Polarization studies of interferences in two-photon excitation of sodium atoms

Two independently tunable, linearly polarized dye lasers are counterpropagated through a heat pipe containing sodium vapor. The frequency of the first laser ω₁ is incremented through both 3p²{P}1/2$,3/2 fine-structure levels while the second laser ${{ω}}₂$ is scanned through the energy increment necessary to reach ns $²S1/2$ and ${nd}²D3/2,5/2 levels (n=5--35). Following excitation, ionization occurs through chemi-ionizing collisions. The polarization (defined as the ionization intensity measured when the two planes of polarization of the laser beams are parallel minus the intensity when perpendicular divided by the sum of the intensities) P=(I?-I_⊥)/(I?+I_⊥) goes from +1 to -1 as ω₁ is tuned between the 3p²{P}1/2$,3/2 fine-structure levels. This novel polarization effect is shown to be due to an interference between the two pathways to the ns $²S1/2$ levels. Upon exciting the unresolved nd $²D3/2$,5/2 levels, the polarization is very close to zero when ${{ω}}₁$ is at or near the 3p $²P3/2$,1/2 levels. A simple few-level atom theory reproduces the experimental observations except for the case when both lasers are in resonance.