In this chapter, we discuss the coupling to the electromagnetic radiation field semiclassically and derive the transition rates for absorption and induced emission in the dipole approximation. We compare with the fully quantized treatment and derive the rate of spontaneous fluorescence. We introduce the Condon approximation for optical transitions. The Franck–Condon-weighted density of states is formulated with the time-correlation function formalism. We discuss the generation of pure Born–Oppenheimer states with very short excitation pulses. Finally, we consider radiationless transitions.
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