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 ${\mathrm{Cs}}_{2}(X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+})+\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{pump}}\ensuremath{\rightarrow}{\mathrm{Cs}}_{2}^{*}\ensuremath{\rightarrow}\mathrm{Cs}(5D)+\mathrm{Cs}(6S)$ 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 $5{D}_{3∕2}\ensuremath{\rightarrow}5{F}_{5∕2}$ 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(5D3∕2) atoms with ground-state Cs atoms has been determined. The result ${k}_{\mathrm{VCC}}=(6.1\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ corresponds to an effective velocity-changing collision cross section of ${\ensuremath{\sigma}}_{\mathrm{VCC}}^{\mathrm{Cs},\mathrm{eff}}=(1.2\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$.