Time Of Flight Experiments Research Articles

The influence of the time-of-flight mobility on the efficiency of solid-state dye-sensitized TiO2 solar cells

The dependence of zero field charge carrier mobilities obtained from time-of-flight (TOF) measurements for a series of doped triphenyldiamines on trap depth and concentration were determined. The influence of these mobilities on the solar cell characteristics have been studied by measuring I–V characteristics and IPCE values using the same doped hole conductors in solid-state dye-sensitized TiO2 solar cells. The traps exhibit only a weak influence on the solar cell characteristics, because the traps which are responsible for low charge transport mobilities in the TOF experiment are filled under steady state illumination conditions. In consequence the effective mobility is raised almost to that of the pristine hole conductor, thus allowing the use of doped hole conductor systems in solar cells without any adverse effects.

Comprehensive study of the surface peak in charge-integrated low-energy ion scattering spectra

Low-energy ion scattering is very surface sensitive if scattered ions are analyzed. By time-of-flight (TOF) techniques, the neutral and the charge-integrated spectra (ions plus neutrals) are obtained, which yield information about deeper layers. It is well known that charge integrated spectra may exhibit a surface peak which is more pronounced for heavier projectiles, e.g., Ne ions. Aiming at a more profound physical understanding of this surface peak, we performed TOF experiments and computer simulations for H, He, and Ne projectiles scattered from a polycrystalline copper target. Measurements were done in the range of 1--9 keV for a scattering angle of $129\ifmmode^\circ\else\textdegree\fi{}$ under UHV conditions. The simulations were performed using the MARLOWE code for the given experimental parameters and a polycrystalline target. In the experiments, a pronounced surface peak was observed at low energies, which fades away at higher energies. This peak is quantitatively reproduced by the simulation. Several atomic layers may contribute to the surface peak, depending on the energy. Analyzing the contributions of the individual outermost atomic layers, one finds that the binary collisions of the projectiles with atoms in the first and the second layer yield a narrow energy distribution, while the contribution from the deeper layers is dominated by multiple scattering and therefore exhibits a very broad energy spectrum. It is shown that the appearance of a more or less pronounced surface peak is due to the relative contributions of single scattering and multiple scattering and thus depends on the projectile energy and mass.

Low Molar Mass and Polymer Discotics: Structure, Dynamics and Opto-Electronic Properties

Columnar discotics display along the columnar axis unusually large mobilities for charge carriers. This fact can be exploited for instance in the areas of photoconductors and organic light emitting diodes. The transport takes place via hopping between localized transport sites. The theoretical analysis based on an analytical approach as well and on simulations predicts that the energetic distribution of the transport sites controls the particular dependence of the charge carrier mobility both on the temperature and the applied electric field. Narrow distributions cause the mobilities to increase and to become independent of temperature and field. A further prediction is that the mobility depends on the length of the columns. We have varied the chemical structure of asymmetrically substituted triphenylenes with the purpose to enhance the spatial order and consequently to reduce the width of the energy distribution of the density of states. Novel methods of macroscopical orientations of the columns were developed. The highly ordered plastic columnar state achieved in this way was found to display a high one dimensional mobility and no field and temperature dependence of the mobility as obvious from Time of Flight experiments. The order within the columnar phase as well as the columnar length was furthermore modified by subjecting the discotics to geometric confinements with dimensions down to a few nm. Confinements imposed in PDLC systems and porous glasses give rise to strong changes not only in spatial structure and molecular dynamics and in the absorption and emission properties but also in LED-properties. Finally we have modified the optical properties of semiconductor quantum well structures by discotic layers.

Coulomb explosion of benzene induced by an intense laser field

Coulomb explosions of benzene induced by an intense femtosecond laser field were investigated by time-of-flight (TOF) mass spectrometry at a laser intensity of 8.0×1016 W cm−2, with a pulse width of 120 fs. The multiply charged ions of Cq+ (q=1–4) and H+ were detected, and their energies were found to be distributed in the range of 0–160 eV. The explosions were concluded to be anisotropic because the kinetic energies of multiply charged carbon ions are highly parallel to the laser electric field. Molecular dynamics simulations, including the effects of tunnel ionization, electron recombination, and the spatial configuration of benzene for laser electric fields, were performed to elucidate the kinetic-energy distributions and the dynamics of exploded benzene ions. The results of the simulations reproduced the essential points of observation concerning the energies and distributions of the ions in the TOF experiments. The simulations suggest that the charge-hopping processes enhance ionization, finally leading to an anisotropic explosion, which events are similar to those found in the case of C60 explosions. Furthermore, our findings have strongly suggested that time evolution of the hopping greatly depends on spatial configuration in the case of planar molecule benzene.

Production of dry powder clots using a piezoelectric drop generator

We have demonstrated that piezoelectrically driven, squeeze mode, tubular reservoir liquid drop generation, originally developed as a “drop-on-demand” method for ejection of microdrops of liquids or suspensions, can successfully operate with dry powder. Spherical silver powder with maximum particle diameter of 20 μm was loaded into and ejected from a 100 μm orifice glass dropper with a flat piezoelectric disk driver. Time of flight experiments were performed to optimize the dropper operation and to determine the size and velocity of the ejected particles. It was found that at certain values of the amplitude, duration, and repetition rate of the voltage pulses applied to the piezoelectric disk, one can eject powder clots of a stable size, comparable with the dropper orifice diameter. In contrast to the operation with a liquid, a clot is not ejected at each pulse, but quasiperiodically with an interval corresponding to thousands of pulses. The application for injection of atoms into helium buffer gas at cryogenic temperatures is discussed.

Dispersive transient current due to hopping at band edge fluctuating with self-affine fractal dimension

Transient current in time of flight experiment (TOF) in the case that electrons or self-trapped (small polaron-like) electrons transfer by hopping at a band edge in the presence of long-range-fluctuations with a self-affine fractal dimension of 3.9, has been investigated by Monte Carlo simulation. The simulation has shown that the electric field dependence of the transit time of the TOF in amorphous semiconductors including a-Si:H is explained as due to the hopping of the self-trapped electrons rather than the electron hopping.

The pyrolysis of methylcyclopentadiene: Isomerization and formation of aromatics

The pyrolysis of methylcyclopentadiene (MCP) was examined in two shock tubes with a variety of techniques. Laser-schlieren (LS) experiments were performed over 1000–2000 K, and 60–600 Torr, in 2% MCP/Kr and 4% MCP/Kr. Incident-shock-quench-tank GC-MS product analyses (using a new method) were obtained over 800–1900 K in 4.5% MCP/Ar, and time-of-flight (TOF) mass spectra were recorded in 2% MCP/Ne over 1250–1600 K and 145–225 Torr. The LS experiments showed an initial endothermic reaction from C-C fission and subsequent exothermic reaction from methyl-radical recombination. Rate constants for the dissociation extracted from early gradients formed a consistent set with an apparent E a of 44 kcal/mol. This highly endothermic dissociation (Δ H 298 o =75 kcal/mol) is thus well into falloff, but the expected pressure dependence was not observed. A routine Gorin-modle Rice-Ramsperger-Kassel-Marus (RRKM) calculation, modified for the fact that only the 5-isomer can fission, produced the magnitude and temperature dependence but predicted detectable pressure variation. TOF and quench-tank product analyses showed benzene, acetylene, naphthalene, and methane as major low-temperature products. The presence of benzene at the lowest temperatures, as well as its rapid formation in the TOF experiments, suggests that H-atom-catalyzed decomposition to benzene is efficient in this pyrolysis, and the naphthalene production supports the c -C 5 H 5 dimerization path, in accord with recent theory. GC-MS analysis of the quench-tank experiments provided the first measurements of the distribution of the three MCP isomers at elevated temperatures, showing that 5-MCP is a very small but increasing fraction with temperature whose enthalpy is about 1.2 kcal/mol above that of the other two isomers.

A test-type hyper-thermal neutron generator for neutron capture therapy – estimation of neutron energy spectrum by simulation calculations and TOF experiments

In order to clarify the irradiation characteristics of hyper-thermal neutrons and the feasibility of a hyper-thermal neutron irradiation field for neutron capture therapy, a “test-type” hyper-thermal neutron generator was designed and made. Graphite of 6 cm thickness and 21 cm diameter was selected as the high temperature scatterer. The scatterer is heated up to 1200°C maximum using molybdenum heaters. The radiation heat is shielded by reflectors of molybdenum and stainless steel. The temperature is measured using three R-type thermo-couples and controlled by a program controller. The total thickness of the generator is designed to be as thin as possible, 20 cm in maximum, in the standing point of the neutron beam intensity. The thermal stability, controllability and safety of the generator at high temperature employment were confirmed by the heating tests. As one of the experiments for the characteristics estimation, the neutron energy spectrum dependent on the scatterer temperature was measured by the TOF (time of flight) method using the LINAC neutron generator. The estimations by simulation calculations were also performed. From the experiment and calculation results, it was confirmed that the neutron temperature shifted higher as the scatterer temperature was higher. The prospect of the feasibility of the “hyper-thermal neutron irradiation field for NCT” was opened from the estimation results of the generator characteristics by the simulation calculations and experiments.

A comparison of time-of-flight and Larmor modulation neutron powder diffraction at a continuous reactor source

A neutron powder diffraction instrument for application at a continuous reactor source based on time-of-flight (TOF) is compared with an instrument based on Larmor modulation (LM). For instruments with a similar wave-vector resolution ( δq/ q=4×10 −3) it is found that the signal-to-noise ratios in a TOF experiment are generally better when using a wide wavelength neutron spectrum. This is caused by the fact that the noise in a LM experiment comes from the integrated neutron flux scattered in the detector, while in TOF the noise stems only from the intensity scattered into individual time channels.

Development of high speed multichannel scalar using line memories

A high speed multichannel scaling mode time analyzer (MCS) for time of flight (TOF) experiments was developed by using high speed serial access memories and a personal computer. The MCS was composed of two memory banks, run alternately for each trigger. The channel width of the MCS was selected from 50 ns to 200 ns (4 stage). The count per channel per trigger signal was one, but the dead time for channel advancing may approach zero. The MCS was tested with linear accelerator (linac)-TOF experiments. The results agreed satisfactorily with the design goal. It was confirmed that the high speed MCS runs normally. A small discrepancy at high energy channels between the time spectrum of the MCS and that of the time-to-pulse-height converter (TPHC)-multichannel pulse height analyzer (PHA) time analyzer was observed. One of the reasons for the discrepancy was the uncertainty of the MCS start time after the trigger signal by linac because of asynchronous crystal oscillations with the linac trigger signal.

Time of flight experiments on amorphous and crystalline boron

Hole transport in amorphous and crystalline boron has been studied by the time of flight method. From the temperature dependence of the drift mobility in crystalline boron, the activation energy of the hole has been estimated to be 240 meV. It is considered that this energy corresponds to the depth of the hole trap in icosahedral B 12 clusters. Dispersive (non-Gaussian) transient transport has been observed for amorphous boron films prepared by evaporation with substrate temperature less than 350°C. The estimated activation energies of the drift mobilities of the films were about 85 meV. In amorphous films prepared with substrate temperatures above 400°C, no dispersive transport was observed.

Modeling of laser induced plasma, spectroscopic and time of flight experiments in pulsed laser deposition

Time of flight and optical emission spectroscopy of laser induced plasmas, typically occurring with Pulsed Laser Deposition, will be compared and interpreted by the results of a complex theoretical model. The model includes the fluid dynamics of the plasma, absorption of the laser energy via inverse bremsstrahlung and the dynamics of ionisation and recombination processes. Experimental results as well as model calculations are shown for different lasers and materials.

Calcul analytique du transport des charges dans le a-Si :H

Multitrapping transport of carriers through exponential band tails is studied for carriers generated at an arbitrary position in the device in presence of a linearly varying electric field, thus generalizing the usual treatment of time of flight experiments. An analytical expression is found for the free carrier density distribution n(x,t) for values of the dispersion parameter 0 < α < 1. In the case α = 1/2, analytical solutions are given for the transient current I(t) and the induced charge Q(t). Comparison with previous calculations is made for the special case of uniform electric field and carriers initially generated at one interface.

A Monte Carlo simulation of the time-of-flight experiment in non-uniformly defected thin crystalline layers

In the present paper we report on the results of the Monte Carlo simulation of the time of flight (TOF) experiment for r and r- epsilon hopping transport in highly defected very thin crystalline layers. The defects are considered as localized states, between which a hopping motion of the injected carriers is possible. The total (integrated over energy) density of defects is assumed to be spatially non-uniform on the macroscopic scale, i.e. the scale comparable with the layer thickness. In particular, we consider an exponential dependence of the total density of hopping centres on the distance from the layer contacts. The results of simulations performed for various defect concentrations, various defect distributions in energy, and various degrees of the layer spatial non-uniformity are discussed. It is shown that both r and r- epsilon hopping transient currents measured in the classical TOF experiment are highly sensitive to the spatial macroscopic scale variations of the total centre concentration. The detailed shape of the transients depends in a complicated way on the system dilution, the energetic centre distribution, and the character of the spatial variations of the total centre density. The existence of the spatial non-uniformity of the layer could be recognized experimentally by observation of the qualitative changes of the current shape with increasing temperature, which leads to lower dispersion, and thus more pronounced characteristic features of the x-dependent total centre density, such as a higher polarity dependence, or the appearance of the current maxima or plateaux just before the effective TOF.

Stellar neutron capture cross sections of the Ba isotopes.

The neutron capture cross sections of $^{134}\mathrm{Ba}$, $^{135}\mathrm{Ba}$, $^{136}\mathrm{Ba}$, and $^{137}\mathrm{Ba}$ were measured in the energy range from 5 to 225 keV at the Karlsruhe 3.75 MV Van de Graaff accelerator. Neutrons were produced via the $^{7}\mathrm{Li}$ (p,n${)}^{7}$ Be reaction by bombarding metallic Li targets with a pulsed proton beam. Capture events were registered with the Karlsruhe 4\ensuremath{\pi} barium fluoride detector. The cross section ratios were determined with an overall uncertainty of \ensuremath{\sim}3%, an improvement by factors of 5 to 8 compared to existing data. Severe discrepancies were found with respect to previous results. As a new possibility in time of flight experiments, isomeric cross section ratios could be determined for $^{135}\mathrm{Ba}$, $^{136}\mathrm{Ba}$, and $^{137}\mathrm{Ba}$. Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT=10 keV and 100 keV. These stellar cross sections were used in an s-process analysis. For the s-only isotopes $^{134}\mathrm{Ba}$ and $^{136}\mathrm{Ba}$ the ${\mathit{N}}_{\mathit{s}}$〈\ensuremath{\sigma}〉 ratio was determined to 0.875\ifmmode\pm\else\textpm\fi{}0.025. Hence, a significant branching of the s-process path at $^{134}\mathrm{Cs}$ can be claimed for the first time, in contrast to predictions from the classical approach. This branching yields information on the s-process temperature, indicating values around ${\mathit{T}}_{8}$=2. The new cross sections are also important for the interpretation of barium isotopic anomalies, which were recently discovered in SiC grains of carbonaceous chondrite meteorites. Together with the results from previous experiments on tellurium and samarium, a general improvement of the ${\mathit{N}}_{\mathit{s}}$〈\ensuremath{\sigma}〉 systematics in the mass range A=120\char21{}150 is achieved. This yields a more reliable separation of s- and r-process contributions for comparison with stellar observations, but reveals a 20% discrepancy with respect to the solar barium abundance.

Displacement currents by propagating charge packets in capacitor structures: Part II: Analysis of the dispersive case

An analytical derivation for the currents induced by charge packet drift ininsulator devices is presented. The theoretical formalism allows for quantization of the time of flight (TOF) current shapes in the special case of general (dispersive) charge spreading. The results are used to extract dispersion parameters of the electrical transport in insulating media from TOF experiments.

The Stokes law radius of the C 60 radical anion in n-hexane by time-of-flight mobility measurements

The time-of-flight (TOF) technique for mobility measurements in the condensed phase has been applied to solutions of C 60 in n-hexane. The C 60 radical anion has a mobility of 5.6 × 10 −4cm 2V −1 s −1, and a Stokes radius of 5.0 Å, which agrees remarkably well with the van der Waals radius of the C 60 molecule (5.0 Å) obtained by electron and X-ray diffraction. The success of the Stokes law, presumably aided by the quasi-spherical nature of C 60 suggests TOF experiments using C 60 for the determination of the “Stokes” viscosities of other liquids.

Time of flight analysis of electrical charge transport in insulating devices

An analytical derivation for the Time of Flight (TOF) induced currents obtained in capacitive devices has been performed. The electric fields and the induced charge on the electrodes have been determined from Poisson's equation. The theoretical current shape is in good agreement with experimental TOF currents to a first approximation. These results contribute to the understanding of the electrical transport inductive processes occurring during TOF experiments in insulator media devices. Although the analysis includes simplifying assumptions, the general method can be used to account for dispersive effects in TOF experimental currents.

Associative desorption of HD and D 2 from a Ni(100) surface: angular and velocity distributions

A time of flight experiment was carried out to measure the angular and velocity distributions of HD and D 2 thermally desorbing from Ni(100). The angular distributions can be described by a cos 3jϑ dependence on polar angle. This is indicative of an activated mechanism of desorption. The angle-energy correlation shows a decrease of the average translational energy of desorbing hydrogen molecules from 〈 E〉 ≅ 2.9 kT s close to the surface normal to 〈 E〉 ≅ 1.7 kT s at 60° off normal. A more detail comparing the data with those available in the literature shows that the results are compatible with two desorption channels acting in parallel, one activated, the other non-activated.

Neutron-scattering studies on CeM2Ge2(M=Ag, Au, and Ru).

The results of elastic, quasielastic, and inelastic neutron-scattering studies on polycrystalline CeM 2 Ge 2 (M=Ag, Au, and Ru) are presented. All compounds reveal long-range magnetic order at low temperatures. Ferromagnetic (M=Ru), antiferromagnetic (M=Au), and incommensurate (M=Ag) structures were detected. Using time-of-flight (TOF) techniques, the crystalline electric-field splittings were determined. With high-resolution TOF experiments the temperature and wave-vector dependence of the magnetic relaxation rate was studied