Dipole-Dipole Broadened Line Shape in a Partially Excited Dense Atomic Gas

We have experimentally investigated the resonance line shape of a partially excited dense atomic rubidium vapor at a density where the binary collision approximation starts to break down. The overall spectral shape is well represented by a simple expression for the dielectric function that includes an excitation-dependent linewidth and shift. The results for the excitation dependence of the linewidth agree with an estimate based on a quasistatic picture of the collisions.