Spectroscopy in the Infrared Region

Analytical spectroscopic methods are based on interaction between radiating energy and matter, wherein the amount of energy emitted or absorbed by atomic or molecular species is measured. The different methods may be classified in accordance with the region of the electromagnetic spectrum involved in the measurement (Fig. 10.1). The interaction of radiation occurs by different mechanisms and supplies different kinds of data. 

Therefore, spectroscopic methods constitute an important tool for elucidating molecular structure and qualitatively and quantitatively determining organic and inorganic compounds. Among these, spectroscopy in the infrared region stands out. This is based on transitions between two vibrational levels of molecules in their fimdamental electronic state, which are normally observed as an absorption spectmm. 

Electromagnetic radiation may be described as a wave with properties such as frequency (i/), wavelength (A), and propagation velocity (c), formed by energy packets called photons or quanta [1-3]. The wave number (T) is another means of describing electromagnetic radiation, and it is equivalent to jX  

The energy content of the photon, in accordance with quantum theory, is expressed by 

Nuclear transition Internal Electronic Transitions Electronic valence Transition Molecular ibration Molecular Rotation Spin orientation Magnetic Field 

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The main barrier to the use of free induction decay for FT spectroscopy in the infrared region is the lack of a convenient powerful source of broadband coherent radiation. In addition, the decay times for the coherently excited polarization in the system tend to be very short at higher frequencies. Coherent terahertz spectrometers operating in the far infrared region are now practical because of the success of ultrafast laser technology in generating broadband terahertz pulses. 

In Section 4.3 we have considered surface polariton spectroscopy in the infrared region. As distinct from that case, SPs in the optical spectral range exist mainly on metal surfaces, i.e., as surface plasmon polaritons (SPP). The SPP dispersion curve extends from the far-infrared up to the far-ultraviolet region and thus it can be excited at any frequency in this range. This allows one to use SPPs as an optical probe of various overlayers on metal surfaces and in-... 

The M.I. described in the previous section has been used very successfully for high-resolution spectroscopy in the infrared region. The basic idea, already discussed above, is the transfer of the high frequencies oj of the incident radiation to audio frequencies Aw = 2(v/c)o) of the interference pattern by the uniform motion of mirror M2 with constant velocity v. 

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