High-resolution laser spectroscopy of the (2) ³∏_g state of Rb₂

High-resolution molecular spectroscopy is a key technology to enable advanced research, e.g., with cold molecules and cold chemistry. As an example, in our recent experiments on state-to-state chemistry we use resonance-enhanced multiphoton ionization (REMPI) to state-selectively detect molecules which have been produced in a cold chemical reaction, see e.g. [1]. In order to achieve high selectivity and efficiency for the REMPI path the resonant intermediate molecular states have to be precisely known and wisely chosen. About two decades ago, the group around William Stwalley at the University of Connecticut set out to explore such REMPI paths for Rb₂ via the state (2) ³∏_g  which correlates to the 5s 4d asymptote [2,3]. For this, they used cold atom-/ cold molecule-based spectroscopy with ns- laser pulses. Recently, we have refined these measurements with a tunable cw-laser, now resolving rotational spectra for both ⁸⁵Rb and ⁸⁷Rb. This allows for an unambiguous assignment of lines to the sub-states Ω = 0, 1, 2. A coupled-channel calculation combined with a fit to the globally available spectroscopic data, shows how the  (2) ³∏_g  state couples to other electronic states and, in particular, allows for determining the strength of spin-orbit coupling.