Intensities in Raman Spectroscopy and Electronic Spectroscopy

The polarized Raman spectra of naphthalene single crystal are recorded with a laser Raman spectrometer. Relative intensites of Raman lines in various polarizations are accurately measured by photon‐counting technique. All the strong lines are polarized in the c′c′ and bb polarizations, and the polarization data alone are not sufficient to decide the symmetry of a Raman active vibration. The oriented gas model is found not adequate to fully understand the results. The polarization data have to be interpreted in terms of the electronic spectrum of naphthalene.

Polarized Raman Spectra X. Polarized Raman Spectra of Naphthalene Single Crystal

Lin¹,

Ting²

1972

Polarized Raman Spectra VIII. Ramam Intensities of Anthracene Excited by Different Laser Light

Ting¹,

Hsien²

1972

An Ar-Kt ion laser was used to excite the Raman spectrum of powdercd anthracene with two laser lines of 4880 and 6471A. There are significant differences in the intensity distribution in these two Raman spectra, These digerences can be best evplaiced by considering that Raman intensities are contributed by two alIomed excited states of anthracene, and their contributions depend upon the laser frequency. From these Ramaa spectra, i t is also possibIe to infer the vibrational structures in :he higher energy, structureless excited state of anthracene.The dependence of Raman intensities on the frequency of the exciting light is well known. The Raman spectrum of a compound was observed to become more and more intense as the frequency of exciting light approached the lowest excited state from below.'' When the exciting light frequency coincides with the absorption band, the scattering process is called resonance Raman scattering*) and its intensity might be loo times more intense than the ordinary Raman scattering when the frequency of exciting light is well below the absorption b a d . Kondilenko and ~t h e r s~)~" had also observed that the overtone vibatiocs also became intense when the frequency of exciting light was increased. In the above investigations, however, it was not mentioned that the relative intensities of various Raman lines showed dependence on the frequency of exciting light.In our previous theoretical considerations,')P) it was expected that all the allowed excited states in a molecule should contribute to the Raman scattering, and their contributions should be additive in terms of the derived polarizability. The contribution by an excited state to a particular Raman line depends upon the foIlowing factors :1. The integrated intensity of the transition between the ground state and the 2. The Franck-Condon integral of the normal vibration in the above transition; and 3. The difference between the transition energy and the energy of incident photons. excited state;

Polarized Raman spectra IX. Low Frequency Vibrations in Crystallne Trans‐Stilbene

Ting¹

1972

Raman spectrum of powdered crystalline trans-stilbene is recorded by a laser Raman spectrometer. The intensity of a low frequency skeIeton bending vibration at 220 cm-'iS observed to be very weak. Its Raman intensity, after corrected for the photocathode response and for the v4 scattering factor, agrees within 10% of the theoretically caIcuIafed value. It is therefore proved that the intensity of a Raman active vibration is proportional to the square of its vibrational frequency, if only one excited state is involved in the scattering process, Lattice vibrations in trans-stilbene crystal are also reported.