Milton Furst

FLUORESCENCE EFFICIENCIES OF ORGANIC COMPOUNDS

The relative fluorescence efficiencies of a large number of organic compounds of different structures were measured in solution under high‐energy and ultraviolet excitations. P/R values of the more efficient solutes under high‐energy excitation are given. Where possible at room temperature the fluorescence under high energy was also determined for the compound in solid form. A considerable parallelism is found between the relative fluorescence efficiencies in solution under both modes of excitation. Oxygen effects were measured and found to be greater on the solvent than on the solute, but do not greatly influence the relative efficiencies. Changes in fluorescence produced by substituents which do not considerably alter the absorption spectrum are discussed.

Energy Migration in Organic Liquids at High Temperatures

The energy‐transfer process in organic liquids has been studied at temperatures from 20°–240°C. The results are consistent with the migration model, according to which electronic excitation energy undergoes a series of hops between neighboring solvent molecules, followed by a short‐range jump to a solute molecule. The mean hopping time τh is related by theory to an experimentally determinable parameter α, which is directly related to the hopping frequency. Values of α, all on the order of 1012 sec−1, were found for a number of solutions. For a temperature increase of 200°C, α was generally found to decrease by about 50%, depending on the solvent utilized. Relationships between the temperature dependence of τh and that of mean intermolecular distance are discussed. Material diffusion in the energy‐transfer processes is estimated to be quite small compared with hopping, at least for efficient solvents. Anomalies in the energy‐transfer properties of anisole are noted.

Short Decay Time Scintillator Solutes

A number of recently synthesized substituted p-oligophenylenes have excellent scintillation properties, including shorter decay times with high pulse heights. The fluorescence properties of four substituted p-oligophenylenes in o- xylene are described; their decay times range from 1.2 x 10/sup -9/ to 2.6 x 10/ sup -9/ sec. (D.L.C.)

Scintillating Solutions Containing Heavy Elements

The use of heavy metals in scintillators (excited by high energy) is limited by two factors: 1) Such materials are very often not sufficiently soluble in solvents exhibiting considerable energy transfer, and 2) they generally quench the excitation energy of the solution and thereby decrease the fluorescent output. The latter effect was found to be due mostly to a quenching of the excited molecules of the solvent which are responsible for transferring energy, and to a lesser extent to the quenching of the fluorescent molecule, although this process also occurs. To circumvent the first difficulty one uses solvents with better solubilities for the metal compounds, but the choice is limited because of poor energy transfer characteristics of many of them. One of the best solvents found up to now is p-dioxane, which exhibits considerable energy transfer and shows better solubility properties than most of the other energy transferring solvents. Another means of overcoming difficulties of solubility is to use organic solvents favorable from the standpoint of solubility, but not so favorable from that of energy transfer. One can effectively induce considerable energy transfer in many of these solutions and thus produce high energy excited fluorescence by adding a relatively small amount, of the order of 10 per cent, of an effective liquid solvent (one which shows energy transfer).

LIQUID SCINTILLATORS AT HIGH TEMPERATURES.

Abstract Effects of temperature above ambient (300–550°K) on liquid scintillators are presented. Under excitation by high energy or light usually a reduction in fluorescence with high temperatures is found. However, when excimers are present, a rise may be found. The energy transfer as a function of temperature is studied by means of a theoretical formula, and is found to be greatly temperature dependent. Its dependence is found to be directly associated with variation in fluorescence properties of the solvent by itself.

Method for Determining Charge Production in Semiconductors and Insulators

A method is described which allows separate determination of charge injection from an electrode into a semiconductor or insulator and separate determination of charge production in the bulk or at surfaces by thermal or radiative means. A basic part of the method is the use of a barrier layer of very high resistance at one or two surfaces of a material such as a crystal of CdS or anthracene. These barriers serve in this way as collecting capacitors for the charges. The amount of charge collected can be determined by quickly discharging these capacitors by light. By this method, in addition to determining very small rates of charge production, the total amount of charge made available in the sample by thermal charge production can be found. Radiative charge production in the presence or absence of external fields can also be determined.

Quenching and Energy Transfer by the Same Substance

The quenching by 0‐terphenyl of the gamma‐ray induced fluorescence of 0‐xylene/9,10‐diphenylanthracene solutions does not give the usual linear dependence of I0/I on quencher concentration. The effect is ascribed to a quencher concentration‐dependent energy transfer from the solvent to 0‐terphenyl which has a finite transfer probability to the fluorescent solute. This interpretation is confirmed by the enhancement of energy transfer caused by the addition of 0‐terphenyl to a solution of a fluorescent solute in tri‐n‐butylphosphate, a solvent having a very low transfer efficiency.

Investigation of Energy Transfer in Liquid Organic Systems

This discussion is concerned mainly with recent investigations of energy transfer in liquid organic systems. The exploration of energy transfer processes goes back to the early work of Franck, Vavilow and others and has continued ever since [1]. New investigations were stimulated when it was found that fluorescence induced by high energy radiation in dilute organic solutions [2, 3, 4] is so great that it can be explained only by assuming transfer of excitation energy from solvent to solute.