Raman cross section measurements in the visible and ultraviolet using an integrating cavity: Application to benzene, cyclohexane, and cacodylate

Abstract

A new technique has been developed for the measurement of absolute Raman scattering cross sections. The Raman scattering of a sample in an integrating cavity is compared to the elastic scattering from a reference scatterer of unit yield. The fundamental advantage of this approach is that it permits the measurement of absolute cross sections throughout the visible and UV regions with a simple experimental apparatus and procedure. The 992 cm−1 mode of benzene was measured in the visible region and compared satisfactorily to the values obtained by Kato and Takuma [J. Chem. Phys. 54, 5398 (1971)] and Abe et al. [J. Raman Spectrosc. 6, 38 (1977)]. The scattering cross section of the 608 cm−1 mode of sodium cacodylate, an aqueous intensity standard, was measured from 647 to 240 nm. Analysis within the A-term formalism provided a functional form for interpolation between measured points and revealed that the 608 cm−1 cacodylate mode derives intensity from states in the vicinity of 83 kK (kilokaiser). The 802 cm−1 skeletal mode of cyclohexane was measured over the same spectral region to provide a nonaqueous intensity standard. In this case the principal contribution to the preresonance scattering arises from electronic states near 115 kK and not from the lowest allowed σ → σ* (CC) states. To compare scattering mechanisms, the CH stretching modes of cyclohexane were referenced to the 802 cm−1 mode. The CH stretching modes of cyclohexane derive A-term intensity from the strongly allowed σ → σ*(CH) state near 85 kK, as expected from simple valence arguments.