ac Stark Splitting in Resonant Multiphoton Ionization with Broadband Lasers

Abstract

An intense laser and a probe laser in near resonance with the sodium $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ and $3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{3}{2}}$ transitions, respectively, produce a three-photon ionization current used to study the broadening and Rabi splitting of the $3{P}_{\frac{1}{2}}$ intermediate state as a function of probe-laser tuning. We find a narrow, intense-laser detuning region where the peak-heights ratio of the Rabi-split $3{P}_{\frac{1}{2}}$ doublet is reversed from that predicted for a monochromatic laser field. This is attributed to the finite (\ensuremath{\sim} GHz) laser linewidth.