Transit Time Dispersion Research Articles

Fractional Model of Multiple Trapping with Charge Leakage: Transient Photoconductivity and Transit–Time Dispersion

The model of multiple trapping into energy-distributed states is a successful tool to describe the transport of nonequilibrium charge carriers in amorphous semiconductors. Under certain conditions, the model leads to anomalous diffusion equations that contain time fractional derivatives. From this perspective, the multiple-trapping model can be used to interpret fractional transport equations, formulate initial and boundary conditions for them, and to construct numerical methods for solving fractional kinetic equations. Here, we shortly review the application of fractional multiple-trapping equations to problems of transient photoconductivity relaxation and transit–time dispersion in the time-of-flight experiment and discuss the connection of the multiple-trapping model with generalized fractional kinetic equations. Different types of charge leakage are discussed. The tempered fractional relaxation is obtained for recombination via localized states and distributed order equations arise for the non-exponential density of states presented as a weighted mixture of exponential functions. Analytical solutions for photocurrent decay in transient photoconductivity and time-of-flight experiments are provided for several simplified situations.

100-fs time-resolved streak tube design based on anisotropy and post-acceleration technology

Reducing the space charge effect and the time dispersion caused by the edge field effect of the scanning deflection system is the key to realizing a 100-fs streak tube. In this paper, a novel fs streak tube is proposed and designed. The factors affecting its temporal resolution are analyzed theoretically and the specifications are given. Parameters including the electric field distribution and electron transmittance of the two common acceleration systems (planar cathode-mesh accelerating electrode and planar cathode-slit accelerating electrode) are compared with each other and analyzed theoretically. The results show that although the electric field distribution formed by the planar cathode (mesh accelerating electrode) can form uniform electric field, the electron transmittance is very low; planar cathode-slit accelerating structure will defocus the photoelectron beam along the scanning direction, but the electron transmittance in the effective detection range of the cathode is as high as 100%. The defocusing of the photoelectron beam can be removed by setting a narrow slit in front of the anode. The focusing electrode adopts two sets of plate-like structures which are vertically positioned in front and back to form a one-dimensional focusing electric fields along the scanning direction and the slit direction, respectively. The spatial focusing electrode is arranged close to the phosphor screen, which is beneficial to pushing back the cross-point of the electron beam along the spatial direction. Thus, the electron transit time dispersion in the condition of large electron density will decrease. At the same time, the anode can provide a post-accelerating voltage of +5000 V, which is beneficial to shortening the transit time and dispersion of the photoelectrons, thereby improving the temporal resolution. Based on the above theoretical analysis, a novel femtosecond streak tube is designed by using the planar cathode-slit accelerating electrode, anisotropic focusing system and post-accelerating method. The influence of the anode slit width on the spatial and temporal resolution is simulated. The results show that the temporal resolution deteriorates with the increase of the anode slot width (10－50 μm), due to the fact that the increase of the anode slit width will lead to the gradual increase of the size of the electron spot along the scanning direction, which will lead to the increase of the technical time dispersion. In addition, this study gives the simulation results of the femtosecond streak tube when the anode slit width is in a range of 10－50 μm. The results show that the static spatial resolution is higher than 100 lp/mm at MTF = 10%, dynamic spatial resolution is higher than 29 lp/mm at MTF = 10%, the temporal resolution is better than 122 fs in the range of 4-mm cathode effective detection length. When the effective detection length of the cathode is increased to 8 mm, the dynamic spatial resolution of the streak tube is higher than 22 lp/mm at MTF = 10%, and the temporal resolution is better than 191 fs.

100 fs time-resolved streak tube design based on anisotropy and post-acceleration technology

Reducing the space charge effect and the time dispersion caused by the edge field effect of the scanning deflection system is the key to realize the 100-femtosecond streak tube. In this paper, a novel femtosecond streak tube is proposed and designed. The factors affecting the temporal resolution are analyzed theoretically and the specifications are given. Parameters including the electric field distribution and electron transmittance on the two common acceleration system structures (planar cathode -mesh accelerating electrode and planar cathode – slit accelerating electrode) are compared and analyzed theoretically. The results show that although the electric field distribution formed by the planar cathode – mesh accelerating electrode could form uniform electric field, the electron transmittance is very low; planar cathode-slit accelerating structure would defocus the photoelectron beam along the scanning direction, but the electron transmittance in the effective detection range of the cathode is as high as 100%. The defocusing of the photoelectron beam can be removed by setting a narrow slit in front of the anode. The focusing electrode adopts two groups of plate-like structures which are vertically placed front and back, forming one-dimensional focusing electric fields along the scanning and the slit direction, respectively. The spatial focusing electrode is placed close to the phosphor screen, which is beneficial to push back the cross-point of the electron beam along the spatial direction. Thus, the electron transit time dispersion in the condition of large electron density would decrease. At the same time, the anode can provide a post-accelerating voltage of +5000 V, which is beneficial to shorten the transit time and dispersion of the photoelectrons, thereby improving the temporal resolution. Based on the above theoretical analysis, a novel femtosecond streak tube is designed by using planar cathode-slit accelerating electrode, anisotropic focusing system and post-accelerating method. The influence of the anode slit width on the spatial and temporal resolution is simulated. The results show that the temporal resolution deteriorates with the increase of the anode slot width (10 μm ~ 50 μm), due to the increase of the anode slit width will lead to the gradual increase of the size of the electron spot along the scanning direction, which would lead to the increase of the technical time dispersion. In addition, this study gives the simulation results of the femtosecond streak tube when the anode slit width is in the range of 10~50 μm. The results show that the static spatial resolution is higher than 100 lp/mm @ MTF=10%, dynamic spatial resolution is higher than 29 lp/mm @ MTF = 10%, the temporal resolution is better than 122 fs in the range of 4 mm cathode effective detection length. When the effective detection length of the cathode is increased to 8 mm, the dynamic spatial resolution of the streak tube tube is higher than 22 lp/mm @ MTF=10%, and the temporal resolution is better than 191 fs.

An inverse problem for a time-fractional advection equation associated with a nonlinear reaction term

Fractional derivative is an important notion in the study of the contemporary mathematics not only because it is more mathematically general than the classical derivative but also it really has applications to understand many physical phenomena. In particular, fractional derivatives are related to long power-law particle jumps, which can be understood as transient anomalous sub-diffusion model (see Sabzikar F, Meerschaert M, Chen J. Tempered fractional calculus. J Comput Phys. 2015;293:14–28; Sokolov IM, Klafter J, Blumen A. Fractional kinetics. Phys Today. 2002;55:48–54; Sokolov IM, Klafter J. Anomalous diffusion spreads its wings. Phys World. 2005;18:19–22; Zhang Y, Meerschaert MM, Packman AI. Linking fluvial bed sediment transport across scales. Geophys Res Lett. 2012;39(20):20404. doi:10.1029/2012GL053476). Based on the models given in Scher H, Montroll EW. Anomalous transit-time dispersion in amorphous solids. Phys Rev B. 1975;12(6):2455–2477 and Zheng GH, Wei T. Spectral regularization method for solving a time-fractional inverse diffusion problem. Appl Math Comput. 2011;218:1972–1990, we study an inverse problem for the advection equation with a nonlinear reaction term in a two-dimensional semi-infinite domain for which we recover the initial distribution from the observation data provided at the final location x = 1. This problem is severely ill-posed in the sense of Hadamard. Thus, we propose a regularization method to construct an approximate solution for the problem. From that, convergence rate of the regularized solution is obtained under some a priori bound assumptions on the exact solution. Eventually, a numerical experiment is given to show the effectiveness of the proposed regularization methods.

Fractal Generalization of the Scher–Montroll Model for Anomalous Transit-Time Dispersion in Disordered Solids

The Scher–Montroll model successfully describes subdiffusive photocurrents in homogeneously disordered semiconductors. The present paper generalizes this model to the case of fractal spatial disorder (self-similar random distribution of localized states) under the conditions of the time-of-flight experiment. Within the fractal model, we calculate charge carrier densities and transient current for different cases, solving the corresponding fractional-order equations of dispersive transport. Photocurrent response after injection of non-equilibrium carriers by the short laser pulse is expressed via fractional stable distributions. For the simplest case of one-sided instantaneous jumps (tunneling) between neighboring localized states, the dispersive transport equation contains fractional Riemann–Liouville derivatives on time and longitudinal coordinate. We discuss the role of back-scattering, spatial correlations induced by quenching of disorder, and spatiotemporal non-locality produced by the fractal trap distribution and the finite velocity of motion between localized states. We derive expressions for the photocurrent and transit time that allow us to determine the fractal dimension of the distribution of traps and the dispersion parameter from the time-of-flight measurements.

Design of a large-format high-resolution streak camera with a planar photocathode

Streak cameras are widely used as ultrafast diagnostic tools in several research fields. With an increase in the complexity of the experiments being conducted, large-format streak cameras with high spatiotemporal resolution are required for wide-band, high-precision spectroscopic recording. This paper presents the design of a high-resolution large-format streak camera with a planar photocathode. To alleviate the effect of field curvature for a large-format photocathode and to improve its spatial resolution, a weak–strong double-focusing lens coupled with a spherical fluorescent screen is used. A high voltage is also used to reduce transit time dispersion in order to improve temporal resolution. Experimental results show that the minimum effective diameter of the planar photocathode is 40 mm, compare to the existing 30 mm photocathode, which can achieve a minimum spatial resolution of 1000 line pairs and a temporal resolution of approximately 4 ps. The dynamic spatial resolution achieved is 20 lp/mm and the deflection sensitivity is approximately 64 kV/mm.

Beta blockade increases pulmonary and systemic transit time heterogeneity: evaluation based on indocyanine green kinetics in healthy volunteers.

Knowledge of factors influencing the heterogeneity of blood transit times is important in cardiovascular physiology. The aim of the study was to investigate the effect of beta-adrenergic blockade on blood transit time dispersion in awake, anxious volunteers. Recirculatory modelling of the disposition of intravascular markers using parametric forms for transit time distributions, such as the inverse Gaussian distribution, provides the opportunity to estimate the systemic and pulmonary transit time dispersion invivo. The latter is determined by the flow heterogeneity in the microcirculatory network. Using this approach, we have analysed indocyanine green (ICG) disposition data obtained in four subjects by frequent early arterial blood sampling before and after beta-adrenergic blockade by propranolol. Propranolol decreased cardiac output from 9·3±2·8lmin-1 to 3·5±0·47lmin-1 (P<0·05). This reduction was accompanied by a 4·5±0·6-fold and 2·1±0·3-fold increase (P<0·001) in the relative dispersion (dimensionless variance) of blood transit times through the systemic and pulmonary circulation, respectively.

The transient distributions of nuclear weapon-generated tritium and its decay product &amp;lt;sup&amp;gt;3&amp;lt;/sup&amp;gt; He in the Mediterranean Sea, 1952–2011, and their oceanographic potential

Abstract. We present a comprehensive account of tritium and 3He in the Mediterranean Sea since the appearance of the tritium generated by the atmospheric nuclear-weapon testing in the 1950s and early 1960s, based on essentially all available observations. Tritium in surface waters rose to 20–30 TU in 1964 (TU = 1018 × [3H]/H]), a factor of about 100 above the natural level, and thereafter declined 30-fold up to 2011. The decline was largely due to radioactive tritium decay, which produced significant amounts of its stable daughter 3He. We present the scheme by which we separate the tritiugenic part of 3He and the part due to release from the sea floor (terrigenic part). We show that the tritiugenic component can be quantified throughout the Mediterranean waters, typically to a ± 0.15 TU equivalent, mostly because the terrigenic part is low in 3He. This fact makes the Mediterranean unique in offering a potential for the use of tritiugenic 3He as a tracer. The transient distributions of the two tracers are illustrated by a number of sections spanning the entire sea and relevant features of their distributions are noted. By 2011, the 3He concentrations in the top few hundred metres had become low, in response to the decreasing tritium concentrations combined with a flushing out by the general westward drift of these waters. Tritium-3He ages in Levantine Intermediate Water (LIW) were obtained repeated in time at different locations, defining transit times from the LIW source region east of Rhodes. The ages show an upward trend with the time elapsed since the surface-water tritium maximum, which arises because the repeated observations represent increasingly slower moving parts of the full transit time spectrum of LIW. The transit time dispersion revealed by this new application of tritium-3He dating is considerable. We find mean transit times of 12 ± 2 yr up to the Strait of Sicily, 18 ± 3 yr up to the Tyrrhenian Sea, and 22 ± 4 yr up into the Western Mediterranean. Furthermore, we present full Eastern Mediterranean sections of terrigenic 3He and tritium-3He age in 1987, the latter one similarly showing an effect of the transit time dispersion. We conclude that the available tritium and 3He data, particularly if combined with other tracer data, are useful for constraining the subsurface circulation and mixing of the Mediterranean Sea.

Cardiac output and systemic transit time dispersion as determinants of circulatory mixing time: a simulation study

A new approach to characterize the kinetics of intravascular mixing process is presented. The mixing time, defined as the time required for achieving 95% homogeneity, is calculated by numerical simulations using a circulatory model applied to the intravascular marker indocyanine green (ICG). The results suggest that the mixing time is determined by cardiac output and the relative dispersion of transit time distribution across the systemic circulation, whereby the rate of mixing increases with increasing cardiac output and decreasing transit time dispersion, and vice versa. The estimation of plasma volume from simulated ICG dilution data using the backextrapolation method shows that slow mixing is accompanied by an overestimation of blood volume. This error may be negligible for mixing times of less than approximately 3 min but high in disease states characterized by low cardiac output and/or high transit time dispersion. In view of the role of transit time dispersion as determinant of intravascular mixing, it would be interesting to know more about the effect of disease states on systemic transit time dispersion.

Transit time dispersion in pulmonary and systemic circulation: effects of cardiac output and solute diffusivity

We present an in vivo method for analyzing the distribution kinetics of physiological markers into their respective distribution volumes utilizing information provided by the relative dispersion of transit times. Arterial concentration-time curves of markers of the vascular space [indocyanine green (ICG)], extracellular fluid (inulin), and total body water (antipyrine) measured in awake dogs under control conditions and during phenylephrine or isoproterenol infusion were analyzed by a recirculatory model to estimate the relative dispersions of transit times across the systemic and pulmonary circulation. The transit time dispersion in the systemic circulation was used to calculate the whole body distribution clearance, and an interpretation is given in terms of a lumped organ model of blood-tissue exchange. As predicted by theory, this relative dispersion increased linearly with cardiac output, with a slope that was inversely related to solute diffusivity. The relative dispersion of the flow-limited indicator antipyrine exceeded that of ICG (as a measure of intravascular mixing) only slightly and was consistent with a diffusional equilibration time in the extravascular space of approximately 10 min, except during phenylephrine infusion, which led to an anomalously high relative dispersion. A change in cardiac output did not alter the heterogeneity of capillary transit times of ICG. The results support the view that the relative dispersions of transit times in the systemic and pulmonary circulation estimated from solute disposition data in vivo are useful measures of whole body distribution kinetics of indicators and endogenous substances. This is the first model that explains the effect of flow and capillary permeability on whole body distribution of solutes without assuming well-mixed compartments.

Achieving 280fs resolution with a streak camera by reducing the deflection dispersion

The factors that limit the temporal resolution in a streak camera operating in an accumulative mode were studied. By controlling the timing jitter, the transit-time dispersion and the technical resolution of the camera on the order of 100fs, the role of the deflection dispersion was investigated experimentally. It was done by changing the electron beam size in the deflection plates with a variable slit in front of the plates. The temporal resolution of the camera reached 280fs when the slit width was a 5μm slit.

Charge carrier mobility in regioregular poly(3-hexylthiophene) probed by transient conductivity techniques: A comparative study

The temperature and electric-field dependence of charge carrier mobility has been studied by a conventional time-of-flight technique in chemically purified, low dark conductivity samples of regioregular poly(3-hexylthiophene). Subsequently, the mobility of doping-induced charge carriers has been determined using the technique of charge carrier extraction by linearly increasing voltage in the same samples exposed to air. The charge carrier mobility determined by both experimental techniques correspond well to each other at temperatures above $130\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating that these experimental techniques are mutually consistent. The study clearly shows that the typical $\mathrm{log}\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\ensuremath{\propto}\ensuremath{\beta}{E}^{1∕2}$, $\ensuremath{\beta}g0$ Poole--Frenkel-like electric-field dependence of the charge carrier mobility diminishes at temperatures around $250--270\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and $\ensuremath{\beta}$ becomes negative at higher temperatures. Such negative electric-field dependence of mobility observed by both experimental techniques is attributed to positional disorder in a random-organic dielectric and analyzed in the framework of the disorder formalism. Finally, the overall agreement indicates that the mode of charge generation has negligible effect on the temperature- and electric-field dependence of mobility except at the lowest temperatures $(l110\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, where transit time dispersion of the photogenerated charge carriers probed by the ToF technique is more pronounced.

パーコレーションクラスターにおける過渡ホッピング輸送

Monte Carlo simulation for percolative hopping transport has been carried out to explain the increase in transit-time dispersion and decrease in drift mobility with increasing applied electric field, found in molecularly doped polymers with low concentrations of charge transporting molecules. These two phenomena cannot be understood in terms of the Gaussian disorder model (GDM), which has been widely accepted for the analysis of the transport data of molecularly doped polymers. It is found that the two phenomena are successfully explained by the present computer simulation. The GDM analysis is applied to the analysis of the simulation data, and it is shown that energetic disorder can be estimated from the GDM analysis even in the case of percolative hopping transport and that the percolative motion of hopping carriers is one of the origins of geometrical disorder. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(2): 1–9, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10014

Kinetic analysis of vascular marker distribution in perfused rat livers after regeneration following partial hepatectomy

Background/Aims: Liver clearance models are based on information (or assumptions) on solute distribution kinetics within the microvasculatory system. The aim was to study albumin distribution kinetics in regenerated livers and in livers of normal adult rats. Methods: A novel mathematical model was used to evaluate the distribution space and the transit time dispersion of albumin in livers following regeneration after a two-thirds hepatectomy compared to livers of normal adult rats. Outflow curves of albumin measured after bolus injection in single-pass perfused rat livers were analyzed by correcting for the influence of catheters and fitting a long-tailed function to the data. Results: The curves were well described by the proposed model. The distribution volume and the transit time dispersion of albumin observed in the partial hepatectomy group were not significantly different from livers of normal adult rats. Conclusions: These findings suggest that the distribution space and the transit time dispersion of albumin ( CV 2) is relatively constant irrespective of the presence of rapid and extensive repair. This invariance of CV 2 implies, as a first approximation, a similar degree of intransinusoidal mixing. The finding that a sum of two (instead of one) inverse Gaussian densities is an appropriate empirical function to describe the outflow curve of vascular indicators has consequences for an improved prediction of hepatic solute extraction.

Transit time dispersion in the pulmonary arterial tree

Knowledge of the contributions of arterial and venous transit time dispersion to the pulmonary vascular transit time distribution is important for understanding lung function and for interpreting various kinds of data containing information about pulmonary function. Thus, to determine the dispersion of blood transit times occurring within the pulmonary arterial and venous trees, images of a bolus of contrast medium passing through the vasculature of pump-perfused dog lung lobes were acquired by using an X-ray microfocal angiography system. Time-absorbance curves from the lobar artery and vein and from selected locations within the intrapulmonary arterial tree were measured from the images. Overall dispersion within the lung lobe was determined from the difference in the first and second moments (mean transit time and variance, respectively) of the inlet arterial and outlet venous time-absorbance curves. Moments at selected locations within the arterial tree were also calculated and compared with those of the lobar artery curve. Transit times for the arterial pathways upstream from the smallest measured arteries (200-micron diameter) were less than approximately 20% of the total lung lobe mean transit time. Transit time variance among these arterial pathways (interpathway dispersion) was less than approximately 5% of the total variance imparted on the bolus as it passed through the lung lobe. On average, the dispersion that occurred along a given pathway (intrapathway dispersion) was negligible. Similar results were obtained for the venous tree. Taken together, the results suggest that most of the variation in transit time in the intrapulmonary vasculature occurs within the pulmonary capillary bed rather than in conducting arteries or veins.

Intrahepatic blood flow distribution in the perfused rat liver: effect of hepatic artery perfusion.

Variations in blood flow to different sinusoids within the liver can prevent uniform uptake of solutes from plasma and contribute to cellular ischemia in low-flow states. However, the degree of variability and the role of hepatic artery perfusion in maintaining uniform flow are poorly defined. We used an indicator dilution technique to compare the distribution of sinusoidal transit times in isolated rat livers perfused through the portal vein alone with livers perfused using both portal vein and hepatic artery. Physiological flow rates were used in each case (1.2 +/- 0.3 ml.min-1.g liver-1), but the second group received 32% of flow through the hepatic artery. Intralobular flow heterogeneity was further assessed by gamma counting of small (approximately 100 mg) pieces of the liver after bolus injection of approximately 5 mCi of a highly extracted compound ([125I])triiodothyronine) into the portal vein. Hepatic artery perfusion had no significant effect on mean sinusoidal transit time or intrahepatic distribution volume for 51Cr-labeled red blood cells or 125I-albumin. Analysis of the outflow profiles indicated that hepatic artery perfusion did not affect transit time dispersion. However, heterogeneity of flow to individual portions of the liver, measured as the coefficient of variation, increased from 19 to 30%. These results indicate relatively uniform perfusion of the sinusoids in the portally perfused rat liver and that additional perfusion of the hepatic artery does not further improve hemodynamics. These results have significance for the design and interpretation of transport studies with the use of the perfused rat liver model.

Tissue distribution kinetics as determinant of transit time dispersion of drugs in organs: Application of a stochastic model to the rat hindlimb

A stochastic theory of drug transport in a random capillary network with permeation across the endothelial barrier is coupled with a model of tissue residence time of drugs assuming radial intratissue diffusion. Axial diffusion is neglected both in tissue as well as in the radially well-mixed vascular phase. The convective transport through the microcirculatory network is characterized by an experimentally determined transit time distribution of a nonpermeating vascular indicator. This information is used to identify three adjustable model parameters characterizing permeation, diffusion, and steady-state distribution into tissue. Predictions are made for the influence of distribution volume, capillary permeability, and tissue diffusion on transit time distributions. The role of convection (through the random capillary network), permeation, and diffusion as determinants of the relative dispersion of organ transit times has been examined. The relationship to previously proposed models of capillary exchange is discussed. Results obtained for lidocaine in the isolated perfused hindleg in rats indicate that although the contribution of intratissue diffusion to the dispersion process is relatively small in quantitative terms, it has a pronounced influence on the shape of the impulse response curve. The theory suggests that the rate of diffusion in muscle tissue is about two orders of magnitude slower than in water.

Arterial, capillary, and venous transit times and dispersion measured in isolated rabbit lungs.

Transit time and relative dispersion of the arterial, capillary, and venous segments of the pulmonary circulation were measured in isolated perfused rabbit lungs. Fluorescence videomicroscopy was used to record the passage of dye through the main pulmonary artery, subpleural microcirculation, and venous outflow. Dye dilution curves were obtained at the main pulmonary artery, subpleural arterioles and venules, and pulmonary vein. Measurements were made at 5-cmH2O airway pressure, at blood flows of approximately 80, 50, and 25 ml.min-1.kg-1, and at left atrial pressures of approximately 0 cmH2O (zone 2) and approximately 12 cmH2O (zone 3). The dye dilution curves were modeled as lagged normal density curves that were used to calculate transit time and relative dispersion between the pulmonary artery and arteriole (artery), arteriole and venule (capillary), venule and pulmonary vein (vein), and pulmonary artery and pulmonary vein (whole lung). In open-chest anesthetized dogs, the passage of dye was recorded from the subpleural arterioles and venules between the seventh and eighth ribs in the left lateral position. At comparable blood flows, capillary transit time was larger in the dog than in the rabbit lung [3.4 +/- 2.4 (SD) vs. 0.87 +/- 0.47 s]. In the rabbit lung, relative dispersion was greater in pulmonary capillaries (average values 0.83-1.6) and veins (0.91-1.6) than in arteries (0.39-0.50), which was similar to the whole lung dispersion (0.47-0.52). A similarly high dispersion (0.93) was measured in the dog's pulmonary capillaries. Thus high dispersion in pulmonary capillaries and veins cannot be detected by whole lung dispersion measurements.

Ultrafast x-ray streak camera for use in ultrashort laser-produced plasma research

In recent years there has been growing interest in energetic (≳100 eV), temporally short (&amp;lt;10 ps) x rays produced by ultrashort laser-produced plasmas. The detection and temporal dispersion of the x rays using x-ray streak cameras has been limited to a resolution of 2 ps, primarily due to the transit time dispersion of the electrons between the photocathode and the acceleration grid. The transit time spread of the electrons traveling from the photocathode to the acceleration grid is inversely proportional to the accelerating field. By increasing the field by a factor of 7, we have minimized the effects of transit time dispersion in the photocathode/accelerating grid region and produce an x-ray streak camera with subpicosecond temporal resolution (≊900 fs). The streak camera has been calibrated using a Michelson interferometer and 100 fs, 400 nm laser light. The characteristics of the streak camera, along with the most recent x-ray streak data will be presented.

Electric-field dependence of the photocarrier hopping mobility in bismuth silicon oxide

The photocarrier mobility of bismuth silicon oxide was measured through a time-of-flight technique. To determine the photocarrier transit time, a method of data analysis was applied that has been developed for transit time dispersion in amorphous solids. The universality of current shape typical of low-mobility amorphous solids was observed and an electric-field dependence of the photocarrier mobility in bismuth silicon oxide is now reported. In accordance with hopping conduction among localized band-gap sites, the photocarrier mobility in bismuth silicon oxide decreases with increasing electric field.