Excited Relaxation Processes Research Articles

Triplet-triplet annihilation-based photon upconversion using nanoparticles and nanoclusters.

The phenomenon of photon upconversion (UC), generating high-energy photons from low-energy photons, has attracted significant attention. In particular, triplet-triplet annihilation-based UC (TTA-UC) has been achieved by combining the excitation states of two types of molecules, called the sensitizer and emitter (or annihilator). With TTA-UC, it is possible to convert weak, incoherent near-infrared (NIR) light, which constitutes half of the solar radiation intensity, into ultraviolet and visible light that are suitable for the operation of light-responsive functional materials or devices such as solar cells and photocatalysts. Research on TTA-UC is being conducted worldwide, often employing materials with high intersystem crossing rates, such as metal porphyrins, as sensitizers. This review summarizes recent research and trends in triplet energy transfer and TTA-UC for semiconductor nanoparticles or nanocrystals with diameters in the nanometer range, also known as quantum dots, and for ligand-protected metal nanoclusters, which have even smaller well-defined sub-nanostructures. Concerning nanoparticles, transmitter ligands have been applied on the surface of the nanoparticles to efficiently transfer triplet excitons formed inside the nanoparticles to emitters. Applications are expanding to solid-state UC devices that convert NIR light to visible light. Additionally, there is active research in the development of sensitizers using more cost-effective and environmentally friendly elements. Regarding metal nanoclusters, methods have been established for the evaluation of excited states, deepening the understanding of luminescent properties and excited relaxation processes.

Bismuth-doped gadolinium oxide films for UV-Vis multicolor conversion: Combined XPS, DFT and photoluminescence study

Results of XPS characterization, theoretical modeling and photoluminescence study of bismuth-implanted gadolinium oxide (Gd2O3:Bi) films are presented. Scenarios for the introduction of bismuth ions into amorphous-crystalline Gd2O3 host and mechanisms for the formation of optically active centers are considered. It has been demonstrated that the optical properties of Gd2O3:Bi films are guided by the interplay of a quite complex system of single Bi2+ ions, (Bi3+–Bi3+) pairs and single Bi3+-ions, oxygen vacancies and "defective" gadolinium ions. Photoinduced excitation-relaxation processes involving various Bi-related centers and defect states of the host are observed. Due to the multicolor emission (red, green and blue) of single Bi2+ ions, (Bi3+- Bi3+) pairs and single Bi3+ ions excited in the ultraviolet spectral range, the UV–VIS energy conversion in a wide spectral range is realized. The advantage of Gd2O3:Bi as a material for energy conversion lies in the fact that due to the polyvalence of bismuth and the variety of different optical centers, a wide range of both absorbed UV light and emitted VIS light is involved in the conversion. This indicates the prospects for practical use of Gd2O3:Bi films as multifunctional material for energy conversion and photo-detection systems.

Preparation of nanoscale [(Ba0.85Ca0.15)0.995Nd0.005](Ti0.9Hf0.1)O3 ceramics via hydrothermal method and effect of grain size on multifunctional performance

Nanoscale [(Ba0.85Ca0.15)0.995Nd0.005](Ti0.9Hf0.1)O3 (Nd-BCTH) ceramics were prepared via the hydrothermal method, and the effects of sintering temperature and holding time on the phase structure, micromorphology, electrical and optical properties of the Nd-BCTH ceramics were explored. All the Nd-BCTH ceramics present rather pure perovskite structure with composition approaching rhombohedral phase around the morphotropic phase boundary (MPB) region. The highest relative density is obtained for the sample sintered at 1220 °C for 10 h. The existence of Ba2+, Ca2+, Ti4+, Hf4+ and Nd3+ is confirmed and the elements distribute rather uniformly detected by X-ray photoelectron spectrometer (XPS) and energy dispersive X-ray (EDX) analysis. The ceramics present nanoscale grain size, which tends to increase with the increase of sintering temperature and holding time, and significantly affects dielectric constant and Curie temperature. Very thin and narrow ferroelectric hysteresis loops are observed, where a considerable energy storage density (173.88 mJ/cm3) and high energy storage efficiency (80.68%) are obtained at low electric field. The increase of sintering temperature and holding time induces a red shift at 400 nm absorption edge and a blue shift at 300 nm absorption edge in the Nd-BCTH ceramics, and all ceramics have a maximum absorption value at around 260 nm. Under the excitation of 269 nm light, the Nd-BCTH ceramics show the strongest fluorescence peak at 473 nm, corresponding to the 4G3/2→4I9/2 transition, emitting indigo blue fluorescence. When the ambient temperature is above 400 °C, grains conduction dominates the conductive mechanism in the nano-sized Nd-BCTH ceramics. Such conduction can be attributed to oxygen vacancies caused due to the evaporation of alkaline-earth metals during high temperature sintering, and show typically thermally excited relaxation process.

Vibration-mediated energy transport in bacterial reaction center: Simulation study.

Exciton energy relaxation in a bacterial Reaction Center (bRC) pigment-protein aggregate presumably involves emission of high energy vibrational quanta to cover wide energy gaps between excitons. Here, we assess this hypothesis utilizing vibronic two-particle theory in modeling of the excitation relaxation process in bRC. Specific high frequency molecular vibrational modes are included explicitly one at a time in order to check which high frequency vibrations are involved in the excitation relaxation process. The low frequency bath modes are treated perturbatively within Redfield relaxation theory. The analysis of the population relaxation rate data indicates energy flow pathways in bRC and suggests that specific vibrations may be responsible for the excitation relaxation process.

Excited-State Dynamics of 2,2′-Bipyridine Moieties Embedded in the Framework of Periodic Mesoporous Organosilica

We investigate the excited-state dynamics of 2,2′-bipyridine (BPy) moieties in a framework of BPy-bridged periodic mesoporous organosilica (BPy-PMO), which is a unique solid chelating ligand that has densely and regularly arranged BPy units that are exposed on the pore surfaces. The steady-state absorption spectrum of BPy-PMO shows broad absorption bands and a prolonged absorption tail compared to its precursor molecule, which suggests that a part of the BPy moieties form dimers and/or aggregates in the ground state. Transient absorption spectra of BPy-PMO after excitation with 295 nm light show mainly intersystem crossing from the lowest singlet excited state (S1) to the lowest triplet excited state (T1) of BPy moieties with a lifetime of 52 ps. Energy transfer (7.2 ps) from the S1 of a BPy monomer to BPy dimers and/or aggregates is also suggested. The lifetime of the excited BPy dimers and/or aggregates is estimated by selective excitation (355 nm) to be 3.4 ns. The lifetime for the T1 of the BPy monomer is estimated to be 700 ns from transient absorption spectra of BPy-PMO after nanosecond 266 nm pulse irradiation. The characteristics of the excitation-relaxation processes of BPy-PMO are discussed.

Application of the Locally Nonequilibrium Diffusion-Drift Cattaneo–Vernotte Model to the Calculation of Photocurrent Relaxation in Diode Structures under Subpicosecond Pulses of Ionizing Radiation

The excitation-relaxation process in electron–hole plasma upon exposure to ionizing radiation for a time shorter than the relaxation time of the mobile carrier energy and momentum is considered. By the example of the calculation of transient ionization processes in a silicon hyperhigh-frequency Schottky diode, local-equilibrium and local-nonequilibrium carrier transport models are compared. It is shown that the local-nonequilibrium model features a wider field of application for describing fast relaxation processes.

Polarized Fluorescence in Tryptophan Excited by Two-Photon Femtosecond Laser Pulses

The decay of polarized fluorescence of tryptophan in water-propylene glycol solution two-photon excited by femtosecond laser pulses was explored. Linearly and circularly polarized light in the wavelength band of 385–510 nm was used for excitation. Processing of the experimental fluorescence signals as functions of time and light polarization provided an opportunity to identify two characteristic lifetimes characterizing the fluorescence intensity decay and the rotational diffusion time τrot. The spectral dependences of the anisotropy parameter Ω and anisotropy parameters r for linearly and circularly polarized excitation have been determined. It is shown that the fluorescence temporal dependence is well described by the sum of two exponents with amplitudes a1 and a2 and characteristic times τf1 and τf2 at all wavelengths in the studied spectral range. The experimental dependences of parameters Ω, r, τf1, and a2/a1 are nontrivial, which indicates the complexity of the excitation relaxation processes. The obtained results can be used for interpretation of the experiments and prove of the theoretical models describing the intensity and polarization of fluorescence of tryptophan molecules in solutions and organic compounds under optical excitation.

Radiation-thermal decomposition of lignin: Products and the mechanism of their formation (Review)

The mechanism of the radiation-thermal conversion of lignin including the formation of aromatic radical cations and their fragmentation resulting in the appearance of phenoxyl radicals is considered. The multipath formation of phenoxyl radicals occurs with the participation of the reactions of molecules with electrons and small radicals (•Н and •СН3) and electronic excitation relaxation processes. Phenoxyl radicals are characterized by smaller thermal stability in comparison with that of parent macromolecules. The further thermally stimulated decomposition of these radicals results in the release of monohydric and dihydric phenols from a polymeric chain. The most effective liberation of phenols takes place on the surface of lignin particles, whereas the formation of wood charcoal with the participation of unsaturated products dominates in the bulk. The formation of dihydric phenols is intensified in the presence of alkanes in the irradiated sample; this fact is indicative of an important role of •Н and •СН3 radicals in the formation of monomeric phenol products.

Применение локально-неравновесной диффузионно-дрейфовой модели Каттанео-Вернотта для описания релаксации фототока в диодных структурах при воздействии субпикосекундных импульсов ионизирующих излучений

AbstractThe excitation-relaxation process in electron–hole plasma upon exposure to ionizing radiation for a time shorter than the relaxation time of the mobile carrier energy and momentum is considered. By the example of the calculation of transient ionization processes in a silicon hyperhigh-frequency Schottky diode, local-equilibrium and local-nonequilibrium carrier transport models are compared. It is shown that the local-nonequilibrium model features a wider field of application for describing fast relaxation processes.

Excitonic and Vibrational Coherence in the Excitation Relaxation Process of Two LH1 Complexes as Revealed by Two-Dimensional Electronic Spectroscopy.

Ultrafast excitation relaxation within a manifold exciton state and long-lived vibrational coherence are two universal characteristics of photosynthetic antenna complexes. In this work, we studied the two-dimensional electronic spectra of two core light-harvesting (LH1) complexes of Thermochromatium (Tch.) tepidum, native Ca2+-LH1 and modified Ba2+-LH1. The role of the vibrational coherence in the exciton relaxation was revealed by comparing the two LH1 with similar structures but different electronic properties and by the evolution of the exciton and vibrational coherence as a function of temperature.

Energy transfer in the chlorophyll f-containing cyanobacterium, Halomicronema hongdechloris, analyzed by time-resolved fluorescence spectroscopies.

We prepared thylakoid membranes from Halomicronema hongdechloris cells grown under white fluorescent light or light from far-red (740nm) light-emitting diodes, and observed their energy-transfer processes shortly after light excitation. Excitation-relaxation processes were examined by steady-state and time-resolved fluorescence spectroscopies. Two time-resolved fluorescence techniques were used: time-correlated single photon counting and fluorescence up-conversion methods. The thylakoids from the cells grown under white light contained chlorophyll (Chl) a of different energies, but were devoid of Chl f. At room temperature, the excitation energy was equilibrated among the Chl a pools with a time constant of 6.6ps. Conversely, the thylakoids from the cells grown under far-red light possessed both Chl a and Chl f. Two energy-transfer pathways from Chl a to Chl f were identified with time constants of 1.3 and 5.0ps, and the excitation energy was equilibrated between the Chl a and Chl f pools at room temperature. We also examined the energy-transfer pathways from phycobilisome to the two photosystems under white-light cultivation.

Quantum Process Tomography Quantifies Coherence Transfer Dynamics in Vibrational Exciton

Quantum coherence has been a subject of great interest in many scientific disciplines. However, detailed characterization of the quantum coherence in molecular systems, especially its transfer and relaxation mechanisms, still remains a major challenge. The difficulties arise in part because the spectroscopic signatures of the coherence transfer are typically overwhelmed by other excitation-relaxation processes. We use quantum process tomography (QPT) via two-dimensional infrared spectroscopy to quantify the rate of the elusive coherence transfer between two vibrational exciton states. QPT retrieves the dynamics of the dissipative quantum system directly from the experimental observables. It thus serves as an experimental alternative to theoretical models of the system-bath interaction and can be used to validate these theories. Our results for coupled carbonyl groups of a diketone molecule in chloroform, used as a benchmark system, reveal the nonsecular nature of the interaction between the exciton and the Markovian bath and open the door for the systematic studies of the dissipative quantum systems dynamics in detail.

Activationless percolating transport of charge carriers in TbMnO3 films at low temperature with low electric field

Au/TbMnO3/YBa2Cu3O7–x capacitors were fabricated on SrTiO3 substrates by pulse laser deposition technique, of which electric properties were investigated in the temperature range from 25 to 300 K. Both current-voltage characteristics and junction resistances with bias voltages showed remarkable temperature dependence, in which obvious thermally excited relaxation processes were found between 150 and 200 K. At the temperatures lower than the activation process, the leakage currents of the capacitors were studied. Interestingly, at high electric field, the mechanism of the leakage was Poole-Frenkel emission. However, at low electric field, the conduction was not Ohmic, and ideal lnJ∝E1/4 characteristics were observed. Analysis showed that the possible origin was related to the inherent inhomogeneous nature of activationless percolating transport.

Femtosecond laser interactions with dielectric materials: insights of a detailed modeling of electronic excitation and relaxation processes

Electronic excitation–relaxation processes induced by ultra-short laser pulses are studied numerically for dielectric targets. A detailed kinetic approach is used in the calculations accounting for the absence of equilibrium in the electronic subsystem. Such processes as electron–photon–phonon, electron–phonon and electron–electron scatterings are considered in the model. In addition, both laser field ionization ranging from multi-photon to tunneling one, and electron impact (avalanche) ionization processes are included in the model. The calculation results provide electron energy distribution. Based on the time-evolution of the energy distribution function, we estimate the electron thermalization time as a function of laser parameters. The effect of the density of conduction band electrons on this time is examined. By using the average electron energy, a new criterion is proposed based on determined damage threshold in agreement with recent experiments (Sanner et al. in Appl. Phys. Lett. 96:071111, 2010).

Slow calcium waves in cultivated postnatal rat skeletal myocytes

A variety of active agents increasing [Ca2+]i in cultivated skeletal myocytes have been investigated. It was shown that, out of the agents such as noradrenaline, carbacholine, caffeine, cyclopiazonic acid, and potassium solution, only the last caused the formation of slow calcium waves in skeletal myocytes. These waves propagated not only near the location of the cell nucleus but also along the whole length of myocytes. It is supposed that this wave process can be related to the modulation of excitation-relaxation processes in skeletal muscles.

Dynamics in the excited electronic state of periodic mesoporous biphenylylene-silica studied by time-resolved diffuse reflectance and fluorescence spectroscopy

The excited state dynamics of periodic mesoporous organosilica (powder) bearing biphenylylene moieties densely in the silica framework (Bp-PMO) is investigated for the first time using femtosecond time-resolved diffuse reflectance (TDR) and picosecond time-resolved fluorescence spectroscopies. The TDR spectra revealed the excitation-relaxation process of the biphenylylene moieties, including the relaxation of the twisted Frank-Condon (FC) state to the lowest singlet excited state (S(1)) with a time constant of 730 ± 95 fs, and efficient quenching of the S(1) state by excimer (E) formations with two time constants of 7.0 ± 0.2 ps (E(1): ca. 64%) and 170 ± 47 ps (E(2) and E(3): ca. 36%). The individual absorption spectra of the FC, S(1), and E states were reconstructed by the TDR spectral analysis. The time-resolved fluorescence spectra showed that the excimers decayed with three time constants of 1.3 ± 0.2 ns (E(1)), 8.2 ± 0.7 ns (E(2)) and 27 ± 2 ns (E(3)). The fluorescence quantum yields of the excimers are suggested to be almost zero for E(1), and unity for E(2) and E(3), which implies that the fluorescence quantum yield of Bp-PMO (Φ(F) = 0.38) can be explained by the fraction of the highly emissive excimers (E(2) and E(3)). The excited state dynamics of Bp-PMO is quite different from those of a solution of 4,4'-bis-(triethoxysilyl)biphenyl precursor and a biphenyl molecular crystal (powder).

Derivation and Application of an Empirical Formula to Describe Interfacial Relaxation Effects in Inhomogeneous Materials

We report an empirically derived formula to describe thermally and electrically excited relaxation processes arising in materials with electrical inhomogeneities. This formula was deduced from the temperature and electric field‐dependent conductivity of calcium copper titanate ceramics and can be used to explain the Schottky barrier behavior of the grain boundary regions that control conductivities. It relates conductivity, activation energy for conduction, breakdown strength, and grain size effects, and is believed to have the potential to explain the high permittivity and breakdown mechanism, as well as the highly nonlinear current–voltage characteristics of these materials of this type.

Magnetic and transport and dielectric properties of polycrystalline TbMnO 3

Polycrystalline bulk perovskite TbMnO 3 were synthesized by conventional solid-state reaction, magnetic and transport and dielectric properties were examined, antiferromagnetism was observed near 10 K, and semiconductor transport behaviour was found within measurable temperature range. Especially, an extraordinarily high low-frequency dielectric constant (∼10 4) near room temperature was observed, but below 200 K the constant sharply decrease to less than 100, two apparent dielectric relaxations appeared in loss tangent, which was interpreted by the hopping of localized carriers and the thermally excited relaxation process.

Can chemistry be derived from quantum mechanics? Chemical dynamics and structure

Following an early suggestion by Dirac, contemporary quantum mechanics (QM) and measurement theory are used to stepwise construct a conceptual framework to chemistry. In QM arbitrary physical quantum states are represented by linear superpositions. Base sets and quantum states are clearly distinguished; the latter are sets of ordered complex numbers. The generator of time evolution $$\fancyscript{H}$$ is shown to have besides the molecular hamiltonian H=H C+K N, the electron–phonon and spin–orbit operators.H is shown diagonal in a product base set formed with generalized electronic diabatic (GED) and nuclear wave functions; the latter represented in a plane wave base set and unique inertial frame; these functions form the GEDM base set. Chemical species are assigned to particular GED base functions and a chemical quantum state is given as a linear superposition in the GEDM base set. Thus, given a fixed number of electrons and nuclei, the GEDM base set includes all electronuclear quantum states from clusters, supermolecule, molecular aggregates, asymptotic states, and ionized states (continuum electron states) to all plasma states. Experimentally measurable quantum states are given by linear superpositions. A chemical process is sensed by amplitude changes involving different electronic states in the linear superposition; vibration states intervene in excitation–relaxation processes; a time evolution of a global quantum 1-system is the feature. If sufficient time is allowed to an isolated system, with a given total energy E, unitary time evolution with $$\fancyscript{H}$$ (but not H) ensures that the system will evolve from the initial state to get amplitudes different from zero for all energy conserving final states. Response towards external dynamic couplings, e.g. electromagnetic fields, is given by changes of amplitudes and expressed in correlation functions. For a quantum state, the amplitude at a given base function reflects the possibility for the system submitted to excitation sources to respond with its spectral signatures; if the experiment is done, and the amplitude is different from zero, then, there will be a response. Algorithms permitting to relate Hilbert space to real space events are discussed. A chemical compound (analyte) is identified to (i) a fixed electronic quantum state; (ii) linear nuclear base states superpositions; (iii) zero amplitude for all other electronic base states. To get this implies either the action of a chemist introducing constraints to bottle the analyte or an accidental change of external (to the box) conditions producing spontaneous separations.

Comparison of temperature-dependent dielectric characteristic in two different (Ba,Sr)TiO3 films

A high dielectric constant of 2500 near room temperature was observed in (Ba０.５Sr０.５)TiO３ (BSTO) film prepared by pulsed-laser deposition (PLD) on Pt/Ti/SiO２/Si (100) substrate at 550℃ in N2 atmosphere. The dielectric constant is weakly temperature dependent above 200 K. The dielectric behavior of this film is different from the BSTO film deposited in the O2 atmosphere, but very similar to that reported for the so-called “colossal” dielectric constant materials, such as CaCu3Ti4O12. The film prepared in O2 atmosphere at 650℃ shows normal ferroelectric phase transition, which are fitted with Curie-Weiss law. However, for the film prepared in N2 atmosphere at 550℃, the temperature dependence of dielectric relaxation can be characterized by a thermally excited relaxation process. Such anomalous dielectric response of the BST films is ascribed to the formation of the Schottky barrier between the metallic electrode and the film surface.