Electrogenerated chemiluminescence induced by sequential hot electron and hole injection into aqueous electrolyte solution

Hole injection into aqueous electrolyte solution is proposed to occur when oxide-coated aluminum electrode is anodically pulse-polarized by a voltage pulse train containing sufficiently high-voltage anodic pulses. The effects of anodic pulses are studied by using an aromatic Tb(III) chelate as a probe known to produce intensive hot electron-induced electrochemiluminescence (HECL) with plain cathodic pulses and preoxidized electrodes. The presently studied system allows injection of hot electrons and holes successively into aqueous electrolyte solutions and can be utilized in detecting electrochemiluminescent labels in fully aqueous solutions, and actually, the system is suggested to be quite close to a pulse radiolysis system providing hydrated electrons and hydroxyl radicals as the primary radicals in aqueous solution without the problems and hazards of ionizing radiation. The analytical power of the present excitation waveforms are that they allow detection of electrochemiluminescent labels at very low detection limits in bioaffinity assays such as in immunoassays or DNA probe assays. The two important properties of the present waveforms are: (i) they provide in situ oxidation of the electrode surface resulting in the desired oxide film thickness and (ii) they can provide one-electron oxidants for the system by hole injection either via F- and F⁺-center band of the oxide or by direct hole injection to valence band of water at highly anodic pulse amplitudes.     

The degradation mechanism of methyl orange under photo-catalysis of TiO2

The properties of photo-generated reactive species, holes and electrons in bulk TiO(2) (anatase) film and nano-sized TiO(2) were studied and their effects towards decomposing pollutant dye methyl orange (MO) were compared by transient absorption spectroscopies. The recombination of holes and electrons in nano-sized TiO(2) was found to be on the microsecond time scale consistent with previous reports in the literature. However, in bulk TiO(2) film, the holes and electrons were found to be on the order of picoseconds due to ultra fast free electrons. The time-correlated single-photon counting (TCSPC) technique combined with confocal fluorescence microscopy revealed that the fluorescence intensity of MO is at first enhanced noticeably by TiO(2) under UV excitation and soon afterwards weakened dramatically, with the lifetime prolonged. Photo-generated holes in nano-sized TiO(2) can directly oxidize MO on the time scale of nanoseconds, while free electrons photo-generated in bulk TiO(2) film can directly inject into MO on the order of picoseconds. Through cyclic voltammetry measurements, it was found that MO can be reduced at -0.28 V and oxidized at 1.4 V (vs. SCE) and this provides thermodynamic evidence for MO to be degraded by electrons and holes in TiO(2). Through comparison of the hole-scavenging effect of MO and water, it was found that in polluted water when MO is above 1.6 × 10(-4) M, the degradation is mainly due to a direct hole oxidation process, while below 1.6 × 10(-4) M, hydroxyl oxidation competes strongly and might exceed the hole oxidation.     

Dynamics of efficient electron-hole separation in TiO2 nanoparticles revealed by femtosecond transient absorption spectroscopy under the weak-excitation condition

The transient absorption of nanocrystalline TiO(2) films in the visible and IR wavelength regions was measured under the weak-excitation condition, where the second-order electron-hole recombination process can be ignored. The intrinsic dynamics of the electron-hole pairs in the femtosecond to picosecond time range was elucidated. Surface-trapped electrons and surface-trapped holes were generated within approximately 200 fs (time resolution). Surface-trapped electrons, which gave an absorption peak at around 800 nm, and bulk electrons, which absorbed in the IR wavelength region, decayed with a 500-ps time constant due to relaxation into deep bulk trapping sites. It is already known that, after this relaxation, electrons and holes survive for microseconds. We interpreted these long lifetimes in terms of the prompt spatial charge separation of electrons in the bulk and holes at the surface.     

Low temperature kinetics and energetics of the electron and hole traps in irradiated TiO2 nanoparticles as revealed by EPR spectroscopy

We have monitored exclusively the dynamics of photogenerated charge carriers trapping in deep traps and trapped electron-hole recombination in UV irradiated anatase TiO2 powders by electron paramagnetic resonance (EPR) spectroscopy at 10 K. The results reveal that the strategy of using low temperatures contributes to the stabilization of the charged pair states for hours by reducing the rate of electron-hole recombination processes. Since only the localized states such as holes trapped at oxygen anions and electrons trapped at coordinatively unsaturated cations are accessible to EPR spectroscopy, the time-dependent population and depopulation of these EPR signals reflect the kinetics and energetics of these trap states. The data support a model of sequential accumulation of deep trap site populations in which the initial fast direct trapping into a deep trap site is followed by slower carrier trap-to-trap hopping until a deep trap is encountered for both photogenerated electrons and holes. Effective modeling of the subsequent decay of trapped-holes is achieved by employing a first-order kinetics, whereas the decay of either surface- or inner-trapped electrons has both a fast and a slow component. The fast component is attributed to a trapped-electron and a free-hole recombination, and the slow component is attributed to trapped electron-hole recombination. The activation energies for the process of diffusion of trapped electrons from their Ti3+ trapping sites are estimated.     

Imaging ultrafast dynamics of molecules with laser-induced electron diffraction

We introduce a laser-induced electron diffraction method (LIED) for imaging ultrafast dynamics of small molecules with femtosecond mid-infrared lasers. When molecules are placed in an intense laser field, both low- and high-energy photoelectrons are generated. According to quantitative rescattering (QRS) theory, high-energy electrons are produced by a rescattering process where electrons born at the early phase of the laser pulse are driven back to rescatter with the parent ion. From the high-energy electron momentum spectra, field-free elastic electron-ion scattering differential cross sections (DCS), or diffraction images, can be extracted. With mid-infrared lasers as the driving pulses, it is further shown that the DCS can be used to extract atomic positions in a molecule with sub-angstrom spatial resolution, in close analogy to the standard electron diffraction method. Since infrared lasers with pulse duration of a few to several tens of femtoseconds are already available, LIED can be used for imaging dynamics of molecules with sub-angstrom spatial and a few-femtosecond temporal resolution. The first experiment with LIED has shown that the bond length of oxygen molecules shortens by 0.1 ? in five femtoseconds after single ionization. The principle behind LIED and its future outlook as a tool for dynamic imaging of molecules are presented.     

Radiation effects, energy storage and its release in solid rare gases

An irradiation of solid argon sample by electrons ionizes the Ar atoms, and part of the beam energy is stored in the solid mainly in the form of self-trapped Ar(2)(+) holes. The pre-irradiated samples are investigated by methods of the so called "activation spectroscopy". During their controlled warm-up three thermally stimulated effects are observed and, in our experiments, simultaneously monitored: a VUV emission resulting from neutralization of the Ar(2)(+) holes by electrons, an anomalous desorption of surface atoms, and an exoelectron emission. A comparison of experiments with linear and step-wise sample heating shows clearly that all three processes are intimately connected. The heating detraps electrons, which neutralize the Ar(2)(+) holes resulting in a bound-free emission of argon dimers, centered around 9.7 eV. The excess energy set free during this process may dislodge surface atoms leading to an anomalous, low temperature, pressure rise. Some of the electrons can also be directly extracted from the sample and detected as an exoelectron current. The experiments provide information about the depth of electron traps, and indicate that there is a nearly continuous distribution of trapping energies.     

Loop currents in chlorophyll a

Loop currents involve the circulation of electrons (or holes) around closed conjugated-bond loops. From the length of the orbital perimeter the kinetic energy per electron can be calculated. In chlorophyll a, four electronic excitations have been analyzed, including: (1) a single positive charge (hole) circling a 19-bond orbit with a Möbius twist (this is proposed as the effective ground state); (2) nine electrons, unevenly spaced, circling the same 19-bond orbit but without the Möbius twist (proposed as the red excited state); (3) nine electrons in the same orbit but evenly spaced and vibrationless (the hopping exciton); (4) ten electrons, unevenly spaced, in a 16-bond orbit (the blue excited state). The fit of theory to experiment is examined.     

Photochemical generation of electrons and holes in germanium-containing ITQ-17 zeolite

Laser flash photolysis of germanium-containing ITQ-17 zeolite (Ge/ITQ-17, a single polymorph of beta zeolite) at 266 nm generates a transient spectrum decaying in the sub-millisecond time scale that is compatible with the formation of two transient species. The shorter lived transient (tau approximately 45 micros under nitrogen) has been assigned to trapped electrons due to the characteristic spectroscopic absorption (single band at 480 nm) and its quenching by typical electron scavengers such as N(2)O and CH(2)Cl(2). The second longer lived transient (lambda(max) = 500, 540, and 600 nm; tau approximately 390 micros) is not quenched by O(2) or electron scavengers, but it is quenched by methanol as hole scavenger and has been assigned to positive holes. Also there is a remarkable similarity of the transient spectrum of the Ge/ITQ-17 with the optical spectrum reported previously for electron-hole pairs in ZSM-5 zeolite. Under the same irradiation conditions, photoejection of electrons and photogeneration of positive holes has not been observed for conventional aluminosilicate zeolites, all-silica zeolites, or GeO(2)-impregnated zeolites. Therefore this photochemical behavior has been ascribed to the presence of framework germanium atoms opening the way for photoresponsive zeolites. The ability of Ge/ITQ-17 to generate photochemically electrons and holes has been confirmed by adsorbing naphthalene and propyl viologen sulfonate as electron donor and acceptor, respectively, and observing the generation of the corresponding radical ions.     

Asymmetric deformation density analysis in carbon nanotubes

Electrons and holes as charge carriers appear when a molecular system is exposed to external electric field. For nonsymmetric molecules, the distribution of charge carriers is also nonsymmetric. Although symmetric distribution of charge carriers is expected when the molecular system is symmetric, as the present work shows, they are not symmetric unexpectedly; that is, electrons and holes are not each other's mirror images. In this respect, asymmetric deformation density analysis is introduced to measure the extent of asymmetric distribution of electrons and holes as charge carriers when the symmetric system is exposed to symmetric external potential. Segments of (5,0) carbon nanotubes with different lengths are selected as symmetric systems, and a linear electric field is applied along the principal axis as symmetric potential. Results show that, at high electric fields, electrons tend to localize at the ends, while holes tend to occupy the middle area of carbon nanotube segments. While charge carriers play a vital role in molecular conductivity, asymmetric distribution of electrons and holes in symmetric systems has not yet been reported.     

Control of a surface photochemical process by fractal electron transport across the surface: O(2) photodesorption from TiO(2)(110)

The photodesorption of O(2) from TiO(2)(110) has been found to exhibit fractal kinetic behavior. The rate coefficient for photodesorption is measured throughout the entire experiment and is shown to decrease by a factor of approximately 100 over a time period of approximately 250 s. A model is proposed in which the electrons associated with O-vacancy defects on the surface percolate from vacancy site to vacancy site via the filled orbitals at these sites to neutralize photoproduced holes. This electron percolation, causing electron-hole recombination, reduces the efficiency of charge transfer between a photoproduced hole and an O(2)(-)(a) species localized at a vacancy defect site, causing the rate of O(2) photodesorption to follow a fractal rate law. We postulate that the fractal electron conduction path across the surface is one-dimensional.     

Quasi-elastic light scattering studies on T7 DNA in the presence of a sinusoidal electric field

The mobilities and lifetimes of electrons and holes as well as the triplet exciton lifetime have been measured in a range of samples originating from a long anthracene crystal grown from the vapour phase in the zone refining tube. The results have demonstrated the influence of chemical and physical imperfections on these parameters. The triplet lifetime has been found to be fairly insensitive to impurities, as opposed to the lifetime of electrons and holes. The measurements of the charge-carrier mobility as a function of temperature have revealed that hole transport in a defective anthracene crystal is controlled by structural traps 0.26–0.27 eV deep, whereas the electron mobility. contrary to theoretical predictions. is lattice controlled over the investigated temperature range. These localized states can be photochemically eliminated. which suggests that paired anthracene molecules (“incipient” dimers) are responsible for the observed trapping asymmetry. These observations, by analogy with dimeric species forming excimers while excited, have been interpreted in terms of new chemical species formed as a result of trapping at “incipient” dimer pairs.     

Intraband relaxation in CdSe nanocrystals and the strong influence of the surface ligands

The intraband relaxation between the 1Pe and 1Se state of CdSe colloidal quantum dots is studied by pump-probe time-resolved spectroscopy. Infrared pump-probe measurements with approximately 6-ps pulses show identical relaxation whether the electron has been placed in the 1Se state by above band-gap photoexcitation or by electrochemical charging. This indicates that the intraband relaxation of the electrons is not affected by the photogenerated holes which have been trapped. However, the surface ligands are found to strongly affect the rate of relaxation in colloid solutions. Faster relaxation (<8 ps) is obtained with phosphonic acid and oleic acid ligands. Alkylamines lead to longer relaxation times of approximately 10 ps and the slowest relaxation is observed for dodecanethiol ligands with relaxation times approximately 30 ps. It is concluded that, in the absence of holes or when the holes are trapped, the intraband relaxation is dominated by the surface and faster relaxation correlates with larger interfacial polarity. Energy transfer to the ligand vibrations may be sufficiently effective to account for the intraband relaxation rate.     

Spatial symmetry holes in many-electron atoms and molecules

When a many-electron system has a spatial symmetry, it is shown that there exist spatial symmetry holes, which imply that two or more electrons are prohibited from being at certain spatial positions simultaneously. Inversion holes, rotation holes, and reflection holes, which result from inversion, twofold rotation, and reflection symmetries, respectively, are discussed in detail. The electron-electron counterbalance hole reported in literature is a particular case of the inversion hole. The spatial symmetry holes are illustrated for simple atoms and molecules.     

Electrical properties of NbO in high magnetic fields

The resistivity and Hall coefficients of polycrystalline samples, single crystals, and doped specimens of NbO_x (0.98 ? x ? 1.02) were measured in magnetic fields up to 160 kG at 4.2, 78, and 295 K. The material is found to be an excellent conductor. The results are interpreted in terms of a nearly free electron model in which holes and electrons contribute jointly to conduction processes. Orbital switching, magnetic breakdown effects, and perturbations arising from impurities are invoked to explain the observed magneto-resistance anomalies.     

Electrical properties of single-crystalline CaAl2Si2

Electrical resistivity and Hall coefficient measurements of single-crystalline CaAl(2)Si(2) revealed that CaAl(2)Si(2) is a metal in which both electrons and holes contribute to the transport properties; its dominant carriers are holes at temperature below 150 K but electrons above that temperature.     

Rate constant of the recombination of free electrons and holes in AgCl at 295 K

The transient kinetics of the loss of electrons generated by light pulses in powdered AgCl has been studied by the microwave photoconductivity method (36 GHz) at 295 K. At high light intensities,I ₀ > 10¹⁴ photon cm^–2 per pulse, the kinetics obeys the second-order law. The rate constant of the recombination of free electrons and holes is equal to 2·10^–12 cm³ s^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2234–2236, September, 1996.     

聚合物单原子光催化剂的载流子分离和表面反应机制

近10年来, 研究者制备了大量的单原子催化剂(SACs), 其在光、 电、 热等催化体系中展现出优异的催化性能及较高的实用性和经济性. 光催化过程的独特性使其在催化本质上明显不同于热催化和电催化过程, 即处于激发态的电子和空穴参与反应, 而非基态的价电子. 本文首先探讨了有机聚合半导体与传统无机金属化合物半导体的区别, 指出聚合物半导体介电常数通常较小且光生电子与空穴的中心距离过短(计算上通常 ^{<1 nm), 导致其界面处几乎不存在明显的能带弯曲. 将金属离子引入聚合物半导体的骨架中可以有效引入给体-受体对, 在提高载流子分离效率的同时延长其寿命. 在高效聚合物基单原子光催化剂的设计过程中, 引入单原子金属位点后的激发态电荷分布及捕获态电子对反应的驱动力是决定催化剂整体性能的关键因素. 时间-空间双因子布局分析法和瞬态吸收光谱可为研究者提供相关信息. 随着人工智能的进一步发展, 建立回归精度接近或达到密度泛函理论水准的能量函数, 从而反推激发态下体系的能量变化, 有望为光催化反应的激发特性与反应活性建立可靠的联系. 此外, 配体和溶剂化效应在今后的研究中也应被仔细考虑.}     

Solid-state electroluminescent devices based on transition metal complexes

Transition metal complexes have emerged as promising candidates for applications in solid-state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. As a result, an efficiency of 10 lm/W--among the highest efficiencies reported in a single layer electroluminescent device--was recently demonstrated. In this article we review the history of electroluminescence in transition metal complexes and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.     

Laser-induced electron impact ionization in a reflectron time-of-flight mass spectrometer

Some details of the generation of electrons by impinging a laser beam on a metal surface are described. It is shown that highly efficient electron generation is observed only during the laser pulse. Therefore, this technique delivers intense pulses of electrons. The process is investigated and different ion source set-ups are discussed. In conjunction with a time-of-flight mass spectrometer this technique can be used to produce mass spectra of different samples ranging from simple organic molecules to peptides.