Measurement of the spectrum of resonance fluorescence from a two-level atom in an intense monochromatic field

We have made measurements of the spectrum of resonance fluorescence emitted by sodium atoms in an intense, uniform, monochromatic field. The transition used in the experiment was between the $3{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ ($F=2\mathrm{}$, $m_F=2\mathrm{}$) ground state and the $3{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ ($F^{\prime }=3\mathrm{}$, $m_F^{\prime }=3\mathrm{}$) excited state. A separate light beam, circularly polarized and resonant with this transition, was used to prepare the sodium atoms in the $F=2\mathrm{}$, $m_F=2\mathrm{}$ magnetic sublevel. This pumping scheme was used to obtain the spectrum for both on- and off-resonance excitations. The measurements are in good agreement with Mollow's calculation. In addition, we describe conditions under which we observed asymmetric spectra similar to those of earlier experiments.