Ab initio study on the low-lying potential energy curves of the diatomic cesium iodide cation (CsI+)

Potential energy curves (PECs) for the low-lying states of the diatomic cesium iodide cation (CsI+) have been calculated using the internally-contracted MRSDCI method with relativistic pseudopotentials. First, we calculate PECs for spin–orbit (SO)-free Λ-S states, 2Σ+ and 2Π, stemming from the triply degenerate 2P state of I. Then we obtain PECs for SO-included Ω states, X3/2, 1/2(I), and 1/2(II), by diagonalizing the matrix of the electronic Hamiltonian plus SO coupling. It is found that all of the three Ω state PECs, X3/2, 1/2(I), and 1/2(II), have shallow wells with the depths of 1.4–4.6 kcal mol−1. It is also predicted that transition dipole moments (TDMs) among these Ω states have values of non-negligible amounts around equilibrium internuclear distances and the TDMs can contribute to the optical transition between the states. In addition, ionization energies from the neutral CsI molecule to the CsI+ cation are estimated by comparing the present PECs with those for CsI obtained in our previous work [J. Chem. Phys. 128 (2008) 024301]. Agreement between theory and experiment at the equilibrium internuclear distance and at the dissociation asymptote is found to be good, suggesting the high reliability of the present calculations.