  We investigate the impact of rotational diffusion on the electrodynamic
coupling of fluorescent dye molecules (oscillating electric dipoles) to a
tunable planar metallic nanocavity. Fast rotational diffusion of the molecules
leads to a rapidly fluctuating mode density of the electromagnetic field along
the molecules' dipole axis, which significantly changes their coupling to the
field as compared to the opposite limit of fixed dipole orientation. We derive
a theoretical treatment of the problem and present experimental results for
rhodamine 6G molecules in cavities filled with low and high viscosity liquids.
The derived theory and presented experimental method is a powerful tool for
determining absolute quantum yield values of fluorescence.
