Observation of L uncoupling in the 5 ¹Δ _g Rydberg state of Na ₂

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 Λ doubling is removed. In our experiment, the intermediate B Πu1 state of Na2 is excited from the thermally populated ground X Σg+1 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 gerade states back to the a Σu+3 state via collision energy transfer. Under our experimental resolution, the splitting of Λ doubling in the 5 Δg1 state of Na2 can be measured. A total of 136 rovibronic levels with ef parities have been assigned to the 5 Δg1 state. The Λ -splitting constants deduced from these data are q0 =0.376(90)× 10-4 cm-1, qv =0.114(6)× 10-4 cm-1, and μ=0.76(33)× 10-8 cm-1. In general, the Λ splitting of the Δ states is considerably smaller than that of the Π states. However, the first-order splitting constants q0 and qv reported here are larger than those in the B Πu1 state. This is due to the L uncoupling of the Rydberg states.