EVOLUTION OF THE 1-SIGMA+ RADIATIVE LIFETIME OF COPPER HALIDES

The observed trend in the radiative lifetimes of the 1SIGMA+ excited states of CuF (1.5 mus), CuCl (0.43 mus), CuBr (0.23 mus), and CuI (0.16 mus) is analyzed and explained in the light of very accurate multiconfigurational self-consistent-field multireference configuration-interaction and second-order Moller-Plesset perturbation theory calculations. Results show that this trend is due to a decreasing ionic (increasing molecular) character of the wave functions of the 1SIGMA+ excited states as the atomic number of the halogen increases. The forbidden X 1SIGMA+:Cu+(d10)X-(s2p6)<--1SIGMA+:Cu+(d9s1)X-(s2p6) (almost atomic 1S<--1D) transition becomes allowed due to the increasing importance of the Cu+(d10)X-(s2p6) and Cu(d10s1)X(s2p5) configurations in the wave function of the 1SIGMA+ excited state, thus reducing the associated radiative lifetime. Spin-orbit effects are found not to play any role in this process since the only large effects are observed in CuI, where the OMEGA = 0+ fine-structure state in question is found to be of pure LAMBDASSIGMA 1SIGMA+ symmetry; this excludes a mixture with the 3SIGMA+ component which would otherwise increase this radiative lifetime because of a spin-forbidden transition towards the X 1SIGMA+ ground state.