ABSTRACT
I n this chapter, we treat the interaction of electromagnetic (EM) radiation with atomic systems using semiclassical techniques. The treatment
is semiclassical because, as mentioned in Chapter 2, “Preliminaries,” the field is treated classically and only the atomic system is quantized. We have already reviewed the relevant features of classical EM theory (in cgs units) in that chapter which we will need for the atom-field interaction Hamiltonian. When treating absorptive or scattering process, we shall not consider the atom as a source of radiation here-instead we shall use the equations for classical EM wave in a vacuum to describe the wave even though it interacts with an atom. Thus we shall not find any change in the intensity of the field as it passes the atom (even if the atom is excited by the field). This is consistent with our classical field approach-a classical field has many photons in it whereas the atomic system can add or subtract only a few. The focus in this chapter will be on transitions that are single photon, we reserve multiphoton processes for the next chapter.
