CHAPTER VI - SCATTERING OF RADIATION

Abstract

This chapter discusses the scattering of radiation. When the frequency of the incident radiation is less than the energy of the first electron excitation, then the electron terms will not be excited in the scattering process. The scattering will be either Rayleigh scattering or Raman scattering because of the excitation of rotational or vibrational levels. The scattering of a photon by a system of electrons consists of the absorption of the initial photon and the simultaneous emission of another photon. The atom may be left either at its initial energy level or at some other discrete energy level. If an atomic energy level is not degenerate, the polarizability and intensity of coherent scattering are determined by the same tensor. The addition of an imaginary part to the energy levels of the intermediate excited states makes the polarizability tensor no longer Hermitian, even at frequencies below the ionization threshold. It contains an imaginary part, which is directly related to the absorption of radiation. After absorbing a photon, the atom will return to the ground state, emitting one or more photons. The absorption cross section is just the total cross section for all possible scattering processes. However, according to the optical theorem, the cross section can be expressed in terms of the anti-Hermitian part of the polarizability tensor.