Effects of an ambient medium on the emission,absorption and scattering of waves by atoms and molecules

A theory describing the interaction between atoms or molecules (or other systems with discrete energy eigenvalues) and waves in an arbitrary mode in an arbitrary ambient medium is developed. Rules for generalizing formulae describing processes for waves in vacuo to include the effects of a medium are stated and the illustrative examples of multipole radiation, the photo-electric effect and Rayleigh and Raman scattering are given.The following specific results are discussed: (1) In an isotropic medium with refractive indexn(), the rate of transitions with frequency isn(),n ³(), ... times that in vacuo for electric dipole, magnetic dipole or electric quadrupole, ..., transitions. (2) The conventional multipole expansion is inadequate when waves with a longitudinal component of polarization exist, but this does not affect the theory of electric and magnetic dipole transitions.A possible astrophysical application of resonant scattering by molecules of electron plasma waves is discussed briefly.