Electronic and vibrational spectra of Cr2O7−− ions in potassium halide matrices

Abstract

Optical and infrared spectra of dichromate ions in KCl, KBr, and KI matrices are reported. Optical spectra of Cr2O7−− consist of three absorption bands that are attributed to the electronic transitions A11(t16)→T21(t15t21); A11(t16)→E1(t15t21) or T21(t15t21); and A11(t16) →T21(t15e1) on the basis of molecular orbitals. Fine structure caused by coupled vibration-electronic transitions is observed in the A11(t16)→T21(t15e1) band. This fine structure further splits at low temperature (77°K) because of the symmetry lowering of the dichromate ion from Td to C3v. Infrared spectra show a number of lines. A model that assumes that the dichromate ion enters the lattice by replacing two halide ions with no charge compensating vacancy satisfactorily explains all the experimental results.