Ab initio and diatomics in molecule potentials for I-2(-), I-2, I-3(-), and I-3

The electronic structure of the I-3(-) molecular anion and its photoproduct s I-2(-), I-2, and I-3 were studied. Ab initio calculations were carried ou t using the multireference configuration interaction (MRCI) method for the valence electrons together with a relativistic effective core potential. Th e ab initio wave functions were also used to compute some spin-orbit coupli ng matrix elements, as well as approximate valence bond wave functions, use d as guidelines in the construction of a 108-state diatomics in molecule (D IM) description of the electronic structure of I-3(-). In the DIM model, sp in-orbit coupling was introduced as a sum of atomic operators. For I-2(-) t he ab initio and the DIM ground-state potentials show excellent agreement w ith the experimental results. The results for I-2 are also in very good agr eement with experimental data. For I-3(-), the MRCI calculations give a ver y good description of the spectroscopic constants and agree with the vertic al excitation energies, provided spin-orbit coupling is included. The DIM d escription fails both quantitively by leading to erroneous spectroscopic co nstants, and qualitatively by not even reproducing the MRCI ordering of the excited-states. The failure of the DIM is attributed to the omission of io nic states. The overall qualitative picture of the excited-state potentials shows a maze of dense avoided crossings which means that all energetically allowed photoproducts will be present in the experiment. The ground electr onic state of I-3 was calculated to be a collinear and centrosymmetric (2)P i (u,3/2). The collinear state is stabilized by spin-orbit coupling relativ e to a bent configuration. Calculated vertical transition energies from the ground to low-lying excited states of the radical are in excellent agreeme nt with the experimental data. The spin-orbit assignment of these states is provided. (C) 2001 American Institute of Physics.