Infrared spectra of sulphate ions trapped in rubidium and cesium selenate: harmonic frequencies and anharmonicity constants for antisymmetric SO_4 stretching vibrations

摘要

Room and low temperature (approx 100 K) IR spectra of the sulphate doped Rb_2SeO_4 and Cs_2SeO_4 were recorded. The positions of the v_3 and v_4 fundamental mode components of the dopant anions, as well as of the v_1 mode, were precisely measured. Nine (out of possible ten) second-order vibrational transitions of the dopant anions were determined. 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 roder (SO_4/K_2SeO_4) > (SO_4/Cs_2SeO_4). According to the measured relative splitting of the v_3 and v_4 mode components, both angular and bond-length distortion of the dopant SO_4~(2-) 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 (SO_4/K_2SeO_4) < (SO_4/Rb_2SeO_4) < (SO_4/Cs_2SeO_4).