The intensity dependent absorption was measured on the D 1 line (6S 1/2 → 6P 1/2 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), t = (1.13 ± 0.20)v rms /D, where v rms is the two-dimensional root-mean-squared speed of the atom and D is the FWHM of the Gaussian laser beam spatial profile.

We report experimental rate coefficients for the energy-pooling collisions Cs(6 P 1/2 ) ϩCs(6 P 1/2 )→Cs(6S 1/2 )ϩCs(nl J Ј ) and Cs(6 P 3/2 )ϩCs(6 P 3/2 )→Cs(6S 1/2 )ϩCs(nl J Ј ) where nl J Ј ϭ7P 1/2 , 7 P 3/2 , 6D 3/2 , 6D 5/2 , 8S 1/2 , 4F 5/2 , or 4F 7/2 . Atoms were excited to either the 6 P 1/2 or 6 P 3/2 state using a single-mode Ti:sapphire laser. The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode ring dye laser beam, tuned to either the 6 P 1/2 →8S 1/2 or 6 P 3/2 →7D 3/2,5/2 transitions, which could be translated parallel to the pump beam. Transmission factors, which describe the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed, were calculated for all relevant transitions. Effective lifetimes of levels populated by energy-pooling collisions are modified by radiation trapping, and these factors were calculated using the Molisch theory. These calculated quantities have been combined with the measured excited-atom densities and fluorescence ratios to yield absolute energy-pooling rate coefficients. It was found that the rate for production, in all cases, is greatest for 6D, but that 1/2-1/2 collisions are significantly more efficient than 3/2-3/2 collisions for populating 7 P. It was also found that 7 P 1/2 is populated two to three times more efficiently than 7 P 3/2 in 1/2-1/2 collisions, but that the 7 P fine-structure levels are approximately equally populated in 3/2-3/2 collisions.

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Abstract-The self-broadening rate coefficients of the cesium D, (62Sy2-6'P1,2] and D, [62S,!,-62P,j2] resonance lines are determined from the 6'P,,, + 6*D,,,,, 6 P,,, + 72D,,, and 62P,:2 -+ 72DSiz transition lineshapes, which are mapped out using single-mode cw lasers. The self-broadening rates are found from Voigt function fits (including hyperfine structure) to the experimental lineshapes. Our measured D, and D, broadening coefficients, kbr(D,) = (5.7 & 1.0) x IO-' cm3 set-' and kbr(DZ) = (6.7 f 1.1) x lo-' cm3 set-I, are compared to previous experimental and theoretical values in the literature. The present D2 broadening coefficient is smaller, and the D, broadening coefficient is larger, than theoretical values.

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