“…These features are similar to those reported previously for excitation on the Cs 852 nm line [3], but here the resolution of the hyperfine components makes visible that the dip is shifted to the red (note that the discussion in [3,4,7] has been limited to the strongest hyperfine component, for which the red side is plagued with the-hardly resolved-other hyperfine components, of a weaker amplitude). It is however very delicate to find a narrow contribution on the fluorescence spectrum itself, as it was already the case in [3,4,7]. However, the FM applied to the laser (at 10 kHz, and with an excursion ~ 10 MHz much smaller than the width of the direct signal) allows to process the signal by a lock-in detector, delivering the frequency-derivative of the initial spectrum, which emphasizes the narrow contributions.…”