A Study Of The Spectroscopic Properties Of The Lowlying Electronic States Of C₂, O₂⁺ Molecules

The potential energy curve (PEC) of diatomic molecule is one of the important branches in atomic and molecular physics. It can completely determine molecular energy, equilibrium geometry, force constants and spectroscopic parameters. At the same time, it is also the potential energy function of core movement, which is the foundation to investigate atomic and molecular collision and reaction and is of special importance in the atom- cluster growth, dissociation and stability analyses.The PECs of eight singlet electronic states (X¹Σ_g⁺, A¹П_u, B¹Δ_g, B′1Σg+, C¹П_g, D¹Σ_u⁺, E1Σg+ and 11Δu) of C₂ radical and six low-lying electronic states (X²Π_g, a₄П_u, A2Πu, b4Σg-, D2Δg and B²Σ_g^-) of O₂⁺ ion are computed by the complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach in combination with the correlation-consistent basis sets of Dunning and co-workers for internuclear separations from 0.07 to 2.0 nm.Fitting the PECs, the spectroscopic parameters (T_e, D₀, D_e, R_e,ω_e,ω_ex_e,ω_ey_e,α_e,β_e,γ_e and B_e) are determined. To obtain more accurate results, the Davidson modification and core-valence correlation and relativistic corrections are included. The way to consider the relativistic correction is to employ the second-order Douglas-Kroll Hamiltonian approximation. For the C₂ radical, the core-valence correlation correction is performed with the cc-pCV5Z basis set, and the relativistic correction is made at the level of cc-pV5Z basis set. For the O₂⁺ ion, the core-valence correlation correction is carried out with the cc-pCVQZ basis set. And the relativistic correction is done at the level of cc-pVQZ basis set. Furthermore, these PECs are extrapolated to the complete basis set (CBS) limit by the two-point total-energy extrapolation scheme.Fitting the PECs including the Davidson modification and core-valence correlation and relativistic corrections, the spectroscopic parameters and molecular constants [vibrational levels G(ν), inertial rotation constants Bνand centrifugal distortion constants Dν] are determined. For the C₂ radical, the first 20 vibrational states of each electronic state are computed when the rotational quantum number J equals zero (J = 0). The G(ν), Bνand Dνof X¹Σ_g⁺, A¹П_u, B¹Δ_g and B′1Σ_g⁺ electronic states are reported when J = 0, which are in excellent agreement with the available measurements. In addition, for the X¹Σ_g⁺ and A¹П_u electronic states of 12C₂, 12C13C and 13C₂ species, the spectroscopic parameters and the complete G(ν), Bνand Dνare calculated for the first time when J = 0. For the non-rotating O₂⁺ ion, the G(ν) and Bνof the first 26 vibrational states are evaluated using the PECs obtained by the MRCI+Q/CV+DK+56 calculations. Comparison with the RKR data shows that the present spectroscopic and molecular results are accurate.