Abstract
The electronic configurations of molecular states of iodine are studied through
the Optical- Optical Double Resonance excitation of the ion pair states in low
vibrational levels. The two photon OODR excitation of ungerade IP states from
the gerade ground state is rationalised and the strength of u/g coupling at the
intermediate step interpreted in terms of a hyperfine interaction first described
by Broyer et al. The potential function of the cl₉(ab) gerade coupling partner
in the hyperfine Hamiltonian is derived, along with the lowest levels of a new IP
state in the second cluster, the H1ᵤ(2) state. An OODR excitation scheme for
populating the Oᵤ⁻(2) IP state is proposed and the dominant component of the
cl₉(ab) state configuration is found for R≈5.5Å.
The radiative lifetimes of nine IP states in low vibrational levels are determined and combined with the relative fluorescent intensities of the IP→Valence
charge transfer transitions to derive the Einstein A-coefficients for all the strong
transitions from these IP states. A theoretical model is developed in terms of a
separated atom description for the electronic configurations of these states and is
used to predict the relative dipole strengths of the IP→Valence transitions. The
Einstein A-coefficients are then interpreted to give the electronic configurations
of the IP states around Rₑᴵᴾ and the relative strengths of the transition dipoles for
Pα ↔ Pα, and pπ ↔ pπ electron transfer between ionic centres. A significant difference from the free ion configurations is found with the lowest energy IP states of a
given symmetry adopting as low a pα occupancy at the cationic centre as the inter-electron repulsion and spin -orbit energies will allow. This stabilisation is driven
by the field gradient due to the anionic charge. The model for the charge transfer
transition accounts for the large difference in the summed dipole strengths that
is observed for some u/g pairs, even though they have closely similar electronic
configurations, and using this model the radiative intensities are shown to be
consistent with results from other techniques that probe the electronic structure of IP states. The transition dipole function for the F0ᵤ⁺ (2) ---> X0₉⁺(aa) transition is established over the range 3.13≤R≤4.12Å and its form interpreted in
terms of the same electron transfer model. The inferred changes in the electronic
configuration of the F0ú (2) state with internuclear separation are shown to be
consistent with experimental results for related transitions and with ab initio
calculations from other research groups.
A rationalisation for the observed energy ordering of the states within an ion
pair cluster is proposed.
