Fluorescence decay rate modification has been observed for molecules in liquid microspheres (3.5 to 25 μm in diameter) where the molecular position and transition moment orientation are well defined. For small sizes ( $<6\mathrm{}$ μm), the decay rates scale roughly as $r^{-1\mathrm{}}$ and agree quantitatively with semiclassical calculations. For larger sizes ( $>15\mathrm{}$ μm) where the cavity storage time and fluorescence lifetime are roughly equal, an anomalous decrease in the decay rate of $\approx 30\%$ is observed which is interpreted as a consequence of intermediate molecule-cavity coupling.

• Received 9 November 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.3931