IR emission spectra

According to the Kirchhoff law, the capability of spectral emission equals the capability of spectral absorption. If the spectral absorptivities are known, it is therefore possible to determine vibrational temperatures from emission spectra. It is important to remember that the absorption coefficient throughout a band in a spectrum often varies considerably with the temperature. Temperatures are usually determined by fitting the calculated to the observed intensities. 

In contrast to the method of line reversal described in the preceding section, the discussed technique can be applied even if the emission bands are very weak. 

It should be noted that the spectral emission is influenced by the. self-absorption of the emitted radiation by the sample. If the temperature distribution is homogeneous, this effect is already included in the determination of the absorptivity. In inhomogenous samples, the self-absorption may be neglected if the absorptivity is below 5%. In this case, the overall emission can be treated as the sum of the emission of all infinitely thin layers into which the sample can be divided (Pepperhoff and Grasz, 1955). Otherwise, the emission of all inner layers must be corrected by transmission factors before summation. For practical calculations, the sample volume can be divided into different layers, each of which is assumed to be in thermal equilibrium. 

If the vibrational temperature is determined by using the intensities of different bands, a distinct value is obtained for each band. These values do not represent the arithmetic mean of all temperatures. Due to the nonlinear increase of the spectral radiance by the black body radiator, the hot zones appear more pronounced than the cold ones. On the other hand, the influence of the more distant zones with respect to the observer is reduced by stronger self-absorption. The vibrational temperatures deduced from bands with high absorption coefficient are therefore lower than those derived from bands with smaller absorption coefficient. Nevertheless, all thus obtained temperature values are between the lowest and the highest temperature of the sample. The method of fitting calculated spectral profiles to the observed ones has been successfully applied in these cases, too. 

The determination of vibrational and rotational temperatures by using emission spectra may be exemplified by vinyl chloride, which can be treated with good approximation 

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The third step consisted of the direct investigation of IR emission spectra for a wide range of concentrations. The investigation showed the tendency of the metals to reduce their coordination number when moving from solid to molten state. This property of the melt depends on the equilibrium between two types of complex ions, MeF72 and MeF6 ... 

Fig. 73. IR emission spectra of KNO3 melt at 450X7. Curve l - Layer thickness - 0.05 mm, reflective surface Curve 2 - Layer thickness - -0.1 mm, absorptive surface Curve 3 - Layer thickness - 0.2 mm, absorptive surface (after Agulyartsky and Sakharov [342]).

Such differences of the thermal IR emission spectra of compounds that consist of oxygen-containing ions and halide ions are related to the relationships between the compounds optical parameters. 

The investigation of IR emission spectra from samples of varying thickness is widely used for solids as well [345,346]. 

Fig. 82 presents IR emission spectra of molten K2NbF7 in an inert atmosphere (sealed retort) and in air. Curves 1 and 2 were obtained in an inert... 

The occurrence of oxyfluoride chains in MF - MNbOF4 molten mixtures was confirmed by IR emission spectra [379]. Fig. 94 presents a typical example, the spectrum of molten LiF - LiNbOF4. The strong band at about 780-800 cm 1 is characteristic of Nb-O-Nb vibrations of the octahedrons that are linked into chains via oxygen bridge atoms. 

Fig. 94. IR emission spectra of molten system LiNbOF4 - LiF at 660°C. Curves 1, 2 and 3 correspond to LiNbOF4 concentrations of 1.0, 0.8 and 0.6 molar fraction, respectively. Reproduced from [379], A. I. Agulyansky, E. L. Tikhomirova, V. T. Kalinnikov, Zh. Neorg. Khim., 33 (1988) 1155, Copyright 1988, with permission of Nauka (Russian Academy of Sciences) publishing.

Ab initio and semiempirical molecular orbital (MO) model calculations have become an efficient way to predict chemical structures and vibrational (i.e., Raman scattering and IR emission) spectra. We and others have used such approaches to better understand certain features of fhe specfra, as explained in the following. The basic principles underlying ab initio model calculations have been described in many textbooks and papers (see for example Refs. 44,47,48). Applications in relation to ILs and similar systems have also been reported, as discussed later. 

Similarly, Cherukuri et al. used the near-IR fluorescence of individualized semiconducting SWNTs to study their blood elimination kinetics and biodistribution in rabbits.127 The pluronic copolymer-coated SWNTs were intravenously administered into rabbits at a dose of 20 pg S WNT per kilogram of body mass. The pluronic coating was found to be displaced by blood protein within seconds, as indicated by changes in the near-IR emission spectra. The nanotube concentration in the blood semm decreased exponentially with a half-life of 1 h, and no adverse effects were observed in terms of the rabbit behavior and the pathological examination. At 24 h post administration, significant concentrations of nanotubes were found only in the liver.127... 

A number of problems arise in connection with the use of emission IR spectroscopy (IRES). One of them arises from the existence of temperature gradients, which can cause self-absorption of the emitted radiation by the colder outer parts of the sample itself another is concerned with the selective reflection that occurs in the vicinity of strong absorption bands. This reduces the absorptance and hence the emittance. Moreover, perturbations can be created by reflections and emission by the cell elements. These problems, however, can in part be overcome so that IR emission spectra can be successfully recorded and are widely used, for example, in the fields of polymer and corrosion science and mineralogy. Some uses of IRES... 

The implications of this result for spectroscopic predictions are considerable. The composition and symmetry of the HOMOs and LUMOs have changed considerably from the NR case, and we see that the HOMO-LUMO transition is now essentially irrelevant with respect to the IR emission spectra. In the simplified one-electron picture, we predict absorption in the extreme IR up to 0.7 eV for C1-, 1.0 eV for Br-, and 0.2 eV for the I-ligand clusters. Dipole allowed one-electron transitions in the IR emission region are predicted to have a threshold at 2.8 eV for C1-, 2.7 eV for Br-, and 1.1. eV for I-ligand clusters. From the DOS diagrams, we can predict intense absorption into the antibonding peak located 4.1, 3.7 and 3.0 eV above Ef for X=C1, Br and I respectively. 

Figure 3.20. Successive FT-IR emission spectra from a thin film of resin from a carbon-epoxy-resin prepreg during cure. Bands that change during cure are marked (George et al., 2006).

Figure 3.22. Time-resolved FT-IR emission spectra (every 13 s) showing the increase in emissivity in the region 1650-1800 cm due to formation of oxidation products. Reproduced with permission from Blakey (2001).

The data set of IR emission spectra consisted of radiometer absorbance values at 146 different wavelengths in the middle-IR band (2 to 5.5 xm). 

Each rocket motor type was fired a number of times (see table 1) and the IR emission spectra were recorded for each test as replicate measurements. The total set of recorded IR emissionspectra thus comprised 420 measurements. The spectra were recorded by Roodt (1998) using a spectral radiometer at varying distances, i.e. 500 m, 350 m, 250 m and 200 m The data were preprocessed in order to compensate for the varying absorbance path lengths and atmospheric conditions (Bouguer s law) as described in Roodt (1998). 

Table 1. The number of middle-IR emission spectra repeat measurements taken from tests for each of the rocket motor types.

In the case of the forward mapping where the IR emission spectra were to be predicted by a given set of rocket motor features, there were 18 input and 146 output variables. Clearly, for a simple linear least squares model, the model requires 19 degrees of freedom (18 inpnit variables plus the bias). However, the situation is more complicated when nonlinear models are fitted to the data... 

The variables (wavelengths) associated with the IR emission spectra were highly correlated. Principal components analysis (PCA), linear and nonlinear PLS showed that at least 86% of the total variance could be explained by the two primary latent dimensions. The forward and reverse modelling results showed that dimensional reduction with a linear model (PLS) produced better models than a nonlinear model (multilayer perceptron neural network trained with the back propagation algorithm) without dimensional reduction. 

The free NF radical produced in two low-lying excited electronic states by various gas-phase reactions is observed by the green and near-IR emission spectra which correspond to the b 2" and a 2 transitions (cf. pp. 272 and 285/7). These two lowest singlet... 

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