In the co-propagating spectrum shown below, the peaks become prominent. The hyperfine intervals [@DAN08] are such that the real peaks are at $-184.390$ MHz, $-120.966$ MHz, and 0, while the additional peaks are at $-63.424$ MHz, $+63.424$ MHz, $+120.966$ MHz, and $+184.390$ MHz. The additional peak within the spectrum (at $-63.424$ MHz) is almost negligible, as was observed for upper-level transitions in $^{87}$Rb. The beam powers are 9 $\mu$W (probe) and 15 $\mu$W (pump).

Spectrum in $^{85}$Rb, $F=2 \rightarrow F'=1,2,3$
-------------------------------------------------

For transitions starting from the lower ground level ($F=2
\rightarrow F'=1,2,3$) shown in Fig. \[rb21\], the hyperfine interval between $F'=1$ and 2 is so small that the crossover resonance in the saturated-absorption spectrum completely swamps the $F'=1$ peak. However, the spectrum with the co-propagating technique shows the peak well resolved. The real peaks are located at 0, $+29.268$ MHz, and $+92.692$ MHz, while the additional peaks are at $-92.692$ MHz, $-63.424$ MHz, $-29.268$ MHz, and $+63.424$ MHz [@DAN08]. The additional peak within the spectrum (at $+63.424$ MHz) is quite prominent as in the case of transitions starting from the lower hyperfine level in $^{87}$Rb, again because the closed $F=2 \rightarrow F'=1$ transition has population trapping in the $m_F = 