Wide‐Angle Interference and Multipole Nature of Fluorescence and Phosphorescence of Organic Dyes

Part of a wave front radiated by an excited molecule interferes with the part of the same wave front reflected by a mirror. Thus in certain directions a strong luminescence is observed, while in others no emission of the same wavelength can be seen. This phenomenon of wide-angle interference has been investigated for the case of monomolecular layers of organic dyes, which are kept in a fixed distance from an evaporated silver mirror. To circumvent the effects of refraction at the air-monolayer interface the substrate carrying the layer system is immersed in a cylindrical cuvette filled with an aqueous solution of calcium thiocyanate. This solution is particularly well suited as immersion fluid for phosphorescence measurements, since its low solubility for oxygen coupled with an increased viscosity prevents the quenching of phosphorescence. A small wavelength range is selected by an interference filter and its intensity measured as a function of the angle with the normal on the mirror surface. The angular characteristics of the emission is an indicator for the nature of the transitions associated with fluorescence and phosphorescence. The well defined distance between the luminescing molecules in the monomolecular layer and the mirror permits a much higher precision compared with earlier investigations. While there is no doubt that the fluorescence is electrical dipole in nature, it might be possible that higher multipole transitions (e.g. magnetic dipole or electric quadrupole) participate in the phosphorescence emission. We have indeed found, that the phosphorescence from the triplet state of monomethin-cyanines at room temperature is not a pure electrical dipole transition, but is partly electric quadrupole in nature.