Vibrational anharmonicity and vibrational Stark effect of sulfate ions trapped in potassium, rubidium and cesium chromate

摘要

Room and low temperature (approximate to 100 K) FT-IR and Raman spectra of the sulfate doped K2CrO4, Rb2CrO4 and Cs2CrO4 were recorded. The positions of the v(1), v(3) and v(4) fundamental mode components of the dopant anions were measured. Nine tout of possible ten) second-order stretch-stretch vibrational transitions of the dopant anions were detected. On the basis of these data, the anharmonicity constants and the corresponding harmonic eigenvalues were calculated for several vibrational transitions using second-order perturbation theory expressions. The anharmonicity of the studied second-order transitions of the type v(1) + v(3i) increases in the order (SO4/K2CrO4)( (SO4/Rb2CrO4) < (SO4/Cs2CrO4), while for those of the type v(3j) + v(3i) it basically follows the trend: (SO4/K2CrO4) > (SO4/Rb2CrO4) < (SO4/Cs2CrO4). The measured relative Stark splittings of the v(3) and v(4) mode components of the dopant SO42- anions, as well as the average X-13i/X-3i3j values decrease in the order (SO4/K2CrO4) > (SO4/Rb2CrO4) > (SO4/Cs2CrO4). In all cases, the splitting is larger for v(3) than for v(4) modes, indicating a smaller angular than the bond length distortion. The theory of vibrational Stark effect suggests that the observed frequency shifts of the v(1), v(3) and v(4) mode components may be attributed to the increase of the field strength at the doped anion site going from K2CrO4 to Cs2CrO4. The Stark splitting of these modes, on the other hand, implies that the internal crystalline field vector is almost parallel to the (hypothetical) C-2 axis of the slightly distorted dopant tetrahedral anions. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 56]