity differences of the respective bands are evident between the transmission and RA spectra. From these data, we calculated temperature dependence of the orientation angle y of the hydro-carbon chain axes of the constituent molecules in the alternating S(PS)9 and S(PS)9-Ba films using Eqs. (2) and (3). The results are shown in Figure 19 [7]. Apparently, the y values of the respective constituents in the S(PS)9-Ba film are much smaller than those of the corresponding molecules in the S(PS)9 film. This reveals that the barium salt molecules are more highly oriented as compared with the...

Although ATR and transmission spectra of the same sample closely resemble each other, differences are observed because of the dependency of the penetration depth on wavelength longer wavelength radiation penetrates further into the sample, so that in an ATR sjjectrum bands at longer wavelengths are more intense than those at shorter ones. 

The appearance of an ATR spectrum is very subtly different to a transmission spectrum. Individual band shapes are slightly unsymmetric-al, but, more importantly, the intensity of the high-wavenumber spectrum (O-H and C-H stretches for instance) is significantly reduced. This cannot easily be avoided, as the ATR effect (equation (10.4)) shows that sampling depth is proportional to wavelength. However, this is not a... 

In many commercially available FT-IR spectrometer systems, software (called the ATR correction) for correcting the band intensities of an observed ATR spectrum for the wavelength-dependent depth of penetration expressed in Equation (13.4) is installed, in order to make the observed ATR spectrum more closely resemble a transmission spectrum. In ATR spectra, peak positions, particularly those of intense bands, tend to have wavenumbers lower than those in corresponding transmission spectra. As described earlier, this is related to the anomalous dispersion of the refractive index in the vicinity of the absorption band. The effect has been discussed and software for correcting ATR spectra to take account of this effect is available [9]. The software for this purpose is also installed in some commercially available FT-IR spectrometers. Input data necessary for this software are the refractive index of the sample and the IRE, the angle of incidence, and the number of internal reflections. 

The ATR technique is a commonly used infrared internal reflection sampling technique. It samples only the surface layer in contact with the ATR element the sampling depth probed is typically of the order of 0.3-3 pm [1]. Unless software corrected, compared with a transmission spectrum, the relative intensity of bands within an ATR spectrum increase in intensity with decreasing wavenumber. Several FTIR instrument companies now supply "ATR-correction" software developed to correct for the different relative intensities of bands observed between ATR and transmission spectra, so that ATR spectra can be more easily compared to and searched against transmission spectra. 

The first manifestation of VCD in the optical train of a spectrometer is the modulation of the intensity of the infrared beam in synchronization with the modulation of the polarization as the beam passes through the circular dichroic sample. The phase of the synchronization is opposite for negative and positive VCD bands. More specifically, in reference to the definition of VCD in Eq. (1), there will be a synchronization between larger transmission (smaller absorbance) and right CP radiation for positive VCD bands, and between larger transmission and left CP radiation for negative VCD bands. 

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