CONSEQUENCES OF PHOTOSELECTION ON INTENSITY AND POLARIZATION OF LUMINESCENT MOLECULES

Optically excited luminescent molecules are often not randomly oriented if rotational relaxation times are slow compared to excited-state lifetimes, i.e., the molecules are photoselected. The properties of the emission are examined and applied to relative intensity measurements. Analysis of the emission intensity shows that relative intensity measurements (e.g., the ratio of fluorescence to phosphorescence quantum yields) are in error whenever the relation of the axes of absorption and emission changes. Analysis of the polarization of the emission shows how to recognize and avoid the errors and how to enhance the sensitivity of relative intensity determinations by suitable selection of the following variables: polarization of the exciting light, angle between the exciting and the emitted beams, and the polarization of the emitted light.