Excitation Kinetics Research Articles

Photophysical properties of Re(I)(CO)3(phen) pendants grafted to a poly-4-vinylpyridine backbone. A correlation between photophysical properties and morphological changes of the backbone

Photochemical and photophysical properties of a polymer, Re-P4VP, consisting of Re(I)(CO)3(phen) pendants grafted to a poly-4-vinylpyridine backbone, P4VP, were interpreted on the bases of morphological transformations. These transformations are responsible of a significant increase of the MLCTRe(I)→phen excited state luminescence lifetime when the polymer concentration is increased. Also a nearly 8-fold increase in the luminescence quantum yield resulted from the protonation of Re-P4VP with consequent changes of the excited state decay kinetics. Results of TEM and AFM morphological studies on P4VP and Re-P4VP in the presence of HClO4 acid, i.e., to form Re-P4VPHnn+, revealed that they have concentration dependent morphologies. From low to large concentrations of the Re-P4VP polymer, the morphology of Re-P4VP varies from a nonhomogeneous distribution of spherical nanoaggregates coexisting with micrometer size fibers to an homogeneous distribution of spherical nanoaggregates with diameters around 25 nm. The Re-P4VP morphology is also altered when the polymer pyridines are protonated. Protonation of diluted solutions of Re-P4VP polymers decrease the sizes of the nanoaggregates and small objects with diameters smaller than 10 nm appear.

Micro-Raman and electronic structure study on kinetics of electronic excitations induced monoclinic-to-tetragonal phase transition in zirconium oxide films

Monoclinic-to-tetragonal phase transformation (PT) in sputtering grown zirconium oxide (ZrO2) films on silicon substrates by electronic excitation (EE) induced by swift heavy ion (SHI) irradiation is reported.

Electronic and atomic kinetics in solids irradiated with free-electron lasers or swift-heavy ions

In this brief review we discuss the transient processes in solids under irradiation with femtosecond X-ray free-electron-laser (FEL) pulses and swift-heavy ions (SHI). Both kinds of irradiation produce highly excited electrons in a target on extremely short timescales. Transfer of the excess electronic energy into the lattice may lead to observable target modifications such as phase transitions and damage formation. Transient kinetics of material excitation and relaxation under FEL or SHI irradiation are comparatively discussed. The same origin for the electronic and atomic relaxation in both cases is demonstrated. Differences in these kinetics introduced by the geometrical effects (μm-size of a laser spot vs nm-size of an ion track) and initial irradiation (photoabsorption vs an ion impact) are analyzed. The basic mechanisms of electron transport and electron-lattice coupling are addressed. Appropriate models and their limitations are presented. Possibilities of thermal and nonthermal melting of materials under FEL and SHI irradiation are discussed.

Simulation of the phosphorescence decay kinetics of donor molecules of matrix-isolated donor—acceptor pairs

The phosphorescence decay kinetics of triplet excitations being energy donors in matrix-isolated donor—acceptor pairs has been analyzed using numerical simulation. It has been noted that the inclusion of states of donor—acceptor pairs in which both molecules are in the triplet state during energy transfer can have an effect on the phosphorescence decay kinetics of donor molecules and its dependence on the excitation power.

Effect of thermal treatment on morphology and luminescence behaviour of potassium- and sodium-yttrium double tungstate nanopowders co-doped with holmium and ytterbium

NaY(WO4)2 and KY(WO4)2 nanopowders co-doped with Ho3+ and Yb3+ ions have been synthesised by a Pechini method. Calcination of the resin as precursor at 500°C for sodium and at 670°C for potassium leads to the creation of suitable double tungstate powders. Calcination at higher temperature leads to increase of size of the crystalline particle. Structural properties were characterised by the X-ray diffraction (XRD), Raman and IR spectroscopy. Sodium yttrium double tungstate belonging to scheelite structure crystallises in the tetragonal space group I41/a whereas the potassium one crystallises in the monoclinic space group C2/c. Spectroscopic properties i.e. absorption, excitation, emission and luminescence kinetics were investigated at room temperature. With excitation at 457nm both compounds exhibit a strong emission band in the green region (545nm) and much weaker emission bands in the red region (650 and 755nm). Emission bands in the region at about 1000nm characteristic for 2F5/2→2F7/2 of Yb3+ electronic transition testify that the energy transfer from holmium to ytterbium ions takes place in both sodium and potassium double tungstate when Ho3+ is excited. With excitation of ytterbium at 975nm both compounds exhibit a strong 650nm emission band in the red region and a much weaker 550nm emission band in the green region.

Monte-Carlo modeling of excitation of the electron subsystem of ZnO and MgO in tracks of swift heavy ions

Monte Carlo code TREKIS is applied to trace kinetics of excitation of the electron subsystem of ZnO and MgO after an impact of a swift heavy ion (SHI). The event-by-event simulations describe excitation of the electron subsystems by a penetrating SHI, spatial spreading of generated electrons and secondary electron cascades. Application of the complex dielectric function (CDF) formalism for calculation of the cross sections of charged particle interaction with a solid target allows to consider collective response of the target to perturbation, which arises from the spatial and temporal correlations in the target electrons ensemble. The method of CDF reconstruction from the experimental optical data is applied. Electron inelastic mean free paths calculated within the CDF formalism are in very good agreement with NIST database. SHI energy losses agree well with those from SRIM and CasP codes. The radial distributions of valence holes, core holes and delocalized electrons as well as their energy densities in SHI tracks are calculated. The analysis of these distributions is presented.

Excitation kinetics of electronic states of hydrogen molecules in nonequilibrium discharges: Electronic ground state

This review offers an analysis of the vibrational kinetics of hydrogen molecules in the singlet ground state X 1∑ g + for various gas discharges. The measured quasi-stationary vibrational-level distribution functions of a hydrogen molecule in the electronic ground state at the corresponding vibrational temperatures of the first level and those calculated in the framework of a semiempirical level collisional–radiative model of low-temperature plasma in hydrogen, developed in this study, are compared. A database of the measured and calculated characteristics of kinetic processes involving vibrational-excited hydrogen molecules, as well as macro and microparameters of hydrogen low-temperature plasma, is created.

EFFECTS OF ACUTE GLOBAL ISCHEMIA ON RE-ENTRANT ARRHYTHMOGENESIS: A SIMULATION STUDY

Sudden cardiac death is mainly caused by arrhythmogenesis. For a functional abnormal heart, such as an ischemic heart, the probability of arrhythmia occurring is greatly increased. During myocardial ischemia, re-entry is prone to degenerate into ventricular fibrillation (VF). Therefore it has important meaning to investigate the intricate mechanisms underlying VF under an ischemic condition in order to better facilitate therapeutic interventions. In this paper, to analyze the functional influence of acute global ischemia on cardiac electrical activity and subsequently on re-entrant arrhythmogenesis, we take into account three main pathophysiological consequences of ischemia: hyperkalaemia, acidosis, and anoxia, and develop a 3D human ventricular ischemic model that combines a detailed biophysical description of the excitation kinetics of human ventricular cells with an integrated geometry of human ventricular tissue which incorporates fiber direction anisotropy and the stimulation activation sequence. The results show that under acute global ischemia, the tissue excitability and the slope of ventricular cellular action potential duration restitution (APDR) are greatly decreased. As a result, the complexity of VF activation patterns is reduced. For the three components of ischemia, hyperkalaemia is the dominant contributor to the stability of re-entry under acute global ischemia. Increasing [K+]o acts to prolong the cell refractory period, reduce the tissue excitability and slow the conduction velocity. Our results also show that VF can be eliminated by decreasing cellular excitability, primarily by elevating the concentration value of extracellular K+.

Kinetics of photoplasma of dense barium vapour

Barium vapour ionisation under laser photoexcitation of the resonance line at a wavelength of is studied numerically. Seed electrons, arising due to the associative ionisation of atoms, gain energy in superelastic collisions and lead to electron avalanche ionisation of the medium. The influence of radiative transfer in a cylindrical gas volume on the excitation kinetics of barium atoms, absorption dynamics of laser radiation and oscillation of ionisation-brightening wave under competition between ionising and quenching collisions of electrons with excited atoms is studied.

PSI–LHCI of Chlamydomonas reinhardtii: Increasing the absorption cross section without losing efficiency

Photosystem I (PSI) is an essential component of photosynthetic membranes. Despite the high sequence and structural homologies, its absorption properties differ substantially in algae, plants and cyanobacteria. In particular it is characterized by the presence of low-energy chlorophylls (red forms), the number and the energy of which vary in different organisms. The PSI–LHCI (PSI–light harvesting complex I) complex of the green alga Chlamydomonas reinhardtii (C.r.) is significantly larger than that of plants, containing five additional light-harvesting complexes (together binding≈65 chlorophylls), and contains red forms with higher energy than plants. To understand how these differences influence excitation energy transfer and trapping in the system, we studied two PSI–LHCI C.r. particles, differing in antenna size and red-form content, using time-resolved fluorescence and compared them to plant PSI–LHCI. The excited state kinetics in C.r. shows the same average lifetime (50ps) as in plants suggesting that the effect of antenna enlargement is compensated by higher energy red forms. The system equilibrates very fast, indicating that all Lhcas are well-connected, despite their long distance to the core. The differences between C.r. PSI–LHCI with and without Lhca2 and Lhca9 show that these Lhcas bind red forms, although not the red-most. The red-most forms are in (or functionally close to) other Lhcas and slow down the trapping, but hardly affect the quantum efficiency, which remains as high as 97% even in a complex that contains 235 chlorophylls.

Rapid fluorescence spectroscopic characterization of salivary DNA of normal subjects and OSCC patients using ethidium bromide.

Recently, deoxyribonucleic acid (DNA) based biomarker(s) detection has been employed for cancer diagnosis. Earlier reports have suggested the presence of more DNA in the saliva of oral squamous cell carcinoma (OSCC) than normal by electrophoresis technique. Based on these, steady state and excited state kinetics of salivary DNA has been performed with 27 normal subjects and 67 OSCC patients saliva using ethidium bromide as a probe to look for the possibility in discrimination between them. On statistical analysis the sensitivity and specificity of 88.9 and 94.0 % has been achieved from the fluorescence emission spectra and 88.9 and 92.5 % with that of fluorescence excitation.

Laser plasma channel formation in barium vapor based on superelastic heating of electrons

Computational study of plasma channel formation kinetics in optically dense barium vapor irradiated by pulsed laser light tuned to the Ba I resonance transition at λ = 553.5 nm has been performed. Seed electrons are produced due to the mechanism of atoms associative ionization, which then gain energy in superelastic collisions and initiate the avalanche ionization of atoms by electron impact. We have studied the influence of radiative transfer effects in cylindrically symmetric gas volume on the excitation kinetics of multilevel barium atoms, dynamics of absorption of laser radiation, and the plasma channel expansion in the form of a halo in condition of competition between the ionizing and quenching electron collisions with excited atoms.

Spectroscopic determination of vibrational and rotational temperatures of NO molecules in N2–O2 mixture microwave discharge

By using an optical emission spectroscopic measurement method, the vibrational and rotational temperatures of nitrogen monoxide molecules are obtained for N2–O2 mixture microwave discharge plasma. We experimentally observed the γ-band (195–340 nm) spectrum of nitrogen monoxide. We also developed a method of theoretical calculation for its vibrational and rotational spectrum, with which we can fit the experimental results theoretically, so that the vibrational and rotational temperatures can be determined. Some experiments are carried out to examine relaxation process of excited states of the N2–O2 gas mixture plasma together with its N2 ratio dependence. We discussed the dominant molecular processes and excitation kinetics for the formation of NO A 2Σ+ state under different discharge conditions, particularly on the N2–O2 mixture ratio.

Unveiling the Eigen-Weller ion pair from the excited state proton transfer kinetics of 3-chloro-4-methyl-7-hydroxycoumarin.

The prototropic reactions of the first excited singlet state of 3-chloro-4-methylumbelliferone (3Cl4MU), in dioxane:water mixtures (Dx:H2O), were revisited using ps-time-resolved fluorescence techniques. The data response to the dielectric constant of the mixtures revealed the presence of an additional fourth kinetic species, kinetically coupled to the neutral (N*), the tautomeric (T*), and anionic (A(-)*) forms of 3Cl4MU, which is assigned to the elusive geminate (A(-)*···H(+)) ion pair. From the data analysis, all rate constants of the prototropic and diffusion processes involved were separately evaluated. The results showed that, whenever the geminate ionic pair is not kinetically detected, the evaluated values for deprotonation and protonation rate constants can substantially deviate from the real ones, depending on the efficiencies of pair recombination and dissociation. Finally, the results provide convincing kinetic evidence for the Eigen-Weller mechanism (intermediacy of the geminate ionic pair) in a quasi-aqueous medium, which to our knowledge had not yet been given.

Chemical kinetics of low pressure high density hydrogen plasmas: application to negative ion sources for ITER

This paper presents a systematic kinetic characterization of a low pressure high power hydrogen plasma. The plasma physics is described with a global model coupled to a homogeneous kinetic model for hydrogen. This model involves reactions which describe the vibrational and electronic excited kinetics of H2, the positive and negative (H−) ion kinetics and the H chemistry. This enables the estimation of the particle density and the electron temperature and their evolutions as a function of power (1–100 kW) and pressure (0.3–4 Pa). These very specific plasma conditions involve physical phenomena not occurring in more usual plasmas, such as gas depletion. To account for this gas depletion, we incorporate in the global model both the H neutral heat equation to calculate the H temperature, and the gas pumping. Indeed, the gas depletion is mainly due to H atom heating leading to a higher pumping loss for H atoms. The consideration of the gas depletion allows us to obtain similar behaviors to the experiments when varying power and pressure. From an accurate analysis of the main formation and destruction pathways for each particle, the species kinetics is discussed and a simplified kinetic model that may be used to describe the non-equilibrium plasma in the negative source for ITER is proposed. Finally, the results point to strong coupling existing between the H atom wall recombination coefficient γH and the gas depletion. An increase of γH reduces the gas depletion, affecting the electron temperature and the electron density as well as the whole plasma kinetics.

A Detailed Analysis of Multiple Photoreactions in a Light-Harvesting Molecular Triad with Overlapping Spectra by Utrafast Spectroscopy

The photoreactions that occur in a molecular triad composed of derivatives of boron-dipyrromethene (BOD), diketopyrrolopyrrole (DPP), and of triphenylamine (TPA) specifically designed for the use as a possible donor material for organic solar cells are studied by ultrafast fluorescence and transient absorption spectroscopy with hyper spectral coverage (300–900 nm). While the latter is often sufficient to reveal the reaction scenario in mono- or bicomponent organic molecules, the overlap of ground state absorption and fluorescence spectra of the components present in the TPA-DPP-BOD triad requires a careful and accurate characterization of the excited state differential absorption spectra and kinetics of the individual isolated moieties. In addition, the absorption spectra of the TPA cation and the DPP and BOD anions are determined by in situ spectroelectrochemical experiments. Picosecond fluorescence demonstrates efficient excited state quenching of both DPP and BOD, with downhill energy transfer from TPA occurring in a subpicosecond time scale, leading to its complete excited state quenching. Additionally, an analysis of the multiexponential fluorescence decay indicates the existence of reactive (quenched) and nonreactive (radiative) subpopulations of DPP and BOD, most probably due to structural heterogeneities. In order to treat this complexity of multiple reaction pathways, the transient absorption data are analyzed by a combination of global fitting, providing decay-associated differential spectra (DADS), and a reconstitution of these DADS by linear combinations of the characteristic difference spectra of all molecular species and excited states. This allows us to disentangle the photoreactions that the TPA, DPP and BOD excited state subpopulations undergo on time scales ranging from 200 fs to a few ns, and to clarify the contribution of TPA as an electron donor in the formation of an intramolecular charge transfer (CT) state. DPP has a dual role as an ancillary light-harvesting unit capable of performing ultrafast energy transfer to BOD, and as an electron acceptor for the CT state. The latter has a remarkably long lifetime of 0.5 ns. We conclude that the triad could act as a very efficient electron-donor material in a blend with commonly used acceptors such as phenyl-C61-butyric acid methyl ester (PCBM).

Interaction between “dissipative solitons” stabilized by aggregation in excitable kinetics

We consider that a population of individuals governed by the Nagumo model is characterized by predisposition towards aggregation. “Dissipative solitons” interacting are solutions for such system. We changed the possibility of extinction, predicted by Nagumo model, by a uniform background of low population’s density and then we observed relevant effect on interaction between “solitons”.

Kinetics of excited states and radicals in a nanosecond pulse discharge and afterglow in nitrogen and air**This paper is closely related to ‘Nitric oxide kinetics in the afterglow of a diffuse plasma filament’ by D Burnette et al (doi:10.1088/0963-0252/23/4/045007) published in this volume, which focuses on the kinetic modelling of the experiments. This paper presents the results of the experiments.

The present kinetic modelling calculation results provide key new insights into the kinetics of vibrational excitation of nitrogen and plasma chemical reactions in nanosecond pulse, ‘diffuse filament’ discharges in nitrogen and dry air at a moderate energy loading per molecule, ∼0.1 eV per molecule. It is shown that it is very important to take into account Coulomb collisions between electrons because they change the electron energy distribution function and, as a result, strongly affect populations of excited states and radical concentrations in the discharge. The results demonstrate that the apparent transient rise of N2 ‘first level’ vibrational temperature after the discharge pulse, as detected in the experiments, is due to the net downward V–V energy transfer in N2–N2 collisions, which increases the N2(X 1Σ, v = 1) population. Finally, a comparison of the model's predictions with the experimental data shows that NO formation in the afterglow occurs via reactive quenching of multiple excited electronic levels of nitrogen molecule, , by O atoms.

Luminescence properties and determination of optimal RE3+(Sm3+, Tb3+and Dy3+) doping levels in the KYP2O7host lattice obtained by combustion synthesis

The β-KYP2O7 powders doped with Sm3+, Tb3+ and Dy3+ were obtained using a one-step urea-assisted combustion method. Their structural properties were characterized by X-ray diffraction (XRD) whereas spectroscopic properties like excitation, emission and luminescence kinetics were investigated at room temperature. β-KYP2O7 doped with Sm3+ and Dy3+ showed concentration quenching above 2 mol% of dopants whereas Tb3+ containing materials could be doped with higher concentrations. The main mechanisms of concentration quenching were identified as dipole–dipole for Sm3+ and Dy3+. Both dopants due to the abundant energy levels are strongly prone to quenching due to the non-radiative CR process whose efficiency strongly increases with the concentration of optically active ions. In the case of Tb3+ emission from both 5D3 and 5D4 levels was recorded. The 5D3 level vastly depopulates due to the CR process upon increase of the Tb3+ content whereas the slight 5D4 emission remains stable up to 5 mol%.

An experimental and TD-DFT theoretical study on the photophysical properties of Methylene Violet Bernthsen

We revisited the photophysics the Methylene Violet (MV) Bernthsen dye in aprotic solvents to rationalize both its peculiar solvatochromism, which cannot be explained by the standard solvatochromic empirical models, and the large dependence of its excited states decay kinetics upon changes on the polarity of the medium.To this end, MV singlet and triplet excited states were characterized by stationary and time-resolved absorption and emission techniques. The experimental information was combined with a detailed theoretical study using TD-DFT to characterize not only the emitting states but also all dark states which would play a role in determining the fate and properties of the experimentally populated excited states. It is concluded that owning to both the high permanent dipole and polarizability of MV, singlet and triplet excited states are stabilized by the medium to a very different extent, leading to an unusual solvatochromism and decay kinetics.