Thermal decomposition of toluene: Overall rate and branching ratio

The two-channel thermal decomposition of toluene, C₆H₅CH₃ → C₆H₅CH₂ + H (1) and C₆H₅CH₃ → C₆H₅ + CH₃ (2), was investigated in shock tube experiments over the temperature range of 1400-1780 K at a pressure of 1.5 (±0.1) bar. Rate coefficients for reactions (1) and (2) were determined by monitoring benzyl radical (C₆H₅CH₂) absorption at 266 nm during the decomposition of toluene diluted in argon and modeling the temporal behavior of the benzyl concentration with a kinetic model. The first-order rate coefficients determined at a pressure of 1.5 bar are expressed by k₁(T) = 2.09 × 10¹⁵ exp (−87510 [cal/mol]/RT) [s⁻¹] and k₂(T) = 2.66 × 10¹⁶ exp (−97880 [cal/mol]/RT) [s⁻¹]. The resulting branching ratio, k₁/(k₁ + k₂), ranges from 0.8 at 1350 K to 0.6 at 1800 K.     

Tunable mid-IR laser absorption sensor for time-resolved hydrocarbon fuel measurements

A wavelength-tunable mid-infrared (mid-IR) laser is used to make time-resolved absorption measurements of methyl-cyclohexane (MCH) and n-dodecane vapor concentration, demonstrating the use of this novel laser source for sensing hydrocarbon fuels. Two sensitive and species-specific diagnostic strategies are investigated: (1) direct absorption at a fixed wavelength, and (2) dual-wavelength differential absorption with two rapidly-alternating wavelengths. The tunable laser light is produced using difference frequency generation by combining two near-infrared diode lasers in a periodically poled lithium niobate crystal, providing a continuous-wave (cw), room temperature mid-IR source with the low intensity noise, and rapid wavelength tunability typical of telecommunications diode lasers. Direct absorption measurements of MCH with a wavelength of 3413.7 nm demonstrate fast time response (1 μs) and low noise in cell (300-675 K) and shock tube (650-1450 K) experiments. The detection limits of MCH range from 0.5 ppm-m at 300 K to 11 ppm-m at 1440 K (pressure = 101 kPa). Next, time-division multiplexing is used to alternately generate two mid-IR wavelengths at 20 kHz, enabling the use of dual-wavelength differential absorption to eliminate interference absorption. Measurements of MCH concentration are first made in a cell, with varying amounts of n-heptane interference absorption. Accurate values of MCH concentration are obtained for n-heptane/MCH ratios as high as 15, demonstrating the utility of this sensor for species-specific hydrocarbon detection in systems with interfering absorption. Finally, time-resolved n-dodecane vapor concentration measurements are made in a shock-heated evaporating aerosol. The dual-wavelength differential absorption diagnostic is sensitive only to the vapor concentration, rejecting droplet extinction. These measurements illustrate the power of the differential absorption strategy for sensitive vapor-phase detection in the presence of particle scattering. The tunability of this new source will allow these concepts to be extended to other hydrocarbon fuels.     

Waves in interplanetary shocks: a wind/WAVES study

We describe results from the first statistical study of waveform capture data during 67 interplanetary (IP) shocks with Mach numbers ranging from approximately 1-6. Most of the waveform captures and nearly 100% of the large amplitude waves were in the ramp region. Although solitary waves, Langmuir waves, and ion acoustic waves (IAWs) are all observed in the ramp region of the IP shocks, large amplitude IAWs dominate. The wave amplitude is correlated with the fast mode Mach number and with the shock strength. The observed waves produced anomalous resistivities from approximately 1-856 Omega.m (approximately 10(7) times greater than classical estimates.) The results are consistent with theory suggesting IAWs provide the primary dissipation for low Mach number shocks.     

Universal set of quantum gates for double-dot spin qubits with fixed interdot coupling

We propose a set of universal gate operations for the singlet-triplet qubit realized by two-electron spins in a double quantum dot, in the presence of a fixed inhomogeneous magnetic field. All gate operations are achieved by switching the potential offset between the two dots with an electrical bias, and do not require time-dependent control of the tunnel coupling between the dots. We analyze the two-electron dynamics and calculate the effective qubit rotation angle as a function of the applied electric bias. We present explicit gate sequences for single-qubit rotations about two orthogonal axes, and a CNOT gate sequence, completing the universal gate set.     

Experimental signature of phonon-mediated spin relaxation in a two-electron quantum dot

We observe an experimental signature of the role of phonons in spin relaxation between triplet and singlet states in a two-electron quantum dot. Using both the external magnetic field and the electrostatic confinement potential, we change the singlet-triplet energy splitting from 1.3 meV to zero and observe that the spin relaxation time depends nonmonotonously on the energy splitting. A simple theoretical model is derived to capture the underlying physical mechanism. The present experiment confirms that spin-flip energy is dissipated in the phonon bath.     

Injectable Capsaicin for the Management of Pain Due to Osteoarthritis

Capsaicin is a potent agonist of the TRPV1 channel, a transduction channel that is highly expressed in nociceptive fibers (pain fibers) throughout the peripheral nervous system. Given the importance of TRPV1 as one of several transduction channels in nociceptive fibers, much research has been focused on the potential therapeutic benefits of using TRPV1 antagonists for the management of pain. However, an antagonist has two limitations. First, an antagonist in principle generally only affects one receptor. Secondly, most antagonists must have an ongoing presence on the receptor to have an effect. Capsaicin overcomes both liabilities by disrupting peripheral terminals of nociceptive fibers that express TRPV1, and thereby affects all of the potential means of activating that pain fiber (not just TRPV1 function). This disruptive effect is dependent on the dose and can occur within minutes. Thus, unlike a typical receptor antagonist, continued bioavailability at the level of the receptor is not necessary. By disrupting the entire terminal of the TRPV1-expressing nociceptive fiber, capsaicin blocks all the activation mechanisms within that fiber, and not just TRPV1 function. Topical capsaicin, an FDA approved treatment for neuropathic pain, addresses pain from abnormal nociceptor activity in the superficial layers of the skin. Effects after a single administration are evident over a period of weeks to months, but in time are fully reversible. This review focuses on the rationale for using capsaicin by injection for painful conditions such as osteoarthritis (OA) and provides an update on studies completed to date.     

Shock tube study of the reaction of CH with N2: overall rate and branching ratio

We have studied the reaction between CH and N2, (1) CH + N2 --> products, in shock tube experiments using CH and NCN laser absorption. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1 nm. The overall rate coefficient of the CH + N2 reaction was measured between 1943 and 3543 K, in the 0.9-1.4 atm pressure range, using a CH perturbation approach. CH profiles recorded upon shock-heating dilute mixtures of ethane in argon and acetic anhydride in argon were perturbed by the addition of nitrogen. The perturbation in the CH concentration was principally due to the reaction between CH and N2. Rate coefficients for the overall reaction were inferred by kinetically modeling the perturbed CH profiles. A least-squares, two-parameter fit of the current overall rate coefficient measurements was k1 = 6.03 x 1012 exp(-11150/T [K]) (cm3 mol-1 s-1). The uncertainty in k1 was estimated to be approximately +/-25% and approximately +/-35% at approximately 3350 and approximately 2100 K, respectively. At high temperatures, there are two possible product channels for the reaction between CH and N2, (1a) CH + N2 --> HCN + N and (1b) CH + N2 --> H + NCN. The large difference in the rates of the reverse reactions enabled inference of the branching ratio of reaction 1, k1b/(k1b + k1a), in the 2228-2905 K temperature range by CH laser absorption in experiments in a nitrogen bath. The current CH measurements are consistent with a branching ratio of 1 and establish NCN and H as the primary products of the CH + N2 reaction. A detailed and systematic uncertainty analysis, taking into account experimental and mechanism-induced contributions, yields a conservative lower bound of 0.70 for the branching ratio. NCN was also detected by continuous-wave, narrow-line-width laser absorption at 329.13 nm. The measured NCN time histories were used to infer the rate coefficient of the reaction between H and NCN, H + NCN --> HCN + N, and to estimate an absorption coefficient for the NCN radical.     

Effect of acoustic resonance on the dynamic lift of tube arrays

The effect of acoustic resonance on the dynamic lift force acting on the central tube in square and normal triangle tube arrays is investigated experimentally. For each array pattern three different tube spacing ratios, corresponding to small, intermediate and large spacing ratios, are tested. The resonant sound field in the tube array is found to cause two main effects. First, it generates a “sound-induced” dynamic lift due to the resonant acoustic pressure distribution on the surface of the tube, and secondly, it synchronizes vorticity shedding from the tubes and thereby enhances the hydrodynamic lift force due to vortex shedding. The combined effect of these two unsteady lift forces depends on the phase shift between them, which is dictated by the frequency ratio of the acoustic mode to the natural vortex shedding frequencies. When the flow velocity is increased during the coincidence resonance range, the phase shift increases rapidly and therefore the effects of the two lift components change from reinforcing to counteracting each other. For the pre-coincidence lock-on range, the frequency ratio remains larger than unity and the two lift components always reinforce each other. Numerical simulations are also performed to compute the sound-induced lift force, and sound-enhancement coefficients are developed to estimate the effect of sound on the vortex shedding forces. The simulation and experimental results are implemented in a simplified design guide, which can be used to evaluate the dynamic lift forces acting on the tubes during acoustic resonances.     

Substituted 1,3-bis(imino)isoindole diols: a new class of proton transfer dyes

A new class of excited-state intramolecular proton transfer (ESIPT) dyes based on a 1,3-bis(imino)isoindole diol motif has been prepared. These molecules exhibit orange emission (～600 nm) with a large apparent Stokes shift (>6000 cm(-1)) and quantum efficiencies up to 45%. Selective modification of the substituents can be used to shift the equilibrium between the enol and keto forms of the molecule in both the ground and excited states.