Analysis and assignment of the optical absorption transitions in CuCl2 with Gaussian density functional calculations

Some vibrational and electronic properties of the CuCl2 molecule are analyzed in order to characterize the electronic states and assign the bands seen on the optical absorption spectra. Equilibrium distances, total energies, electronic structure and harmonic vibrational frequencies of the low-lying states are calculated with the functional density method, in the unrestricted spin formalism, at two levels of theory for the exchange and correlation potential, the local and so-called non-local spin density approximations. All the results are compared with recent experimental data. The ground state is confirmed to be 2Πg, with a linear coordination as for the excited d→d states (2Σ+g, 2Δg) and charge-transfer states (2Πu, 2Σ+u). These low-lying states are in the following sequence: 2Σ+g, 2Πu, 2Δg, 2Σ+ u, in order of increasing energies, different from those of previous Hartree-Fock results. Their harmonic vibrational frequencies are given and the oscillator strengths of the absorption transition from the ground state to these states are calculated. Therefore we propose a new assignment for the optical near infrared transitions (absorption and emission) to Σ+g↔2Πg, which would explain the lack of spin—orbit components of the emitting level in the experimental spectra, while the red ones could be 2Πu↔2Πg as already proposed in earlier works