where is, in general, a function of position, momentum, and spin operators.

Let us initiate the system in the eigenstate of the unperturbed Hamiltonian, , and switch on the harmonic perturbation at . It follows from Equation (796) that

where

This formula is analogous to Equation (803), provided that

(854) |

Thus, it follows from the analysis of Section 8.6 that the transition probability is only appreciable in the limit if

Clearly, (855) corresponds to the first term on the right-hand side of Equation (851), and (856) corresponds to the second term. The former term describes a process by which the system gives up energy to the perturbing field, while making a transition to a final state whose energy level is less than that of the initial state by . This process is known as

By analogy with Equation (816),

Equation (857) specifies the transition rate for stimulated emission, whereas Equation (858) gives the transition rate for absorption. These equations are more usually written

It is clear from Equations (852)-(853) that . It follows from Equations (857)-(858) that

(861) |

In other words, the rate of stimulated emission, divided by the density of final states for stimulated emission, equals the rate of absorption, divided by the density of final states for absorption. This result, which expresses a fundamental symmetry between absorption and stimulated emission, is known as