Determining the cesium $7d \, ^2 \! D_j$ hyperfine structure using two-photon resonant spectroscopy of a thermal beam

The hyperfine structures of the $7d \, ^2 \! D_{3/2}$ and $7d \, ^2 \! D_{5/2}$ states of $^{133}$Cs are determined through two-photon, laser-induced-fluorescence spectroscopy of a thermal beam. Two single-mode external-cavity diode lasers provide narrow band radiation for resonant two-step excitation of the $7d \, ^2 \! D_j$ states. A servo-feedback circuit locks one laser to the $6s \, ^2 \! S_{1/2} (F) \rightarrow 6p \, ^2 \! P_{3/2} (F')$ hyperfine transitions. Optical pumping of the ground hyperfine manifold is minimized by phase modulating this laser at 9.193 GHz. The second laser is scanned over the $6p \, ^2 \! P_{3/2} (F') \rightarrow 7d \, ^2 \! D_j (F'')$ transitions. Using various combinations of the ground and intermediate hyperfine levels (i.e., $F$ and $F'$), all hyperfine intervals of the $7d \, ^2 \! D_j$ states are observed. The scanned laser's relative frequency is calibrated through phase modulation; the resulting sidebands cause atomic features to be repeated at precise intervals. High accuracy is achieved by directly referencing the modulation frequency to the $^{87}$Rb $5s \, ^2 \! S_{1/2} (F=1) \leftrightarrow 5s \, ^2 \! S_{1/2} (F=2)$ ground state hyperfine transition using an atomic frequency standard.