Wavenumber dependent sensitivity

The time resolution of the instrument determines the wavenumber-dependent sensitivity of the Fourier-transformed, frequency-domain spectrum. A typical response of our spectrometer is 23 fs, and a Gaussian function having a half width... 

Because the time scale of the Raman scattering event ( 3.3 x 10-14s for a vibration with wavenumber shift 1000 cm-1 excited in the visible) is much shorter than that of the fastest conformational fluctuations in biomolecules, the ROA spectrum is a superposition of snapshot spectra from all the distinct chiral conformers present in the sample. Together with the dependence of ROA on chirality, this leads to an enhanced sensitivity to the dynamic aspects of biomolecular structure. The two-group model provides a qualitative explanation since it predicts ROA intensities that depend on absolute chirality in the form of a sin x dependence... 

Chasnov (1994) has carried out detailed studies of inert-scalar mixing at moderate Reynolds numbers using direct numerical simulations. He found that for decaying scalar fields the scalar spectrum at low wavenumbers is dependent on the initial scalar spectrum, and that this sensitivity is reflected in the mechanical-to-scalar time-scale ratio. Likewise, R is found to depend on both the Reynolds number and the Schmidt number in a non-trivial manner for decaying velocity and/or scalar fields (Chasnov 1991 Chasnov 1998). 

For recording of the emission spectrum, the emitted radiation is focussed on the slit of a monochromator and intensities measured attach wavelength. Since sensitivities of photocells or photomultipliers are wavelength dependent, a standardization of the detector-monochromator combination is necessary for obtaining true emission spectrum This can be done by using a standard lamp of known colour temperature whose emission characteristics is obtained from Planck s radiation law. The correction term is applied to the instrumental readings at each wavelength. Very often substances whose emission spectra have been accurately determined in the units of relative quanta per unit wavenumber intervals are... 

This equation allows contact-free determination of the sample temperature. In order to evaluate it, the measured intensities have to be transformed into absolute values, and they must be corrected with respect to the spectral sensitivity of the spectrometer (see Secs. 3.5.4 and 6.8.4.2.3). As shown in Fig. 2.4-2 the optimum range of the Raman bands employed for this purpose is depending on the expected temperature range. The intensity ratio has to be of a value which can be measured with a large signal-to-noise ratio. For the measurement of sample temperatures near room temperature Raman lines in the range 0. .. 500 cm are useful, for temperatures of about 100 K Raman lines with a wavenumber of less than 200 cm should be employed. 

Generally, the assumption is made that scattering does not depend on the wavenumber so that the conversion of the measured reflectance spectrum R by means of the Kubelka-Munk function F R), results in an absorption-proportional representation. As for ATR and reflection-absorption spectroscopy, also the diffuse-reflectance spectmm does not consist of dispersion features but band-like structures. For changes in low absorption, the sensitivity of diffuse reflectance is greater than the one of transmittance, while strong absorption bands are less pronounced in the diffuse-reflection (see Fig. 6.4-18). Therefore, diffuse-reflection spectra resemble poorly resolved transmittance spectra. For diffuse reflectance spectra where R is in the order of 0.01 or below, the function -log R or just I / R is equally well suited for conversion (Olinger and Griffiths, 1988). Such level are found with compact samples such as polymer foams or varnishes with filler (Otto, 1987 Korte and Otto, 1988). 

The properties of MCT detectors depend on their composition, that is their Hg Cd ratio. Narrow-band MCT detectors are typically about 50-fold more sensitive than DTGS, but do not respond to radiation below 750cm . The cut-off can be extended to a lower wavenumber, but at the expense of sensitivity. Thus, midband MCT detectors have a cut-off of about 600cm , while their sensitivity is about half that of the narrow-band detector. Wide-band detectors cut off at -450 cm but are even less sensitive. Fortunately, few spectra of organic samples contain useful bands below 700cm , as a result, FT-IR microscopes are almost invariably equipped with narrow-band MCT detectors. 

Here, Re and Im denote the real and imaginary parts of the qtiantity in brackets, respectively, c is the mass sensitivity of plate mode velocity, and )9 Is the APM wavenumber y9 - ((<<>n/ n) (n /b) ] -The velocity shift arises from mass loading by the entrained liquid layer, explaining the dependence of this effect on c attenuation arises from power dissipation in the liquid. 

In certain phospholipid membranes that contain unsaturated acyl chains, the typical lamellar liquid crystalline phase converts to a micellar non-lamellar phase upon heating [14]. Such a thermally induced transition involves a major structural rearrangement. Temperature studies of the infrared spectra of phospholipids provide a sensitive means of studying such transitions in lipids. Figure 7.2 shows the temperature-dependence of the wavenumber of the symmetric CH2 stretching... 

These days, almost all IR measurement uses Fourier transform IR (FTIR), which is superior to the dispersive IR in the sensitivity, sampling time, and resolution. Measurable wavenumber region depends upon the detector used the most popular detectors are TGS (triglycine sulfate), 350-7,800 cm and semiconductor-type detector MCT (HgCdTe), the low detection limit around 650 cm ... 

The dependence upon strain of the wavenumbers for the Raman modes of several different substituted polydiacetylene single crystal fibres has been measured by various groups of workers [10,12,14]. Most attention has been paid to the behaviour of the mode which is essentially the symmetrical stretching mode of the C C triple l nd, as this is the most sensitive to applied strain. The dependence of the wavenumber of this band upon applied strain for a polyTSHD single crystal [11] is shown in Figure 8.2, and the dependemx of the position of this Raman band upon strain for four polydiacetylenes [14] with different... 

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