Chapter 13 - Rayleigh and Raman Scattering

Abstract

The term “scattering” is applied to Rayleigh and spontaneous Raman signal generation because Compton-style arguments describing a collision between a photon and a molecule are often used to explain these phenomena. Rayleigh signal generation is termed “elastic scattering” because the photon leaves with the same energy that it had before the interaction. A green laser beam, for example, is visible to the eyes as it traverses through air owing to elastic scattering. If one looks toward the laser, one can see point scattering from dust and a much more uniform but weaker beam that is generated by molecular Rayleigh scattering. If one then goes to the laser and views the beam as it traverses away from the source, most of the dust appears to have vanished but the Rayleigh signal remains. This is because scattering from large particles favors the forward direction, while Rayleigh scattering is more isotropic. If the laser is vertically polarized and one looks down at the beam, it nearly disappears. If one views it from the side, Rayleigh scattering is very strong. Raman scattering, on the other hand, is “inelastic” in the sense that the signal is shifted in wavelength from the incident light; Raman photons leave with energy that is different from that of the incoming photons.