  The interaction of a two-level atomic ensemble with a quantized single mode
electromagnetic field in the presence of optical collisions (OC) is
investigated both theoretically and experimentally. The main accent is made on
achieving thermal equilibrium for coupled atom-light states (in particular
dressed states). We propose a model of atomic dressed state thermalization that
accounts for the evolution of the pseudo-spin Bloch vector components and
characterize the essential role of the spontaneous emission rate in the
thermalization process. Our model shows that the time of thermalization of the
coupled atom-light states strictly depends on the ratio of the detuning and the
resonant Rabi frequency. The predicted time of thermalization is in the
nanosecond domain and about ten times shorter than the natural lifetime at full
optical power in our experiment. Experimentally we are investigating the
interaction of the optical field with rubidium atoms in an ultra-high pressure
buffer gas cell under the condition of large atom-field detuning comparable to
the thermal energy in frequency units. In particular, an observed detuning
dependence of the saturated lineshape is interpreted as evidence for thermal
equilibrium of coupled atom-light states. A significant modification of
sideband intensity weights is predicted and obtained in this case as well.
