Non-radiative relaxation of the excited Ã 1B2u state of benzene

— The influence of excitation wavelength, pH, oxygen and solvents, upon fluorescence quantum yields, were measured for insole Indole in neutral aqueous solution exhibits the same wavelength dependence as tryptophan, which indicates that COO- absorption is not responsible for the effect. Parameters such as pH and oxygen influence only the absolute fluorescence quantum yields but not their relative variation with wavelength, indicating competition with fluorescence emission for S1 deactivation. without any influence upon deactivation of higher excited states. In contrast, solvents exhibit a specific influence upon the wavelength dependence; for indole, the decrease of fluorescence yield excited around 215 nm, compared with the yield in the first absorption band is about 40% in water, 10% in acetonitrile, 70% in n-hexane and cyclohexane, whereas no appreciable decrease occurs in methanol, ethanol or n-butanol. These observations, together with the Stokes shifts of the emission spectra, may be well correlated with Kosower's Z-values, expressing microscopic solvent-solute interactions.

Influence of the environment on the excitation wavelength dependence of the fluorescence quantum yield of indole.

The S3 → S1 internal conversion efficiency of benzene is found to depend critically on the effectiveness of environmental perturbation in mixing the S3 and S1 states. Evidence for this derives from the experimental correlation of the conversion efficiency with the intensities of the 0–0 absorptive and emissive transitions in solutions with the solvents benzene, methanol, isopropyl alcohol, tetrahydrofuran, ethyl ether, acetonitrile, hexane, cyclohexane, methylcyclohexane, decalin, isooctane, and perfluorinated hexane. Additionally, the importance of S3 − S1 mixing is shown to explain the marked increase in the efficiency of internal conversion observed upon methyl substitution of benzene.

Effect of Solvent Perturbation on the S3 → S1 Internal Conversion Efficiency of Benzene, Toluene, and p‐Xylene

This paper reports the emission spectra, pulse heights, and decay curves of CsI(TI) crystals with a Tl/sup +/ concentration ranging from 10/sup -6/ to 10/sup -2/ mol under electron and alpha particle excitations. Larger pulse heights from the crystals with Tl/sup +/ concentration of more than 10/sup -3/ mol were observed for a higher ionization density of alpha particle excitation compared with those of electron excitation. This enhancement may be explained in terms of the higher scintillation efficiency for higher Tl/sup +/ concentration. Decay curves under electron and alpha particle excitations were compared with the decay curve under pulsed UV excitation. The observed decay curves were explained in terms of the sum of the three energy transfer processes to create excited states (Tl/sup +/)*: the prompt process, the V/sub k/ diffusion process to Tl/sup 0/, and the electron release process from Tl/sup 0/ to Tl/sup ++/. Here, Tl/sup 0/ and Tl/sup ++/ were produced through (Tl/sup +/+electron)/spl rarr/Tl/sup 0/ and [Tl/sup +/+hole (or V/sub k/)]/spl rarr/Tl/sup ++/.

Dependence of scintillation characteristics in the CsI(Tl) crystal on Tl/sup +/ concentrations under electron and alpha particles excitations

The Borexino Solar Neutrino Experiment will observe the monoenergetic (862 keV) 7Be neutrinos, produced in the solar reaction 7Be+e − →7 Li+νe. These neutrinos are the second most abundant species of solar neutrinos, with an expected flux at earth of 5 × 109/cm2/s. Using ν − e scattering in an aromatic liquid scintillator, Borexino will make the first real time measurement of the solar neutrino flux at energies less than 1 MeV. In addition to checking Standard Solar Model and neutrino oscillation predictions at low energies, Borexino will test the MSW vacuum-matter transition, luminosity constraint, and non-standard theories such as mass varying neutrinos. The Borexino detector will also be sensitive to supernova neutrinos, geoneutrinos, reactor neutrinos, and pep solar neutrinos. The pep measurement will tightly constrain the primary pp solar neutrino flux whose energy is below the Borexino threshold. With an expected rate of 35 events per day from solar 7Be neutrinos, the maximum tolerable background rate is one count per day. Removal of radioactive isotopes from the liquid scintillator is essential for the experiment’s success and will be achieved with purification techniques including filtration, distillation, water extraction, nitrogen stripping, and silica gel adsorption. Results from small-scale purification efficiency tests are presented. Water extraction showed moderate but inadequate removal of 210Po which is a dominant background. Distillation reduced 210Po by a factor of more than 500. Online purification involves cycling over 300 m3 of scintillator from the detector though the purification plants. Flow patterns within the detector that influence the purification efficiency were determined with numerical simulations. Poor flow in the prototype Counting Test Facility showed effectively stagnant volumes within the detector. These are not present in the larger Borexino detector. Surface contamination in Borexino arises primarily from contact with contaminated liquids and the deposition of airborne radon progeny. Measurements of desorption rates showed that surface contaminants are transferred to the scintillator logarithmically with time. Partitioning constants between the scintillator and surfaces were measured and airborne deposition rate of radon progeny in a clean room environment are analyzed. The efficiency of various surface cleaning techniques was also tested.

The Borexino Solar Neutrino Experiment: Scintillator purification and surface contamination

The fluorescence excitation spectra of liquid benzene, toluene, p-xylene, mesitylene, 2-methylnaphthalene, 1,6-dimethylnaphthalene, naphthalene, fluorobenzene and fluoronaphthalene, and of cyclohexane solutions of the first six compounds, were observed in deoxygenated systems at wavelengths not less than 195 nm. The observed dependence of the relative fluorescence yield on excitation wavelength and concentration is analysed in terms of a model kinetic scheme involving the formation of higher excimer states D** from the higher excited molecular states X**. Data are obtained on the relative efficiencies of internal conversion from X** and D** to the corresponding fluorescent states. The nature of the competing processes is discussed.

http://iopscience.iop.org/article/10.1088/0022-3700/1/5/322/pdf

The fluorescence excitation spectra of aromatic liquids and solutions

Excimer (excited dimer) formation occurs in all the aromatic hydrocarbon solvents used in liquid scintillators. The influence of this process on solvent-solute energy transfer has been studied for a model system: 9, 10-diphenyl anthracene (DPA) in 1, 6-dimethyl naphthalene (DMN), diluted by n-heptane (Hp). Data on the fluorescence lifetimes, spectra and quantum yields and the viscosity of DMN in Hp are used to analyze the results. It is found that the energy transfer rate parameters of the excited monomer and excimer differ, but that they are independent of Hp dilution. The influence of diffusion and solvent energy migration on solvent-solute energy transfer are discussed, and a migration mechanism involving excimer formation and dissociation is proposed.

The Influence of Excimer Formation on Solvent-Solute Energy Transfer in Organic Liquid Scintillators

The scintillation process in impurity-activated alkali halide crystals is considered by analogy to that in binary solid organic solutions. A general relation is proposed for the differential scintillation efficiency dL/dE as the product of the efficiencies of the primary and secondary scintillation processes. The primary efficiency is influenced by electron-hole recombination and ionization quenching and is a function of the specific energy loss dE/dr. The secondary efficiency is influenced by host-activator energy transfer and concentration quenching and is a function of the activator concentration c . Experimental data in support of the model are cited. An extension of the model is proposed to account for the temperature dependence of the scintillation efficiencies and decay times of pure and impurity-activated alkali halides, and this is compared with experimental data.

J. B. Birks

Papers

The fluorescence excitation spectra of aromatic liquids and solutions

The Influence of Excimer Formation on Solvent-Solute Energy Transfer in Organic Liquid Scintillators

The Scintillation Process in Alkali Halide Crystals