Spin-Orbit Effects in the Spectroscopy of the X 2 Π and a 4 Σ - Electronic States of Carbon Iodide, CI

Highly correlated ab initio calculations have been performed to describe the potential energy curves (PECs) and the spectroscopic properties of the X Π state and of the first excited state of the CI radical. Multi Reference configuration interaction calculations with Davidson correction (MRCI+Q) and relativistic effective core potential for the iodine atom have been performed. It is found that the two lowest electronic states, the X Π and the a Σ states, are stable against dissociation and well separated from the other electronic states. Spectroscopic constants of these two states have been evaluated using their calculated PECs. Because of the presence of the iodine atom in this molecular system, spin-orbit (SO) interactions are playing an important role in the molecular and in the dissociation regions. The excitation energy of the a Σ state is calculated 1.67 eV (MRCI) above the X Π ground state and 1.70/1.62 eV (MRCI with SO correction) for the Ω = / and / transitions, respectively. The dissociation energy D of the X Π ground state is evaluated 2.66 eV (MRCI calculation) without SO correction and 2.46/2.36 eV with SO correction for the Ω = / and / components, respectively. The dissociation energy D of the a Σ state is evaluated 0.99 eV (MRCI calculation) without SO correction and 0.83/0.72 eV with SO correction for the Ω = / and / components, respectively. This work should help for the identification of this radical in laboratory and in atmospheric media.