signal remains flat (or Doppler free) until the pump also comes into resonance with a transition for the same zero-velocity atoms. Thus there are three transmission peaks at the locations of the hyperfine transitions, with no crossover resonances in between. The hyperfine peaks are located at $-423.600$ MHz, $-266.657$ MHz, and 0 [@DAN08], all measured with respect to the frequency of the locked probe laser. The linewidth of the peaks is about 19 MHz, which is only 50% larger than the linewidth obtained in the saturated-absorption spectrum. The primary cause for the transparency peaks is the phenomenon of EIT in this V-type system. The pump laser causes an [*AC Stark shift of the ground level*]{} (creation of dressed states [@COR77]) and hence reduces probe absorption at line center. In addition, there are effects of saturation and optical pumping, but these are less important than the EIT effects.

Since the experiments are done in a vapor cell with the full Maxwell-Boltzmann distribution of velocities, we have to consider that there will be two additional velocity classes that absorb from the locked probe: both moving in the same direction as the probe but with velocities such that one drives transitions to the 