A CASSCF/MRCI study of the low-lying Rydberg states of CIO

An ab initio calculation has been performed on the lowest seven doublet and six quartet Rydberg states of CIO at the CASSCF/MRCI level and with basis sets suitable for the extended molecular orbitals of such states (aug-cc-pVTZ with up to eleven extra Gaussian functions). Calculations on the quartet states reveal the energy ordering of Rydberg orbitals to be 4sσ, 4pπ, 4pσ;, 3dδ, 3dσ and 3dπ. The calculated doublet ab initio potential curves confirm experimental assignments of the C²Σ- and F²Σ- states but require reassignments for the symmetries of the D (²Δ), E (²Π) and H (²Δ) Rydberg states. These revisions are supported by spin-orbit coupling calculations that suggest the separation between the Ω components is small. In addition, a ²Σ⁺ state has been identified as the likely upper state for two previously unassigned vibronic bands recorded in absorption studies.