  We observe interaction-induced broadening of the two-photon 5s-18s transition
in 87Rb atoms trapped in a 3D optical lattice. The measured linewidth increases
by nearly two orders of magnitude with increasing atomic density and excitation
strength, with corresponding suppression of resonant scattering and enhancement
of off-resonant scattering. We attribute the increased linewidth to resonant
dipole-dipole interactions of 18s atoms with spontaneously created populations
of nearby np states. Over a range of initial atomic densities and excitation
strengths, the transition width is described by a single function of the
steady-state density of Rydberg atoms, and the observed resonant excitation
rate corresponds to that of a two-level system with the measured, rather than
natural, linewidth. The broadening mechanism observed here is likely to have
negative implications for many proposals with coherently interacting Rydberg
atoms.
