Observation of L uncoupling in the 5 1Δg Rydberg state of Na2.

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 ¹Π_u state of Na₂ is excited from the thermally populated ground X ¹Σ_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 gerade states back to the a ³Σ_u⁺ state via collision energy transfer. Under our experimental resolution, the splitting of Λ doubling in the 5 ¹Δ_g state of Na₂ can be measured. A total of 136 rovibronic levels with e/f parities have been assigned to the 5 ¹Δ_g state. The Λ-splitting constants deduced from these data are q₀=0.376(90)×10^-4 cm^-1, q_v=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 q_v reported here are larger than those in the B ¹Π_u state. This is due to the L uncoupling of the Rydberg states. [ABSTRACT FROM AUTHOR]