Ab initio calculation on the low-lying excited states of BCl radical

The internally contracted multi-reference configuration interaction method (MRCI) is used to calculate 22 Λ–S electronic states of BCl radical at the level of aug-cc-pVQZ basis set, where Davidson modification with the Douglas–Kroll scalar relativistic correction is also taken into account. This is the first time multi-reference configuration calculation on the excited state of BCl radical. Spin–orbit coupling effect is introduced by the state interaction approach with the full Breit–Pauli Hamiltonian operator for the two lowest excited and ground Λ–S states. The potential energy curves of the Λ–S and Ω electronic states are depicted according to the avoided crossing rule of the same symmetry. The spectroscopic constants for the bound Λ–S and Ω states are fitted, which are in good agreement with the corresponding observed values. The calculated excitation energy separation of a3Π0−−a3Π0+ is only 0.22 cm−1, while the energy intervals of a3Π0+−a3Π1 and a3Π1−a3Π2 are 47.85 cm−1 and 48.06 cm−1, respectively. In addition, transition dipole moments (TDMs) followed by Franck–Condon factors and radiative lifetimes for the transitions from a3Π0+, a3Π1 and A1Π1 to the ground state X 1 ∑ 0 + + are predicted as well.