Classical transmission

For IR sensing, three transducer principles are standard classical transmission for (sufficiently) transparent samples, (diffuse) reflection for opaque samples, in particular solids and strongly turbid liquids and attenuated total reflection (ATR), in particular for strongly absorbing samples and fluids with varying amounts of suspended solids or gas bubbles. 

In Equation 6.2 T(T) is equal to unity in classical TST, g(T) is a measure of the deviation from the assumption that reactant molecules are locally equilibrated, and k(T) describes the contribution from non-classical transmission through the barrier. k(T) is usually dominated by tunneling but also includes nonclassical reflections. 

Diffuse reflectance differs from classical transmission in which no particulate matter exists to scatter the beam of radiation. It is necessary to contrast correlation spectroscopy (correlation analytical techniques based on spectroscopic measurements) to a classical, one wavelength, monochromatic application of Beer s law. The use of multiple wavelengths produces a multiterm analytical equation in reflectance R of the general type ... 

Transmission arrangements for SI applications follow the classical transmission layout, with a radiation source on one side of a sample and the SI analyser on the other side. The information depth is either constant or can easily be measured, which is an advantage in particular for (semi-)quantitative analysis. Although occasionally applied, such transmission systems suffer from the restriction that the sample must either be freely suspended or rest on a support that is transparent in the investigated spectral range. 

The second major assumption related to the dynamics is that recrossings of the saddle point are absent, i.e., the classical transmission probability is given by... 

The objective is now to modify this equation such that quantum dynamical corrections to the classical transmission probability, Eq. (6.15), are introduced. 

In the present context, it is relevant to consider the barrier penetration that is associated with the traditional (one-dimensional) picture of tunneling. When we consider the time-dependent description of tunneling where a (broad) wave packet hits, e.g., a rectangular barrier, one finds that the center of the wave packet moves as a classical particle. The part of the packet that penetrates the barrier and tunnels through is not slowed down, i.e., it has exactly the same position and velocity as a wave packet that did not experience a barrier (see also [7] for a general discussion of the time-dependent picture of tunneling). The classical transmission probability of Eq. (6.15) is now replaced by the quantum mechanical transmission probability Pqm(E) (see Fig. 6.4.2). Thus, as a natural extension of the conventional formulation based on classical mechanics, in the derivation above we replace Pc by Pqm. That is, we can replace Eq. (6.21) by... 

The main problem in IR measurements in situ is a strong IR absorption by the solvent water. Therefore the measurements in the classical transmission mode are practically excluded (the path length must not exceed 15 tan). Short path lengths are provided by the attenuated total reflection (ATR) mode. The probing light beam undergoes multiple total internal reflection in the ATR element (e.g. a cylindrical rod made of ZnSe, with two cone-sharpened ends) and each reflection... 

Fletcher. The classical transmission line is illustrated in Fig. 1.13(a). Note that the distributed capacitance connects a resistive line to a wire of zero resistance. This circuit element is appropriate when there are two types of charge carrier, one of which is very much more mobile than the other. In this model, the more mobile carrier is modeled by the wire of zero resistance, whereas the resistive line describes the less mobile carrier. However from a more general viewpoint, it is preferable to... 

For the classical transmission line to be an appropriate model, one of the carriers, electrons or ions, must be much more mobile than the other. A more general model is shown in Fig. 4.4. In this transmission line there... 

We now use this model to explain our AC impedance data from polyvinylferrocene. " Our experimental results are similar to those obtained by Rubinstein et who interpreted their data in terms of the classical transmission line with a single resistance, as shown in Fig. 4.1. However, we show that it is possible to measure both / and Rx as... 

When neither transmission nor reflection infrared spectra are satisfactory, spectra can generally be obtained by photoacoustic spectroscopy. The IR spectrum that is obtained is ratioed against an IR spectrum of carbon-black. PAS spectra resemble normal IR spectra with the same absorbance peak wavenumber locations as classic transmission spectra. Because the signal-to-noise ratio of PAS is very low, several thousand scans are needed to obtain a spectrum. The nature and interpretation of the spectra are markedly influenced by the thermal and acoustic properties of the sample. 

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