The radiative decay of Cs atoms excited into the 5${\mathrm{D}}_{5/2}$ level and passing between two metallic mirrors spaced by a 1.1-μm gap is observed to depend upon their angular momentum. Spontaneous emission at a wavelength of 3.49 μm is suppressed for the substates with maximum angular momentum normal to the mirrors, which survive without substantial decay during ∼13 natural lifetimes. The radiation rate is modified by application of a magnetic field which mixes sublevels having lifetimes in the gap. This experiment illustrates the anisotropy of the vacuum field in confined space.

• Received 11 November 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.666