Optical–optical double resonance studies of the halogen ion-pair states. Part 3.—The F(0+u)→ X(0+g) transition dipole moment function in I2

The F(0+u)(2) second cluster ion-pair state has been populated by the OODR scheme F(0+u)(2)â†� B(0+u)/c(1+g)â†� X1Σ+g, using hyperfine g/u mixing at the intermediate stage. Fluorescence from the ion-pair → valence transition, F(v′)→ X 1Σ+g, for v′= 0, 4, 9, 19 has been dispersed. Using the upper and lower state potentials from previous RKR analyses, the transition dipole function µFX(r) is obtained between 3.1 and 4.2 A by fitting the envelope of the highly oscillatory structure and is placed on an absolute footing from the known lifetime of the v′= 0 level of the F state. The F → X 1Σ+g and f(0+g)(2)→ B(0+u) transition dipoles show a very different behaviour over the range r= 3.1–3.6 A. This is interpreted in terms of the rapidly changing electronic configurations of the ion-pair states near their equilibrium separations.