Ab initio MRCI + Q calculations on the low-lying excited states of the MgBr radical including spin–orbit coupling

• The PECs of the 14 Λ–S states have been calculated by MRCI + Q method.
• The low-lying ion-pair state B2Σ+ has been studied for the first time.
• The PDMs and SO matrix elements were computed to reveal the sudden changes at RACP.
• The PECs of the 30 Ω states generated from 14 Λ–S states have been calculated.
• The abrupt changes have been explained with the wavefunctions.

Accurate theoretical calculations on the MgBr radical have been carried out by using the high-level relativistic multireference configuration interaction method with Davidson correction (MRCI + Q) using correlation-consistent Quintuple-ζ quality basis set. The potential energy curves (PECs) of the 14 Λ-S states of MgBr have been computed. In order to improve the PECs, the core–valence correlation, scalar relativistic effect, and spin–orbit coupling effect are taken into account in the computations. The spectroscopic constants of the bound states have been determined from the computed PECs. The results of the ground state X2Σ+ and the first excited state A2Π are in good agreement with those from the available experiments, while spectroscopic constants of the other electronic states are firstly reported. The low-lying ion-pair state B2Σ+ correlated to ion-pair dissociation limit Mg+ (2Sg) + Br− (1Sg) is characterized. The permanent dipole moments (PDMs) of Λ-S states and the R-dependent spin–orbit (SO) matrix elements are computed. The results indicate that the abrupt changes of PDMs and the SO matrix elements are attributed to the changes of the electronic configurations near the avoided crossing point. After taking the SOC effect into account, the 14 Λ-S states split into 30 Ω states, and the SOC splitting for the A2Π is calculated to be 102.58 cm− 1. The SOC effect, leading to the double-well potential of the Ω = (3)1/2 state, is found to be substantial for MgBr. In order to further illustrate the SOC effect and the avoided crossing phenomenon of the PECs, the Λ-S compositions in the Ω state wavefunctions are analyzed in detail. Finally, the transition dipole moments (TDMs) of several transitions from upper Ω states to the ground X2Σ + 1/2 state and the corresponding radiative lifetimes have been studied. It is shown that the (1)3/2–X2Σ + 1/2 and (2)3/2–X2Σ + 1/2 are particularly important to the observed transitions A2Π–X2Σ+ and C2Π–X2Σ+. The present study should shed more light on the electronic structures and transition properties of electronic states of the MgBr radical.

The PECs of the Λ-S states of MgBr molecule.Figure optionsDownload as PowerPoint slide