The spontaneous decay of an atom located between two parallel plane mirrors of reflectivity $R$ is calculated using the Weisskopf-Wigner method, both when the two mirrors are very large and when they are small. When the mirrors are large and of high reflectivity, the atom radiates more rapidly than in free space and into the Fabry-Perot modes of the mirror system. When the mirrors are small, their effect on the total radiation rate is small, but radiation into the few modes defined is enhanced if the reflectivity is high. It is concluded that the effect of the presence of a laser etalon is insufficient to justify omitting the coupling of the atom to non-Fabry-Perot modes from the Hamiltonian of an atomic system in a laser etalon.

• Received 7 October 1969

DOI:https://doi.org/10.1103/PhysRevA.2.102