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 st ates of the copper halides, CuX (X = Cl, Br, I) are studied by means o f two different relativistic computational approaches. One is based on the CASSCF/CASPT2 approach with operators accounting for scalar relat ivistic effects evaluated as a first order correction to the CASSCF en ergy. The other is a fully relativistic four component SCF-CI treatmen t based on the Dirac-Coulomb Hamiltonian and hence accounts intrinsica lly for spin-orbit coupling as well as for scalar effects. The lowest excited states ((1,3)Sigma(+), (1,3)Pi, (1,3)Delta) are all closely re lated to the formal ionic configuration Cu+(3d(9)4s(1)) X-(ns(2)np(6)) . The agreement between calculated and measured transition energies an d transition dipoles and their trends in the series strengthens recent assignments of the observed bands. Unobserved 'neutral' states, domin ated by the configuration Cu(3d(10)4s(1)) X(ns(2)np(5)), 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 o f significance only for the (1) Sigma(+) states, judging from the weig hts 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 w ith an increase in the calculated transition dipoles for the (1) Sigma (+) states. This is consistent with the observed decrease of the lifet imes. The magnitudes of the spin-orbit splittings in the ionic states are governed by the splitting in Cu+ (2000 cm(-1)) as expected.