Fluorescence decay and quenching of pyrene labeled on sulfonate polyelectrolytes in salt-free aqueous solutions

Fluorescence decay and quenching of pyrene labels on copolymers of 2-acrylamido-2-methylpropanesulphonic acid (AMPS) and N,N-dimethylacrylamide were observed in dilute salt-free aqueous solutions as a function of the mole fraction F_AMPS of AMPS from 0 to 0.896. Monoexponential decay was found for the samples of F_AMPS<0.35 and biexponential decay for the samples of F_AMPS>0.35. The fast decay component is 80%, and the averaged lifetime 〈τ〉 and lifetime τ₁ of the fast decay is decreased with increasing F_AMPS. Quenching efficiency of Cu²⁺, I⁻, CH₃NO₂, and dinitrobenzene to the pyrene label was investigated in the framework of Stern-Volmer plot. With increasing F_AMPS the quenching efficiency of Cu²⁺ is increased while that of I⁻ decreased. For the neutral quenchers, the quenching rate constant k_q increases when F_AMPS<0.449 then decreases, showing a decline of accessibility to the pyrene label. These results were interpreted consistently with the counterion condensation concept, where condensed counterions caused the polyelectrolyte chains to aggregate. The existence of less-polar “temporal aggregated domain” in highly charged polyelectrolytes appears to lead to the slower decay and lower accessibility of the pyrene labels.     

Temperature dependence of the spin-lattice relaxation rates in the triplet state of pyrazine at low temperatures

The electron spin-Lattice relaxation, (SLR) rate constants have been measured for the triplet state at pyrazine-d₄ in a cyclohexane matrix from 1 to 8 K by resolving the phosphorescent decay curve into three exponential components. A matrix kinetics is presented that gives the relationship between the SLR rate constants and the observed decay component rate constants and the decay component intensities of the three zero-field levels. An exact numerical procedure based on the Newton-Raphson technique is described in detail for systems of two and three coupled levels. This procedure was used to find the SLR rate constants starting from either the decay component rate constants or the decay component intensities. Both the matrix kinetics and the numerical procedure should be useful in many other studies involving multilevel systems whose sublevels are connected by some combination of SLR and microwaves. The observed SLR rate constants are best explained as a function of temperature as the sum of a direct process (linear in T) and a Raman process (T² dependence). Their relative magnitudes indicate that the Raman process is probably antharmonic. Vancus mechanisms for SLR in this system are examined, and a short review of SLR theory and experiment is given. The nature of the zero-field sublevels in pyrazine-J₄ is also discussed as is the effect of the choice of matrix solvent upon SLR rate constants and the phosphorescent spectrum.     

Lifetime studies of the HgNH3 complex

The luminescence induced by the Hg-photosensitized reaction of NH₃ was studied by repetitive fast pulsed excitation. From observations of the decay of the luminescence, the emitter of the 290 nm fluorescence was found to be the precursor of the 340 nm emission (the stabilized complex). The first-order decay rate of the stabilized complex was found to be given by 1/τ_O + k[NH₃], where τ₀ = 2.3 μs and k = 3.5 × 10^?13 cm³ molecule^?1 s^?1.     

Reaction rate for isomerization in a molecular beam

It is shown that in an ideal gas (molecular beam), at temperature T, consisting of molecules that can isomerize and whose molecular dynamics can be non-ergodic, the observed decay rate of the concentration can be greater than that. k_TST, predicted by transition state theory. The “observed” rate constant is then given by (Q/Q_≠⁽¹⁾)k_TST where Q_≠⁽¹⁾ is the contribution to the canonical partition function Q arising from only activated and irregular states. It is also shown that in the presence of strong collisions the long-time decay rate of the concentration provides no information about the intramolecular rate process, but at low collision rates it is sensitive to the non-ergodic dynamics.     

Singlet exciton interactions in crystalline naphthalene

The decay of prompt fluorescence in crystalline naphthalene at 300 K, excited by a picosecond 266 nm pulse, has been studied as a function of excitation intensity. Experimental decay curves can be fitted only when the exponential distribution in depth of excitation and the radial (gaussian) intensity profile of the excitation are both taken into account. From an analysis of the decay at early time (?5 ns) a best fit value of the singlet—singlet annihilation rate constant is found γ_SS = (4 ± 1) × 10^?10 cm³ s^?1. If the reaction is diffusion-limited, this rate implies an average singlet diffusivity D_S = (2 ± 1) × 10^?4 cm² s^?1.     

A time-resolved EPR study of the magnetic and decay properties of short-lived non-phosphorescent 3nπ* pyridazine

The properties of ³nπ^* pyridazine were investigated by time-resolved EPR. The zfs (X = 0.092 cm⁻¹, Y = −0.162 cm⁻¹, Z = 0.071 cm⁻¹), the sublevel decay rate constants (k_x ≈ k_z ≈ 2.0×10⁵ s⁻¹, k_y = 1.7×10⁶ s⁻¹) and the relative populating rates (P_x: P_y: P_z ≈ 0.1 : 1 : 0.1) were determined. The lack of phosphorescence is ascribed to a very large radiationless decay rate constant (k^nr ≈ 10⁶ s⁻¹).     

The lowest excited triplet state of triphenylboron and of the isoelectronic triphenylcarbenium ion

Triphenylboron BPh₃ and the triphenylcarbenium salts C⁺Ph₃/SbCl₆^? and C⁺Ph₃/BF₄^? have been investigated by ODMR and emission spectroscopic methods. The zero-field splitting (ZFS) parameters D and E and the decay rate constants of the triplet zero-field levels (ZFL) as well as the phosphorescence spectra were measured. The non-zero E values indicate a symmetry lower than D₃ for the Jahn-Teller unstable triplet state of all compounds. The radiative decay of T₁ shows a strong delocalization of the triplet wavefunction for C⁺Ph₃, but a strong localization on the benzene rings for BPh₃. This is in agreement with MO calculations.     

Decay of H<Subscript>2</Subscript>O<Subscript>2</Subscript> under the action of the colloidal catalyst based on iron(III) oxides in a neutral medium

The catalytic activity of the colloidal catalyst based on iron(III) hydroxide was studied in the decomposition of H₂O₂ in a neutral medium (pH 6.7). A colloidal micellar solution of iron(III) hydroxide after preparation was kept at 19–20 °С for 2 or 20 h without additives or with C₂H₅OH additives. The decomposition of H₂O₂ under the action of the colloidal catalyst (20 h) proceeds via the first-order reaction with the decay rate constant k_d = 1.26?10^–4 s^–1, whereas the decay rate of the first-order reaction is k_d = 0.77?10^–4 s^–1 for the colloidal catalyst (2 h) prepared in the presence of C₂H₅OH.     

Non-radiative relaxation of Ho3+ in various tellurite glasses

Non-radiative relaxation rates from ⁵F₄, ⁵S₂ to ⁵F₅ of Ho³⁺ in three tellurite glasses were measured as a function of temperature. The experimental behaviour of the nonradiative decay rate W_nr(T) with the temperature lies between two theoretical curves fitted to 4 and 5 phonons. Phonon frequencies were obtained from Raman measurements, a correlation was found between the magnitude of W_nr(0) and the amount of disorder in glass.     

Decay of atomic systems in a strong electric field. Negative ion and hydrogen atom

An exact transcendental equation was obtained determining the energy levels of a particle bound by a short-range force and placed in uniform electric field. At high intensities there is a considerable difference between the values of decay probabilities calculated by previous theory and by this one. The results obtained were used to determine the electron binding energy and the polarizability of negative ions by employing experimental data on ion decay during a flight through the field region. Concrete calculations were carried out for the C^? ion.The calculation of the resonance energy E_r and the decay probability 2Γ of the quasistationary state n = S, n₁ = 3, n₂ = 0, and m = 1 of the hydrogen atom in a uniform electric field was performed by means of the direct integration of the time-independent Schrödinger equation for field intensities ranging from 0.8 × 10⁶V/cm to 1.7 × 10⁶V/cm. The calculated values fo E_r perfectly agree with those of the Rayleigh—Schrödinger perturbation energies calculated up to the fourth power in the field strength and differ substantially from the resonance energies obtained earlier by Hirschfelder and Curtiss. Our results concerning the rate of spontaneous ionization also differ substantially from the results of Hirschfelder and Curtiss and agree rather well with the results of Bailey, Hiskes and Riviere who used the general WKB method of Rice and Good.     

Metastable decay of rare gas cluster ions: mechanism and kinetics

Metastable decay of cluster ions has been discovered only recently. It was noted that one has to take this metastable decay into account when using mass spectrometry to probe neutral clusters, because ion abundance anomalies in mass spectra of rare gas and molecular clusters are caused by delayed metastable evaporation of monomers following ion production. Moreover, it was found that(i) the individual metastable reaction rates k depend strongly on cluster size and cluster ion production pathways and that(ii) there exists experimental evidence (k=k(t)) and a theoretical prediction that a given mass selected cluster ion generated by electron impact ionization of a nozzle expansion beam will comprise a range of metastable decay rates. In addition, it was discovered that metastable Ar cluster ions which lose two monomers in the μs time regime decay via sequential decay series Ar _n ⁺ *→Ar _n?1 ⁺ *→Ar _n?2 ⁺ * with cluster sizes 7≤n≤10 andn=3 (similar results were obtained recently in case of N₂ cluster ions). Conversely, the dominant metastable decay channel of Ar ₄ ⁺ * into Ar ₂ ⁺ was found to proceed predominantly via a single step fissioning process.     

Polyol-mediated solvothermal synthesis and luminescence properties of CeF3, and CeF3:Tb nanocrystals

CeF₃ and CeF₃:Tb³⁺ nanocrystals were successfully synthesized through a facile and effective polyol-mediated route with ethylene glycol (EG) as solvent. Various experimental techniques including X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and photoluminescence (PL) spectra as well as decay dynamics were used to characterize the samples. The results indicated that the content of NH₄F and reactant concentrations were key factors in the product shape and size. Excessive NH₄F was necessary for the formation of hexagonal nanoplates. The specific morphology of product can be controlled by changing the NH₄F content and reactant concentrations. In addition, Tb³⁺ doped-CeF₃ sample shows strong green emission centered at 544 nm corresponding to the ⁵D₄-⁷F₅ transition of Tb³⁺. Due to the decrease of nonradiative decay rate, the lifetime of ⁵D₄ level of Tb³⁺ become longer gradually upon increasing the size of product.     

Temperature dependence of trapped electrons in crystalline D2O ice from 77 to 6 k

The temperature dependence of the abundance of trapped electrons which absorb in the visible (e^?_vis) and infrared (e^?_IR) in crystalline D₂O ice has been studied by pulse radiolysis between 77 and 6 k. The yield (G) and decay of e^?_vis show little dependence on temperature or doping with NH₄F. At 6 K G(e^?_vis) is 0.54 and the electron decays by half within 5 μs. These observations are consistent with e^?_vis being mainly located in spurs. The yield and decay of e^?_IR, on the other hand, show a more marked dependence on temperature. In the pure crystal G(e^?_IR) increases more than tenfold from ≈ 0.1 at 77 K to 1.3 at 6 K and its decay rate is greatly decreased at the lower temperature. Doping with NH₄F increases G(e^?_IR) to 0.85 to 77 K and to 1.8 at 6 K and some decay is observed at 6 K but not at 77 K. These results are interpreted on the basis that geminate recombination between electrons and holes is very fast at 77 K but becomes sufficiently slow for the electrons to be observed at 6 K. It is also inferred that the hole is more mobile than e^?_IR. The mechanisms causing the decays of e^?_vis and e^?_IR are discussed.     

Excited state dynamics of C2O(A3Πi)

The fluorescence decay and bimolecular electronic quenching behavior of C₂O ($\text{A}$³Π_i) is reported. C₂O($\text{X}$³Σ^?) is produced by laser photolysis of C₃O₂ at 266 nm and is subsequently excited by a tunable flashlamp pumped dye laser. The fluorescence decay is highly nonexponential and dominated by both short (≈ 15 μs) and long (50–250 μs) decay components. The long-lived emission, itself, is nonexponential. The fluorescence decay is modeled as the sum of three exponential components. The short-lived emission is quenched by C₃O₂ at higher than the gas kinetic rate while the long-lived fluorescence is quenched much less efficiently. Fluorescence quenching measurements are also reported for collisions with Ar, N₂ and O₂.     

Lifetime of the 4D 3/2 and 4D 5/2 metastable states in Sr II

Sr⁺ ions were confined in a r.f. quadrupole trap for times of the order of 30 min. The metastable 4D states were populated via laser excitation of the 5P states. The weak quadrupole transition rate into the 5S _1/2 ground state at 674 and 687 nm was deduced from observation of the exponential decay. At background pressures above 10^?7 mbar the radiative decay is dominated by collisional quenching. Extrapolation of the observed decay rate to zero background pressure yields the radiative lifetimes. At pressures around 10^?6 mbar fine structure mixing collisions between the 4D states have been observed, which lead to corrections of the extrapolated lifetimes. As the final result we obtain 395±38 ms for 4D _3/2 and 345±33 ms for 4D _5/2. These results are somewhat higher than theoretical predictions.     

An apparent isotope anomaly in the reaction CH3 + H/D

The reaction between CH₃ and D has been studied by laser flash photolysis, using absorption and resonance fluorescence to monitor decay of CH₃ and D, with [CH₃] ⪢ [D]. A rate constant k₂ = (1.75 ± 0.045) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ was obtained for 50 ⩽ P ⩽ 600 Torr, 289 ⩽ T ⩽ 401 K. Absence of a pressure dependence in k₂ demonstrates that the reaction is at its high-pressure limit, because of preferential fragmentation of CH₃D into CH₂D + H. Decay of D is shown to be free from complications at 300 K, but at 400 K regeneration of D by reaction between OD and D₂ has to be included explicitly. Results were analysed by a numerical method according to a scheme which includes this and other, less important, reactions, k₂ shows no dependence on [D₂] over a tenfold range. The value calculated from k₂ for the limiting high pressure rate constant for CH₃ + H is at least a factor of two lower than that obtained by extrapolation of rate data from similar experiments on CH₃ + H.     

ESR study of free radicals in UHMW-PE fiber irradiated by gamma rays

ESR spectra of the trapped radicals in an ultra-high molecular weight polyethylene (UHMW-PE) fiber irradiated by gamma rays showed well-resolved hyperfine splitting at room temperature since the c-axis of the crystallites is aligned with the fiber direction and the radicals are trapped in crystallites. The alkyl radical (?CH₂?^?CH?CH₂?) was the major product after irradiation in vacuum and in air at room temperature. Some of the alkyl radicals converted to allyl radicals (?CH₂?^?CH?CH=CH?) and polyenyl radicals (?CH₂?^?CH?(CH=CH)_n?CH₂?) during storage in vacuum. Upon storage in air atmosphere, the alkyl radicals decayed by reaction with oxygen. Of particular interest is the very slow decay rate of the alkyl radical trapped in UHMW-PE fiber, the half-life is 26 days in vacuum, and 13 days in air at room temperature, which is about 1/30 and 1/100 of that reported for high density polyethylene (HDPE), respectively. The extremely long lifetime of the alkyl radical is supposed to be caused by the large size of crystallites in UHMW-PE fiber. The rate of radical decay was accelerated by annealing at elevated temperature.     

The kinetics of the gas phase reaction of O(3P) with N2O5

The kinetics of the gas phase reaction between O(³P) atoms and N₂O₅ have been examined in a discharge-flow mass spectrometer at 4.5 torr (N₂) total pressure. At O(³P) concentrations (5–10) × 10¹⁴ molecules/cm³, the decay of N²O⁵ was very small and only slightly greater than the data scatter. From these data, upper limits to the rate constant of this reaction was obtained at 223 and 300 K: k²²³ and k³⁰⁰ ? 3 × 10^?16 cm³/molecule s.     

Experimental evidence for the influence of excition interactions on the temporal decay of crytalline tetracene flourescence

Crytalline tetracene fluorescence was excited at room temperature by high intensity picosecond pulses at 531.5 nm (ca. 10¹⁵ photons cm^?2 per pulse). The analysis of the decay curves during the first 1.5 ns at high excitations, shows a decay faster than the singlet lifetime measured at low intensity, due to singlet quenching by S–S and S–T interactions, and the appearance of a slow decay due to singlets generated by T–T interactions. Numerical fitting is obtained with rate constants γ_SS and γ_SS equal to 5 × 10 ^?9 cm³ s^?1 within a factor of approximately two.     

Rate Coefficient for Self-Association Reaction of CF2 Radicals Determined in the Afterglowof Low-Pressure C4F8 Plasmas

We have analyzed decay kinetics of CF₂ radicals in the afterglow of low-pressure, high-density C₄F₈ plasmas. The decay curve of CF₂ density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10^–4. This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF₂ radicals (CF₂+CF₂C₂F₄). The rate coefficient for this reaction has been evaluated as (2.6–5.3)×10^–14 cm³/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr.