Theoretical MRCI + Q study on electronic structure and spectroscopic properties of the HX⁺(X = F, Cl, Br, I) cations

In this work, we have systematacially investigated the electronic structure and spectroscopic properties of the HX + (X = F, Cl, Br, I) cations by using the highly correlated MRCI + Q approach. This is the first comprehensive ab initio study on the electronic states of the HX + cations. The spin–orbit coupling (SOC) effect is introduced with the state interaction approach. There are total 10 Λ-S and 18 Ω states obtained in the calculation. The results show that the potential energy curves’ (PECs) shapes and structure of the electronic states of different HX + cations exhibit the significant distinction due to the X-dependent energy-level order of the unique dissociation channel H + ( 1 S g ) + X( 2 P u ). From PECs, the spectroscopic constants of the bound electronic states are determined, which are in good agreement with available observed values. Regarding HBr + and HI + , the good agreement has been achieved only when the SOC effect is considered. The predissociation for the first excited A 2 Σ + state of HCl + , HBr + , and HI + is analyzed based on computed spin–orbit matrix elements. Around the equilibrium position, the energy splittings of X 2 Π are calculated to be 288, 661, 2691, and 5137 cm −1 for HF + , HCl + , HBr + , and HI + , respectively. It has been demonstrated that SOC is substantial for HX + , leading to significant changes in PECs’ shapes as well as in electronic structure. Finally, the transition properties are predicted, including transition dipole moments, Franck–Condon factors, and radiative lifetimes. Both the Ω transitions A 2 Σ+ 1/2-X 2 Π 3/2 and A 2 Σ+ 1/2-X 2 Π 1/2 of the HX + cations are determined to possess the radiative lifetimes at the microsecond (µs) level.