Interaction of Electromagnetic Radiation with Matter

Before studying interaction of electromagnetic radiation with matter, it is necessary to understand what electromagnetic radiation is. 

Electromagnetic radiation shows both wave nature as well as particle nature. 

According to Plank, electromagnetic radiation is made of tiny energy packets, known as photons, which comes continuously from source of radiation. 

Mathematically, the energy associated with the electromagnetic radiation is given by 

Wavelength It is the difference between two adjacent trugs or crests. It is represented as X and measured in nm (nanometer) or in A [Angstrom]. 

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As discussed in more detail elsewhere in this encyclopaedia, many optical spectroscopic methods have been developed over the last century for the characterization of bulk materials. In general, optical spectroscopies make use of the interaction of electromagnetic radiation with matter to extract molecular parameters from the substances being studied. The methods employed usually rely on the examination of the radiation absorbed. 

The interaction of electromagnetic radiation with matter can be explained using either the electric field or the magnetic field. For this reason, only the electric field component is shown in Figure 10.2. The oscillating electric field is described by a sine wave of the form... 

The plane of polarization is conventionally taken to be the plane containing the direction of E and that of propagation in Figure 2.1 this is the xy plane. The reason for this choice is that interaction of electromagnetic radiation with matter is more commonly through the electric component. 

The physical basis of spectroscopy is the interaction of light with matter. The main types of interaction of electromagnetic radiation with matter are absorption, reflection, excitation-emission (fluorescence, phosphorescence, luminescence), scattering, diffraction, and photochemical reaction (absorbance and bond breaking). Radiation damage may occur. Traditionally, spectroscopy is the measurement of light intensity... 

The field of science that studies the interaction of electromagnetic radiation with matter is known as spectroscopy. Spectroscopic studies on the wavelength, the intensity of the radiation absorbed, emitted, or scattered by a sample, or how the intensity of the radiation changes as a function of its energy and wavelength, provide accurate tools for studying the composition and structure of many materials (Davies and Creaser 1991 Creaser and Davies 1988). 

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. 

As already discussed, the interaction of electromagnetic radiation with matter leads to absorption only if a dipole moment is created as a result of such interaction. During the process of emission the dipole is destroyed. This may be stated symbolically as d /dt is positive and d fdt 0. 

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. 

Collision-induced absorption is a well developed science. It is also ubiquitous, a common spectroscopy of neutral, dense matter. It is of a supermolecular nature. Near the low-density limit, molecular pairs determine the processes that lead to the collision-induced interactions of electromagnetic radiation with matter. Collision-induced absorption by non-polar fluids is particularly striking, but induced absorption is to be expected universally, regardless of the nature of the interacting atoms or molecules. With increasing density, ternary absorption components exist which are important especially at the higher temperatures. Emission and stimulated emission by binary and higher complexes have also... 

A simple thought experiment due to Einstein gives a basic idea of the interaction of electromagnetic radiation with matter. Consider a space surrounded on all sides by perfectly reflecting mirrors (Figure 2.9). Inside this cavity there is a hot material body which is in thermal equilibrium with the radiation which fills the cavity. This radiation is then isotropic, as it fills, in a random manner, all the space of the cavity its intensity can be defined as an energy per unit volume. 

Interaction of electromagnetic radiation with matter occurs over a broad range of frequencies and usually in a highly specific way (Table 9.1). Study and use of these interactions is in the domain of spectroscopy, which provides information ranging from the electronic structure of atoms to dynamics of polymeric chains. 

Although Doug Hutchinson s original interest was in semiempirical methods, some of his research at Queen s was concerned with the theory of interaction of electromagnetic radiation with matter,162 163 an interest aroused during his postdoctoral time with Hank Hameka at Penn and stimulated further... 

Photoredox processes include both photoreduction and photo-oxidation of the excited species. An electron transfer that results from an electronic state produced by the resonant interaction of electromagnetic radiation with matter is called photoinduced electron transfer (PET) [30-32]. This can be done by either a direct or a photosensitized process (see below and Figure 6.6). 

Spectroelectrochemistry, reflection mode — The interaction of electromagnetic radiation with matter (-> spectroscopy) may occur by absorption or scattering when radiation impinges on matter or passes through matter. In the latter case (transmission mode) the radiation before and after passage is evaluated in order to obtain the desired spectrum. In studies of opaque materials or of surfaces interacting with matter inside the (bulk)... 

Most interactions of electromagnetic radiation with matter contain a geometric, as well as an energetic component. For visible and ultraviolet absorption this is because the fundamental relationship governing the absorption of light is the transition moment integral, (6.1), in which r is the transition moment vector defining... 

Spectroscopy involves the study of the interactions of electromagnetic radiation with matter. In the case of liquids, radiation of a wide range of frequencies, and thus energies, has been used, all the way from radio-frequency waves to X-rays. Experiments involving neutrons, which are associated with very short wavelengths, are also important. In the spectroscopic experiment the incident radiation may be either absorbed or scattered and the experimental information is obtained by examining the intensity and direction of the radiation after it has passed through the sample. 

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. 

These practical aspects are governed by the interaction of electromagnetic radiation with matter at the molecular level this interaction polarizes the charge distribution and alters the propagated field. The linear response is described by the polarizability (a) and the non-linear response (the subject of this article) is described by the hyperpolarizabilities (P, y, etc.). The word hyperpolarizability was first used by Coulson, Macoll and Sutton [1] some 40 years ago. 

There are probably several reasons why electrochemical methods are not as popular as chromatographic or optical methods. One is that electrochemistry and electrochemical methods are not emphasized in typical college curricula. One can cite the nearly universal disappearance of fundamental electrochemistry from beginning general and physical-chemistry courses, whereas the interaction of electromagnetic radiation with matter and the energy levels concerned is covered in many first-year courses. Electrochemical theory is really no more complex or abstruse, but probably not so well unified at present, as spectrochemical theory. 

Spectroscopic methods are used in the structural characterization of biomolecules (Bell, 1981 Campbell and Dwek, 1984 Greve et al, 1999 Hammes, 2(X)5). These methods are usually rapid and noninvasive, require small amount of samples, and can be adapted for analytical purposes. Spectroscopy is defined as the study of the interaction of electromagnetic radiation with matter, excluding chemical effects (photochemistry refers to the interaction with chemical effects). The electromagnetic spectrum covers a wide range of wavelengths (Figure 7.1). 

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