Doppler-free spectroscopy of launched laser-cooled atoms, in preparation for Rydberg excitation

We investigate Doppler-free spectroscopy of cold ⁸⁵Rb atoms launched from a magneto-optical trap (MOT).  Each cold-atom cloud travels upward with vertical speed v_y = 5.9±0.4 m/s and transverse speed v_x = 0.4±0.1 m/s, passing through a near-resonant probe beam.  We observe three spectral features corresponding to the D2 manifold of ⁸⁵Rb with minimal Doppler broadening.  The prominent feature for F = 3 → F' = 4 yields transmission T = 0.94.  For comparison, we build on a pre-existing model of absorption in warm atoms to predict the spectrum of cold atoms transitioning from the 5S_1/2 state to the 5P_3/2 state.  These results confirm and add to current understanding of cold atom absorption of near-resonant laser light in real experimental conditions, laying the groundwork for further exciting experiments such as multi-photon absorption to reach selected sublevels of the Rydberg state with n = 35.