Symmetry adapted cluster-configuration interaction study on the excited and ionized states of TiBr4 and TiI4

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Abstract
The symmetry adapted cluster-configuration interaction (SAC-CI) method is briefly reviewed and applied to the excitation and ionization spectra of TiX4 (X=Br, I). The valence excited states of these molecules are investigated systematically and compared with the previous study on TiCl4. The experimental spectra are well reproduced and assigned by the SAC-CI calculation including spin–orbit interaction of the ligand p atomic orbital (AO) and Ti d AO. Nine A1, ten A2, 20 E, and 30 T1 and T2 states are calculated for the excited states, and the oscillator strengths are distributed among the transitions to 30 T2 states, which cause the excitation spectra to be very complicated, especially for the TiI4 molecule. The ordering of the ionized states in the outer valence region is (1t1)−1<(3t2)−1<(1e)−1<(2t2)−1< (2a1)−1, which is the same as that of TiCl4. The spin–orbit splittings in the 2T2 states of TiBr4 and TiI4 are estimated to be smaller than those of the previous studies, and we propose a new assignment for the experimental photoelectron (PE) spectra.