The Interaction of Radiation with Matter

Controlled radiation sources provide the most important modem tools for studying molecular structure and chemical dynamics. Virtually everything we know about the ways atoms interact has been deduced or confirmed by irradiation at a wide variety of wavelengths, from radiowaves to X-rays. In fact, protein and DNA structural determinations were the most important driving force in creating the modem chemical and molecular basis for the biological sciences. 

Two important applications of radiation to determine molecular structure—X-ray crystallography and magnetic resonance—were discussed in Chapters 3 and 5. In this chapter we will discuss a variety of other techniques. Microwave absorption usually forces molecules to rotate more rapidly, and the frequencies of these absorptions provide a direct measure of bond distances. Individual bonds in a molecule can vibrate, as discussed classically in Chapter 3. Here we will do the quantum description, which explains why the greenhouse effect, which overheats the atmosphere of Venus and may be starting to affect the Earth s climate, is a direct result of infrared radiation inducing vibrations in molecules such as carbon dioxide. 

Molecular absorption of visible or ultraviolet light usually excites electrons into higher energy states. Chemicals used as dyes absorb only a portion of the visible spectrum, and thus appear colored. Many of these dyes also generate spontaneous emission 

Absorption is an intuitively reasonable process. If you have a large number of photons with the right energy, it makes sense that molecules can absorb some of those photons, and in the process move from a lower state L to an excited state U, in accord with Bohr s relation 

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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]... 

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. 

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. 

Instrumental analytical methods are based on well-known physical laws concerned with the interaction of radiation with matter, and measurement of the resulting phenomena (radiation or particles). Often, the laws governing this interaction are reasonably well understood but were deduced from simple systems, usually one- or maximally two-component systems, not on complex samples. In practice they are often too general and too approximate for their straightforward use in analytical chemistry. 

Facility in a subset of these techniques is essential for the fundamental examination of microstructured materials such as polymers. Note that all of these techniques demand fundamental knowledge of optics and the interaction of radiation with matter that is not routinely dispensed in a chemical engineering education. 

The interaction of radiation with matter can have profound effects. Whether in solid, solution, or gaseous states, radioactivity can impact the environment and therefore change the molecular speciation of the actinides. To put this into perspective, three examples are discussed below plutonium metal, americium crystals, and an aqueous solution of plutonium. 

Knowledge of the properties of nuclear radiation is needed for the measurement and identification of radionuclides and in the field of radiation protection. The most important aspect is the interaction of radiation with matter. 

The interaction of radiation with matter can cause redirection of the radiation or transitions between energy levels of the atoms and molecules, or both. More subtle effects involve not only the color or wavelength of the radiation but its change in intensity and in the polarization of the light. It is by spectroscopy that we are able to... 

Atomic nuclei are much heavier than electrons and can, in general, be treated accurately using a classical approach. Electrons, of course, must be treated quantum mechanically, and they are considered to move via the equations of quantum mechanics within the fixed external potential of the positively charged nuclei. Because of the relative speed of the motion of the electrons compared to that of the nuclei, their motion is, to an excellent approximation, separate from that of the nuclei in what is called the Born-Oppenheimer approximation. Moreover, excited electronic states are usually irrelevant at temperatures of interest to chemical engineers (<10,000 K), so only their ground state (minimum energy state) needs to be considered. (1 do not consider here the interaction of radiation with matter, the treatment of which is not readily possible at this time using Car-Parrinello methods.)... 

In a solid, the interatomic distances are of the order of an Angstrom (iA = iO i m = 100 pm). For example, the interatomic distance for a C-H bond is 1.08 A, for a single C-C bond the distance is 1.54 A, and for a metal-oxygen bond it is about 2 A, Thus, in order to distinguish two neighboring atoms, a microscope must use radiation with a wavelength of the order of 1 A. The image obtained with such a microscope obviously depends on the interaction of radiation with matter. 

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