  The Dicke model is of fundamental importance in quantum mechanics for
understanding the collective behaviour of atoms coupled to a single
electromagnetic mode. In this paper, we demonstrate a Dicke-model simulation
using cavity-assisted Raman transitions in a configuration using
counter-propagating laser beams. The observations indicate that motional
effects should be included to fully account for the results and these results
are contrasted with the experiments using single-beam and co-propagating
configurations. A theoretical description is given that accounts for the beam
geometries used in the experiments and indicates the potential role of motional
effects. In particular a model is given that highlights the influence of
Doppler broadening on the observed thresholds.
