Thermalization of fast cesium 5D_3∕2 atoms in collisions with ground-state cesium atoms

We have investigated collisions involving fast, excited Cs atoms produced by photodissociating Cs₂ molecules with a pulsed dye laser. The velocities of the atoms in the 5D state formed by the process Cs₂(X ¹Σ_g⁺)+ℏω_pump→Cs₂^*→Cs(5D)+Cs(6S) are much greater than typical thermal velocities associated with the cell temperature. Using a narrow-band cw probe laser to observe the increased Doppler broadening of the 5D_3∕2→5F_5∕2 excitation line shape, we are able to monitor the time evolution of the velocity distribution of these 5D atoms. We analyze the data using a model that predicts the time-dependent excitation line shape of the fast atoms. Because the photons used to dissociate the molecules have a well-defined energy, the velocity distribution of the excited atoms in the early time after they are produced can be fairly well determined. Over time, velocity-changing collisions with ground-state Cs atoms cause the velocity distribution of excited atoms to approach the thermal limit. An analysis based on the strong-collision model leads to a prediction that the observed line shape at intermediate times will be a linear combination of contributions from distinct “fast” and “thermalized” atomic populations. By fitting our data to this model, a rate coefficient for velocity-changing collisions of fast Cs(5D_3∕2) atoms with ground-state Cs atoms has been determined. The result k_VCC=(6.1±1.2)×10⁻¹⁰ cm³ s⁻¹ corresponds to an effective velocity-changing collision cross section of σ_VCC^Cs,eff=(1.2±0.2)×10⁻¹⁴ cm².