摘要:

Certain of the presently known aromatic molecules, for example, p-terphenyl and PBD, have relatively fast fluorescence rise and decay times. However, to keep pace with recent developments in fast electronic circuitry and in photomultiplier tubes, scintillators are needed with even faster decay times. Some factors relevant to a short lifetime will be discussed—mainly from an empirical viewpoint.

ISSN:0369-1152    

DOI:10.1080/15421406808082907

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The time distribution of the light output of several liquid scintillators has been measured. Attempts were made to shorten the time distributions of the scintillations by the addition to the liquids of dissolved oxygen and various organic chemicals. Bromobenzene and oxygen were found to be the most effective in quenching the tail of the scintillation and shortening the decay time.

ISSN:0369-1152    

DOI:10.1080/15421406808082908

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The ratio of excimer to molecular fluorescence intensities in both the delayed and total emission spectra of pyrene in ethanol has been measured as a function of solute concentration and temperature from 150–350°K. An analysis of the data in terms of triplet-triplet annihilation leading directly to excimer formation (process 1) or production of the lowest excited molecular singlet state (process 2) shows that the relative probabilitiesk1/k2of these processes increases from zero at low temperatures to a maximum of 1.8 at higher temperatures.

ISSN:0369-1152    

DOI:10.1080/15421406808082909

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

Our work on the delayed electroluminescence of anthracene crystal has shown that the recombination of holes and electrons produces the normal fluorescence of anthracene, and that time dependence of the fluorescence can be modified by an external electric field. We have also shown that the recombination of holes and electrons produces what appears to be an ion-pair or charge-transfer (CT) exciton, that this CT exciton diffuses through the anthracene crystal, and can form a dimer with another CT-exciton; the decay of the dimer produces one photon, at most. The normal decay of the nearest neighbor CT exciton, and the CT-exciton dimer produces a tail that extends into the microsecond region. Some estimates are given for the lifetime of the CT exciton; ionic energy levels for anthracene crystal are also given.

ISSN:0369-1152    

DOI:10.1080/15421406808082910

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The mechanism of energy transport in organic molecular crystals can be studied by sensitized fluorescence. One can analyze nearly all of the experimental results on sensitized fluorescence so far reported by a hopping model.1In this model, energy is hopping from molecule to molecule with a characteristic time of about 10−13sec (incoherent exciton). Since one does not observe an activation energy for energy migration in pure organic crystals, there is no firm theoretical foundation for the hopping model, and it seems more satisfactory to explain energy transfer by a free exciton (coherent exciton) mechanism.

ISSN:0369-1152    

DOI:10.1080/15421406808082911

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The fluorescence decay time of purest naphthalene crystals has been measured as a function of temperature (300°K-4°K) and thickness. The radiative lifetimes of the two Davydov-components are measured as τa= 115 ± 5 nsec and τb= 33 ± 5 nsec.

ISSN:0369-1152    

DOI:10.1080/15421406808082912

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

Upon illumination of pure molten aromatic compounds like naphthalene a photocurrent is observed which is due to the generation of the lowest excited triplet state by light. A model is proposed basing on the assumption that triplet excitons can migrate over large distances from the excitation region to the anode where they can produce charge carriers. Numerical calculations support the model.

ISSN:0369-1152    

DOI:10.1080/15421406808082913

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

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.

ISSN:0369-1152    

DOI:10.1080/15421406808082914

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

The phenomenon of triplet-triplet annihilation in the following mixed crystal systems is discussed: phenanthrene-h10in biphenyl, phenanthrene-d10in biphenyl, naphthalene-h8in biphenyl, naphthalene-d8in biphenyl and chrysene in biphenyl. The kinetics of the phenomenon are elaborated, particular attention being paid to tthe effects of (i) host-guest triplet energy gaps; (ii) random walk of triplet host excitons; and (iii) the influence of adventitious impurity or host crystalline defects. The topics discussed and concerning which experimental information is presented are: (a) the decay behavior of phosphorescence and delayed fluorescence; (b) the temperature dependence of the above decays; (c) the growth characteristics of phosphorescence and delayed fluorescence; (d) the temperature dependence of phosphorescence and delayed fluorescence intensities; (e) the concentration dependence of delayed fluorescence intensity; and (f) the incident intensity dependence of phosphorescence and delayed fluorescence intensities. It is shown that all data, with the single exception of delayed fluorescence decay at T ∼ T 77°K, are in accord with the kinetic model developed. The decay behavior of delayed fluorescence at 77°K is interpreted in terms of a crystal defect model. Exception is taken to the model of Hadley, Rast and Keller for the temperature dependence of phosphorescenc decay; it is shown that the annihilative model is in better accord with fact.

ISSN:0369-1152    

DOI:10.1080/15421406808082915

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor

摘要:

A knowledge of the quantum yields of fluorescence, φf, and phosphorescence φp, alone does not make it possible to separate the two non-radiative processes, φF1, and φP1, which quench respectively the first excited singlet state, S1, and the lowest triplet state, T1. To do this, the quantum yeield of triplet formation, φT, must be known. We have measured the extinction coefficients, φT, of triplet-triplet absorption for a series of aromatic hydrocarbons in EPA at 77°K. From the φTdata plus chemical actinometry, we have obtained values of φT. In addition, we have made accurate absolute measurements of fluroescence yield, φF, in solution at 25°C. From our φFdata and room-temperature values of φTreported by Lamola and Hammond, we can calculate values of φF1. Benzene, naphthalene, phenanthrene, chrysene and 1, 2-benzanthracene have significant values of φF. Perdeuteration causes no significant change in the value of φF1. We conclude, therefore, that internal conversion from S1to S0is determined largely by processes other than coupling of the electronic excitation to C-H vibrations. At 77°K, φFdata to combine with our φTvalues are available only for anthracene. Depending on which of two φFvalues is used, the value obtained for φF, is either 0.20 or 0.12 with an uncertainty of ±0.10. Clearly there is a great need for accurate low-temperature measurements of fluorescence yield in order to determine whether significant internal conversion from S1to S0occurs at 77°K.

ISSN:0369-1152    

DOI:10.1080/15421406808082916

出版商:Taylor & Francis Group    

年代:1968

数据来源: Taylor