Submicrosecond Time Scale Research Articles

Slowing molecules by optical microlinear deceleration

We study the creation of stationary cold molecules by rapid deceleration of supersonically cooled molecules in a high-intensity pulsed optical lattice. Using the heavy molecule ${\mathrm{I}}_{2}$ as an example we predict the evolution of the velocity distribution function of the ensemble during the deceleration period, and show that stationary cold molecules with temperatures below 1 K and densities of approximately ${10}^{12}{\mathrm{cm}}^{\ensuremath{-}3}$ can be produced over length scales of hundreds of micrometers on submicrosecond time scales.

Mechanisms of photorelease of carboxylic acids from 1-acyl-7-nitroindolines in solutions of varying water content

1-Acyl-7-nitroindolines have been found to be useful photoactivated protecting groups for rapid release of carboxylates in aqueous solution. Mechanistic details of carboxylic acid photorelease from model compounds in solutions of varying H2O-CH3CN composition are now reported, using data from product studies, deuterium isotope effects, kinetic studies (UV-Vis), and nanosecond laser flash photolysis. Our data support a mechanism (via T1) that involves reaction of a critical photogenerated intermediate 21 (an acetic nitronic anhydride), obtained via photochemical transfer of the acetyl group from the amide nitrogen to one of the oxygen atoms of the nitro group. This mode of transfer has been reported in the photochemistry of a variety of N-acetyl-o-nitrodiphenyl-amines. Two competing pathways of reaction from 21 that differ in how the acyl group is cleaved are proposed to account for the products observed. In solutions of higher water content, the predominant reaction pathway of 21 is via an A(AL)1-like cleavage that results in formal intramolecular redox reaction of the aromatic ring system, to give the released carboxylic acid and 7-nitrosoindole 3 (after tautomerisation of an initially formed nitroso-3H-indole 9). In solutions of low water content, the major pathway for hydrolysis of 21 is via the standard addition-elimination mechanism (A(AC)2) with water as the nucleophile, that releases the carboxylic acid and nitroindoline 4. Laser flash photolysis studies in wholly aqueous medium gave a transient (within the 20 ns laser pulse) at lambda(max) 450 nm, assignable to 21. Single exponential decay of this species in water (k(obs) = 5 x 10(6) S(-1)) is assigned to the release of the carboxylic acid and formation of the nitroso-3H-indole 9, which is supported by time-resolved measurements of acid release using bromothymol blue. Therefore, 1-acyl-7-nitroindolines photorelease their protected functionalities at rates in the submicrosecond time scale.

Inactivation of microorganisms using pulsed high-current underwater discharges

This article describes the design, construction and operation of an experimental facility to produce pulsed high-current underwater discharges for inactivation of bacteria. Pulsed high-pressure shock waves were produced in a sub-microsecond time scale. Measurements using a piezoelectric probe showed pressures ranging from 80 to 100 MPa. Applying a capacitor energy of 800 J, up to 7.5 log reduction was observed for E. coli after application of five pulses. Four to 5 log reductions were observed for Staphylococcus aureus and Lactic acid bacteria as well as Saccharomyces cerevisiae populations, using same conditions. An increase in the number of applied shock wave pulses resulted in a nearly exponential reduction in the population of the above microorganisms.

The secondary response distortion in transient absorption spectroscopy

Transient absorption spectroscopy has been used for many years to record the kinetics and spectra of transient chemical species following a short photolysis or radiation pulse. Linearity of the detector is always important, but cannot always be assumed for experiments on a submicrosecond timescale, and should never be assumed on a subnanosecond timescale. In addition, fast photodiode detectors often exhibit a delayed secondary response, which depends on the wavelength of excitation. In this article we demonstrate the distortions introduced by secondary detector response with some examples from pulse radiolysis. A method for calibration is demonstrated using the broad visible/near-infrared absorption of solvated electrons in water. A standard decay shape is constructed from stroboscopic measurements at short time, and from the (400–800 nm) response of an EG&G FND100Q silicon diode at times greater than 10 ns. The calibration is applied to the FND100Q silicon diode, and to GMP566 germanium and GAP520 InGaAs diodes manufactured by Germanium Power Devices, Inc. Correction of the latter diode is needed to record a visible/near infrared spectrum of solvated electrons in heavy water at elevated temperatures.

Stepwise cycloreversion of oxetane radical cations with initial C-O bond cleavage.

2,4,6-Triaryl(thia)pyrylium salts have been used as electron-transfer photosensitizers for the cycloreversion of the oxetane ring system. The radical cation of 2,3-diphenyl-4-hydroxymethyloxetane (1) undergoes stepwise splitting via initial O-C2 cleavage. Spin and charge in the resulting intermediate are located in the oxygen and carbon atoms, respectively. Subsequent intramolecular nucleophilic attack produces 2,3-diphenyl-4-hydroxytetrahydrofuran (4a). Formation of this product occurs in the submicrosecond time scale, competing with C3-C4 cleavage to the detectable (lambdamax = 470 nm) trans-stilbene radical cation.

Acyl/aroylperoxyl radicals: a comparative study of the reactivity of peroxyl radicals resulting from the α-cleavage of ketones

Employing 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS2−) as a selective radical probe, nanosecond laser flash photolysis (LFP) and competitive kinetic methods have been used to investigate the reactions of acyl/aroylperoxyl radicals derived from the addition of oxygen to the α-cleavage products of various ketones in methanol. The selectivity of ABTS2− for acyl/aroylperoxyl radicals (as opposed to alkylperoxyl radicals), under the experimental conditions used, is supported by the oxygen concentration dependence of the ABTS˙− radical ion absorption amplitudes for acyl radicals that undergo decarbonylation on sub-microsecond timescales. The oxygen concentration dependence of the ABTS˙− transient absorption reflects the competition between decarbonylation and oxygen addition and therefore this data also provides information on decarbonylation and oxygen addition rates. Further evidence in support of the selectivity of ABTS2− for acyl/aroylperoxyl radicals is the agreement between results obtained from using different ketone precursors for specific acyl/aroylperoxyl radicals. The order of reactivity of the peroxyl radicals towards ABTS2− is found to be aroylperoxyl > acylperoxyl ≫ alkylperoxyl. The use of ABTS2− as a monitor for acyl/aroylperoxyl radicals has been exploited by using competitive kinetics to obtain rate constants for the reactions of various acyl/aroylperoxyl radicals with vitamin E and trolox.

Striking stabilization of Arc repressor by an engineered disulfide bond.

A solvent-exposed Cys11-Cys11' disulfide bond was designed to link the antiparallel strands of the beta sheet both in the Arc repressor dimer and in a single-chain variant in which the Arc subunits are connected by a 15-residue peptide tether. In both proteins, the presence of the disulfide bond increased the T(m) by approximately 40 degrees C. In the single-chain background, the disulfide bond stabilized Arc by 8.5 kcal/mol relative to the reduced form, a significantly larger degree of stabilization than caused by other engineered disulfides and most natural disulfides. This exceptional stabilization arises from a modest effective concentration of the Cys11-Cys11' disulfide in the native state (71 M) and an anomalously low effective concentration in the denatured state (40 microM). Disulfide cross-linking of the two beta strands in the single-chain Arc background accelerated refolding by a factor of 170 into the sub-microsecond time scale. However, the major energetic effect of the disulfide occurs after the transition state for Arc refolding, slowing unfolding by 200 000-fold.

C30 Self-Assembled Monolayers on Silica, Titania, and Zirconia: HPLC Performance, Atomic Force Microscopy, Ellipsometry, and NMR Studies of Molecular Dynamics and Uniformity of Coverage

C30 self-assembled monolayers (SAMs) have been prepared on zirconia, titania, and two different silica gels by reacting C30 trichlorosilane with the humidified surfaces. 13C solid-state NMR spectroscopy indicated higher alkyl chain order on titania and zirconia materials than on the silica C30 phases. Order is inferred from the relative intensity of the main methylene carbon resonance assigned to an all-trans conformation. Carbon longitudinal relaxation time (T1C) data reveal that these ordered alkyl chains still have large-amplitude motions on submicrosecond time scales at ambient temperature. Since fast diffusional rotation about the chain axis is compatible with an all-trans conformation, T1C, carbon chemical shift, and proton line width data for the alkane rotator phase (C19) and the C30 phases were compared. Proton spin diffusion experiments were also conducted using an initial polarization gradient based on mobility differences. These experiments indicated both a higher mobility for the free end of ...

Strong exciton-erbium coupling in Si nanocrystal-doped SiO2

Silicon nanocrystals were formed in SiO2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO2 layer was implanted with Er to a peak concentration of 1.8 at. %. Upon 458 nm excitation the sample shows a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm. By measuring the excitation spectra of these features as well as the temperature-dependent intensities and luminescence dynamics we conclude that (a) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism, (b) the Er excitation process at room temperature occurs at a submicrosecond time scale, (c) excitons excite Er with an efficiency >55%, and (d) each nanocrystal can have at most ∼1 excited Er ion in its vicinity.

Flexibility of Duplex DNA on the Submicrosecond Timescale

Using a site-specific, Electron Paramagnetic Resonance (EPR)-active spin probe that is more rigidly locked to the DNA than any previously reported, the internal dynamics of duplex DNAs in solution were studied. EPR spectra of linear duplex DNAs containing 14–100 base pairs were acquired and simulated by the stochastic Liouville equation for anisotropic rotational diffusion using the diffusion tensor for a right circular cylinder. Internal motions have previously been assumed to be on a rapid enough time scale that they caused an averaging of the spin interactions. This assumption, however, was found to be inconsistent with the experimental data. The weakly bending rod model is modified to take into account the finite relaxation times of the internal modes and applied to analyze the EPR spectra. With this modification, the dependence of the oscillation amplitude of the probe on position along the DNA was in good agreement with the predictions of the weakly bending rod theory. From the length and position dependence of the internal flexibility of the DNA, a submicrosecond dynamic bending persistence length of around 1500 to 1700 Å was found. Schellman and Harvey (Biophys. Chem. 55:95–114, 1995) have estimated that, out of the total persistence length of duplex DNA, believed to be about 500 Å, approximately 1500 Å is accounted for by static bends and 750 Å by fluctuating bends. A measured dynamic persistence length of around 1500 Å leads to the suggestion that there are additional conformations of the DNA that relax on a longer time scale than that accessible by linear CW-EPR. These measurements are the first direct determination of the dynamic flexibility of duplex DNA in 0.1 M salt.

Microsecond motions of the lipids associated with trypsinized Na,K-ATPase membranes. Progressive saturation spin-label electron spin resonance studies.

The microsecond motions of spin-labeled lipids associated with the Na(+)/K(+)-transporting ATP hydrolase (Na,K-ATPase) in native and tryptically shaved membranes from Squalus acanthias have been studied by progressive saturation electron spin resonance (ESR). This includes both the segmental mobility of the lipid chains and the exchange dynamics of the lipids interacting directly with the protein. The lipids at the protein interface display a temperature-dependent chain mobility on the submicrosecond time scale. Exchange of these lipids with those in the bulk bilayer regions of the membrane takes place on the time scale of the nitroxide spin-lattice relaxation, i.e., in the microsecond regime. The off-rates for exchange directly reflect the specificity of ionized fatty acids relative to protonated fatty acids for interaction with the Na,K-ATPase. These essential features of the lipid dynamics at the intramembranous protein surface, namely, a temperature-dependent exchange on the microsecond time scale that reflects the lipid selectivity, are preserved on removing the extramembranous parts of the Na,K-ATPase by extensive trypsinization.

Determination of the distance between Y ox⋅Z and Q −⋅A in photosystem II by pulsed EPR spectroscopy on light-induced radical pairs

Out-of-phase electron spin echo envelope modulation (ESEEM) spectroscopy was used to determine the distances within two consecutive radical pair states initiated by a laser flash in photosystem II membrane fragments at pH 11. The distance between the spin density centers of the primary electron donor cation radical, P +⋅ 680, and the reduced plastoquinone acceptor, Q −⋅ A, has been found to be 27.7±0.7 Å in agreement with previous results. Near room temperature and at high pH, P +⋅ 680 is reduced by Y Z, a redox active tyrosine residue, on a sub-microsecond timescale. As a consequence, the subsequent radical pair state, Y ox⋅ ZQ −⋅ A, could be investigated after almost complete reduction of P +⋅ 680 by Y Z. The determined dipolar electronic spin-spin coupling within the radical pair Y ox⋅ ZQ −⋅ A corresponds to a distance of 34±1 Å between the two molecules.

Fast photomultiplier tube gating system for photon counting applications

A normally “on” linear-focused 14-stage end-window photomultiplier tube (PMT) (Electron Tubes Limited 9893Q/100B), designed for fast photon counting, has been gated through control of the voltage applied to the first dynode. The gating circuit reduces the gain of the PMT during a laser pulse, in order to discriminate against the detection of scattered light, and then increases the gain promptly to observe extremely low levels of laser-induced fluorescence (LIF). An extinction factor for the laser scattered photons of >105 was observed, and has enabled count rates for photons due to LIF as low as 1 Hz to be measured for a laser pulse-repetition frequency of 7 kHz. The rise of the PMT gain is monitored directly by observation of the fluorescence using time-resolved photon counting, and the PMT turn-on time is 30 ns. No significant distortion of the temporal profile of the fluorescence was observed during PMT turn-on. The system, which can also be operated in ungated mode, is rugged and reliable, and has been employed outdoors during atmospheric field measurements of the hydroxyl radical using LIF. The gating circuit is suitable for the rejection of laser scattered light in applications with extremely low levels of fluorescence on the submicrosecond timescale.

Features and technology of ferroelectric electron emission

Spontaneous electrical polarization of ferroelectric materials can be changed either by reversal or by phase transition from a ferroelectric into a nonpolar state or vice versa. If spontaneous polarization changes are induced at a submicrosecond time scale, strong uncompensated surface charge densities and related fields are generated, which may lead to the intense self-emission of electrons from the negatively charged free surface areas of the ferroelectric cathode. The nature of this self-emission differs essentially from other methods of ferroelectric electron emission and from conventional electron emission in that the latter methods are only achieved by extracting electrons with externally applied electric fields. When electron guns are constructed with ferroelectric cathodes, new design criteria have to be taken into account. The intensity, the energy, the temporal and spatial distribution and the repetition rate of the emitted electron beams can be adjusted within wide limits. The advantages of ferroelectric cathodes and the technological difficulties arising during their production, preparation, and operation are identified and discussed and solutions to the problems are proposed. Experiences with a few applications of ferroelectric electron emission are reported and suggestions for further applications are made.

Short-Time-Scale Thermal Mapping of Microdevices Using a Scanning Thermoreflectance Technique

The performance and reliability of microdevices can be strongly influenced by the peak temperature rise and spatial temperature distribution during brief electrical overstress (EOS) phenomena, which can occur at sub-microsecond time scales. The present study investigates short-time-scale laser reflectance thermometry of micro devices by examining the impact of passivation overlayers on the thermoreflectance signal and by demonstrating a calibration method suitable for metallization. This manuscript also describes a scanning laser thermometry facility that captures temperature fields in microdevices with 10 ns temporal resolution and 1 μm spatial resolution. The facility combines scanning laser optics with electrical stressing capability to allow simultaneous interrogation of the thermal and electrical behavior of devices. Data show the transient temperature distribution along the drift region of silicon-on-insulator (SOI) power transistors and along metal interconnects subjected to brief electrical stresses. The theory and experimental capability developed in this study are useful for studying short-time-scale thermal phenomena in microdevices and verifying models employed for their simulation.

Time-Resolved Friction with Applications to High-Speed Machining: Experimental Observations

Pressure-shear plate impact experiments are conducted to study the kinetics of friction on a submicrosecond time scale. The configuration offers the simplicity of allowing the interpretation of the experimental results by using the framework of one-dimensional plane wave analysis. To address the problem of high-speed machining, one of the impacting plates is chosen to correspond to a wear-resistant grade of tungsten-carbide tool material and the other to a commercial-grade titanium alloy (Ti-6Al-4V). The interfacial conditions are varied from sticking to slipping by varying the angle of impact and/or the surface roughness. Also, within a single experiment, transitions from conditions of fully sticking to intermittent, to full sliding are introduced by subjecting the interface to step changes in normal pressure. The experimentally obtained time-resolved frictional response of the interface is used to examine the role of the interfacial normal pressure, slip velocity, slip distance, and surface roughness in governing the interfacial slip resistance. Presented at the 52nd Annual Meeting In Kansas City, Missouri May 18–22, 1997

Pressure Effects on Submicrosecond Phospholipid Dynamics Using a Long-Lived Fluorescence Probe

The effects of applied external hydrostatic pressure on submicrosecond lipid motions in DPPC4 bilayers have been examined using coronene (a long-lived planar fluorescent molecule) and DPH. Steady-state fluorescence emission anisotropy (EA) values ( ) obtained for probe-labeled DPPC SUVs measured at different fixed temperatures above Tc as a function of increasing hydrostatic pressure reveal pressure-induced lipid phase transition profiles. For coronene-labeled samples, the observed lipid “melt” profiles are broad and shifted to higher midpoint EA pressure values (P1/2) compared with corresponding DPH-labeled SUVs at the same temperature. The data suggest lipid motions occurring on the submicrosecond time scale, detected only by using a long-lived fluorescence probe, which occur well above the normally reported “fluid–gel” lipid phase transition. Slopes of the pressure-to-temperature equivalence plots (dP1/2/dT = 39 bar/K) obtained for DPH-or coronene-labeled DPPC SUVs are identical within experimental error and reflect probe independence. For DPH, the slope of the P1/2(T) plot provides the expected phase transition phospholipid volume change. However, intercept values (at P1/2 = 1 bar) or apparent phase transition temperatures obtained from the equivalence plots for the two probes are not equal. Differences appear to arise due to the very disparate fluorescence lifetime values of the two probes, which result in rotational sensitivity of coronene to gel lipid volume fluctuations occurring during the extended time window provided by coronene fluorescence.

Ferroelectric electron emission: Principles and technology

The spontaneous electrical polarization of ferroelectric materials can be changed either by reversal or by phase transition from a ferroelectric into a non-ferroelectric state or vice versa. If spontaneous polarization changes are induced with fast heat, mechanical pressure, laser or electric field pulses on a submicrosecond time scale, strong uncompensated surface charge densities and related polarization fields are generated, which may lead to the intense self-emission of electrons from the negatively charged free surface areas of the ferroelectric sample. Hence, electron guns can be built with extraction-field-free ferroelectric cathodes, which may be easily separated from the high-field regions of post-accelerating gap structures. The intensity, the energy, the temporal and spatial distribution, and the repetitition rate of the emitted electron beams can be controlled within wide limits via the excitation pulses and external focusing and accelerating electromagnetic fields. The technological advantages and difficulties of ferroelectric cathodes during production and in practical operation are identified and discussed. A new design of electron guns with ferroelectric cathodes is described. Experience with a few applications of ferroelectric electron emission is reported and suggestions for further applications are made.

Submicrosecond phospholipid dynamics using a long-lived fluorescence emission anisotropy probe

The use of the long-lived fluorescence probe coronene (mean value of tau(FL) approximately 200 ns) is described for investigating submicrosecond lipid dynamics in DPPC model bilayer systems occurring below the lipid phase transition. Time-resolved fluorescence emission anisotropy decay profiles, measures as a function of increasing temperature toward the lipid-phase transition temperature (T(C)), for coronene-labeled DPPC small unilamellar vesicles (SUVs), are best described in most cases by three rotational decay components (phi(i = 3)). We have interpreted these data using two dynamic lipid bilayer models. In the first, a compartmental model, the long correlation time (phi(N)) is assigned to immobilized coronene molecules located in "gel-like" or highly ordered lipid phases (S-->1) of the bilayer, whereas a second fast rotational time (phi(F) approximately 2-5 ns) is associated with probes residing in more "fluid-like" regions (with corresponding lower ordering, S-->0). Interests here have focused on the origins of an intermediate correlation time (50-100 ns), the associated amplitude (beta(G)) of which increases with increasing temperature. Such behavior suggests a changing rotational environment surrounding the coronene molecules, arising from fluidization of gel lipid. The observed effective correlation time (phi(EFF)) thus reflects a discrete gel-fluid lipid exchange rate (k(FG)). A refinement of the compartmental model invokes a distribution of gel-fluid exchange rates (d(S,T)) corresponding to a distribution of lipid order parameters and is based on an adapted Landau expression for describing "gated" packing fluctuations. A total of seven parameters (five thermodynamic quantities, defined by the free energy versus temperature expansion; one gating parameter (gamma) defining a cooperative "melting" requirement; one limiting diffusion rate (or frequency factor: d(infinity))) suffice to predict complete anisotropy decay curves measured for coronene at several temperatures below the phospholipid T(C). The thermodynamic quantities are associated with the particular lipid of interest (in this case DPPC) and have been determined previously from ultrasound studies, thus representing fixed constants. Hence resolved variables are r(O), temperature-dependent gate parameters (gamma), and limiting diffusion rates (d(infinity)). This alternative distribution model is attractive because it provides a general probe-independent expression for distributed lipid fluctuation-induced probe rotational rates occurring within bilayer membranes below the phospholipid phase transition on the submicrosecond time scale.

Measurement of fast high-voltage transients on electrostatic precipitators

A simple, inexpensive, capacitive divider has been designed for measuring impulse voltages on a sub-microsecond time scale used to produce advanced energisation of electrostatic precipitators. The design is compact, and overcomes the problem of stray capacitance errors and parasitic oscillations in the measurement cables. The frequency response of the divider allows the accurate measurement of the detail of voltage pulses with rise times of 50–100 ns, and durations up to 100 μs, with very high input voltages. A simple calibration procedure has been developed which enables quick, accurate calibration of this divider, in situ, at the correct frequency, using the capacitors of the pulse generator, and the capacitance of the precipitator.