Interactions of Radiation with Matter

The high-energy X-rays when passing through matter are able to excite the electrons of the inner shells of an atom. When the excited electrons fall back to their original position they release the energy absorbed. This can be measured and is the principle used in X-ray fluorescence. 

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. 

XRD is used for the identification of crystalline materials such as pigments, metal powders, organic materials and salts. Non-crystalline materials lacking a regular crystal lattice, such as glass, do not produce a clear pattern. 

The group frequency region falls approximately between 4000 to 1400 cm-1, and the absorption bands in it may be assigned to vibration of pairs of two (or sometimes three) atoms. The frequency is characteristic of the masses of the atoms involved and the nature of their bond, ignoring the rest of the molecule. Therefore, IR spectra are useful for determining the presence of functional groups in organic compounds alcohols (—OH), ketones (=CO), amines 

In the fingerprint region (approximately 400-1400 cm ) the absorption bands correspond to the vibrations of the molecule as a whole, and are therefore characteristic for each molecule, and can serve as unambiguous identification by comparison with a standard. However, long polymers of a given family, such as long chains of fatty acids can give almost identical spectra. 

The discussions of the equation of transfer and the solution of this equation in Chapter 2 rest entirely on concepts of classical physics. Such treatment was possible because we considered a large number of photons interacting with a volume element that, although it was assumed to be small, was still of sufficient size to contain a large number of individual molecules. But with the assumption of many photons acting on many molecules we have only postponed the need to introduce quantum theory. Single photons do interact with individual atoms and molecules. The optical depth, r (v), depends on the absorption coefficients of the matter present, which must fully reflect quantum mechanical concepts. The role of quantum physics in the derivation of the Planck function has already been discussed in Section 1.7. Both the optical depth and the Planck function appear in the radiative transfer equation (2.1.47). 

The interaction of radiation with matter can take many forms. The photoelectric effect, the Compton effect, and pair generation-armihilation are processes that occur at wavelengths shorter than those encountered in the infrared. Infrared photons can excite rotational and vibrational modes of molecules, but they are insufficiently energetic to excite electronic transitions in atoms, which occur mostly in the visible and ultraviolet. Therefore, a discussion of the interaction of infrared radiation with matter in the gaseous phase needs to consider only rotational and vibrational transitions, while in the solid phase lattice vibrations in crystals must be included. 

In the following sections on the interaction of radiation with gas molecules we begin with an overview of the physical principles of radiative transitions in molecules in Sections 3.1 and 3.2, proceed to discussions of the properties of diatomic and polyatomic molecules in Sections 3.3 and 3.4, and, finally, examine line strengths in Section 3.5 and line shapes in Section 3.6. Interactions of radiation with solid and liquid surfaces, as well as cloud particles, are the subject of Sections 3.7 and 3.8. For further information on molecular spectroscopy we refer the reader to text books, such as Pauling Wilson (1935), Herzberg (1939,1945,1950), Townes Schawlow (1955), or Steinfeld (1974). The book by Murcray Goldman (1981) is 

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Interaction of Radiation with Matter Energy Transfer from Fast Charged Particles... 

Concepts Interaction of radiation with matter Light and matter interact by scatter and absorption. Absorption only occurs when the colour of the radiation is the same as an energy separation within the molecule, but scatter occurs at all wavelengths... 

The fundamentals of the interaction of radiation with matter have been considered in several texts (1-8). 

Measurements of the intensity and wavelength of radiation that is either absorbed or emitted provide the basis for sensitive methods of detection and quantitation. Absorption spectroscopy is most frequently used in the quantitation of molecules but is also an important technique in the quantitation of some atoms. Emission spectroscopy covers several techniques that involve the emission of radiation by either atoms or molecules but vary in the manner in which the emission is induced. Photometry is the measurement of the intensity of radiation and is probably the most commonly used technique in biochemistry. In order to use photometric instruments correctly and to be able to develop and modify spectroscopic techniques it is necessary to understand the principles of the interaction of radiation with matter. 

Within the semiclassical, perturbational treatment of the interaction of radiation with matter [77,78] and within the dipole approximation [79], the total energy absorption cross section may be written in the form [11,12,20,80]... 

The primary interaction of radiation with matter leads to the formation of positive ions and excited molecules. Thus, in a medium constituted of molecules of a substance AB, the primary radiation-chemical events can be written ... 

We have seen that two important lithographic parameters of a resist are sensitivity and contrast. This leads to a consideration of the design features that must be incorporated into the resist in order to optimize these parameters and, in turn, requires a fundamental understanding of the interaction of radiation with matter and how the polymer molecular parameters affect lithographic response. These aspects have been extensively covered in the literature, (5,6) and only the conclusions relating to lithographic performance will be summarized. 

We now consider the effect of exposing a system to electromagnetic radiation. Our treatment will involve approximations beyond that of replacing (3.13) with (3.16). A proper treatment of the interaction of radiation with matter must treat both the atom and the radiation field quantum-mechanically this gives what is called quantum field theory (or quantum electrodynamics). However, the quantum theory of radiation is beyond the scope of this book. We will treat the atom quantum-mechanically, but will treat the radiation field as a classical wave, ignoring its photon aspect. Thus our treatment is semiclassical. 

Figure 2.9 In the thought-experiment of the basic interaction of radiation with matter, a material body at vanishing density is in thermal equilibrium with monochromatic electromagnetic radiation held within a perfectly reflecting enclosure...

At this point we have described nuclear transitions and reactions that produce various forms of nuclear rad iation. The radiation propagates out from the originating nucleus and interacts with other matter along its path. These interactions with external matter allow us to observe the radiation, and its effects, and to determine the nature of the transition inside the nucleus. The interaction of radiation with matter is also the cause of chemical, physical, and biological changes that concern the public at large. We will specifically address the operating principles of radiation detectors in the next chapter, but first we will consider the fundamental interactions of nuclear radiation with matter. 

INTERACTION OF RADIATION WITH MATTER we find that... 

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