Measurement and modelling of intensity dependent absorption and transit relaxation on the cesium line

The intensity dependent absorption was measured on the line (6 transition) in atomic cesium. The magnetic field applied to the vapour and the spatial cross section of the laser beam were controlled and varied during data collection. A three-level rate equation model is presented in an attempt to explain the results. We show that this well known approach does not sucessfully model the data obtained in the absence of a magnetic field. Hence, a more complex and complete model that explicitly includes all of the hyperfine magnetic sublevels (a multilevel model) is presented. This approach accurately models all of the data collected. The good agreement betweeen this model and the data allows the determination of the transit relaxation rate (due to atomic time of flight through the laser beam), , where is the two-dimensional root-mean-squared speed of the atom and D is the FWHM of the Gaussian laser beam spatial profile.