Ab initio calculation of the ground and first excited states of the lithium dimer

Abstract

Based on a high level ab initio calculation which is carried out with the multireference configuration interaction (MRCI) method under the aug-cc-pVXZ (AVXZ) basis sets, X = T, Q, 5, the accurate potential energy curves (PECs) of the ground state Χ¹Σ_g⁺ and the first excited state A¹Σ_u⁺ of Li₂ are constructed. By fitting the ab initio potential energy points with the Murrell-Sorbie potential function, the analytic potential energy functions (APEFs) are obtained. The molecular bond length at the equilibrium (R_e), the potential well depth (D_e), and the spectroscopic constants (B_e, ω_e, α_e, and ω_eχ_e) for the Χ¹Σ_g⁺ state and the A¹Σ_u⁺ state are deduced from the APEFs. The vibrational energy levels of the two electronic states are obtained by solving the time-independent Schrödinger equation with the Fourier grid Hamiltonian (FGH) method. All the spectroscopic constants and the vibrational levels agree well with the experimental results. The Franck-Condon factors (FCFs) corresponding to the transitions from the vibrational level (v'=0) of the ground state to the vibrational levels (v"=0-74) of the first excited state have been calculated. The FCF for the vibronic transition of A¹Σ_u⁺(v"=0) ←Χ¹Σ_g⁺ (v'=0) is the strongest. These PECs and corresponding spectroscopic constants provide reliable theoretical references to both the spectroscopic and the molecular dynamic studies of the Li₂ dimer.