Observations of high vibrational levels of the 4fσ 4¹Σu+ state of H2.

Resonantly enhanced multiphoton ionization via the EF¹Σg+, v' = 6 double-well state has been used to probe the energy region below the third dissociation limit of H2 where several high vibrational levels of the 4¹Σu+ state are expected. Theoretical ab initio potential energy curves for this state predict a deep inner well and shallow outer well where vibrational levels above v = 8 are expected to exhibit the double-well character of the state. Since the 4¹Σu+ state has f -state character, transitions to it from the ground state are nominally forbidden. However, the d character of the outer well of the EF1S+ g state allows access to this state. We report observations of transitions to the v = 9-12 levels of the 4¹Σu+ state and compare their energies to predicted energies calculated from an ab initio potential energy curve with adiabatic corrections. Assignments are based on measured energies and linewidths, rotational constants, and expected transition strengths. The amount of agreement between the predicted values and the observations is mixed, with the largest discrepancies arising for the v = 9 level, owing to strong nonadiabatic electronic mixing in this energy region. [ABSTRACT FROM AUTHOR]