Perturbation-facilitated all-optical triple resonance spectroscopy of the Na₂ b ³Π_u state

The Na₂ b ³Π_u state has been studied by the continuous wave (cw) all-optical triple resonance (AOTR) technique. The AOTR technique used here corresponds to a perturbation-facilitated optical-optical double resonance (PFOODR) excitation through A ¹Σ_u⁺∼b ³Π_u mixed intermediate levels from the ground state to the 2 ³Π _g state and stimulated emission pumping (SEP) of the 2 ³Π_g→b ³π_u transition. This sub-Doppler high-resolution PFOODR-SEP technique has allowed us to reach many b ³Π_u levels (Ω=0, 1, 2, perturbed and unperturbed, including low v below the A ¹Σ_u ⁺ potential minimum and higher vibrational levels up to v=57), Based on these new high resolution data and previous results from high resolution spectroscopy, we have determined a new set of deperturbed molecular constants from the b ³Π_u state up to v=57, the corresponding RKR potential energy curve and the A ¹Σ_u ⁺∼b ³Π_u spin-orbit interaction constants. This represents an example of a powerful and general technique for observing a "dark" (e.g., triplet) perturbing state when only the "bright" (e.g., singlet) perturbed state is well known from single photon spectroscopy.