We describe radiative-decay-induced four-level crossover signals in Doppler-broadening-free saturation spectroscopy. A four-level crossover signal occurs when atoms in a segment of the velocity distribution absorb light from the saturating beam, radiatively decay in part to a different lower level, and cause enhanced absorption from the probe beam on a transition that shares neither a common upper nor a common lower level with the transition of the first absorption. We interpret a signal in a saturation spectrum published by Hänsch, Shahin, and Schawlow as a four-level crossover signal induced by radiative decay.

• Received 3 February 1978

DOI:https://doi.org/10.1103/PhysRevA.17.2099