Thermal Relaxation Rate Research Articles

Dynamics on Multiple Potential Energy Surfaces: Quantitative Studies of Elementary Processes Relevant to Hypersonics.

The determination of thermal and vibrational relaxation rates of triatomic systems suitable for application in hypersonic model calculations is discussed. For this, potential energy surfaces for ground and electronically excited state species need to be computed and represented with high accuracy, and quasiclassical or quantum nuclear dynamics simulations provide the basis for determining the relevant rates. These include thermal reaction rates, state-to-state cross sections, and vibrational relaxation rates. For exemplary systems (i.e., [NNO], [NOO], and [CNO]), all individual steps are described, and a literature overview for them is provided. Finally, as some of these quantities involve considerable computational expense, for the example of state-to-state cross sections, the construction of an efficient model based on neural networks is discussed. All such data is required and being used in more coarse-grained computational fluid dynamics simulations.

Modification of the spectra of correlated charm-anticharm quark pairs in the quark-gluon plasma * * This work was supported by Chinese government scholarship (CSC) for foreigner graduate students and by NSFC (11675079)

Heavy quarks play an important role in probing the properties of strongly interacting quark-gluon plasma (QGP) created in ultra-relativistic heavy-ion collisions. We study the interactions of single heavy (charm) quarks and correlated charm and anticharm ( ) quark pairs with the medium constituents of QGP by performing fireball+Langevin simulations of the pertinent Brownian motion with elastic collisions. Besides studying the traditional observables, the nuclear modification factor and the elliptic flow of single heavy quarks in QGP for different thermal relaxation rates, we also study the broadening of the azimuthal correlations of charm and anticharm quark pairs in the QGP medium for different relaxation rates and transverse momenta classes. We quantified the smearing of pair azimuthal correlations with an increasing thermal relaxation rate: while the (nearly) back-to-back correlations among pairs are almost completely washed out at low transverse momentum (pT), these correlations at high pT largely survive the pair diffusion. This provides a novel observable for diagnosing the properties of QGP.

The N(4S) + O2(X3Σ −g) ↔ O(3P) + NO(X2Π) reaction: thermal and vibrational relaxation rates for the 2A′, 4A′ and 2A′′ states

The N(4S) + O2(X3Σ −g) ↔ O(3P) + NO(X2Π) reaction: thermal and vibrational relaxation rates for the 2A′, 4A′ and 2A′′ states

Kinetics of photo-isomerization of azobenzene containing surfactants

We report on photoisomerization kinetics of azobenzene containing surfactants in aqueous solution. The surfactant molecule consists of a positively charged trimethylammonium bromide head group, a hydrophobic spacer connecting via 6 to 10 CH2 groups to the azobenzene unit, and the hydrophobic tail of 1 and 3CH2 groups. Under exposure to light, the azobenzene photoisomerizes from more stable trans- to metastable cis-state, which can be switched back either thermally in dark or by illumination with light of a longer wavelength. The surfactant isomerization is described by a kinetic model of a pseudo first order reaction approaching equilibrium, where the intensity controls the rate of isomerization until the equilibrated state. The rate constants of the trans-cis and cis-trans photoisomerization are calculated as a function of several parameters such as wavelength and intensity of light, the surfactant concentration, and the length of the hydrophobic tail. The thermal relaxation rate from cis- to trans-state is studied as well. The surfactant isomerization shows a different kinetic below and above the critical micellar concentration of the trans isomer due to steric hindrance within the densely packed micelle but does not depend on the spacer length.

The N(4S) + O2(X3Σ) ↔ O(3P) + NO(X2Π) reaction: thermal and vibrational relaxation rates for the 2A', 4A' and 2A'' states.

The kinetics and vibrational relaxation of the N(4S) + O2(X3Σ-g) ↔ O(3P) + NO(X2Π) reaction is investigated over a wide temperature range based on quasiclassical trajectory simulations on 3-dimensional potential energy surfaces (PESs) for the lowest three electronic states. Reference energies at the multi reference configuration interaction level are represented as a reproducing kernel and the topology of the PESs is rationalized by analyzing the CASSCF wavefunction of the relevant states. The forward rate matches one measurement at 1575 K and is somewhat lower than the high-temperature measurement at 2880 K whereas for the reverse rate the computations are in good agreement for temperatures between 3000 and 4100 K. The temperature-dependent equilibrium rates are consistent with results from JANAF and CEA results. Vibrational relaxation rates for O + NO(ν = 1) → O + NO(ν = 0) are consistent with a wide range of experiments. This process is dominated by the dynamics on the 2A' and 4A' surfaces which both contribute similarly up to temperatures T ∼ 3000 K, and it is found that vibrationally relaxing and non-relaxing trajectories probe different parts of the potential energy surface. The total cross section depending on the final vibrational state monotonically decreases which is consistent with early experiments and previous simulations but at variance with other recent experiments which reported an oscillatory cross section.

AC elastocaloric effect as a probe for thermodynamic signatures of continuous phase transitions.

Studying the response of materials to strain can elucidate subtle properties of the electronic structure in strongly correlated materials. Here, we focus on the elastocaloric coefficients, forming a second rank tensor quantity describing the relation between entropy and strain. In contrast to the better-known elastoresistivity, the elastocaloric effect is a thermodynamic quantity. Experimentally, elastocaloric effect measurements are demanding since the thermodynamic conditions during the measurement have to be well controlled. In this work, we present a technique to measure the elastocaloric effect under quasiadiabatic conditions. The technique is based on oscillating strain, which allows for increasing the frequency of the elastocaloric effect above the thermal relaxation rate of the sample. We apply the technique to Co-doped iron pnictide superconductors and show that the thermodynamic signatures of second order phase transitions in the elastocaloric effect closely follow those observed in calorimetry experiments. In contrast to heat capacity, elastocaloric effect measurements allow for the electronic signatures to be measured against a small phononic background even at high temperatures and in addition give information on the symmetry of the involved order parameters. This establishes the technique as a powerful complimentary tool for extracting the entropy landscape as a function of strain proximate to a continuous phase transition.

PH‐Induced Fluorescence and Thermal Relaxation Rate Modulation in a Hydrazone Photoswitch

AbstractA recently developed family of bistable hydrazone‐based photochromic compounds open new possibilities for molecular machinists with their unique combination of properties. Herein we report on the effect of acid/protonation on the switching and fluorescence emission of a recently reported dimethyl amino hydrazone derivative (1). The fluorescence of 1 can be turned ON/OFF upon deprotonation/protonation cycles, and protonated 1 photoswitches efficiently. Meanwhile, acid catalysis leads to a dramatic acceleration of its thermal relaxation rate (from years to hours).

Mapping the Conditions for Hydrodynamic Instability on Steady-State Accretion Models of Protoplanetary Disks

Abstract Hydrodynamic instabilities in disks around young stars depend on the thermodynamic stratification of the disk and on the local rate of thermal relaxation. Here, we map the spatial extent of unstable regions for the Vertical Shear Instability (VSI), the Convective Overstability (COS), and the amplification of vortices via the Subcritical Baroclinic Instability (SBI). We use steady-state accretion disk models, including stellar irradiation, accretion heating, and radiative transfer. We determine the local radial and vertical stratification and thermal relaxation rate in the disk, which depends on the stellar mass, disk mass, and mass accretion rate. We find that passive regions of disks—that is, the midplane temperature dominated by irradiation—are COS unstable about one pressure scale height above the midplane and VSI unstable at radii >10 au. Vortex amplification via SBI should operate in most parts of active and passive disks. For active parts of disks (midplane temperature determined by accretion power), COS can become active down to the midplane. The same is true for the VSI because of the vertically adiabatic stratification of an internally heated disk. If hydrodynamic instabilities or other nonideal MHD processes are able to create α-stresses (>10−5) and released accretion energy leads to internal heating of the disk, hydrodynamic instabilities are likely to operate in significant parts of the planet-forming zones in disks around young stars, driving gas accretion and flow structure formation. Thus, hydrodynamic instabilities are viable candidates to explain the rings and vortices observed with the Atacama Large Millimeter/submillimeter Array and Very Large Telescope.

Mitigating Thermoelastic Dissipation of Flexural Micromechanical Resonators by Decoupling Resonant Frequency from Thermal Relaxation Rate

Mitigating Thermoelastic Dissipation of Flexural Micromechanical Resonators by Decoupling Resonant Frequency from Thermal Relaxation Rate

Determination of photoisomerization parameters for a PMMA thin film doped with disperse orange 3

An analytic absorption function was derived to investigate the dynamics of photoisomerization in a poly(methyl methacrylate) thin film doped with disperse orange 3 (DO3). The temporal absorption measured using the pump-probe method was well-behaved, following an analytic function with a single exponential term. The isomerization and thermal-relaxation rates were evaluated by curve-fitting. The analytic absorption function was validated by measuring the thermal-relaxation rates with the pump separately on and off. Using this function, an analytic method for determining the photoisomerization parameters was demonstrated: for DO3 in poly(methyl methacrylate), the photoisomerization quantum yields for trans-cis and cis-trans processes were determined to be ϕTC=0.12±0.01 and ϕCT=0.77±0.06, respectively. The absorption cross section for the cis isomer was measured to be σC=(0.92±0.03)×10−16cm2. This analytic method can thus be used to characterize the dynamics of photoisomerization in dye doped thin films.

Phonon transport in nanocomposites

The transport properties of phonons in a nanocomposite have been investigated in this paper. In our model, the nanocomposite consists of nanoparticles embedded in a host dielectric material. The phonon dispersion relation of the nanocomposite is characterized in terms of a defined phonon refractive index and a filling factor parameter. This phonon refractive index depends on its velocity whereas the filling factor is determined by the diameter of the nanoparticles. Consequently, at zero filling, only the host material is present. Our model calculations also include point defects such as impurities, vacancies and the mass and size variations of nanoparticles. The phonon conductivity is determined by employing the Kubo formulism. We also report our calculated results for the phonon relaxation rates due to point defects and boundaries scattering where the latter is due to phonons interacting with the surface boundaries of nanoparticles and the host material. The thermal conductivity, dispersion relation, velocity, density-of-states and relaxation rates of phonons are determined by the filling factor and phonon refractive indices of the nanoparticles. There is a good agreement between our numerical results and the data obtained experimentally for the conductivity of AlN-polyimide nanocomposite. Our results also demonstrate that as the filling factor is increased the phonon conductivity also increases. Possible applications are new types of nanocomposites with variable thermal conductivity by adjusting the filling factor and refractive indices of the nanoparticle and host material. These nanocomposites may be important components in the fabrication of thermal nanodevices used for heating and cooling.

Modification of the thermal relaxation kinetics of the photoinduced (at T = 425 K) metastable dark conductivity of a-Si:H films by weak illumination during the initial stage of relaxation

The effect of weak illumination during the initial stage of relaxation of the dark metastable conductivity of an undoped a-Si:H film, photoinduced at T = 425 K, on the rate of its subsequent thermal relaxation is studied. It is found that the kinetics of relaxation upon illumination or in the absence of illumination is described by stretched exponents with the parameters τ0 and β, which are smaller in the case of illumination. It is shown that a decrease in these parameters increases the rate of thermal relaxation of the dark conductivity of the film. Because the temperature and the illumination intensities at which the study is carried out are low, the changes in the relaxation rate of the metastable conductivity are unlikely associated with significant restructuring of the amorphous network. This may be due to changes in the system of hydrogen bonds, which can result, in particular, from the generation and relaxation of slow photoinduced defects under the influence of illumination.

Functional Mode Hot Electron Transfer Theory

Charge carriers that have been driven out of thermal equilibrium by their excessive vibrational energies are termed hot carriers. A theory has been developed to model the injection of these vibrationally excited electrons by explicitly accounting for the time-dependent thermal relaxation of the electron-transfer driving vibrational mode, as ascertained using functional mode analysis. Specifically, the thermal relaxation rate of the driving mode is first determined through the so-called frozen-phonon approach after which the energy-dependent line shape function is revisited to include memory effects for the vibrational quanta within the framework of Fermi’s golden rule. As shown by the numerical simulations of a 6-methyl-azulene-2-carboxylic acid dye molecule bound to an anatase TiO2[101] surface, our new theory not only yields persistently faster electron injection rates with higher incident photon energy but also exhibits a sharp increase when the vibrational quanta of the photoexcited dye molecule chang...

Dynamics of a chlorophyll dimer in collective and local thermal environments

We present a theoretical analysis of exciton transfer and decoherence effects in a photosynthetic dimer interacting with collective (correlated) and local (uncorrelated) protein-solvent environments. Our approach is based on the framework of the spin-boson model. We derive explicitly the thermal relaxation and decoherence rates of the exciton transfer process, valid for arbitrary temperatures and for arbitrary (in particular, large) interaction constants between the dimer and the environments. We establish a generalization of the Marcus formula, giving reaction rates for dimer levels possibly individually and asymmetrically coupled to environments. We identify rigorously parameter regimes for the validity of the generalized Marcus formula. The existence of long living quantum coherences at ambient temperatures emerges naturally from our approach.

Red-Shifting Azobenzene Photoswitches for in Vivo Use.

Recently, there has been a great deal of interest in using the photoisomerization of azobenzene compounds to control specific biological targets in vivo. These azo compounds can be used as research tools or, in principle, could act as optically controlled drugs. Such "photopharmaceuticals" offer the prospect of targeted drug action and an unprecedented degree of temporal control. A key feature of azo compounds designed to photoswitch in vivo is the wavelength of light required to cause the photoisomerization. To pass through tissue such as the human hand, wavelengths in the red, far-red, or ideally near infrared region are required. This Account describes our attempts to produce such azo compounds. Introducing electron-donating or push/pull substituents at the para positions delocalizes the azobenzene chromophore and leads to long wavelength absorption but usually also lowers the thermal barrier to interconversion of the isomers. Fast thermal relaxation means it is difficult to produce a large steady state fraction of the cis isomer. Thus, specifically activating or inhibiting a biological process with the cis isomer would require an impractically bright light source. We have found that introducing substituents at all four ortho positions leads to azo compounds with a number of unusual properties that are useful for in vivo photoswitching. When the para substituents are amide groups, these tetra-ortho substituted azo compounds show unusually slow thermal relaxation rates and enhanced separation of n-π* transitions of cis and trans isomers compared to analogues without ortho substituents. When para positions are substituted with amino groups, ortho methoxy groups greatly stabilize the azonium form of the compounds, in which the azo group is protonated. Azonium ions absorb strongly in the red region of the spectrum and can reach into the near-IR. These azonium ions can exhibit robust cis-trans isomerization in aqueous solutions at neutral pH. By varying the nature of ortho substituents, together with the number and nature of meta and para substituents, long wavelength switching, stability to photobleaching, stability to hydrolysis, and stability to reduction by thiols can all be crafted into a photoswitch. Some of these newly developed photoswitches can be used in whole blood and show promise for effective use in vivo. It is hoped they can be combined with appropriate bioactive targets to realize the potential of photopharmacology.

Spatially close azo dyes with sub-nanosecond switching speeds and exceptional temporal resolution.

Photoswitchable bis-azo dyes with an outstanding temporal resolution of 10(15) times between the thermal relaxation rates of their two constituting photochromes are reported. Remarkably, the close spatial proximity of both azo photochromes in these molecular assemblies translates in an unprecedented 10(3) -fold acceleration of the thermal isomerization rate of their faster azo unit compared to the one displayed by the isolated counterpart. Indeed, the relaxation time of the fast-isomerizing platform of the herein reported bis-azobenzenes is as low as 200 ps under ambient conditions. In the wake of these results, the bis-azo dyes described herein are invaluable chromophores for the design of multifunctional light-addressable materials in which simultaneous switching in two very different timescales might be essential.

Photochromic organic nanoparticles as innovative platforms for plasmonic nanoassemblies.

The fabrication of hybrid core-shell nanoassemblies involving a nondoped azo photochromic core coated with a dense shell of gold nanoparticles is reported to investigate the influence of localized plasmons onto the azo core photoisomerization. Photochromic organic nanoparticles, regarded as a novel class of high-density photoswitchable nanomaterials, are first elaborated upon precipitation in water of push-pull azo molecules, containing sulfur-terminated units to chelate gold nanoparticles. Photoisomerization studies of the azo nanoparticles reveal significantly higher E → Z photoconversion yields and Z → E thermal back relaxation rate constants compared to those of dyes processed as thin films and in solution, respectively. These unexpected results are ascribed to the large surface-to-volume ratio and cooperative effects encountered in nanoparticles that deform without disassembling under polarized illumination as a result of the weak change in the azo dipole moment. UV-vis spectroscopy and Raman microscopy of the hybrid nanoassemblies show strong optical coupling between both photoactive constituents, confirming that gold nanoparticles are tightly positioned on the azo core surface. Such coupling causes partial quenching of the azo photoisomerization but does not impact the thermal back relaxation. Longer sulfur-terminated chains provide reduced quenching of the photoreaction by the localized plasmons, thereby opening perspectives toward plasmon-mediated deformation of nano-objects for light-controlled nanomechanics.

Red, far-red, and near infrared photoswitches based on azonium ions.

Azonium ions formed by p-amino substituted azo compounds with both ortho- and meta-methoxy substituents exhibit strong absorbance in far-red and near infrared spectral region. The compounds undergo robust photoswitching in aqueous solution and exhibit a range of thermal relaxation rates from 10 μs-100 ms.

The Role of Criticality on the Horizontal and Vertical Scales of Extratropical Eddies in a Dry GCM

Abstract This paper discusses the sensitivity of the horizontal and vertical scales of extratropical eddies when criticality is varied in a dry, primitive-equation, general circulation model. Criticality is a measure of extratropical isentropic slope and when defined appropriately its value is often close to 1 for Earth’s climate. The model is forced by a Newtonian relaxation of temperature to a prescribed temperature profile, and criticality is increased by increasing the thermal relaxation rate on the mean flow. When criticality varies near 1, it is shown that there exists a weakly nonlinear regime in which the eddy scale increases with criticality without involving an inverse cascade, while at the same time the Rossby radius may in fact decrease. The quasigeostrophic instability of the Charney problem is revisited. It is demonstrated that both the horizontal and vertical scales of the most unstable wave depend on criticality, and simple estimates for the two scales are obtained. The authors reconcile the opposite trends of the eddy scale and Rossby radius and obtain an estimate for the eddy scale in terms of the Rossby radius and criticality that is broadly consistent with simulations.

Nonlinear dynamics in aluminum nitride contour-mode resonators

In this work, we discuss the self-heating nonlinear dynamics of aluminum nitride contour mode resonators. The self-heating introduces a quadratic coupling between the resonator motion and its temperature. This coupling produces a nonlinear frequency response identical to the weakly nonlinear Duffing resonator in steady-state. However, since the thermal relaxation rate is much smaller than the mechanical relaxation rate, the dynamics of the resonator differ from the conventional Duffing resonator.