Naphthalene decay time

Organic scintillation phosphors include naphthalene, stilbene, and anthracene. The decay time of this type of phosphor is approximately 10 nanoseconds. This type of crystal is frequently used in the detection of beta particles. 

This proposal regarding the dependence of the short decay time component on viscosity is supported by the fact that the 530-nm pulse excites preferentially the H, hydrazone, form of the 1PA2N molecule and specifically the trans isomer of the H form (HT). In addition, the cis form is known to be unstable, quite possibly because of the steric hindrance between the naphthalene and benzene components of the 1PA2N, and therefore of the very low ground state population. 

Attempts to directly detect the triplet state of benzene under these or similar conditions by transient spectroscopy techniques have so far been unsuccessful. Thomas and Mani (244) found a transitory species with an absorption maximum and a decay time of 112 ns in benzene and benzene cyclohexane mixtures. The results from addition of naphthalene, blacetyl, oxygen, and plperylene suggest that this is a product of reaction between triplet and ground state benzenes. Build-in times for this species, which are 3 ns for pure benzene and 20 ns for 10 percent benzene/cyclohexane mixture are temperature independent. Laser... 

As an example of excitation energy transfer studied by time-resolved fluorescence, let us take again the case of the inclusion complex of the multichromophoric cyclodextrin CD-St with oxazine 725 described in Section 7.2.4.2 [15]. Figure 7.9 shows the fluorescence decay of CD-St the very first part of the decay is due to energy transfer [13] from the steroidic naphthalene fluorophores to oxazine 725. Data analysis led to an average decay time for transfer of about 25 ps, which is quite fast, as expected from the short average distance between donor and acceptor ( 9-10 A). 

Fig. 9. Decay of luminescence with time. Ordinate In (luminescence intensity) one division = 0.25. Abscissa time one division = 0.0003 sec. for curves (a) and (6) 0.0005 sec. for curve (c) 1.0 sec. for curve (d) and 0.1 sec. for curve (e). (a) and (6) Delayed fluorescence of pyrene monomer and dimer in ethanol at +23°C. (c) Delayed fluorescence of naphthalene in ethanol at —23°C. (d) Triplet-singlet phosphorescence of 10-W phenanthrene in EPA at 77°K. (e) Delayed fluorescence of 10-lAf phenanthrene in EPA at 77°K.

The kinetics of recombination of the tetramethyl-p-phenylenediamine cation radical TMPD with etr and with the naphthalene anion radical Nh in vitreous squalane was studied in ref. 57. The studies were carried out at temperatures of 77 - 150K in two time ranges 10 4 to 10 1 s and 102 to 10 s. At low temperatures (e.g. at 77 K), for both recombination processes the decay of the luminescence intensity for both time ranges was found to be described by eqn. (7) with m = 1 (see the data for the reaction of TMPDf with et7 in Fig. 13), which is characteristic of the tunneling mechanism of recombination. At higher temperatures, however, the kinetics of the luminescence decay for the reactions with et and Nh" turned out to be different. Thus, for example, at 98 K the kinetics for both reactions is described by eqn. (7) as before. But while for the reaction... 

Table I shows that in either dioxane or acetonitrile the quantum yield for degradation of I, is unaffected by the presence of 0.1 M of triplet quencher, either sorbic acid, naphthalene or cyclohexadiene. In ethanol, triplet quenchers reduce < >d from 0.34 to 0.14. Quantum yields for intersystem crossing, as determined by a laser opto-acoustic technique ( ), were 0.36 in ethanol and 0.59 in dioxane. These results agree with our earlier report (3), and indicate that significant reactivity occurs from St of I in protic solvents, and that reaction occurs exclusively from Sx in aprotic solvents. While triplet quenching experiments cannot rigorously exclude participation by short-lived higher triplet states, Palm et al (9) have obtained conclusive evidence from CIDNP experiments for singlet-state participation in a series of aryloxy-acetophenones. Note that the triplet state of I is formed in aprotic solvents, and that in deaerated solutions at room temperature it decays by first-order kinetics with a lifetime of 200 ns (3). Remarkably, despite having lifetimes about 100 times longer than other, differently-substituted, aryloxyacetophenones (the longer lifetimes may...

Figure 16 shows the absorption spectrum obtained by additive-free polyethylene [67], At ambient temperature the absorption observed on nanosecond time-scale increased continuously from 500 to 200 nm without showing any maximum. The absorption in UV is similar to that obtained by y-irradiation. Considering the results obtained by liquid alkanes, the absorption seems to be comprised of several different free radicals. At 95 K additional absorption due to the trapped electron was observed at wavelengths longer than 600 nm the band was observable even at ambient temperature in the picosecond time-domain [96]. The electron decays presumably by the hole-electron recombination. The decay of the trapped electron was independent of the presence of carbon tetrachloride, suggesting that the additives reacted with a mobile electron but not with the trapped electron. On adding naphthalene, the radiation-induced spectrum showed the bands due to the first excited triplet state and the radical... 

The transient Q-band EPR experiments provide direct evidence for sequential electron transfer from the primary to the secondary radical pair of the triplet channel in a triad consisting of a zinc-9-desoxo-meso-methylpyrochlorophyllide donor (ZC), a pyromellitimide primary acceptor (PI), and a naphthalene-1,8 4,5-diimide secondary acceptor oriented in a liquid crystal (Heinen et al., 2002). At room temperature this process occurs with an exponential time constant of tT = 50 + 1 ns. In the singlet-initiated channel, the intramolecular electron-transfer rates are too fast for direct EPR detection. The species decay with a time constant of tS = 36 1 ns by charge recombination to the singlet ground state. 

The transient T-T absorption in the gas phase has been measured recently for aromatic molecules such as naphthalene (119,211) and anthracene (80,81) using flash kinetic spectroscopy and tandem laser pulse absorption techniques. Particularly, the later technique (211) provides time-dependent absorption spectra of the "isolated" unrelaxed triplet molecules because of its capability for rapid monochromatic excitation and detection. It will certainly provide a wealth of Important kinetic and spectroscopic information about the evolution and decay of triplet states. Direct observation of the formation of transient hot ground-state (Sq) molecules through an internal conversion process has also been achieved with laser excitation and laser... 

Charge-transfer complexes of neutral molecules in zeolites have also been examined. Transient experiments with 1,2,4,5-tetracyanobenzene (TCNB) as acceptor and arene donors have been reported. For naphthalene, transient absorption bands centered at 470 and 680 nm due to TCNB and naphthalene radical were observed [138]. The decay was found to be biphasic and was 10 times slower in dehydrated zeolite Y than in the hydrated sample, indicating a strong interaction with the framework. 

Nature of the Lower Temperature Transition. The complex nonexponential phosphorescence decays, apparent under the higher time resolution afforded by use of the modified 199 spectrometer are a consequence of interjection by the polymer matrix in the photophysies experienced by the chromophore. Non-exponential decays of triplet naphthalene (and other chromophore) emissions have been observed in PMMA. Horie et al.(18-20) ascribe such effects to dynamic, intermolecular quenching of the excited state by the polymer whereas MacCallum et al(21-23) invoke an energy migrative process within the polymer following quenching of the triplet state of the naphthalene. 

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