Fig. 2

An experimental Fortrat diagram. a A series of two-colour resonant four-wave mixing (TC-RFWM) spectra ordered by the experimentally determined rotational quantum number J″ of the ground state \({\mathrm{X}}0_{\mathrm{g}}^ +\) is shown. The regions of unambiguously assigned lines of the ⁶⁵Cu⁶³Cu isotopologue are coloured based on the excited electronic state (blue: \(G_{68}0_{\mathrm{u}}^ +\), green: O1 and red: \({\mathrm{J}}0_{\mathrm{u}}^ +\) (v = 2)). b The corresponding rovibronic levels are plotted, based on molecular constants fitted to the assigned line positions. For clarity, the energy scale was reduced by taking into account the rotational constant B of one state to straighten the parabolas