Charge transfer and relativistic e ects in the low-lying electronic states of

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 di erent relativistic computational approaches. One is based on the CASSCF/CASPT2 approach with operators accounting for scalar relativistic e ects evaluated as a ® rst 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 e ects. The lowest excited states (1,3 R +, 1,3 P, 1,3 n) are all closely related to the formal ionic con ® guration 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 ǹeutral ’ states, dominated by the con ® guration Cu(3d104s1) X(ns2np5), are situated mostly far above the ìonic ’ states. Particular attention was given to the mixing of these states, i.e. to the importance of charge transfer e ects in the description of the observed states. These seem to be of signi ® cance only for the 1 R + states, judging from the weights of the charge transfer con ® gurations 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 1 R + 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. 1