Computer simulation of decay of trapped electrons in glassy matrices by tunnelling to scavenger molecules

The kinetics of decay of trapped electrons via tunnelling to acceptor molecules in a glassy matrix was computed by the method of stochastic simulation. The kinetic curves of electron decay were obtained for different concentrations of electron acceptor in the matrix. The results of the simulation are compared with the corresponding results calculated on the basis of certain kinetic models of the electron tunnelling in low-temperature glasses.     

Decay of trapped electrons by tunnelling to scavenger molecules in low-temperature glasses

The decay of trapped electrons by tunnelling to scavenger molecules is investigated theoretically. An exact expression has been derived which gives an exponential decrease of the trapped electron yield with the scavenger concentration at any given time.     

Electron tunnelling in alkaline glasses studied by luminescence emitted after γ-irradiation at 4 and 77°K

ITL kinetics of pure and phenol-containing alkaline glasses obey a reciprocal time law which can be derived by differentiation from the decay kinetics of trapped electrons. It is compatible with a tunnelling mechanism.     

Dynamics of surface-localised electronic excitations studied with the scanning tunnelling microscope

The decay rates of electron and hole excitations at metal surfaces as determined by a scanning tunnelling microscope are presented and discussed. Surface-localised electron states as diverse as Shockley-type surface states and quantum well states confined to ultrathin alkali metal adsorption layers are covered. Recent developments in the analysis of the experimental procedures that are used to determine decay rates with the scanning tunnelling microscope, namely the analysis of line shapes and the spatial decay of standing wave patterns, are discussed.     

A note on H-atom transfer and alkyl radical migration in polyethylene crystallites: MNDO saddle-point energies in a model n-heptane crystal

MNDO estimates are given for activation energies of intra-chain and inter-chain radical migrations in a model alkane crystal. All the estimates are higher than expected, due to the constraints of the model and the computational method used to acquire them. By extrapolation from published kinetic studies and calculations on low temperature H-atom tunnelling, it is reasoned that only inter-chain H-atom migration is energetically favourable. This is in keeping with experimental data presented by Kashiwabara, concerning the temperature dependent decay of radicals trapped in a urea-polyethylene complex.     

Radiation Chemistry of Organic Molecules in Solid Rare Gas Matrices: 2. Selective Deprotonation of the Primary Radical Cations upon Irradiation of Oxygen-Containing Molecules in Xenon Matrices

The composition of radical products trapped after irradiation of various ethers (dimethyl ether, tetrahydrofuran, methylal, 1,3-dioxolane) or acetaldehyde in xenon matrices at 15–17 K in the presence of electron scavengers was studied by an ESR technique. It was shown that the primary radical cations give corresponding deprotonation products (carbon-centered radicals), rather than stabilize in xenon, under the given experimental conditions. The deprotonation process is characterized by extremely high selectivity; i.e., the only type of radicals resulting from deprotonation at maximum spin density position was observed in each of the cases. The possible mechanism of the reactions and the nature of their selectivity are discussed.     

Spectra of trapped electrons in irradiated rigid media

In the irradiated polar systems, the blue shifts of the absorption spectra of trapped electrons (e _t ⁻ ) have as yet been observed by pulse radiolysis in the milisecond time range. This fact has been ascribed to the reorientation of the matrix polar molecules under the influence of the coulombic electron field. In order to check whether the spectral changes of e _t ⁻ are limited to very short times only, the spectra of e _t ⁻ in frozen 8M NaOH, 95% ethylene glycol (EG), 95% methanol, 95% ethanol, 95% n-propanol, 95% isopropanol, 95% butanol (+5% H₂O) were examined at the liquid nitrogen temperature by applying the stationary γ-radiolysis. Except for 8M NaOH and 95% EG, in the remaining matrices the shifts of e _t ⁻ spectra towards the short wavelengths were observed within 24 hrs after irradiation. The less polar the matrix, the larger was the spectral shift. The observed effect seems to indicate that the reorientation process takes place in times much longer than these measured by pulse radiolysis. After the addition of scavengers the lowering of the initial absorption occured in all investigated systems. This was ascribed to the reaction of “dry” electrons with the scavengers. Moreover, the results obtained by the pulse radiolysis of ethanol indicate that the presence of scavengers accelerates the decay of the infrared part of the trapped electron spectrum. The “infrared electrons” (located in shallow traps) neither react with the scavenger nor deepen their traps by reorientation. Most probably in the presence of scavengers these electrons recombine with positive ions via tunneling.     

Kinetics of geminate-ion recombination by electron tunnelling

A phenomenological model is developed for geminate-ion recombination by electron tunnelling. Experimentally observed decay kinetics of recombination luminescence is well described by this model.     

硫增感AgBrI T颗粒乳剂光电子行为研究

利用微波相敏技术,获得了硫增感AgBrIT颗粒乳剂中自由光电子和束缚光电子时间衰减信号,分析了光电子衰减时间、电子陷阱效应、光电子寿命、有效陷阱深度及束缚电子转移时间与增感时间的关系,获得了最佳的增感时间、衰减时间、电子陷阱效应、光电子寿命、有效陷阱深度和转移时间数值.     

Further evidence of anion formation in rigid matrix photoionization and bleaching kinetics implications

Thermoluminescence studies on tetramethylparaphenylenediamine photoionized in a methylcyclohexane glass at 77°K have confirmed the presence of anions as well as trapped electrons. The analysis of bleaching kinetics has shown that trapped electron decay is associated with a sum of first order processes while anion decay is better described by a second order law.     

Trapping and reactivity of electrons in methanol-isopropanol glasses irradiated at 77 K

Mixtures of methanol and isopropanol in a ratio (v/v) varying from 91 to 19 have been gamma-irradiated at 77 K and relative yields of trapped electrons have been measured. The reactivity of the trapped electrons towards acetone and uranyl nitrate has been studied at various methanol isopropanol ratios. On the basis of the results, an attempt has been made to assess the relative importance of the reaction of the electron with the solute prior to trapping with respect to the tunnelling reaction of the trapped electron with the solute.     

A way for evaluating parameters of electron transport in non-polar molecular liquids derived from analysis of the trapped electron recombination kinetics

The geminate recombination kinetics of electron-ion pairs produced by high energy radiation in liquid hydrocarbons is considered in the two state model of electron transport. The purpose of the study is to relate the trapped electron transient optical absorption, observed in the pulse radiolysis experiments, to fundamental parameters of electron transport in liquid. It is shown that measurements of the half-life time and amplitude of the trapped electron decay curve allow one to find the electron life time in a localized state.     

Hydride-exchange reactions between NADH and NAD+ model compounds under non-steady-state conditions. Apparent and real kinetic isotope effects

The kinetics of the hydride exchange reaction between NADH model compound 10-methyl-9,10-dihydroacridine (MAH) and 1-benzyl-3-cyanoquinolinium (BQCN+) ion in acetonitrile were studied at temperatures ranging from 291 to 325 K. The extent of reaction-time profiles during the first half-lives are compared with theoretical data for the simple single-step mechanism and a 2-step mechanism involving initial donor/acceptor complex formation followed by unimolecular hydride transfer. The profiles for the reactions of MAH deviate significantly from those expected for the simple single-step mechanism with the deviation increasing with increasing temperature. The deviation from simple mechanism behavior is much less pronounced for the reactions of 10-methyl-9,10-dihydroacridine-10,10-d2 (MAD) which gives rise to extent of reaction dependent apparent kinetic isotope effects (KIEapp). Excellent fits of the experimental extent of reaction-time profiles with theoretical data for the 2-step mechanism, in the pre-steady-state time period, were observed in all cases. Resolution of the kinetics of the hydride exchange reaction into the microscopic rate constants over the entire temperature range resulted in real kinetic isotope effects for the hydride transfer step ranging from 40 (291 K) to 8.2 (325 K). That the reaction involves significant hydride tunnelling was verified by the magnitudes of the Arrhenius parameters; Ea D - EaH = 8.7 kcal mol-1 and AD/AH = 8 x 10(4). An electron donor acceptor complex (lambda max = 526 nm) was observed to be a reaction intermediate. Theoretical extent of reaction-time profile data are discussed for the case where a reaction intermediate is formed in a non-productive side equilibrium as compared to the case where it is a real intermediate on the reaction coordinate between reactants and products. The common assumption that the two cases are kinetically indistinguishable is shown to be incorrect.     

Configurations of the free radicals in irradiated polypropylene and the relation of their decay reactions to the molecular motion

Free radicals produced in irradiated polypropylene were studied by the electron spin resonance method. Two temperature regions in which the free radicals decay rapidly were found at around 170°K. and 260°K. The first temperature region corresponds to the γ-dispersion of polypropylene and the second to the β-dispersion. Steric configurations of the free radicals were investigated, and it was concluded that the free radicals trapped in polymer, conformation of which is appreciably twisted from the stable 3₁-helical structure, decay with small-scale motion of the matrix polymer. The decay of free radicals trapped in polymer of less twisted conformation is associated with the large-scale motion of the matrix polymer. Activation energies of decay were found to be 11 kcal./mole at the lower temperature and 48 kcal./mole at the higher temperature. Time constants of the decay reactions were compared with those for molecular motion of the matrix, with results reflecting the relations of the decay of the polymer radicals to molecular motion in the matrix.     

Metastable decay of rare gas cluster ions — the (Ar3)+ system

The decay of (Ar ₃ ⁺ * → Ar ₂ ⁺ + Ar is studied from a dynamical viewpoint by employing an effective, spherical interaction and constructing the complete set of bound and metastable wavefunctions for the full range of rotational states of the triatomic complex. In agreement with the experimental findings, the present calculations show that the effect of populating the high rotational states of the overall cluster gives rise to a wealth of new resonances (predissociative tunnelling) and can cover a very broad range of metastable dissociation rates, with lifetimes ranging from 10^?11 s to 10 s. Such a dynamical mechanism could therefore provide a simple explanation for the observed behaviour of rare gas cluster ions and could indicate further ways of improving the modelling of such processes.     

ESR信号的衰减与超增感机理的研究

众所周知,超增感机理在感光化学中起着极其重要的作用,因此成为广大感光科学工作者研究的一大热点,先后提出了各自的设想。     

Riboflavin-sensitized photochemical radical ion chain reaction of sulfo-group substitution for halogen in halogenated hydroxynaphthalenes

It was shown that riboflavin (RF) is a sensitizer for the radical ion chain reaction of sulfo group substitution for halogen in halogenated hydroxynaphthalenes. The initaition mechanism involves the electron transfer reaction between the sulfite ion and excited riboflavin. The quantum yields of RF radical anions from the singlet and the triplet states upon excitation in an aqueous sodium sulfite solution are 0.01 and 0.15, respectively, as determined by means of flash photolysis. The principal decay reaction for RF radical anions is their recombination with sulfite radical anions in the bulk of solution at a rate constant of (3.8 ± 0.5) × 10⁹ dm³ mol^?1 s^?1. The quantum yields of the riboflavin-sensitized substitution reaction increases in the presence of electron scavengers (chloranil, dinitrobenzene) and inorganic salts in the system.     

TRAPPING OF NITRIC OXIDE FORMED DURING PHOTOLYSIS OF SODIUM NITROPRUSSIDE IN AQUEOUS AND LIPID PHASES: AN ELECTRON SPIN RESONANCE STUDY

Abstract— Photolytic decomposition of sodium nitroprusside (SNP), a widely used nitrovasodilator, produced nitric oxide (NO), which was continuously monitored by electron spin resonance (ESR) spectroscopy. The NO present in the aqueous or the lipid phase was trapped by either a hydrophilic or a hydrophobic nitronyl nitroxide, respectively, to form the corresponding imino nitroxide. The conversion of nitronyl nitroxide to imino nitroxide was monitored by ESR spectrometry. The quantum yield for the generation of NO from SNP, measured from the rate of decay of nitronyl nitroxide, was 0.201 ± 0.007 and 0.324 ± 0.01 (¯± SD, n = 3) at 420 nm and 320 nm, respectively. The action spectrum for NO generation was found to overlap the optical absorption spectrum of SNP closely. A mechanism for the reaction between SNP and nitronyl nitroxide in the presence of light is proposed and computer-aided simulation of this mechanism using published rate constants agreed well with experimental data. The methodology described here may be used to assay NO production continuously during photoactivation of NO donors in aqueous and lipid environments. Biological implications of this methodology are discussed.     

Development of a novel and high performance visible-light-induced Cd3OSO4 nanophotocatalyst for degradation of diazinon

One of the most important challenge in recent years is the removal and even better than that the degradation of poisons from environmental systems. Nanostructures due to their interesting properties are very suitable for this purpose. Herein, Cd₃OSO₄ nanomaterial was synthesized with a simple co precipitation method, characterized and was used, as a visible-light-driven photocatalyst in degradation of diazinon. The experimental conditions were also optimized. The results showed degradation efficiency is about 90%. Moreover, some kinetic studies were performed and showed this system obeys first order kinetics. Photo- oxidation mechanism was investigated in the presence of hole and electron scavengers. This is the first report on the investigation of the photocatalytic properties of Cd₃OSO₄ nanomaterial. The calculations of band gap of this new photocatalyst (1.85ev) showed its capability for application as a new visible light driven photocatlyst.