Raman Spectroscopy

Abstract

Infrared and Raman spectroscopy are closely related as both originate from transitions in vibrational and rotational energy levels of the molecule on absorption of radiations. Since different methods of excitation are used, the spectroscopic selection rules* are different. The intensity of IR absorption depends on the change in dipole moment of the bond, whereas Raman intensity depends on the change in polarizability of the bond accompanying the excitation. Thus, an electrically symmetrical bond (i.e. having no dipole moment) does not absorb in IR region (i.e. the transition is forbidden) but it does absorb in Raman scattering (i.e. the transition is allowed). In other words, an electrically symmetrical bond is Raman active but IR inactive. However, an electrically unsymmetrical bond may be IR active and Raman inactive or both IR and Raman active.