  Here we report on measurements of the absolute absorption spectra of dense
rubidium vapour on the D1 line in the weak-probe regime for temperatures up to
170 C and number densities up to 3 \times 10^14 cm^-3. In such vapours,
modifications to the homogeneous linewidth of optical transitions arise due to
dipole-dipole interactions between identical atoms, in superpositions of the
ground and excited states. Absolute absorption spectra were recorded with
deviation of 0.1% between experiment and a theory incorporating resonant
dipole-dipole interactions. The manifestation of dipole-dipole interactions is
a self-broadening contribution to the homogeneous linewidth, which grows
linearly with number density of atoms. Analysis of the absolute absorption
spectra allow us to ascertain the value of the self-broadening coefficient for
the rubidium D1 line: \beta/2\pi = (0.69 \pm 0.04) \times 10^-7 Hz cm^3, in
excellent agreement with the theoretical prediction.
