John L. Kropp

Internal‐Energy‐Transfer Efficiencies in Eu3+ and Tb3+ Chelates Using Excitation to Selected Ion Levels

Fluorescence yields of solutions of several chelates of europium and terbium have been measured: (a) upon excitation in the ligand absorption bands and (b) upon selective excitation to individual upper levels of the rare‐earth ion. Yields with upper‐ion‐level excitation are lower than when the emitting level is excited directly and yields with ligand excitation are lower still. Efficiencies of energy transfer to the emitting level from upper ion levels and from the ligands, for the solutions studied, are calculated from the data.The fluorescence yield observed when the emitting level is excited directly, together with the fluorescence lifetime, allows both radiative and radiationless rate constants for deactivation of the emitting level to be calculated. The enhanced fluorescence yields of chelates compared to unchelated rare‐earth ions are found to be due primarily to the enhancement of the radiative transition by the chelate environment rather than to a protective influence against quenching provided by...

Energy Transfer in Solution between UO22+ and Eu3+

Energy transfer from UO22+ to Eu3+ has been observed in several solutions. Quantitative data for the quenching of UO22+ fluorescence at various [Eu3+] and the corresponding build up of Eu3+ fluorescence has been obtained in D2O solutions. Rate constants for the transfer of energy from UO22+ to Eu3+ are derived, the value is determined to be about 107M−1·sec−1. This value is less than the diffusion‐controlled reaction rate. The low value can be related to mutual ion—ion repulsion.

Temperature‐Dependent Quenching of Fluorescence of Europic‐Ion Solutions

The fluorescence lifetime of Eu(NO3)3 in CH3OH, CH3OD, and CD3OD and of EuCl3 in D2O has been determined as a function of temperature. The temperature‐dependent quenching of fluorescence from the 5D0 level is attributed to radiationless deactivation through the 5D1 level. From these and other results it is concluded that the 5D0 level of Eu3+ does not possess a mechanism for radiationless conversion to the ground level that does not involve the upper levels or solvent vibrations.

Effect of Added Quenchers in Organic Scintillator Solutions: Organometallics

Quenching constants (γs and γx) are given for a variety of perphenyl metals (Si, Ge, Sn, Pb, Sb, and Hg) and for mercury dimethyl acting on benzene and cyclohexane solvents and p‐terphenyl and DPA scintillators. Specific rates are calculated for the quenching processes and are found to break into the following classes: <7×109 M—1 sec—1 corresponding to spin‐perturbation‐induced quenching (with a probability factor less than unity, according to Umberger—LaMer calculations) and ≧3×1010 M—1 sec—1 (exceeding such calculated values), corresponding to highly favored excitation‐transfer processes. The latter high values can be explained in terms of the domain theory of liquid scintillator solutions. Specific rates in the intermediate range for GePh4, and perhaps for SiPh4, acting on solvents are consistent with resistance to high‐energy irradiation and with the attendant probability that long‐lived excited states of such quenchers actually transfer excitation to the scintillator.Anomalously high quenching consta...

Effect of Added Quenchers in Organic Scintillator Solutions: Aromatic Halides

Luminescence‐intensity measurements, as affected by quencher concentration, solvent and scintillator, yield information on quenching constants, on specific rates of quenching, on mechanism of quenching, on contrast between quenching of solvent and quenching of scintillator, and on contrast between quenching in benzene as solvent and quenching in cyclohexane as solvent. For a number of aromatic bromides (and for iodobenzene), excitation transfer appears to be involved in the quenching process; in those cases the specific rates are higher than may be expected for simple diffusion‐controlled reactions. Typical specific rates (kq) are ≃ or >2.5×1010 M—1 sec—1 in benzene. The donor states from which excitation is transferred are not identified. In cyclohexane, calculated kq values are very much higher with an implication that the mechanism of the quenching process is quite different in such cases. Some of the results can be explained in terms of the theory of solvent domains; the data for cyclohexane suggest t...

Observations of Phosphorescence in Plastic Scintillators Excited by 60Co γ Irradiation

The γ‐induced luminescence of chrysene‐d12, coronene‐d12, and p‐terphenyl has been studied in plastic hosts (DER‐332 and polymethylmethacrylate). Both phosphorescence and fluorescence are observed. The variation of phosphorescence and fluorescence as a function of solute concentration is determined. From the concentration dependence, Q parameters can be calculated for the phosphorescence (QP) and fluorescence (QF) of the compounds studied. The ratio of phosphorescence to fluorescence under γ irradiation (Rγ) is compared to that observed under uv irradiation (Ruv) and a kinetic scheme is developed that relates (Rγ/Ruv) to QP and QF assuming only excitation transfer. From the fit of the kinetic treatment to the experimental points, it is concluded that triplet coronene‐d12 is formed mainly via excited solvent triplets. Triplet p‐terphenyl is not formed via excited solvent but is formed by other processes probably involving ions.