ABSTRACT
This chapter considers an atom as a quantum mechanical system. But the electromagnetic radiation will be treated as classical waves described by the Maxwell equations. The treatment is called semi-classical. Quantum mechanics describes an atom as a quantized system with a set of discrete energy levels. Each energy level is designated by a set of quantum numbers like the principal quantum of the Rydberg level and the quantum numbers representing the orbital, spin and total angular momentum. A molecule will have a different set of quantum numbers arising from its rotational and vibrational motions. Atoms in a solid have energy levels in the form of a band. The observed spectrum due to spontaneous emission from a collection of atoms is not monochromatic, but it is spread over a certain range of frequencies. The absorption spectrum also shows that the atom does not absorb at a single frequency, but it is spread over a band of frequencies around the resonance frequency.
