The 31Πu state in Na2

We report the first experimental observation of the 31Πu state in the Na2 molecule. Polarisation labelling spectroscopy technique is applied to determine energies of about 500 levels in this state. We deduce a set of Dunham coefficients fitting levels with ν ⩽ 21. The spectroscopic data are used to construct a Rydberg–Klein–Rees potential for the state. Its main molecular constants, Te, ωe, and Be, are found to be 31697.61(2), 103.169(7) and 0.105698(12) cm−1, respectively.