Fluorescence matter/radiation interactions

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... 

Abstract Photochemistry is concerned with the interaction between light and matter. The present chapter outlines the basic concepts of photochemistry in order to provide a foundation for the various aspects of environmental photochemistry explored later in the book. Electronically excited states are produced by the absorption of radiation in the visible and ultraviolet regions of the spectrum. The excited states that can be produced depend on the electronic structure of the absorbing species. Excited molecules can suffer a variety of fates together, these fates make up the various aspects of photochemistry. They include dissociation, ionization and isomerization emission of luminescent radiation as fluorescence or phosphorescence and transfer of energy by intramolecular processes to generate electronic states different from those first excited, or by intermo-lecular processes to produce electronically excited states of molecules chemically different from those in which the absorption first occurred. Each of these processes is described in the chapter, and the ideas of quantum yields and photonic efficiencies are introduced to provide a quantitative expression of their relative contributions. 

In this chapter, we discuss the basic principles that are necessary to understand measurements made with electromagnetic radiation, particularly those deeding with the absorption of UV, visible, and IR radiation. The nature of electromagnetic radiation and its interactions with matter are stressed. The next four chapters are devoted to. spectroscopic instruments (Chapter 25), molecular absorption spectroscopy (Chapter 26), molecular fluorescence spectroscopy (Chapter 27), and atomic spectroscopy (Chapter 28). 

Figure 4.4 Jablonski-type energy diagrams for possible excited energy states when light interacts with matter, (a) Three possible transition pathways for return to ground state without radiation, (b) Two possible transition pathways with fluorescent light emission as final step on return to ground state, (c) Two possible transition pathways with phosphorescent light emission as final step on return to ground state.

The mass absorption coefBdent// plays a very important role in quantitative XRF analysis. Both the exciting primary radiation and the fluorescence radiation are attenuated in the sample. To relate the observed fluorescence intensity to the concentration, this attenuation must be taken into account As illustrated in Fig. 11.1, the absorption of radiation in matter is the cumulative effect of several types of photon—matter interaction processes that take place in parallel. Accordingly, in the X-ray range the mass attenuation coefficient of element i can be expressed as ... 

Figure 18 Energy scheme for radiation-matter interaction processes. A-SR = anti-Stokes Raman scattering, RH = Rayleigh scattering, SR = Stokes Raman scattering, AB = absorption, FL = fluorescence, lA = infrared absorption, IE = infrared emission.

Since sensitivity is dependent on the excitation source intensity, it would seem that one should be able to increase the source intensity and thereby improve the sensitivity of the fluorescence technique for analysis. This is valid with low radiative fluxes. However, there is one more interaction between radiation and matter that must be introduced into this discussion to explain the limitation of this assumption stimulated emission. 

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