some non-zero-velocity group, the pump drives one transition while the probe drives the other. In earlier work [@BAN03], we have shown that the use of co-propagating pump and probe beams overcomes the problem of crossover resonances. Closely-spaced levels that are not resolved in SAS can be resolved by this technique. Probe transmission in such multilevel atoms is caused by the phenomenon of electromagnetically induced transparency (EIT) [@HAR97; @DAN05], and population depletion due to optical pumping. Furthermore, by scanning only the pump beam, the signal appears on a flat background without the underlying Doppler profile seen in SAS. This is advantageous for applications such as laser locking or laser frequency measurement.

In this work, we present a complete study of the spectra taken with the co-propagating technique in the $D_2$ line of the two isotopes of Rb. In particular, we show that the effect of non-zero velocity groups is to cause additional peaks. However, in contrast to SAS, only one of these spurious peaks appears within the spectrum, and the real peaks are unaffected. Interestingly, the spurious peak within the spectrum is almost negligible for transitions starting from the upper ground hyperfine level, but highly prominent for transitions starting from the 