Wavelength dependence of the preresonance Raman cross sections of CH3CN, SO42−, ClO4−, and NO3−

Abstract

The Raman scattering cross sections of vibrations of SO₄⁻², NO₃⁻, ClO₄⁻, and CH₃CN have been determined between 220–640 nm. The cross sections of the symmetric stretching vibrations of SO₄⁻² and ClO₄⁻ as well as the 918 cm⁻¹ C–C stretching vibration of CH₃CN display almost a ν⁴ excitation frequency dependence. The intensity of the 2249 cm⁻¹ C≡N stretching vibration increases somewhat faster than ν⁴, but significantly slower than that which would be expected if a dipole allowed C≡N π→π* transition at ∼150 nm dominated the Raman intensity. The intensity of the NO₃⁻ symmetric stretch increases with a frequency dependence close to that expected from an Albrecht A term contribution from the ∼200 nm π→π* transition. The fact that the 981 cm⁻¹ SO₄⁻², 932 cm⁻¹ ClO₄⁻, and 918 cm⁻¹ CH₃CN vibrations show essentially only ν⁴ intensity dependencies indicates that no particular dipole allowed transition dominates the preresonance Raman intensities. These results suggest that conventional preresonance Raman expressions do not accurately model the Raman intensities. The small excitation frequency dependencies of SO₄⁻², ClO₄⁻, and CH₃CN Raman intensities show that these species are ideal internal standards for Raman intensity measurements in the UV spectral region.