Infrared radiation, absorption mechanism

Interpretation of the results requires a knowledge of the orientation of the water molecules on the platinum surface and the mechanism of infrared radiation absorption. 

In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation is transmitted without a loss in intensity. At selected frequencies, however, the radiation s intensity is attenuated. This process of attenuation is called absorption. Two general requirements must be met if an analyte is to absorb electromagnetic radiation. The first requirement is that there must be a mechanism by which the radiation s electric field or magnetic field interacts with the analyte. For ultraviolet and visible radiation, this interaction involves the electronic energy of valence electrons. A chemical bond s vibrational energy is altered by the absorbance of infrared radiation. A more detailed treatment of this interaction, and its importance in deter-... 

The radiative mechanism can be summarized by the reaction scheme in Equation (23) where and represent the rate constants for absorption and spontaneous emission of the infrared radiation. 

Not all vibrations and rotations are infrared-active. If there is no change in dipole moment, then there is no oscillating electric field in the motion, and there is no mechanism by which absorption of electromagnetic radiation can take place. An oscillation, or vibration, about a center of symmetry, therefore, will not be observed in the infrared spectrum (absorption) but can be observed in the Raman spectrum (scattering). 

In the near and the mid infrared, the absorption of light by matter originates from the interaction between the radiation from a light source and the chemical bonds of the sample. More precisely, if the atoms situated at the two extremes of a bond are different, they form an electric dipole that oscillates with a specific frequency. If such a non-symmetrical bond is irradiated by a monochromatic light source whose frequency is the same as the dipole, then an interaction will occur with the bond. Thus, the electrical component of the wave can transfer its energy to the bond on condition that the mechanical frequency of the bond and the electromagnetic frequency of the radiation are the same (Figure 10.1). This simplified approach can be used to rationalize that in the absence of a permanent... 

Several recent overviews of principles and applications of Raman, FTIR, and HREELS spectroscopies are available in the literature [35-37, 124]. The use of all major surface and interface vibrational spectroscopies in adhesion studies has recently been reviewed [38]. Infrared spectroscopy is undoubtedly the most widely applied spectroscopic technique of all methods described in this chapter because so many different forms of the technique have been developed, each with its own specific applicability. Common to all vibrational techniques is the capability to detect functional groups, in contrast to the techniques discussed in Sec. III.A, which detect primarily elements. The techniques discussed here all are based in principle on the same mechanism, namely, when infrared radiation (or low-energy electrons as in HREELS) interacts with a sample, groups of atoms, not single elements, absorb energy at characteristic vibrations (frequencies). These absorptions are mainly used for qualitative identification of functional groups in the sample, but quantitative determinations are possible in many cases. 

It is also important to consider that the color of a material affects its absorption of infrared radiation when the material is exposed to sun radiation. Figure 3.23 shows that the darker the color the more extensive the absorption and the higher the temperature. The temperature of products has many further implications on its performance, such as degradation, flexibility, change of shape, mechanical strength, creep, chemical resistance, etc. 

One of these methods was the vibrational spectroscopy which had its roots in the late 1920 s and early 1930 s. One of them was the fundamental understanding of molecular vibrations on the basis of quantum mechanics it was first put in evidence by the absorption of infrared radiation and later also found in the modulations of scattered visible light in the Raman effect. The two... 

Spectroscopy developed rapidly after the achievements of Bunsen and Kirchhoff, but a knowledge of the real cause and origin of spectral hnes was long in coming. It had to wait for the introduction of quantum theory and quantum mechanics. Two Danes took the decisive steps. In 1912 Niels Bjerrum studied the absorption of infrared radiation in gases. He showed that molecules absorb vibration and rotation energy in distinct quanta. Niels Bohr solved the problem of atomic spectra. 

One of these methods is the vibrational spectroscopy which has its roots in the late 1920 s and early 1930 s. One of them was the fundamental understanding of molecular vibrations on the basis of quantum mechanics it was first put in evidence by the absorption of infrared radiation and later also found in the modulations of scattered visible light in the Raman effect. The two methods complement each other dramatically because of their different response to the selection rules which control the transition probabilities between different vibrational states of a molecular framework. The other root was a gradual and substantial improvement of the experimental techniques, such as stronger and more uniform sources of the primary radiations, higher resolution in the spectroscopic part of the equipment, and, perhaps most of all, more sensitive and reliable receivers. 

In a quantum mechanical description, the simple spring-like picture of chemical bonds, of course, breaks down and the molecule has to be described as a many-body system of interacting particles including electrons and nuclei. Nevertheless, the normal mode vibrations have their counterpart in the fundamental excitations of the nuclear vibrational degrees of freedom (DOF) of the molecule. The fundamentals can be excited by infrared radiation (IR) and characteristic absorption bands in the IR spectra immediately point to the existence of certain chemical bonds or to functional groups and hence IR (and Raman) spectroscopy are powerful tools to investigate and study the chemical structure of molecules. 

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