Theoretical Investigations On The Electronic Structures And Spectroscopic Properties Of Several Diatomic Molecules Containing Main Group VIA Elements

The diatomic molecules containing main group VIA elements widely exist in atmosphere of earth, planets, and interstellar space, which are closely related to the fields of atmospheric photochemistry, planetary photochemistry, and so forth. Meanwhile, they have standout contributions to technological application for investigatons of laser materials. Hence, their investigations on diatomic molecules containing main group VIA elements have always attracted lots of attentions. Since the electronic configurations of main group VIA elements (viz. O-Te) are ns2np4, they have so many valence electrons that not only active chemistry properties forming stable-or metastable-states diatomic molecules but also making the structure of electronic states more complicated. To our knowledge, the electronic states have various of coupling interaction each other, which may lead to predissociation and diffuse bands causing some difficulties on identifications of spectra of the excitation states. Therefore, the theoretical prediction of electronic state structures of the diatomic molecules containing main group VIA elements are extremely valuable.In this work, the calculated molecular systems involve O2, SO, S2, Se2, Te2.Update to today, a great deal of achievements on experimental and theoretical studies (especially, O2, SO, and S2) have been acquired, which are mainly focus on the strong B3∑u-X3∑g-bands similar to the famous Schumann-Runge spectral bandslying in the1750-5350 A region(Schumann-Runge bands of O2),2400-7000 A(S2),3250-6400 A(Se2),4250-6300 A(Te2)), respectively. Meanwhile, the predissociation phenomenon of the B3∑u- state has widely been observed.Hitherto, however despite lots of ab initio calculation methods have been carried out for the computations of the species, in comparison with achieved experimental data, there are certain development spaces in the theoretical approaches. The information on the rovibrationalenergy levels of the low-lying electronic states of S2 is still limited; moreover, as for heavier Se2 and Te2 dimers, the low-lying electronic state structure and spectral properties are still inadequate, of which core-valance correlation effects from d orbitals should be considered in the calculations of potential energy curves; furthermore, the remarkable spin-orbit coupling effect is also introduced into the calculations whereby a suitable method; additionally, the predissociation models of the B3∑u- states of the Se2 and Te2 dimers are ambiguous, our theoretical works are thereforegradually developed for solving these problems. Investigations and conclusions in this thesis are:1) For simple systems(O2, SO, S2) containing main group VIA elements, we try to employ a new high-level method, that is, multireference configuration interaction approach involved explicity correlation term, for calculation of potential energy curves of the X3∑g- (X3∑- for SO) and alAg (a1A for SO). Meanwhile, scalar relativistic correction, Davidson modification, and core-valence correlation correction are all included in our computations. By virtue of potential energy curves, we estimated the spectroscopic constants of the bound states, and the influence from the different corrects on spectroscopic parameters are discussed. For S2 dimer, the detailed information on rovibrational energy levels is provided on the base of the potential energy curves. In addition, it is illustrated that Davidson modification, core-valence correlation correction and scalar relativistic effect have certain influence on the spectroscopic constants, which cannot be neglected in high-level calculations for molecular spectral structure.2) Regarding the heavier Se2 and Te2 molecules containing main group VIA elements, we employedrespectively Hund Case a and Hund Case c rules for investigations of the electronic structures and potential energy curves. We carried out multireference configuration interaction method (MRCI) for the potential energy curves’calculations of low-lying 20 A-S states (Hund Case a) together with scalar relativistic effect, Davidson(+Q) correction, and core-valence correlation correction from the d orbitals. Furthermore, on the basis of the bound potential energy curves, the spectroscopic constants are determined. Furthermore, spin-orbit coupling effect is introduced into the MRCI(+Q) by utilizing Breit-Pauli operator, and the potential energy curves of Ω states (Hund Case c) are obtained.The spectroscopic parameters of the bound Ω states are estimated based on the Ω-state potential energy curves. Moreover, by connection with the experimental measurements, the complete predissociation picture of the B3∑u- state in the Se2 dimer is developed on the basis of the spin-orbit coupling effect. For Se2 and Te2 dimers, we analyzed the Ω-state wavefunctions considering spin-orbit coupling interaction, especially, the variations of the dominant A-S composition of the (2)0u+ and (4)1u states original from the B3∑u-(A-S) state near the avoid crossing points and the changes of the transition dipole moments (TDMs) of the (2)u+-X1Og+ and (4)1u-X21g subsystems are evaluated along with molecular internuclear distance R, which illustrate the influence onthe strong B3∑u--X3∑g- transition and the physical mechanism of strong emission bands of B3∑u--X3∑g-. Finally, it is indicated in our calculations that spin-orbit coupling interaction is crucial for the structure electronic state, spectroscopic properties, and predissociation dynamical processes of the Se2 and Te2 dimers.