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

Abstract

Room and low temperature (∼100K) IR spectra of the sulphate doped Rb2SeO4 and Cs2SeO4 were recorded. The positions of the ν3 and ν4 fundamental mode components of the dopant anions, as well as of the ν1 mode, were precisely measured. Nine (out of possible ten) second-order vibrational transitions of the dopant anions were detected. The anharmonicity constants and the harmonic eigenvalues were calculated for several second-order transitions on the basis of the second-order perturbation theory expressions. Comparison with our previous results show that the anharmonicity of practically all of the studied second-order transitions decreases in the order (SO4/K2SeO4)>(SO4/Rb2SeO4)>(SO4/Cs2SeO4). According to the measured relative splitting of the ν3 and ν4 mode components, both angular and bond-length distortion of the dopant SO42− anions decrease in the same order. 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 order transitions vs. the corresponding anharmonicity constants suggests that the significance of the electrical anharmonicity increases in the order (SO4/K2SeO4)<(SO4/Rb2SeO4)<(SO4/Cs2SeO4).