Ab initio calculations on low-lying electronic states of TeO_2 and Franck-Condon simulation of the (1)~1B_2 -X~1A_1,TeO_2 absorption spectrum including anharmonicity

摘要

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-quintuple-delta quality.Equilibrium geometrical parameters,harmonic vibrational frequencies,and relative electronic energies of the X ~1A_1,~1B_l,~1B_2,~1A_2,~3A_1,~3B_1,~3B_2,and ~3A_2 states of TeO_2 have been calculated.Potential energy functions (PEFs) of the X ~1A_1 and the (1) ~1B_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-delta quality.Franck-Condon factors (FCFs) for the electronic transition between the X~1A_1 and (1)~1B_2 states of TeO_2 were calculated with the above-mentioned ab initio PEFs.The (1)~1B_2<-X ~1A_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 (LIF) spectrum of Hullah and Brown [J.Mol.Spectrosc.200,261 (2000)].The ab initio results and spectral simulation reported here confirm the upper electronic state involved in the LIF spectrum to be the (1) ~1B_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)~1B_2<- X~1A_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)~1B_2 electronic surface with higher ~1B_2 states in a region of large TeO bond length (>=2.0 A) and OTeO bond angle (52=135.0~°).