Charge transfer and relativistic effects in the low-lying electronic states of CuCl, CuBr and CuI

The spectral transitions and the character of the low-lying excited states of the copper halides, CuX (X = Cl, Br, I) are studied by means of two different relativistic computational approaches. One is based on the CASSCF/CASPT2 approach with operators accounting for scalar relativistic effects evaluated as a first order correction to the CASSCF energy. The other is a fully relativistic four component SCF-CI treatment based on the Dirac-Coulomb Hamiltonian and hence accounts intrinsically for spin-orbit coupling as well as for scalar effects. The lowest excited states (1,3Σ+, 1,3Π, 1,3Δ) are all closely related to the formal ionic configuration Cu+ (3d94s1) X- (ns2np6). The agreement between calculated and measured transition energies and transition dipoles and their trends in the series strengthens recent assignments of the observed bands. Unobserved `neutral’ states, dominated by the configuration Cu(3d104s1) X(ns2np5), are situated mostly far above the `ionic’ states. Particular attention was given to the mixing of these states, i.e. to the importance of charge transfer effects in the description of the observed states. These seem to be of significance only for the 1Σ+ states, judging from the weights of the charge transfer configurations in the total wave functions and the character of the open shell orbitals. The calculated increase in charge transfer on going from Cl to I in the series goes together with an increase in the calculated transition dipoles for the 1Σ+ states. This is consistent with the observed decrease of the lifetimes. The magnitudes of the spin-orbit splittings in the ionic states are governed by the splitting in Cu+ (2000 cm-1) as expected.