<i>Ab initio</i> study of the spectroscopy of AgI: A complete active space self-consistent field+averaged coupled pair functional approach to the lowest excited states

Abstract

The seven lowest-lying singlet and triplet (X and 2 1Σ+ states, the first Σ+3, Π3,1, and Δ3,1) electronic states of AgI were studied through extensive complete active space self-consistent field+averaged coupled pair functional calculations, with relativistic effective core potentials and optimized Gaussian basis sets for both atoms. The 2 1Σ+ and Σ+3 states show relative minima very near the equilibrium geometry of the ground state, while the former also has another minimum around 8.1 a.u. and is attractive up to 20 a.u. The lowest Π1 state was found to be totally repulsive. The Δ3,1 and Π3 states present very shallow minima, around 5.2 a.u. for the Δ states and at 7.4 a.u. for the Π3 state. The calculated spectroscopic constants for the ground and excited states are compared with the available experimental data and results are discussed in light of the known spectroscopy of the other silver halides. Even though the Π3 state is mostly repulsive, it is almost degenerate with the 2 1Σ+ one near the equilibrium geometry of the ground state, and lies within 4500 cm−1 from the latter in the 5–9 a.u. range, thus a strong 2 1Σ+–3Π mixture through the spin–orbit interaction is predicted to occur that will lead to the fine-structure (0+) single B state (dominated by the 2 1Σ+ component), recently Rydberg–Klein–Rees fitted to explain the revised B←X transitions [Stueber et al., J. Chem. Phys. 109, 9831 (1998)]. These results unequivocally assign the A←X transition as arising from Σ+3←X 1Σ+. In this way accurate spectroscopic data have been derived that lead to new assignments and predictions for electronic states unobserved so far.