Observations of the 41<m:msubsup> Σ u + state of H2

Resonantly-enhanced multiphoton ionization via the EF¹Σ_g⁺ , v^' = 6 double-well state has been used to probe the energy region of the high vibrational levels of the 4¹Σ_u⁺ state of H₂ . Theoretical ab initio potential energy curves for this state predict a deep inner-well and shallow outer well due to an avoided crossing with the B^'' B ¹Σ_u⁺ curve. Transitions to the 4¹Σ_u⁺ state have not been assigned previously because absorption from the ^{(1 sσ) 2} ground state is forbidden due to the f character of the inner well. However, transitions from the EF¹Σ_g⁺ state with inner well s character and combined doubly-excited and d character outer well are allowed. The high vibrational levels converging on the third dissociation limit should exhibit rotational constant values dependent on the varying amounts of inner and outer-well character for a given v. We report experimental energies for the v = 8 - 12 levels and compare favorably to the predicted adiabatic rovibrational energies¹. The v = 9 level is the exception since it lies just above the avoided crossing, which makes predicting its energy difficult.