Independent Time Constants Research Articles

Rydberg state dynamics and fragmentation mechanism of N,N,N',N'-tetramethylmethylenediamine.

The non-adiabatic relaxation processes and the fragmentation dynamics of Rydberg-excited N,N,N',N'-tetramethylmethylenediamine (TMMDA) are investigated using femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. Excitation at 208nm populates TMMDA in a charge-localized 3p state. Rapid internal conversion (IC) to 3s produces two charge-delocalized conformers with independent time constants and distinct population ratios. As the system explores the 3s potential surface, the structural evolution continues on a 1.55ps timescale, followed by a slower (12.1ps) relaxation to the ground state. A thorough comparison of the time-dependent mass and photoelectron spectra suggests that ionization out of the 3p state ends up with the parent ion, the vibrational energy of which is insufficient for the bond cleavage. On the contrary, by virtue of the additional energy acquired by IC from 3p, the internal energy deposited in 3s is available to break the C-N bond, leading to the fragment ion. The fragmentation is found to occur on the ion surface instead of the Rydberg surface.

Single-molecule analysis reveals rotational substeps and chemo-mechanical coupling scheme of Enterococcus hirae V1-ATPase

V1-ATPase (V1), the catalytic domain of an ion-pumping V-ATPase, is a molecular motor that converts ATP hydrolysis–derived chemical energy into rotation. Here, using a gold nanoparticle probe, we directly observed rotation of V1 from the pathogen Enterococcus hirae (EhV1). We found that 120° steps in each ATP hydrolysis event are divided into 40 and 80° substeps. In the main pause before the 40° substep and at low ATP concentration ([ATP]), the time constant was inversely proportional to [ATP], indicating that ATP binds during the main pause with a rate constant of 1.0 × 107 m−1 s−1. At high [ATP], we observed two [ATP]-independent time constants (0.5 and 0.7 ms). One of two time constants was prolonged (144 ms) in a rotation driven by slowly hydrolyzable ATPγS, indicating that ATP is cleaved during the main pause. In another subpause before the 80° substep, we noted an [ATP]-independent time constant (2.5 ms). Furthermore, in an ATP-driven rotation of an arginine-finger mutant in the presence of ADP, −80 and −40° backward steps were observed. The time constants of the pauses before −80° backward and +40° recovery steps were inversely proportional to [ADP] and [ATP], respectively, indicating that ADP- and ATP-binding events trigger these steps. Assuming that backward steps are reverse reactions, we conclude that 40 and 80° substeps are triggered by ATP binding and ADP release, respectively, and that the remaining time constant in the main pause represents phosphate release. We propose a chemo-mechanical coupling scheme of EhV1, including substeps largely different from those of F1-ATPases.

Photo-induced enhancement of the dark conductivity in nanocrystalline germanium/amorphous silicon composite thin films

A light-induced increase in the dark conductivity is observed in composite thin films consisting of hydrogenated amorphous silicon (a-Si:H) containing nanocrystalline germanium (nc-Ge) inclusions, synthesized by co-deposition in a dual-chamber plasma enhanced chemical vapor deposition system. Unlike the Staebler-Wronski effect or persistent photoconductivity observed in amorphous semiconductors, this photo-induced excess conductivity is observed even in composite nc-Ge/a-Si:H films with a minimal photosensitivity. The decay of the excess conductivity follows a single-exponential time dependence, with a temperature independent time constant. We propose that spatial separation of photo-excited charge carriers by the compositional morphology present in the film, with tunneling of photo-excited holes into the germanium nanocrystalline inclusions, is involved in the creation and removal of this effect.

Single molecule recordings of lysozyme activity.

Single molecule bioelectronic circuits provide an opportunity to study chemical kinetics and kinetic variability with bond-by-bond resolution. To demonstrate this approach, we examined the catalytic activity of T4 lysozyme processing peptidoglycan substrates. Monitoring a single lysozyme molecule through changes in a circuit's conductance helped elucidate unexplored and previously invisible aspects of lysozyme's catalytic mechanism and demonstrated lysozyme to be a processive enzyme governed by 9 independent time constants. The variation of each time constant with pH or substrate crosslinking provided different insights into catalytic activity and dynamic disorder. Overall, ten lysozyme variants were synthesized and tested in single molecule circuits to dissect the transduction of chemical activity into electronic signals. Measurements show that a single amino acid with the appropriate properties is sufficient for good signal generation, proving that the single molecule circuit technique can be easily extended to other proteins.

Studies on oxygen chemical surface exchange and electrical conduction in thin film nanostructured titania at high temperatures and varying oxygen pressure

We report on oxygen surface exchange studies in ∼450-nm-thick nanocrystalline titania films with an average grain size of ∼13 nm by electrical conductivity relaxation along with the conductivity measurements at varying temperatures and oxygen partial pressures (pO(2)s). By electrochemical impedance spectroscopy technique, the high temperature conductivity was measured in the pO(2) range from ∼10(-16) to ∼10(-6) Pa at temperatures from 973 to 1223 K and activation energy, ΔE(a), for conduction was estimated as ∼3.23 eV at pO(2) ∼10(-11) Pa. Under reducing atmosphere (pO(2) < 10(-6) Pa), two distinct n-type conduction regimes were observed and corresponding predominant defects are discussed while, at high pO(2) regime (pO(2) >10(-6) Pa), ionic conduction appears dominant leading to a conductivity plateau. The surface relaxation was observed to have two independent time constants likely originating from microstructural effects. The surface exchange coefficients are measured as ∼10(-8)-10(-7) m∕s and ∼10(-9)-10(-8) m∕s for each contribution with ΔE(a)s of 2.79 and 1.82 eV, respectively, without much pO(2) dependence across several orders of pO(2) range of ∼10(-16)-10(-6) Pa in the temperature range between 973 and 1223 K. The results are of potential relevance to understanding the near-surface chemical phenomena in nanocrystalline titania which is of great interest for energy and environmental studies.

After-pulsing and cross-talk in multi-pixel photon counters

Multi-pixel photon counters (MPPC) are pixelated Geiger-mode photon manufactured by Hamamatsu photonics. They will be used for reading out all the scintillator elements within the near detector of the T2K experiment. Their performances photo-detection efficiency, dark noise and gain, and their insensitivity to magnetic field fulfill the ND280 requirements. On the other hand, two known issues, cross-talk and after-pulsing may adversely impact the detector response. In this paper, cross-talk and after-pulsing are precisely measured by recording waveforms and identifying all the avalanche pulses. At an over-voltage of 2 V, an avalanche generates on an average 0.5 additional avalanches due to after-pulsing, whereas it generates 0.13 at 1 V over-voltage. After-pulses follow two independent time constants of 15 and 82 ns. The cross-talk probability is a factor of 3 smaller than the after-puling probability. Simulating the MPPC response, we find that after-pulsing and cross-talk do not degrade the detector performance significantly within the expected operating conditions of the T2K near detector.

Measurement of oxygen transport kinetics in epitaxial La 2NiO 4+ δ thin films by electrical conductivity relaxation

The oxygen transport kinetics of mixed ionic and electronic conducting La 2NiO 4 thin films made by pulsed laser deposition (PLD) were measured using the electrical conductivity relaxation (ECR) technique. Since the film thickness is ∼ 3000 Å, the oxygen transport kinetics are controlled by the surface exchange rate. The experimental data are not well described by the usual single time constant model for oxygen surface exchange, but a good fit is obtained using two independent time constants. This model implies that the La 2NiO 4 film consists of two independent regions with different exchange rates that correspond to two different film microstructures.

Calcium Dependence of Exocytosis and Endocytosis at the Cochlear Inner Hair Cell Afferent Synapse

Release of neurotransmitter at the inner hair cell (IHC) afferent synapse is a fundamental step in translating sound into auditory nerve excitation. To study the Ca 2+ dependence of the underlying vesicle fusion and subsequent endocytosis, we combined Ca 2+ uncaging with membrane capacitance measurements in mouse IHCs. Rapid elevations in [Ca 2+] i above 8 μM caused a biphasic capacitance increase corresponding to the fusion of ∼40,000 vesicles. The kinetics of exocytosis displayed a fifth-order Ca 2+ dependence reaching maximal rates of >3 × 10 7 vesicle/s. Exocytosis was always followed by slow, compensatory endocytosis (τ ≅ 15 s). Higher [Ca 2+] i increased the contribution of a faster mode of endocytosis with a Ca 2+-independent time constant of ∼300 ms. These properties provide for rapid and sustained transmitter release from this large presynaptic terminal.

Investigation of GaAs MESFET equivalent circuits using transient current-continuity equation solutions

Transient solutions of Poisson and electron current-continuity equations are made to investigate the dependence of the equivalent-circuit parameters C ds, τ, and R i on the gate and drain biases. We find that C ds is related by a single time constant to R ds. A new model for the Y 21 matrix element is proposed that does not involve the parameter τ. At high frequencies, the frequency-dependent parts of the Y matrices are better modeled using four independent time constants. Comparisons of the simulations with equivalent-circuit parameters extracted for 0.22 μm and 1.15 μm gate-length MESFETs are made.

Mechanism of interaction of the cyanine dye diS-C3-(5) with renal brush-border vesicles.

The equilibrium binding mechanism and kinetics of binding of diS-C3-(5) (3,3'-dipropylthiodicarbocyanine iodide) to rabbit renal brush-border membrane vesicles (BBMV) were examined using steady-state and time-resolved fluorescence, and fluorescence stopped-flow methods. In aqueous solution, diS-C3-(5) exists as a monomer at concentrations less than 5 microM with fluorescence emission peak at 670 nm (excitation 622 nm), anisotropy r = 0.102, and lifetime tau = 1.2 nsec (23 degrees C). Upon addition of increasing BBMV (voltage clamped to 0 mV using K+/valinomycin), the 670 nm emission peak decreases, corresponding to formation of a nonfluorescent membrane dimer, and subsequently a new emission peak at 695 nm increases, corresponding to membrane monomer. Dynamic depolarization studies show that aqueous diS-C3-(5) rotation is unhindered with a rotational rate R = 0.57 nsec-1 while membrane monomer is hindered with steady-state anisotropy r = 0.190, lifetime tau = 2.1 nsec, R = 0.58 nsec-1 and limiting anisotropy r infinity = 0.11. Based on equilibrium fluorescence titrations, the membrane monomer-dimer (M-D) dissociation constant, Kd = [M]2/[D][BBMV], is 0.0013, where BBMV is expressed as membrane phospholipid concentration. Three distinct kinetic processes are identified by stopped-flow experiments in which BBMV are mixed with diS-C3-(5). There is rapid binding of diS-C3-(5) to the membrane to form bound monomer with a 6-msec exponential time constant. The membrane monomer at the membrane outer surface then aggregates to form bound dimer at the outer surface with a concentration independent time constant of 30 msec. The overall dimerization reaction probably consists of a rate-limiting reorientation process (30 msec) followed by a rapid dimerization which occurs on a nanosecond time scale. Finally, there is a 0.8 to 1 sec translocation of membrane dimer between symmetric sites at the inner and outer membrane surfaces. The translocation reaction is the step which is probably sensitive to changes in transmembrane electrical potential.

High-speed integrated circuits based on InP-MISFETs with plasma SiO 2 gate insulator : K. P. Pande and D. Gutierrez. Solid St. Electron.28 (10), 1045 (1985)

Transient solutions of Poisson and electron current-continuity equations are made to investigate the dependence of the equivalent-circuit parameters Cds, τ, and Ri on the gate and drain biases. We find that Cds is related by a single time constant to Rds. A new model for the Y21 matrix element is proposed that does not involve the parameter τ. At high frequencies, the frequency-dependent parts of the Y matrices are better modeled using four independent time constants. Comparisons of the simulations with equivalent-circuit parameters extracted for 0.22 μm and 1.15 μm gate-length MESFETs are made.

Is there a future for TAB? : Donald B. Brown and Martin G. Freedman. Solid St. Technol., 173 (September 1985)

Transient solutions of Poisson and electron current-continuity equations are made to investigate the dependence of the equivalent-circuit parameters Cds, τ, and Ri on the gate and drain biases. We find that Cds is related by a single time constant to Rds. A new model for the Y21 matrix element is proposed that does not involve the parameter τ. At high frequencies, the frequency-dependent parts of the Y matrices are better modeled using four independent time constants. Comparisons of the simulations with equivalent-circuit parameters extracted for 0.22 μm and 1.15 μm gate-length MESFETs are made.

Color Center Studies in Alkali Halides by Pulsed Electron Beam Irradiation

The formation and decay of color centers in alkali halides have been studied using a pulsed electron beam. The F and V -type bands reach maximum heights in optical density at the end of the radiation pulse. V k and V 1 bands in KBr appear to show the first order decay kinetics; their room temperature life times are 1.3 and 3.8 µsec, respectively. A fraction of F and F ' bands bleach by the annihilation with migrating V k centers. The activation energies for the disappearance of H , V 1 and V k centers in KBr are found to be 0.09, 0.25 and 0.39±0.02 eV, respectively. A fraction of H centers in KCl decays with a temperature independent time constant (∼0.13 µsec) which is due to the tunneling with nearby F centers. In crystals containing U -centers (H - ions) the F band continues to grow for several microseconds after the pulse. Absorption bands which decay with the same time constant as the F band growth have been found at 775 mµ (KBr) and 725 mµ (KCl).

Similarity and dimensional relationships in control systems

The essential theorems of dimensional analysis are reviewed and related to the familiar process of normalizing or ‘non-dimensionizing’, by which the results of calculation or observation of relationships between varying quantities may be expressed so as to be applicable to many possible observations. In recording or cataloguing the behaviour of systems of given kinds having various combinations of values of parameters, it is desirable to express the behaviour in terms of the fewest possible composite parameters. It is shown that, for transfer functions, this is achieved by the usual process of normalizing applied to the transfer function when expressed in the form of a ratio of polynomials in p. In systems with feedback this number of parameters may be smaller than the number of the ratios of independent time-constants of the equations. This implies that systems with feedback may have dynamically similar behaviour, even though their basic time-constants are in different ratios. Consideration is given to systems involving non-linear relationship. In such cases the choice of the products, having dimension zero, in terms of which results may most effectively be recorded is particularly important and may be less obvious. A procedure is outlined, using the concepts of sets of affine curves and representative values, by which the appropriate composite variables may be discovered.

Stabilization of carrier-frequency servomechanisms. II. Design formulae for proportional-derivative networks

In an alternating current servomechanism, the error is proportional to the modulation envelope of a modulated-carrier error signal. It is shown in part I that for stability and fidelity of the servo, it is highly desirable that the effect of the controller includes a proportional-derivative action on the modulation envelope. This action may be obtained with various forms of RC networks, including the parallel “T,” bridge “T,” and Wien Bridge forms. This part contains detailed design procedures and tables of values for the various types of proportional-derivative networks. Several forms of parallel “T” networks arise from the fact that there are five independent time constants in the network, while in order to realize the desired transfer characteristic it is necessary to impose only four conditions. It is indicated how the remaining degree of freedom may be used to obtain the most suitable input and output impedances for the source and load impedances with which the parallel “T” is to be used. The derivations for the parallel “T” formulae are given in an Appendix. Tolerance requirements on the components of parallel “T” and bridge “T” networks are derived. If ±1 per cent components are used at 60 cycles, the resonant frequency will lie between 56.4 and 63.6 cycles, and the notch width (rejection band width) will be within ±0.99 cps. of the correct value. In order to guarantee that the phase shift at 60 cycles is within ±10°, the percentage deviation of each part must be less than ( 9.0 T dω 0 ), where ω 0 is the carrier angular frequency, T d the derivative time constant.