Mean Trapping Time Research Articles

Femtosecond laser direct processing in wet and dry silica glass

The infrared femtosecond laser damage threshold is found to be independent of OH content in pure silica glass. Additionally, the density and the mean trapping time of electrons excited in the conduction band are also found to be independent on OH concentration.

Derivation of a Fokker–Planck equation for generalized Langevin dynamics

A Fokker–Planck equation describing the statistical properties of Brownian particles acted upon by long-range stochastic forces with power-law correlations is derived. In contrast with previous approaches (Wang, Phys. Rev. A 45 (1992) 2), it is shown that the distribution of Brownian particles after release from a point source is broader than Gaussian and described by a Fox function. Transport is shown to be ballistic at short times and either sub-diffusive or super-diffusive at large times. The imposition of occasional trapping events onto the Brownian dynamics can result in confined diffusion ( d / d t 〈 x 2 〉 → 0 ) at long times when the mean trapping time is divergent. It is suggested that such dynamics describe protein motions in cell membranes.

Transport of optical excitations on dendrimers in the continuum approximation

We study the incoherent transport of optical excitations created at the rim of a dendritic molecule to a trap occurring at the core. The corresponding discrete random walk is treated in a continuum approximation, resulting in a diffusion-like process which admits semi-analytical solutions. The thus obtained arrival time distribution for the excitation at the trap is compared with the one for the original, discrete problem. In the case of an inward bias or even a weak outward one, the agreement is very good and the continuum approximation provides a good alternative description of the energy transfer process, even for small dendrimers. In the case of a strong outward bias, the mean trapping time, which sets the time scale for the entire distribution, depends exponentially on the number of generations in both approaches, but with a different base. The failure of the continuum approximation for this case is explained from the peaked behavior of the excitation density near the rim.

Electrophoretic Separation of Long Polyelectrolytes in Submolecular-Size Constrictions: A Monte Carlo Study

We use a bond-fluctuation Monte Carlo method to study the motion of long polyelectrolytes inside an array of microscopic entropic traps. The molecules are pulled through the array by an electric field and forced into submolecular-size constrictions between the larger trap regions. We numerically solve the Laplace equation inside the structure to obtain realistic field lines for our simulations. We find that the mobility of the molecules increases with molecular size and that the size-separation mechanism relies mainly on the overall deformation of the molecules as they approach the narrow constrictions. We also investigate specific aspects of the separation mechanism, namely the conformational behavior of the molecule, the hernia nucleation process, and the trapping time statistics as a function of molecular size and field strength. Our simulation results for the mobility, the critical hernia nucleation size, the mean trapping time, and the resolution are consistent with the experimental data and model previously published by Han et al.1,2 Finally, we predict that such microfluidic structures could be used to separate topoisomers bearing the same molecular size.

Trapping of minority carriers in multicrystalline silicon

Abnormally high effective carrier lifetimes have been observed in multicrystalline silicon wafers using both transient and steady-state photoconductance techniques. A simple model based on the presence of trapping centers explains this phenomenon both qualitatively and quantitatively. By fitting this model to experimental data acquired with a quasi-steady-state photoconductance technique, it is possible to determine the trap density, trap energy, and the ratio between the mean-trapping time and mean-escape time. A correlation between trap density and dislocation density in the material has been found.

Stochastic resonance and symmetry breaking in a one-dimensional system

We derive and discuss properties of an exact solution for the average time for trapping of a Brownian particle driven by a random, asymmetric but unbiased, telegraph signal. The particle moves along a line segment terminated by either two traps or a trap and a reflecting point. Numerical results suggest that stochastic resonance, defined as a nonmonotonic behavior of the mean trapping time, is absent in the first case but present in the second. This generalizes a result obtained earlier by Doering and Gadoua [Phys. Rev. Lett. 69, 2318 (1992)] and implies that symmetry breaking alone does not necessarily create stochastic resonance.

Mobility of atomic hydrogen in solid krypton and xenon

Atomic hydrogen is produced in xenon and krypton matrix by in situ x-ray induced photolysis of the dopants water, butane, acetone, or methane and trapped interstitially forming the “caged hydrogen” with characteristic VUV absorption bands. Their thermal bleaching as investigated between 8 K and 45 K cannot be described by a first-order process. A random walk model is introduced considering the hydrogen atoms to move in a thermally activated diffusive motion to traps where they become “invisible.’’ The temperature dependence of the mean trapping time is Arrhenius-type and characteristic only for the matrix. Infrared experiments have confirmed a partial recovery of the dopant molecule only in krypton. Annealing-induced IR absorption bands are assigned to a new complex which involves two hydrogen atoms.

The range of random walk on trees and related trapping problem

This paper treats of the range of the simple random walk on trees and a related trapping problem. The strong law of large numbers and the central limit theorem for the range, and some asymptotic behaviour for the mean trapping time and survival probability are presented.

希ガスおよび大気成分中のアルカリイオンの移動度とクラスター反応

The interaction potential between the Li+ ion and neutral atoms or molecules (Ar, Kr, Xe, N2, and O2) can be estimated from the ionic mobilities measured in gases. Assuming Langevin reaction coefficient as a limiting value, we have plotted experimental reverse Li+ ion-clustering reaction coefficients as Arrhenius plot and the activation energies for the reverse ion-clustering reactions were estimated. We found a good relationship between the activation energies and the well depth of the potential between the Li+ ion and the neutral atoms or molecules estimated from the mobility measurement. We also found a relationship for the forward reaction coefficients, which can be deduced from the Langevin reaction coefficient and the mean trapping time calculated from the interaction potential. The quadrupole moment for the diatomic molecule such as N2 and O2, plays an important role in forward ion-clustering reaction of Li+ ions.

Space-time observation of an electron gas in SiO2.

We report the first space-time observation of the ultrafast excitation and relaxation of an electron gas pumped into the conduction band of a wide band-gap insulator by an intense subpicosecond laser pulse. The experiment is based on a new phase-sensitive technique which allows a resolution of 120 fs in time and 5 \ensuremath{\mu}m in space. In crystalline and amorphous Si${\mathrm{O}}_{2}$, no evidence of gas expansion has been observed and a mean trapping time of 150 fs for the photogenerated carriers is measured.

Statistics of firn closure: a simulation study

AbstractThe trapping of bubbles of air in polar ice has provided a unique record of past atmospheric composition. However, the interpretation of measured concentrations depends on the statistics of the trapping process. Measurements of trace atmospheric constituents whose concentrations are changing steadily can be interpreted in terms of an “effective age” of the gas which differs from the age of the ice by a delay which corresponds to the mean trapping time. The statistics of bubble trapping can be modelled as a percolation model which is one of a class of models whose transitions are characterized by large critical fluctuations. These critical fluctuations cause an intrinsic sample-to-sample variability in the delay time and thus in the effective age. Monte Carlo simulations using a lattice model of the firn are presented, showing the effect of finite sample size on the age distribution of trapped gas. For samples containing more than about 103–104 bubbles, the simulations indicate that the range of variability is small compared to the average duration of the trapping process.

Statistics of firn closure: a simulation study

AbstractThe trapping of bubbles of air in polar ice has provided a unique record of past atmospheric composition. However, the interpretation of measured concentrations depends on the statistics of the trapping process. Measurements of trace atmospheric constituents whose concentrations are changing steadily can be interpreted in terms of an “effective age” of the gas which differs from the age of the ice by a delay which corresponds to the mean trapping time. The statistics of bubble trapping can be modelled as a percolation model which is one of a class of models whose transitions are characterized by large critical fluctuations. These critical fluctuations cause an intrinsic sample-to-sample variability in the delay time and thus in the effective age. Monte Carlo simulations using a lattice model of the firn are presented, showing the effect of finite sample size on the age distribution of trapped gas. For samples containing more than about 103–104bubbles, the simulations indicate that the range of variability is small compared to the average duration of the trapping process.

Trapping of point defects in alloys.

Based on the random walk of point defects in alloys, neglecting the interaction between the components and their interaction with the point defects, the kinetics of the annihilation of point defects in fcc alloys is studied by computer simulations. Particularly, the dependence of the mean trapping time in random alloys on various parameters, including the structure of traps, the composition of alloy, and the mobility ratio of the components, is investigated and reported here.

Charge carrier transport properties in thallium bromide crystals used as radiation detectors

The authors report on measurements of the two most important transport parameters, the mobility mu and the mean trapping time tau for electrons and holes, in TlBr crystals prepared in the laboratory. The results using the transient charge technique are presented along with the data obtained by the pulse height spectrum analysis. The values of ( mu tau )/sub e/ and ( mu tau )/sub h/ measured for TlBr are still lower than those reported for more established detector materials such as CdTe and HgI/sub 2/. It is noted, however, that purer TlBr exhibited somewhat improved transport characteristics, implying that mu tau may still be impurity-limited. For this reason, future improvements in purification will probably yield detectors having improved charge carrier transport.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Excitation trapping efficiency and kinetics in Rb. sphaeroides R26.1 whole cells probed by photovoltage measurements on the picosecond time-scale

Light-gradient photovoltage measurements in whole cells of the purple bacterium Rb. sphaeroides R26.1 were used to probe the trapping kinetics and the trapping efficiency resulting from 30 ps excitation at 532 nm. The time-course of the photovoltage is described by two distinct phases. The slower phase is characterized by a time constant of 190 ± 10 ps, contributing to about 60% of the total amplitude. It is independent of the excitation energy and of the initial fraction of reaction centers (RC) in the open state (primary donor reduced). This phase is assigned to the forward electron transfer from the bacteriopheophytin intermediary acceptor to the quinone. The fast phase is assigned to the appearance of the primary charge separated state (i.e., trapping). If all the RCs were in the open state before excitation, the time constant of the fast phase decreased from 55 ± 10 ps to less than 40 ps upon increasing the energy of the flash from less than 0.5 to more than 3 photons absorbed per RC. At the latter energy, 90 ± 5% of the RCs are closed by the flash. If the initial concentration of RCs in the closed state was increased from 0 to about 100%, either by background illumination or by preexcitation with another picosecond flash delivered 20 ns before the measuring flash, the mean trapping time increased from 55 ± 10 to 90 ± 15 ps for a probe flash energy corresponding to 0.5 photon absorbed per RC. The data are analyzed and described in terms of the competition between trapping and exciton-exciton annihilation in the lake model of the photosynthetic membrane of purple bacteria (Deprez, J., Paillotin, G., Dobek, A., Leibl, W., Trissl, H.-W. and Breton, J. (1990) Biochim. Biophys. Acta 1015, 295–303).

Time-resolved photoluminescence measurement of carrier trapping time of InGaAs/InP quantum wells

We have measured the diffusion and trapping of photoexcited hot carriers in a InGaAs/InP heterostructure using an optical time-of-flight technique with picosecond time resolution. The efficiency of trapping of carriers into the well is found to increase rapidly between 4 K and room temperature. A mean trapping time of 4 ps is measured for a 50 Å well.

Optical time-of-flight measurement of carrier diffusion and trapping in an InGaAs/InP heterostructure

We have measured the diffusion and trapping of photoexcited hot carriers in an InGaAs/InP heterostructure using an optical time-of-fight technique with picosecond time resolution. The ambipolar diffusivity is found to decrease by an order of magnitude between 4 K and room temperature, and the efficiency of trapping of carriers into the well increases rapidly in the same temperature range. A mean trapping time of 4 ps is measured for a 50 Å well.

II. Primary electrogenic reactions in chloroplasts probed by picosecond flash-induced dielectric polarization

Excitation of chloroplasts by non-saturating flashes is known to generate an electric polarization of the suspension which can be measured as a photovoltage with electrodes positioned alongside the propagation axis of the light. The polarization originates from the light-gradient in the suspension and the anisotropic excitation of the primary charge separation in the reaction centers of both photosystems. By using a capacitative measuring cuvette and light pulses of 30 ps duration we could achieve a time resolution on the order of 200 ps, even at low ionic strengths. At the excitation wavelengths of 532 and 694 nm, a photovoltage developed with the rise-time of the apparatus when the reaction centers of both photosystems were open. Closing of the reaction centers of Photosystem I by oxidizing P-700 resulted in a photovoltage with the same fast rise, but of half amplitude. This shows that (i) the primary charge separation in both types of reaction centers and (ii) the mean trapping time of excitions from the antenna system by P-680 and P-700 are shorter or equal to 175 ps. When the traps of Photosystem I were closed and the primary quinone acceptor Q in the reaction centers of Photosystem II was reduced, there was almost no photovoltage response around 1 ns. The only primary reaction known under these conditions is the charge separation in Photosystem II between the primary donor P-680 and the intermediary pheophytin acceptor. Assuming this reaction, the observed lack of a photovoltage indicates that the intermediary acceptor Phe lies on the same membrane side as P-680.

Diffusion-mediated localization on membrane surfaces

Using the model of a cell membrane of a spherical surface in which membrane components may diffuse, the rate of localization due to trapping under diffusion control has been estimated by computing an analytical expression for the mean trapping time including the possibilities of a trapping probability less than 1 and/or the establishment of an equilibrium at the trap boundary.

The yield of chlorophyll a fluorescence as a means to test the various deexcitation mechanisms in the antenna system of green plants

With the aid of measurements of the fluorescence yield, the efficiency of the various deexcitation mechanisms of an exciton in the light-harvesting system has been determined. For this purpose, the fluorescence of dark-adapted as well as of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated and preilluminated leaves of Zea mays L. was excited by single ultrashort laser pulses of different energies. The experimental results have served for the fitting of solutions of rate equations, which describe the deexcitation by linear relaxation processes like fluorescence and radiationless transitions, by annihiation of excitons, and by traps both in the ground state and in an excited state. We have obtained the following results: a ratio of antenna chlorophyll molecules to Photosystem II traps of 600:1, an annihilation constant γ = 2·10 −8 cm 3·s −1, a mean trapping time of t ̄ =0.5 ns , a trapping probability for traps in the ground state of 2·10 −8 cm 3·s −1, and 6·10 −9 cm 3·s −1 for traps in an excited state.