The Normal Raman Effect

Abstract

From the derivation of an element of the scattering tensor it is seen that the normal Raman effect is a second order effect. If the scattering particle interacts with one photon of energy hv o , then the elements of the tensor for Rayleigh scattering are obtained from expressions which involve the wave functions of the unperturbed atom, ion or molecule (first order effect). The appropriate expressions for normal Raman scattering involve perturbed and unperturbed wave functions (second order effect), and for higher order Raman scattering (the hyper Raman effect) only perturbed wave functions play a role. The wave functions referred to in the above are of electronic origin even if the Raman transition takes place between either rotational or vibrational levels of the electronic ground state. Only for electronic Raman scattering does the (average) value of an element of (α ϱσ ) kn occur between two electronic states (k and n) and the vibrational and rotational Raman effects may be looked upon as being electronic Raman effects because they are related to the electronic polarizability (α ϱσ ) kk . Throughout the development of Raman spectroscopy, the vibrational effect has been most important, followed by the rotational Raman effect. A considerable amount of attention was paid to these directions during the pre-laser period, but only two electronic Raman effects were reported. After the invention of the laser and its introduction as source for excitation of Raman spectra, attention shifted first to vibrational Raman scattering.