IR emission

Some samples are not amenable to transmission/absorption or reflectance spectroscopy. Samples can be characterized by their IR emission spectrum under certain conditions. If the sample molecules are heated, many will occupy excited vibrational states and will emit radiation upon returning to the ground state. The radiation emitted is characteristic of the vibrational levels of the molecule, that is, the IR spectrum, and can be used to identify the emitting sample. The IR emission from the sample is directed into the spectrometer instead of the usual IR light source. Very small 

---

The eombination in a compact system of an infrared sensor and a laser as excitation source is called a photothermal camera. The surface heating is aehieved by the absorption of the focused beam of a laser. This localisation of the heating permits a three-dimensional heat diffusion in the sample to be examined. The infrared (IR) emission of the surface in the neighbourhood of the heating spot is measured by an infrared detector. A full surface inspection is possible through a video scanning of the excitation and detection spots on the piece to test (figure 1). 

IRE Infrared emission IR emission spectrum observed High resolution, long... 

In principle, emission spectroscopy can be applied to both atoms and molecules. Molecular infrared emission, or blackbody radiation played an important role in the early development of quantum mechanics and has been used for the analysis of hot gases generated by flames and rocket exhausts. Although the availability of FT-IR instrumentation extended the application of IR emission spectroscopy to a wider array of samples, its applications remain limited. For this reason IR emission is not considered further in this text. Molecular UV/Vis emission spectroscopy is of little importance since the thermal energies needed for excitation generally result in the sample s decomposition. 

When samples are heated, they emit infrared radiation with a characteristic spectrum. The IR emission of ceramics, coals, and other complicated solids and thin films can be studied. Also, if conditions make it difficult to use an infrared source... 

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 ... 

The simplest technique is the IR emission spectroscopy method, as presented schematically in Fig. 72. The method allows the successful investigation of aggressive molten materials and is generally applicable over a very wide range of temperatures and concentrations. 

The spectra obtained using the apparatus described in [342] is appropriate for IR emission measurements of both thin and thick layers of melts. 

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. 

There is no doubt that IR emission spectroscopy of thick layers is preferable for investigations of fluoride melts containing tantalum and niobium. 

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

The melting process of potassium fluorotantalate, K2TaF7, was investigated by IR emission spectroscopy using thick layers of the melt [356]. It should be mentioned that in some cases, if the temperature of the sample is high enough, the above method enables to obtain spectra of the material in solid state as well. 

Fig. 74 shows the IR emission spectrum of the transition of K2TaF7 from solid to molten state. 

IR spectra of the molten system K2TaF7 - KF are more sensitive to the concentration of KF (Fig. 76, b). Molten K2TaF7 is characterized by a strong band at 605 cm"1 and a shoulder at 540 cm 1, as shown before (see Fig. 74, curve 3). The addition of KF to molten K2TaF7 leads to a decrease in the intensity of the above band and to the appearance of a band at 540 cm 1. When the KF concentration equals or exceeds 0.9 mol fraction, a band is observed at 540 cm 1 only. This spectral transformation indicates that the equilibrium in Equation (80) shifts to the left with the increase in KF concentration, and that at KF concentrations above 0.9 mol fraction, only TaF72 complexes are present, while TaF6 ions are not observed by IR emission spectral methods. 

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.

Efficient IR-emitting phosphors excited by a direct current electroluminescence (DCEL) device were first reported in 1969. Sixty/40 CdS/ZnS doped with 10-4gg-1 Cu was shown to be an efficient converting phosphor giving a maximum output at 900nm.20 The near-IR emission of thin-film electroluminescence (TFEL) devices containing ZnS Er (at 980 nm), ZnS Nd (at 900 nm), and ZnS Tm (at 805 nm) have been reported.21... 

Flowever, there is a trade-off in using near-IR emissive lanthanides, in that luminescence lifetimes are shorter, and quantum yields lower, compared to complexes of Tb and Eu. This arises because the near-IR emissive lanthanides are quenched by lower harmonics of the O-H oscillator, increasing the Franck-Condon overlap with the metal excited state. For neodymium, matters are further complicated by the manifold of available metal-centered excited states, which leads to particularly effective quenching by C-H oscillators. Thus, complexes in which there are few C-H oscillators close to the metal are desirable if the luminescence lifetime is to be optimized (e.g. 44).76 97-101... 

Usually, when referring to infrared spectroscopy, IR absorption spectroscopy is meant. Although this is the dominant method, occasionally it can be useful for sensing applications to also use IR emission spectroscopy. 

IR emission spectroscopy makes use of the reciprocal effect of IR absorption spectroscopy. At temperatures above 0 °K, molecules undergo a number of vibrational, vibrational-rotational or purely rotational movements. The relaxation of these excited states leads to the emission of thermal radiation, primarily in the IR region. 

Fig. 1. Rate coefficients for the low-pressure region of the unimolecular decomposition of water Circles represent measurements by ir emission (2.8 / ) lower curve at higher temperatures, ki upper curve at lower temperatures, 2/c,. Triangles represent measurements by uv absorption (3100 A), evaluated according to a rate law c = l—exp(A 1[Ar]/) A Ar = 0.5-1 xlO-2 mole.l-1 A Ar = 2-3 x 10-2 mole.l-1. (From Olschewski et al. )...

Furthermore, Olschewski et al. showed that results from ir emission and OH absorption are comparable only above 5000 °K, where the formation of OH is governed by reaction (1). 

CIR-FTIR spectroscopy provides a direct technique for studying in situ hydrous metal oxide surfaces and molecules adsorbed on these surfaces (37). By itself, FTIR spectrometry is a well established technique which offers numerous advantages over dispersive (grating) IR spectrometry (1) improved accuracy in frequency measurements through the use of a HeNe laser (2) simultaneous frequency viewing (3) rapid, repetitive scanning which allows many spectra to be collected in a small time interval (4) miriimal thermal effects from IR beam and (5) no detection of sample IR emissions (38). 

Cyanine dyes are in principle capable of shifting the emission into the near-IR region [157], however, their ionic character makes it difficult to dope them into films by vacuum vapor evaporation. Other materials investigated for IR emission are complexes based on rare earth ions (Nd3+, Er3+), which are also used in inorganic amplifiers and lasers [158]. 

---