Infrared spectra of sulphate ions trapped in rubidium and cesium selenate:harmonic frequencies and anharmonicity constants for antisymmetric SO4 stretching vibrations

Room and low temperature ( similar to 100 K) IR spectra of the sulphate dop ed Rb2SeO4 and Cs2SeO4 were recorded. The positions of the nu(3) and nu(4) fundamental mode components of the dopant anions, as well as of the nu(1) m ode, were precisely measured. Nine (out of possible ten) second-order vibra tional transitions of the dopant anions were detected. The anharmonicity co nstants and the harmonic eigenvalues were calculated for several second-ord er transitions on the basis of the second-order perturbation theory express ions. Comparison with our previous results show that the anharmonicity of p ractically all of the studied second-order transitions decreases in the ord er (SO4/K2SeO4) > (SO3/Rb2SeO4) > (SO4/Cs2SeO4). According to the measured relative splitting of the nu(3) and nu(4) mode components, both angular and bond-length distortion of the dopant SO42- anions decrease in the same ord er. In all cases, the angular distortion seems to be smaller than the bond length one. The correlation of intensity of the bands due to the second ord er transitions vs. the corresponding anharmonicity constants suggests that the significance of the electrical anharmonicity increases in the order (SO 4/K2SeO4) < (SO4/Rb2SeO4) < (SO4/Cs2SeO4). (C) 1999 Elsevier Science B.V. A ll rights reserved.