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

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.