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. It is evaluated for the case of pure electric dipole absorption by substates of levels with J values in the range 1/2 less-than-or-equal-to J less-than-or-equal-to 2. The results are examined in an attempt to identify polarizations for which the transition probabilities of the substates of a level are equal. It is shown that such a polarization exists only if J = 1/2, in which case it is linear polarization. For other J values, the polarization which minimizes the spread in transition rates is identified and results tabulated. The minimum spread is least for transitions J --> J + 1. Use of these transitions and polarizations in experiments on the optogalvanic effect might improve the agreement with rate-equation modelling of these processes.