Harmonic Perturbations

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

Let us initiate the system in the eigenstate of the unperturbed Hamiltonian, , and then switch on the harmonic perturbation at . It follows from Equation (8.59) that

where

Equation (8.114) is analogous to Equation (8.66), provided that

(8.114) |

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

Clearly, criterion (8.118) corresponds to the first term on the right-hand side of Equation (8.114), whereas criterion (8.119) corresponds to the second. 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 is less than that of the initial state by . This process is known as

By analogy with Equation (8.79),

Equation (8.120) specifies the transition rate for stimulated emission, whereas Equation (8.121) gives the transition rate for absorption. These transition rates are more usually written [see Equation (8.80)]

where it is understood that the previous expressions must be integrated with to obtain the actual transition rates.

It is clear from Equations (8.115) and (8.116) that . It follows from Equations (8.120) and (8.121) that

(8.121) |

In other words, the rate of stimulated emission, divided by the density of final states for stimulated emission, is equal to 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