The hyperfine structure of the Na243Σ+gstate

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Abstract
The fine and hyperfine structure in the Na2 4 3Σ+ g state is partially resolved by Perturbation Facilitated Optical Optical Double Resonance (PFOODR) spectroscopy via A 1Σ+ u ∼ b 3Π u mixed intermediate levels. The hyperfine splitting and intensity patterns are shown to be consistent with the Hund's case b βS limit, when the 4 3Σ+ g state is free of perturbations by nearby vibrational levels of the case a β 2 3Π g and 3 3Π g states. Perturbed levels of 4 3Σ+ g show case b βJ hyperfine structure. The crucial difference between cases b βJ and b βS is the absence, in b βS , of fine structure intervals EN, J - E N, J ± 1 large relative to ΔN = 0, ΔJ = ± 1 J-destroying matrix elements of the hyperfine hamiltonian. It appears that case b βS coupling will be the rule rather than the exception for triplet Rydberg states of Na2, Li2, and H2 built on the ion-core X 2Σ+ g electronic ground state. The fine and hyperfine structure will be dominated by an nlγ-invariant and N-independent Fermi Contact contribution from the singly occupied σ g ns valence molecular orbital. For Na2 4 3Σ+ g , the bI. S coupling constant is b = 214 ± 50 MHz, in satisfactory agreement with semi-empirical values of 221 or 150 MHz naively derived from the free Na atom 3 2 S F = 2 - F = 1 interval.