Polarized Fluorescence

Abstract

The emission from a fluorescent solution is always polarized to some degree. By this we understand that an observer who sees the light emitted in a direction at right angles to the direction of the excitation through a polarizer observes changes in the intensity of the emission as he rotates the polarizer (Figure la). This polarization, first discovered by Weigert (1920) has received considerable attention; its origin and significance are well understood in the case of fluorescent solutions. Briefly, the polarization results from the existence of a preferential orientation in the molecules at the time of the emission. The exciting light produces a selection of the orientations because the molecules preferentially excited are those in which the transition moment in absorption is parallel or at a small angle to the electric vector of the polarized excitation. This original orientation is preserved if the molecules undergo negligible motion between excitation and emission, in which case the polarization is a maximum, called the fundamental or limiting polarization P ₀ .