Dynamic symmetry lowering of the sulfate ion in aqueous solution probed by polarization-resolved CARS spectroscopy

The effect of ion–ion interactions on the structure and dynamics of the sulfate ion in aqueous MgSO4 solution has been studied by polarization-resolved CARS spectroscopy. Depolarization ratio of the totally symmetric S–O stretch band of the SO42− ion was determined accurately for three different MgSO4 concentrations, 0.8, 1.3, 2.5 mol dm−3. At 0.8 mol dm−3, the observed depolarization ratio was zero as expected from Td symmetry. On the other hand, small but meaningful non-zero depolarization ratios were obtained at higher concentrations. These non-zero depolarization ratios are interpreted in terms of symmetry lowering of the sulfate ion, which is due to dynamic ion pair formation between Mg2+ and SO42− ions.