Neutral atoms in ionic lattices: Excited states of KCl : Ag-0

The optical-absorption spectrum of a cationic Ag-0 atom in a KCI crystal ha s been studied theoretically by means of a series of cluster models of incr easing size. Excitation energies have been determined by means of a multico nfigurational self-consistent field procedure followed by a second-order pe rturbation correlation treatment. Moreover results obtained within the dens ity-functional framework are also reported. The calculations confirm the as signment of bands I and TV to transitions of the Ag-Ss electron into deloca lized states with mainly K-4s,4p character. Bands II and III have been assi gned to internal transitions on the Ag atom, which correspond to the atomic Ag-4d to Ag-Ss transition. We also determine the lowest charge transfer (C T) excitation energy and confirm the assignment of band VI to such a transi tion. The study of the variation of the CT excitation energy with the Ap-Cl distance R gives additional support to a large displacement of the Cl ions due to the presence of the Ag-0 impurity. Moreover, from the present resul ts, it is predicted that on passing to NaCl:Ag-0 the CT onset would be out of the optical range while the 5s-5p transition would undergo a redshift of 0.3 eV. These conclusions, which underline the different character of invo lved orbitals, are consistent with experimental findings. The existence of a CT transition in the optical range for an atom inside an ionic host is ex plained by a simple model, which also accounts for the differences with the more common 3d systems. The present study sheds also some light on the R d ependence of the s(2)-sp transitions due to s(2) ions like Ti+.