ABSTRACT
The light absorbed by an atom or molecule can be reemitted at lower spectral frequencies. This phenomenon is known as fluorescence; its fundamental and basic features are described in depth in this chapter. In the first part, the following (and other similar) questions are addressed: What factors control fluorescence intensity, spectral structure, polarization, lifetime, and quantum yields of the different processes affecting the fate of the excited state? What is the relation of these observables with respect to the fluorophore structure and its surrounding medium? These and other similar questions are explained in the first part of the chapter. Thereafter, attention is turned to the quantitative description of the phenomenon of quenching based on the Stern–Volmer law and to one of its most interesting cases: the so-called Förster resonance energy transfer that satisfactorily describes the energy transfer, e.g., in proteins. The chapter concluded with a section in which the basic principles of the modern field of single molecule spectroscopy are introduced.
