Knowledge Bank

The phenomenon of electronic orbital angular momentum $L$ uncoupled from its internuclear axis has been observed in the sodium dimer using high resolution cw optical-optical double resonance spectroscopy. When $L$-uncoupling occurs, the degeneracy of $\Lambda$-doubling is removed. In our experiment, the intermediate $B1\Pi u$ state of Na$2$ is excited from the thermally populated ground $X1\Sigma g+$ state by a single line Ar$+$ laser. Then, a single-mode dye laser is used to probe the Rydberg states from the intermediate state. The signals are detected by monitoring the UV fluorescence from the triplet \emph{gerade} states back to the $a3\Sigma u+$ state via collision energy transfer. Under our experimental resolution, the splitting of $L$-uncoupling and $\Lambda$-doubling in both the $51\Delta g$, $51\Pi g$ states of Na$2$ can be measured.Total of 136 rovibronic levels with $e/f$ parities have been assigned to the $51\Delta g$ state. The $\Lambda$-splitting constants deduced from these data are : $q0$=,$0.376(90)\times 10-4$,cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ , $qv$=,$0.114(6)\times 10-4$,cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ , $\mu$=, $ 0.76(33)\times10^{-8}$cm$^{-1}$. In general, the $\Lambda$-splitting of the $\Delta$ states is considerably smaller than that of the $\Pi$ states. However, the first order splitting constants $q0$ and $qv$ reported here are larger than those in the $B1\Pi u$ state. This is due to the $L$-uncoupling of the Rydberg states. Total of 230 rovibrational levels are assigned to the Na$2$ $51\Pi g$ state, and the following are its correspondent $\Lambda$-splitting constants : $q0$=,$0.1758320898(0.017)\times 10-3$,cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ , $qv$=,$-0.7124318162(0.071)\times 10-5$,cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ , $\mu$=,$ -0.9608208561(0.8)\times10^{-8}$cm$^{-1}$. The splitting of $\Lambda$-doubling increases quadratically with the rotational quantum number $J$ and weakly depends on the vibrational quantum number $v$. These splitting constants are much larger than those in the Na$2$ $B1\Pi u$ state. This indicates that the splitting of $\Lambda$-doubling in the $51\Pi g$ state is affected by both the perturbations by adjacent $\Sigma$ states and the $L$-uncoupling. (grand no. NSC-95-2112-M-012-MY3).