A theoretical evaluation of the effect of photoselection on the measurement of the circular polarization of luminescence

摘要

The orientation of molecules excited by a beam of light is not isotropic as a rule. If rotatory Brownian motion is not fast during the lifetime of the excited state the polarization properties of the light emitted from the excited molecules will depend as a rule on the mode of photoselection of the molecules by the excitation beam. A general theoretical method is developed for proper weighting in the process of summing of the contributions of the various photoselected molecules to the emitted light when Brownian rotational motion is frozen. A formula is obtained for the calculation of the probabilities of emission of light of a specified polarization from the photoselected system. To use this formula average values of multiples of elements of the Euler matrix are to be evaluated first; the polarization properties of the emitted light are then obtained by insertion of the components of the electric and magnetic transition dipole moments characterizing the absorption and emission of light by the molecule under discussion. By use of this formula, the Perrinhyphen;Jablonski formula for the linear polarization of emitted light is derived in a simple fashion. It is shown that for asymmetric molecules the degree of circular polarization of the emitted light is affected by photoselection if the electric transition moments responsible for absorption and emission are not parallel. It is of much interest that under such circumstances the angle between the emitting electric and magnetic dipole moments can in principle be obtained.