THE DOUBLE - MINIMUM (2) ${^{1}}\Sigma^{+}_{u}$ STATE OF $Na_{2}$

Author Institution: Laboratoire Aim\'{e} Cotton, C.N.R.S. II; Laboratoire de Spectrom\'{e}trie Ionique et Mol\'{e}culaire, 43 boulevard du 11 Novembre 1918; Physical Chemistry Laboratory, Oxford UniversityThe spectroscopic properties of a new double-minimum state, (2) ${^{1}}\Sigma^{+}_{u}$, of $Na_{2}$ have been determined by studies of infra-red fluorescence recorded at high resolution by Fourier transform spectrometry. Some levels (v,J) of the inner well, and a few upper levels (i.e. levels above the potential maximum) are populated by u.v. lines ($\sim 350$ nm) of krypton-ion and argonion lasers. All vibrational levels of the outer well and many upper levels form the lower states in fluorescence from Rydberg states, e.g. (3s,5s) ${^{1}}\Sigma^{+}_{g}$ populated in two-photon processes (3s,5s) ${^{1}}\Sigma^{+}_{g} \leftarrow A{^{1}}\Sigma^{+}_{u} \leftarrow X{^{1}}\Sigma^{+}_{g}$ by a ring dye-laser. A potential curve has been derived for (2) ${^{1}}\Sigma^{+}_{u}$: the two potential minima lie at $28454.6 cm^{-1}, 3.688 {\AA}$ and $27879.0 cm^{-1}, 6.715 {\AA}$ respectively, and the energy at the top of the barrier is about $29220 cm^{-1}$ (energies are given relative to the minimum in $X{^{1}}\Sigma^{+}_{g}$)