Quantum-Mechanical Systems in Monochromatic and Nearly Monochromatic Fields

It is shown how Rabi's treatment of the response of a two-level quantum-mechanical system to a monochromatic external driving field may be rigorously placed within the framework of the time-dependent quantum mechanics of a general system by using unitary transformations to separate out slowly varying contributions in closed form. The method is extended to include a certain class of nonmonochromatic fields and the concept of a linearly time-dependent (induced) transition probability is discussed.