The effects of saturation and velocity selective population in two-step 6S1/2 → 6P3/2 → 6D5/2 laser excitation in cesium

Excited states population distributions created by two-step 6S1/2 → 6P3/2 → 6D5/2 laser excitation in room temperature cesium vapor were quantitatively analyzed applying absorption and saturation spectroscopy. A simple method for the determination of the excited state population in a single excitation step that is based on the measurements of the saturated and unsaturated absorption coefficients was proposed and tested. It was shown that only ≈ 2% of the ground state population could be transferred to the first excited state by pumping the Doppler broadened line with a single-mode narrow-line laser. With complete saturation of the second excitation step, the population amounting to only ≈ 1% of the ground state can be eventually created in the 6D5/2 state. The fluorescence intensity emerging at 7P3/2 → 6S1/2 transition, subsequent to the radiative decay of 6D5/2 population to the 7P3/2 state, was used to assess the efficiency of the population transfer in the chosen two-step excitation scheme. The limitations imposed on the sensitivity of such resonance fluorescence detector caused by velocity-selective excitation in the first excitation step were pointed out and the way to overcome this obstacle is proposed.