groups causes additional peaks, but only one of them appears within the spectrum. These observations are again supported by density-matrix calculations taking the thermal velocity distribution into account. An important difference between transitions starting from the upper ground level and transitions starting from the lower ground level, is that the additional peak is almost negligible in the first case and quite prominent in the second case. This difference arises because of the difference in number of magnetic sublevels for the closed transition in each set.

We have adapted this technique to make measurements of hyperfine intervals by using one laser along with an AOM to produce the scanning pump beam. We measure intervals in the $D_2$ lines of Rb and Cs with 20 kHz precision. By measuring the entire spectrum and looking at the symmetry of the line shape, we avoid several potential sources of systematic error. The measurements are consistent with earlier results from our laboratory obtained by locking the AOM to the frequency difference, and show that our earlier error budget was reasonable.

This work was supported by the Department of Science and Technology, India. V.N. acknowledges support from the Homi Bhabha Fellowship Council and A.K.S. from the 