The absorption of arbitrarily polarized light by atoms and molecules

The theory of the incoherent absorption of light by quantum systems is examined to determine how the transition probabilities of the substates of an initial energy level vary as the polarization of the exciting radiation is altered. The aim is to identify polarizations for which the spread of transition rates among the substates of an energy level is a minimum. To achieve this, a new set of quantization axes is proposed for the case of elliptical polarization. The definition of these new axes is based on a consideration of Poincare's sphere. As the polarization is varied, the axes move smoothly between the sets of axes conventionally used for linear and circular polarization. Using these axes, expressions are derived for the transition probability for the absorption of a photon of specified polarization by a specified substate of the initial level. This expression applies to any multipolarity of transition and covers the case of partial polarization.