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Starting from an ultracold sample of ground-state ^23 Na ^87 Rb molecules, we investigate the lowest ro-vibrational level of the b ^3 Π state with high resolution laser spectroscopy. This electronic spin-forbidden X ^1 Σ ^+ ↔ b ^3 Π transition features a nearly diagonal Franck–Condon factor and has been proposed useful for probing and manipulating the ultracold molecular gas. We measure the transition strength directly by probing the ac Stark shift induced by near resonance light and determine the total excited-state spontaneous emission rate by observing the loss of molecules. From the extracted branching ratio and the theoretical modeling, we find that the leakage to the continuum of the a ^3 Σ ^+ state plays the dominant role in the total transition linewidth. Based on these results, we show that it is feasible to create optical trapping potentials for maximizing the rotational coherence with laser light tuned close to this transition.

We study, both theoretically and experimentally, the dynamical polarizability $\alpha (\omega )$ of ${\mathrm{Rb}}_{2}$ molecules in the rovibrational ground state of ${a}^{3}{\Sigma }_{u}^{+}$ . Taking all relevant excited molecular bound states into account, we compute the complex-valued polarizability $\alpha (\omega )$ for wave numbers up to 20 000 ${{\rm{cm}}}^{-1}$ . Our calculations are compared to experimental results at $1064.5\;\mathrm{nm}$ ( $\sim 9400\;{{\rm{cm}}}^{-1}$ ) as well as at $830.4\;\mathrm{nm}$ ( $\sim 12\;000\;{{\rm{cm}}}^{-1}$ ). Here, we discuss the measurements at $1064.5\;\mathrm{nm}$ . The ultracold Rb _2 molecules are trapped in the lowest Bloch band of a 3D optical lattice. Their polarizability is determined by lattice modulation spectroscopy which measures the potential depth for a given light intensity. Moreover, we investigate the decay of molecules in the optical lattice, where lifetimes of more than $2\;{\rm{s}}$ are observed. In addition, the dynamical polarizability for the ${X}^{1}{\Sigma }_{g}^{+}$ state is calculated. We provide simple analytical expressions that reproduce the numerical results for $\alpha (\omega )$ for all vibrational levels of ${a}^{3}{\Sigma }_{u}^{+}$ as well as ${X}^{1}{\Sigma }_{g}^{+}$ . Precise knowledge of the molecular polarizability is essential for designing experiments with ultracold molecules as lifetimes and lattice depths are key parameters. Specifically the wavelength at $\sim 1064\;\mathrm{nm}$ is of interest, since here, ultrastable high power lasers are available.