states of alkali-metal atoms, plays an important role in fine-tuning atomic wavefunctions used in theoretical calculations. This is because comparison between theoretical and experimental determinations of hyperfine structure provides a stringent test of atomic calculations in the vicinity of the nucleus [@SJD99]. In addition, hyperfine structure in these multielectron atoms is sensitive to core polarization and core correlation effects [@AIV77].

Many of the alkali-metal atoms have transitions to the first-excited state (so-called $D$ lines) which are in the near infrared, and therefore accessible with diode lasers. They also have a high-enough vapor pressure near room temperature that the spectroscopy can be done in a vapor cell. The thermal motion of the atoms inside the cell causes [*Doppler broadening*]{}, which is typically 100 times larger than the natural width of the hyperfine transitions. The standard technique to overcome the first-order Doppler effect is to use a [*counter-propagating*]{} pump beam to saturate the transition for zero-velocity atoms, in what is called saturated-absorption spectroscopy (SAS) [@DEM82].

Most atoms also have several closely-spaced hyperfine levels within the Doppler profile. In these cases, it is well known that SAS also produces spurious crossover resonances in between each pair of hyperfine transitions. They occur because, for 