The Na2 3 3Πg state adiabatically dissociates to the 3s+4p atomic limit. The 3 3Πg levels below the 3s+3d atomic limit have been observed by perturbation facilitated optical–optical double resonance (PFOODR) fluorescence excitation spectroscopy. Energy levels above the 3s+3d atomic limit cannot be observed by detecting molecular fluorescence but are observable by detecting atomic 3d→3p fluorescence. The 3 3Πg state interacts with the 3s+3d 2 3Πg state. At lower energy this interaction is responsible for many perturbations between bound vibrational levels of these two states. Above the 3s+3d limit, as a result of this interaction, the 3 3Πg state is strongly predissociated and the linewidths of levels close to the 3s+3d limit are about 22 cm−1. The predissociation interaction is due to an avoided crossing between the adiabatic 2 3Πg and 3 3Πg states at R≈2.85 Å. Weak predissociation of the 4 3Σg+ and 2 3Δg states are also observed and discussed.

The hyperfine spectra of the Na 2 1 3 ⌬ g state have been recorded with sub-Doppler continuous wave ͑CW͒ perturbation facilitated optical-optical double resonance ͑PFOODR͒ spectroscopy viaThe rotational lines into the Nϭ14-51 levels of the 1 3 ⌬ g state observed previously ͓J. Mol. Spectrosc. 134, 50 ͑1989͔͒ have four components for transitions between symmetric rotational levels or five components for transitions between antisymmetric rotational levels and the hyperfine coupling belongs to Hund's case b ␤S . For the low-N levels observed later, however, the hyperfine spectra are much more complicated. We have worked out the matrix elements of the molecular Hamiltonian in the case b ␤S basis. After taking into consideration spin-orbit, spin-spin, and spin-rotation interactions, we obtained a set of molecular constants for the Na 2 1 3 ⌬ g state, with which we can reproduce the hyperfine spectra of both highand low-N rotational levels.

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