On the Dependence of Vibrational Raman Intensity on the Wavelength of Incident Light

Experiments on the wavelength dependence across the visible region of the intensity of the 1334‐cm−1 Raman line of p‐nitroaniline (PNA) are analyzed from a general theoretical viewpoint. When theory is simplified to correspond to the PNA example, two obvious sources of Raman intensity are apparent. One is based on a possible shift of equilibrium nuclear position along the 1334‐cm−1 mode upon exciting PNA into its first excited singlet state (in the near ultraviolet). The other source deals with vibronic coupling of the first excited electronic state with a whole family of states in the deep ultraviolet having similar polarizations. It is seen how this second mechanism appears to be the dominant one in this case. The absolute scattering cross section for the 1334 cm−1 is given and is found to agree with the order of magnitude estimates calculated theoretically.