Matter, interaction with infrared radiation

When dealing with low-energy infrared radiation, the interaction with matter is limited to the absorption of light by the outer shell electrons, i.e. those used in forming compounds. Hence, particular bonds will absorb particular wavelengths. This is the principle used for infrared spectroscopy. There are equivalent techniques for ultraviolet radiation and visible radiation, but they are mostly used to provide information about concentration of a given compound, rather than for identification purposes such as XRF or IR techniques. 

Light, which is a form of electromagnetic radiation, can interact with matter in a number of different ways. The interaction of light of a given wavelength with a particular object depends on the molecular structure of the objeet. Ineident light may be transmitted, refleeted, absorbed, or scattered by the molecules. Infrared (IR) speetroseopy is a particular type of absorption spectroscopy whereas Raman speetroseopy arises via the inelastie scattering of photons by molecules of the object. Both the strueture and the eleetronie distribution of the molecule determine the intensity of a vibrational transition for eaeh teehnique. In this sense, the methods may be considered to be complementary, and in some eases a eombination of both may prove to be especially useful [14,15]. 

The vibrational motions of the chemically bound constituents of matter have fre-quencies in the infrared regime. The oscillations induced by certain vibrational modes provide a means for matter to couple with an impinging beam of infrared electromagnetic radiation and to exchange energy with it when the frequencies are in resonance. In the infrared experiment, the intensity of a beam of infrared radiation is measured before (Iq) and after (7) it interacts with the sample as a function of light frequency, w[. A plot of I/Iq versus frequency is the infrared spectrum. The identities, surrounding environments, and concentrations of the chemical bonds that are present can be determined. 

Most of what we know about the structure of atoms and molecules has been obtained by studying the interaction of electromagnetic radiation with matter. Line spectra reveal the existence of shells of different energy where electrons are held in atoms. From the study of molecules by means of infrared spectroscopy we obtain information about vibrational and rotational states of molecules. The types of bonds present, the geometry of the molecule, and even bond lengths may be determined in specific cases. The spectroscopic technique known as photoelectron spectroscopy (PES) has been of enormous importance in determining how electrons are bound in molecules. This technique provides direct information on the energies of molecular orbitals in molecules. 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

The interaction of electromagnetic radiation with matter in the domain ranging from the close ultraviolet to the close infrared, between 180 and 1,100 nm, has been extensively studied. This portion of the electromagnetic spectrum, called UV/Visible because it contains radiation that can be seen by the human eye, provides little structural information except the presence of unsaturation sites in molecules. However, it has great importance in quantitative analysis. Absorbance calculations for compounds absorbing radiation in the UV/Visible using Beer-Lambert s Law is the basis of the method known as colorimetry. This method is the workhorse in any analytical laboratory. It applies not only to compounds that possess absorption spectra in that spectral region, but to all compounds that lead to absorption measurements. 

Spectroscopy is concerned with the interaction of light with matter. This monograph deals with collision-induced absorption of radiation in gases, especially in the infrared region of the spectrum. Contrary to the more familiar molecular spectroscopy which has been treated in a number of well-known volumes, this monograph focuses on the supermolecular spectra observable in dense gases it is the first monograph on the subject. 

Note that spectrochemical methods that use not only visible but also ultraviolet and infrared radiation are often called optical methods in spite of the fact that the human eye is sensitive to neither of the latter two types of radiation. This somewhat ambiguous terminology arises as a result of both the many common features of instruments for the three spectral regions and the similarities in the way in which we view the interactions of the three types of radiation with matter. 

Spectroscopy is in general terms the science that deals with the interaction of electromagnetic radiation with matter in particular, it can be said to be the investigation of the optical properties, i. e. the transmission or reflection, of a sample within a certciin spectral range. These properties are studied as a function of the wavelength or frequency of the incident electromagnetic radiation. The spectral range mainly under consideration here is the infrared. 

We can understand the interactions of infrared radiation with matter in terms of changes in the molecular dipoles associated with vibrations and. rotations. We will not go into great depth about the classical and quantum theories of infrared spectroscopy— such detail is really beyond The scope of this present book. Those interested in gaining more in-depth knowledge of the background theory will find that most standard, Physical Chemistry texts provide a detailed coverage of this topic. 

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