Applying the cw laser absorption and fluorescence method the cross sections for the fine-structure mixing and quenching of the caesium 5D J states have been measured in pure caesium vapour. Caesium atoms were optically excited to the 5D 5/2 state via the quadrupole-allowed 6S 1/2 → 5D 5/2 transition. The ground-state caesium density N 0 , obtained from the absorption at the pumped quadrupole transition, and the fluorescence intensity I 689 of the sensitized 5D 3/2 → 6S 1/2 emission were

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... simultaneously in the range 3 × 10 14 cm −3 N 0 1 × 10 16 cm −3 at constant temperature T = 585 K. It was found that the quantity N 2 0 /I 689 exhibited a parabolic dependence on N 0 , confirming that the quenching of the Cs 5D J states is due to collisions with Cs ground-state atoms, not molecules. The coefficients of the second-order polynomial fitted through the measured data yielded the cross sections σ 5/2→3/2 = (57 ± 19) × 10 −16 cm 2 and σ D = (35 ± 10) × 10 −16 cm 2 for the Cs 5D J fine-structure mixing and quenching, respectively, due to collisions with caesium ground-state atoms. Using recently calculated Cs * + Cs potentials we performed an analysis which shows good agreement between the measured values and the theoretical predictions.