Ab initio calculations on low-lying electronic states of TeO2 and Franck-Condon simulation of the (1)1B2←X̃ 1A1 TeO2 absorption spectrum including anharmonicity

Abstract

Ab initio calculations have been carried out on low-lying singlet and triplet states of TeO 2 at different levels of theory with basis sets of up to the augmented-polarized valence-quintuplequality. Equilibrium geometrical parameters, harmonic vibrational frequencies, and relative electronic energies of the X 1 A 1 , 1 B 1 , 1 B 2 , 1 A 2 , 3 A 1 , 3 B 1 , 3 B 2 , and 3 A 2 states of TeO 2 have been calculated. Potential energy functions ͑PEFs͒ of the X 1 A 1 and the (1) 1 B 2 states were computed at the complete-active-space self-consistent-field multireference configuration interaction level, with a basis set of augmented-polarized valence-quadruple-quality. Franck-Condon factors ͑FCFs͒ for the electronic transition between the X 1 A 1 and (1) 1 B 2 states of TeO 2 were calculated with the above-mentioned ab initio PEFs. The (1) 1 B 2 ←X 1 A 1 absorption spectrum of TeO 2 was simulated employing the computed FCFs, which include Duschinsky rotation and anharmonicity, and compared with the recently published laser-induced fluorescence ͑LIF͒ spectrum of Hullah and Brown ͓J. Mol. Spectrosc. 200, 261 ͑2000͔͒. The ab initio results and spectral simulation reported here confirm the upper electronic state involved in the LIF spectrum to be the (1) 1 B 2 state of TeO 2 and also confirm the vibrational assignments of Hullah and Brown. However, our simulated spectrum suggests that the reported LIF spectrum from 345 to 406 nm represents only a portion of the full (1) 1 B 2 ←X 1 A 1 absorption spectrum of TeO 2 , which extends from ca. 406 to 300 nm. Another dye other than the two used by Hullah and Brown is required to cover the 345-300 nm region of the LIF band. Ab initio calculations show strong configuration mixing of the (1) 1 B 2 electronic surface with higher 1 B 2 states in a region of large TeO bond length (у2.0 Å) and OTeO bond angle (у135.0°).