Fluorescence of Solutions Bombarded with High Energy Radiation (Energy Transport in Liquids) Part II

In organic solutions containing fluorescent molecules a considerable part of the high energy radiation absorbed in the bulk material is transformed into light emitted by the fluorescent molecules. There is a transfer of excitation energy from the bulk material to these molecules which reaches its maximum at concentrations of approximately 1 g/l and higher. Experiments with mixed solvents are described where relatively small concentrations (several mole percent) especially of naphthalene and $o$-diphenylbenzene act in some way as a second solvent. The excitation energy can be localized by investigating the fluorescence of these mixed solvents when different fluorescent solutes are added. It is found that the energy migrates from the original solvent to the second solvent and then to the fluorescent molecule. This intermediate energy transfer can change the light emission curve completely. From this change the dependence of the light emission on internal and self-quenching could be determined. Besides this internal energy transfer, an energy transfer via radiation occurs which is described in detail by studying the absorption of the fluorescent light inside the solution by adding small amounts of a second solute of the order of 0.1 g/l. Such small amounts change the light intensity considerably and shift the spectrum to the spectrum of the second solute. Photographs and data of the spectral distribution of such solutions are presented, which show that such drifts already occur with very small amounts of the second solute.