The Raman Effect

Raman scattering is widely used to study the vibrational and rotational properties of molecules. Normally infrared spectroscopy is needed to probe these levels but because of the Raman effect it is possible to excite these levels using visible light. In some cases Raman scattering is the only way to study these transitions. An example of this is the molecule H2. Since this molecule has no electric dipole (even in excited vibrational states) it will not radiate and we will see no emission line for this transition. Raman scattering is very closely related to Rayleigh scattering. In both cases light with more then enough energy to excite any vibrational or rotational states, but not enough energy to bring it out of the ground electronic state, is shined on the molecule. This light then excites the molecule to a virtual state that then decays back down to lower energy states. In Rayleigh scattering the molecule decays back to the initial state, figure (1), and in Raman scattering it decays to different state. When the final energy state is higher then the original state it is called a Stokes transition, figure (2), and when the final state is lower it is called a anti-Stokes transition. The light that is emitted in the de-excitation can then be studied.