Pre-resonance Raman scattering in nitrobenzene vapour

Abstract

For the measurement of absolute Raman scattering cross-sections in the vapour phase, a sample cell was designed and the procedure for measurements and the details of its design are described. The absolute Raman cross-sections of the 1347 cm−1 mode of nitrobenzene vapour were measured using both the 992 cm−1 mode of benzene and the 802 cm−1 mode of cyclohexane as internal intensity standards for excitation wavelengths in the range 406–230 nm. The results show a maximum in the excitation profile, which is attributed to the charge-transfer state of the molecule at around 240 nm. The long data averaging time needed for signal retrieval reduces the Raman intensity by progressively reducing the concentration of the sample through photochemical processes at resonant excitations. The Raman excitation profile at pre-resonance wavelengths shows more than two orders of magnitude enhancement over that predicted by the λ0−4 dependence of the cross-section. A non-linear regression analysis of the data points to the Albrecht A-term pre-resonance approximation (AATPA) theory was carried out. For the two wavelength ranges 406–290 and 406–260 nm, values of the resonant state were found to be 235 and 232 nm, respectively. Considering the assumptions made in the derivation of the theory and the large measurement errors (±20 absolute and ±10% relative), the theory is considered to be applicable to the present data. Although the analysis shows that the contribution to the enhancement of Raman scattering by electronic states lying higher than the charge-transfer state is negligible, the validity of the wavelength range for the application of the theory to the present data points remains uncertain. Copyright © 2000 John Wiley & Sons, Ltd.