Spectroscopic study of the F1Σg+ outer well state in H2, HD and D2

Two-photon UV-photolysis of hydrogen sulfide molecules is applied to produce hydrogen molecules in highly excited vibrational levels in the X${}^1{\Sigma }_g^{+}$ electronic ground state, up to the dissociation energy and into the quasibound region. Photolysis precursors H₂S, HDS and D₂S are used to produce vibrationally hot H₂, HD and D₂. The wave function density at large internuclear separation is excited via two-photon transitions in the F${}^1{\Sigma }_g^{+}$ - X${}^1{\Sigma }_g^{+}$ system to probe ro-vibrational levels in the first F${}^1{\Sigma }_g^{+}$ outer well state of gerade symmetry. Combining with accurate knowledge of the X${}^1{\Sigma }_g^{+}$ ($v,J$) levels from advanced ab initio calculations, energies of rovibrational levels in the F${}^1{\Sigma }_g^{+}$ state are determined. For the H₂ isotopologue a three-laser scheme is employed yielding level energies at accuracies of $4\times 10^{-3}$ cm⁻¹ for F($v=0,J$) up to $J=21$ and for some low $J$ values of F($v=1$). A two-laser scheme was applied to determine level energies in H₂ for F($v=0-4$) levels as well as for various F levels in HD and D₂, also up to large rotational quantum numbers. The latter measurements in the two-laser scheme are performed at lower resolution and the accuracy is strongly limited to 0.5 cm⁻¹ by ac-Stark effects. For H₂ a new quasibound resonance X${}^1{\Sigma }_g^{+}$ ($v=6$, $J=23$) is detected through the Q(23) and O(23) transitions in the F0-X6 band. Also a quasi-bound resonance in D₂ is assigned, for the first time in this species: X${}^1{\Sigma }_g^{+}$ ($v=17$ , $J=15$). The experimental results on F($v,J$) level energies are compared with previously reported theoretical results from multi-channel quantum-defect calculations as well as with results from newly performed non-adiabatic quantum calculations.