Measurements of the absolute Raman cross sections of benzene

Abstract

The absolute Raman cross sections of the 992 cm−1 (ring breathing) and 3060 cm−1 (C–H stretching) modes of benzene are presented at excitation wavelengths of 514.5, 488.0, 441.6, and 325.0 nm. The technique employs a straightforward determination of the total (unrestricted energy bandwidth) scattering cross section along with a simple relative measurement. The effect of the dispersion of the local field correction (via the frequency dependence of the index of refraction) is found to significantly affect the preresonance benzene cross sections and can distort the normally employed ‘‘A-term’’ analysis. When the local field effect is incorporated into the data analysis, the results involving neat benzene and previous studies involving 1% benzene in acetonitrile are brought into reasonable agreement. The ‘‘n2’’ solid angle correction is also verified experimentally using air, water, and benzene as test substances. The preresonance enhancement of the 992 cm−1 mode is found to be consistent with coupling to electronic excitations centered at ∼80 kK which presumably involve the π-electron system. In contrast, the 3060 cm−1 mode is coupled to electronic states centered in the deep ultraviolet, at ∼150 kK, which presumably involve the more energetic σ-electron excitations.