Introduction to Wave Scattering by Atoms and Molecules in the Free State

Abstract

Let us imagine an incident plane wave source of radiation impinging on a fixed array of scattering centers. We graphically represent such a situation by drawing lines of maximum wave amplitude. The incident source originating to the readers left and traveling or propagating toward the readers right will be represented by a series of parallel vertical lines with the inter line spacing corresponding to the wavelength. As the incident wave arrives at a particular scattering center a disturbance or scattering occurs, and the part of the incident wave impinging on the scattering point is re-emitted in the form of an outgoing spherical wave. This outgoing spherical wave is represented by a series of concentric circles again with each circle representing a maxima of the wave amplitude. The inter circle spacings will be constant and will correspond to the wavelength of the diffracted wave. This wavelength of the scattered wave does not have to be the same as that for the incident plane wave. If it is the same the wave diffraction is said to be elastic, that is no energy transfer takes place between the incident wave and the diffraction object. If the diffracted wave length is different the incident wave is said to be scattered inelastically. In Fig. 1 a schematic picture of the situation for elastic scattering is shown.