Autler-Townes splitting in a sodium molecular-ladder scheme*

We report results from studies of the Autler-Townes (AT) splitting observed in sodium dimer using optical-optical double resonance spectroscopy. In our experiment, a single line Ar$^+$ laser at 4727 $\AA$ pumps the thermally populated ground state level $X\,^1\Sigma_g^+(v''=1,\,J''=37)$ of Na$_2$ to the intermediate $B\,^1\Pi_u(v'=9,\,J'=38)$ level. Then, a single mode cw Ti:sappher or dye laser couples to the higher Rydberg states. The signals are detected by monitoring the UV fluorescence from the triplet \emph{gerade} states back to the $a\,^3\Sigma^+_u$ state via collision energy transfer. Unlike all the other intermediate levels pumped by different Ar$^+$ laser lines, this strong transition populated the $B\,^1\Pi_u (v'=9,\,J'=38)$ level leads to an AT splitting on all the levels observed in the higher Rydberg states, for example, 3, 5, 6, 7\,$^1\Pi_g$ states and 3, 5, 6\,$^1\Delta_g$, states.