Energy Dissipation Channels Research Articles

Contrasting the Excited-State Dynamics of the Photoactive Yellow Protein Chromophore: Protein versus Solvent Environments

Wavelength- and time-resolved fluorescence experiments have been performed on the photoactive yellow protein, the E46Q mutant, the hybrids of these proteins containing a nonisomerizing “locked” chromophore, and the native and locked chromophores in aqueous solution. The ultrafast dynamics of these six systems is compared and spectral signatures of isomerization and solvation are discussed. We find that the ultrafast red-shifting of fluorescence is associated mostly with solvation dynamics, whereas isomerization manifests itself as quenching of fluorescence. The observed multiexponential quenching of the protein samples differs from the single-exponential lifetimes of the chromophores in solution. The locked chromophore in the protein environment decays faster than in solution. This is due to additional channels of excited-state energy dissipation via the covalent and hydrogen bonds with the protein environment. The observed large dispersion of quenching timescales observed in the protein samples that contain the native pigment favors both an inhomogeneous model and an excited-state barrier for isomerization.

Frictional properties of alkanethiol self-assembled monolayers with different thermal annealing

We have studied the thermal annealing effect on friction of alkanethiol self-assembled monolayers (SAMs) using atomic force microscopy and frictional force microscopy. The friction is found to increase with thermal annealing time once the annealing temperature is high enough. The change in friction is well correlated with the change in the SAM structure. From a densely packed (∛×∛)R30° phase to a (p×∛) stripe phase with lower density, the magnitude of friction is found to increase by ∼10 times. Such an increase in friction is proposed as being due to the less compact surface structure of the stripe phase, which opens up additional energy dissipation channels to the film.

The efficiency of formation of primary radiation defects in LiF and MgF2 crystals

Processes of radiation formation of primary defects—F centers and self-trapped excitons—in lithium and magnesium fluorides, which have crystal lattices of different types and similar widths of the band gap and valence band, have been studied in a wide temperature range (11–500 K). It is shown that, along with qualitative similarity of the regularities of formation of the defects under study, LiF and MgF2 crystals are characterized at low temperatures (11–100 K) by different relationships between the energy dissipation channels for self-trapping electronic excitations and the types of self-trapped excitons arising.

Ultraviolet luminescence of single crystals and single-crystal films of YAlO3

The nature of intrinsic emission bands of yttrium orthoaluminate in the UV spectral region at max=220 nm (5.63 eV) and 330 nm (4.13 eV) is studied on the basis of the luminescence of single crystals and single-crystal films of YAlO3 and Ce: YAlO3 excited by synchrotron radiation sources with an energy of 3–25 eV at 9 and 300 K. The single crystals and single-crystal films were obtained, respectively, from solution and solution-melt by liquid-phase epitaxy and are characterized by considerably different concentrations of substitutional and vacancy defects. It is found that only the luminescence band at 300 nm, which has the decay time τ=4.1 ns and is excited in a band shifted from the range of interband transitions by 0.25 eV, has exciton-like character. The luminescence band at 220 nm with τ=0.1 µs at 9K, which is observed only for YAlO3 single crystals and is absent in the luminescence of single-crystal films, is associated with antisite defects of the YAl3+ type, which are a specific type of cationic isoelectronic impurities. It is shown that the phosphors based on single-crystal films of YAlO3 have a simpler scintillation decay kinetics than their bulk analogues due to the absence of channels of excitation energy dissipation associated with the antisite defects of YAl3+ type and vacancy defects.

Relaxation of a 2D MHD flow across a magnetic field (2D hydrodynamic flow) in a bounded region

The problem on magnetohydrodynamic (MHD) flow of a solitary vortex across a magnetic field in a volume confined by rigid walls is solved numerically for large Reynolds numbers (including magnetic Reynolds numbers) and small Alfven-Mach numbers MA. In this case, the MHD problem is reduced to that of two-dimensional hydrodynamic turbulence. It is shown that sound is not generated by a turbulent medium for small values of MA; consequently, this kinetic energy dissipation channel is closed in this case. Calculations show that, in contrast to 3D turbulence, kinetic energy dissipation for 2D turbulence occurs, as expected, over time periods on the order of L2/v(L is the characteristic size of the system and v is the kinematic viscosity). In our calculations with numerical viscosity v∼vΔx (Δx is the unit cell size), this corresponds to time values on the order of ∼(L/Δx)(L/v). In the kinetic energy spectra for a turbulent flow in a bounded region in the inertial interval (lying between the energy-carrying and viscosity regions), the values of E(k) decrease with increasing wave numbers k at a higher rate than in proportion to k−3. The volume distribution of vorticity becomes narrower with time (the characteristic values of curlv decrease) and is blurred; for large time periods, the distribution approximately retains its shape as well as asymmetry with respect to positive and negative values, which is associated with the asymmetry of the initial conditions.

Effect of C60 molecular rotation on nanotribology.

The effect of C60 molecular rotation on the nanotribological properties of C60 single crystal surfaces has been studied by atomic/frictional force microscopy. The orientational order-disorder phase transition, in which the high temperature C60 free rotation is reduced to a low temperature hindered rotation, is shown to give rise to an abrupt change in friction and adhesion. This change in frictional force is quantitatively consistent with the observed change in adhesion. The similar slopes of the friction versus load curves in both phases indicate that the friction coefficient in the two phases remains about the same. Hence the C60 rotation does not provide an additional energy dissipation channel in the friction process.

Size-dependent hot-electron dynamics in small Pdn−-clusters

Using time-resolved photoelectron spectroscopy we show that electron relaxation processes via inelastic electron–electron scattering are efficient energy dissipation channels not only in bulk metals but also in extremely small transition metal clusters. The photoelectron spectra of optically excited Pd3−, Pd4−, and Pd7− reveal effective electron relaxation times of less than 100 fs. Moreover the relaxation times vary with cluster size. In comparison to simple metal clusters the bulklike inelastic scattering rates in open d-shell transition metal clusters are attributed to the larger valence electron level density. An energy transfer to the vibrational degrees of freedom occurs within 10 ps.

Ultrafast Diffraction of Transient Molecular Structures in Radiationless Transitions

Radiationless transitions in molecules are ubiquitous in photophysical, chemical, and biological systems. Because such transitions are usually ultrafast in nature, the determination of transient structures is extremely difficult, particularly for complex molecules with many degrees of freedom. Here, we report the direct determination of the molecular structure during the ultrafast, nonradiative transition in aromatic pyridine, excited above the so-called channel three threshold, which marks the onset of an anomalous decay. The approach we invoked is ultrafast electron diffraction (UED), developed in this laboratory to image transient molecular structures in real time. The diffraction results reveal the breakage of “old bonds” near 1.4 and 2.4 A and the formation of “new bonds” near 1.3 and 1.5 A and at distances greater than ∼3.5 A, thus permitting the dominant intermediate species to be identified. Unexpectedly, a ring-opened diradical structure resulting from C−N bond scission was observed and was found to form with a rate of (17 ps)-1. This hitherto unknown intermediate must be considered as a new channel for energy dissipation in pyridine and possibly analogous molecules. These results represent a clear demonstration of the potential of UED for studying structural dynamics of radiationless transitions in complex molecular systems.

Relationship between energy dissipation rate and channel morphology in the development of the model braided channel

A certain pattern of channel is the product of its self-adjustment under given boundary, discharge and sediment conditions. Based upon the principle of process-response model, an experimental study with 18 runs is carried out in LESRC. This paper is focused on the variation of the energy dissipation versus the channel morphology during and after the bedmaking process of braided channel. The results show that there exists a good empirical relationship between the energy dissipation rate and channel morphology. According to this relationship and the theory of minimum rate of energy dissipation, the authors explain the metamorphosis of the model channel with the development of the braided river.

Dissipation of the spin wave energy in magnetic multilayer films

It is shown that the penetration of standing spin waves into a layer with high damping is one of the channels of energy dissipation of these waves. The line broadening of spin wave modes due to this layer increases with the mode index and may be much larger than the natural linewidth associated with a layer having low damping. The linewidths of spin wave modes are found to be anisotropic, which is due to the dependence of the penetration depth of spin waves into the high-damping layer on the orientation of the external magnetic field with respect to the film. A theoretical model is proposed, which is consistent with the experimental data.

Dynamics and drag of a vortex in type II superconductor with weak inhomogeneities

The dynamics of the Abrikosov vortex lattice and a single vortex in a type II superconductor with defects is studied taking into account inertial as well as gyroscopic (Hall) properties. The spectrum of normal modes in the absence of defects has two branches. In the limit of small k, one of the branches is gapless and has a quadratic dispersion relation, while the other branch has a finite gap. In the limit of large k, the dispersion relations for both modes become linear (acoustic). It is shown that the interaction with defects in a moving vortex or a vortex lattice excites oscillations corresponding to these modes. This creates an additional energy dissipation channel for translation motion of the vortex. In the case of a single vortex, the corresponding drag force diverges as V−1/2 for V→0, i.e., prevails over the regular force of viscous friction for small velocities.

Soliton formation in a resonant amplifying—absorbing medium

Self-formation of one or several soliton-like pulses from a nonsoliton powerful laser pulse during its propagation through a two-component resonant amplifying—absorbing medium with an energy dissipation channel is studied numerically. Both components of the medium are simulated by two-level particles. Homogeneous mixtures of these components and media with consecutive amplifying and absorbing layers are considered. A qualitative analysis of the results is based on a simplified model of the process of soliton-like pulse propagation.

On the bunching effect of electrons in spatially periodic resonance fields

The spatial relaxation of the electron distribution function (EDF) in uniform and spatially periodic electric fields is considered. The analysis is based on the Boltzmann kinetic equation containing spatial gradients and operators of elastic and inelastic collisions. We demonstrate that the presence of a discrete spectrum of excited states due to the process of electron back scattering leads to the appearance of a quasicontinuous flux of energy in the elastic region, similar to the flux created by means of energy loss in elastic impacts. In the absence of channels of energy dissipation connected with energy loss in elastic impacts and in the presence of several excited states, information about the initial EDF injected in a field of arbitrary configuration, can be transmitted over an unlimited distance.The process of the relaxation has the nature of undamped oscillations. The introduction of channels of energy dissipation results in a damped oscillatory relaxation character of the EDF injected into a uniform field and in the establishment of a homogeneous EDF in this field. The relaxation of an EDF injected into a spatially periodic resonance field results in the distribution function having specific maxima which change in energy and coordinates along the resonance paths in accordance with the potential distribution (the so-called bunching effect). This effect can be used for the interpretation of EDF formation in S- and P-striations in inert gas discharges at low pressures and currents.

High‐latitude ionospheric electrodynamics as determined by the assimilative mapping of ionospheric electrodynamics procedure for the conjunctive SUNDIAL/ATLAS 1/GEM period of March 28–29, 1992

During the conjunctive SUNDIAL/ATLAS 1/GEM campaign period of March 28–29, 1992, a set of comprehensive data has been collected both from space and from ground. The assimilative mapping of ionospheric electrodynamics (AMIE) procedure is used to derive the large‐scale high‐latitude ionospheric conductivity, convection, and other related quantities, by combining the various data sets. The period was characterized by several moderate substorm activities. Variations of different ionospheric electrodynamic fields are examined for one substorm interval. The cross‐polar‐cap potential drop, Joule heating, and field‐aligned current are all enhanced during the expansion phase of substorms. The most dramatic changes of these fields are found to be associated with the development of the substorm electrojet in the post midnight region. Variations of global electrodynamic quantities for this 2‐day period have revealed a good correlation with the auroral electrojet (AE) index. In this study we have calculated the AE index from ground magnetic perturbations observed by 63 stations located between 55° and 76° magnetic latitudes north and south, which is larger than the standard AE index by about 28% on the average over these 2 days. Different energy dissipation channels have also been estimated. On the average over the 2 days, the total globally integrated Joule heating rate is about 102 GW and the total globally integrated auroral energy precipitation rate is about 52 GW. Using an empirical formula, the ring current energy injection rate is estimated to be 125 GW for a decay time of 3.5 hours, and 85 GW for a decay time of 20 hours. We also find an energy‐coupling efficiency of 3% between the solar wind and the magnetosphere for a southward interplanetary magnetic field (IMF) condition.

Modeling Species and Energy Transport in Moderate Pressure Diamond Deposition H2 Plasmas

A one-dimensional diffusion model for H 2 plasma obtained under moderate pressure discharge conditions is presented. The transport equations resulting from this model are solved and enable the estimation of the axial profiles of the plasma species densities and of plasma temperatures on the axis of a diamond deposition microwave plasma reactor. The comparison between the calculated results and experimental measurements shows that the temperatures are well predicted by the model, while some discrepancy was observed for H-atom mole fraction. The analysis of the energy dissipation channels shows that, at low pressure, most of the power lost by the electron is dissipated in activating electron-impact H 2 dissociation reaction. The model is also used to investigate the effect of the microwave power density on the plasma composition and temperatures. It shows that the increase of the power density leads to a strong and non-linear increase of gas temperature and H-atom mole fraction. This result is also in a good agreement with measurements and is consistent with the high quality of the diamond films obtained at high power density discharge conditions.

A minimum energy dissipation model for drainage basins that explicitly differentiates between channel networks and hillslopes

A channel initiation process has been incorporated into the minimum energy dissipation models for channel networks. When a scale parameter λ becomes very large, the model introduced here becomes identical to the standard minimum energy dissipation model of Rigon et al. The structure of the channel networks obtained from this model are more similar to natural river networks on small length scales than those from previous models. The drainage densities D d of the minimum energy dissipation drainage basins are found to be independent of the size A of the basin but proportional to the average hillslopes inclination σ in the basin. The Strahler version of Horton's relationships among the number of streams N S , the average channel length L S and the average drainage area A S are found to be hold well for the minimum energy dissipation channel networks and the Strahler bifurcation ratio is very close to that found in nature. The scaling relationships characterizing the main streams and the boundaries of the drainage networks generated from the minimum energy dissipation model with hillslope processes are similar to those of the drainage networks generated from the standard minimum energy dissipation models without hillslope processes. A universal power law area distribution is also found for the drainage basins obtained from this minimum energy dissipation model.

QUANTUM FIELD APPROACH TO THE THEORY OF DEVELOPED TURBULENCE OF CONDUCTING FERROFLUIDS

Universality of a principle of the renormalization group (RNG) enables to apply it to a wide spectrum of physical problems, including stochastic problems in the mechanics of continuum. In this paper we study using RNG method statistic properties of a randomly stirred ferrohydrodynamic systems with weak magnetization m. It is shown that also in these systems, similarly to magnetohydrodynamic systems, a steady state asymptotic regime exists in which the spectrum of the pulsation energy has Kolmogorov character k−5/3. In such a regime Lorentz force has no influence on the behavior of a turbulent system on large scales with the result that the magnetic field together with the field of magnetization behave as a passive admixture in the velocity field. It was also shown that strong nonlinear interactions of the velocity field with magnetization generate in the equation for dynamics of the field of magnetization term wν∇2m, which opens a new channel for energy dissipation. Dimensionless "magnetization" Prandtl number w−1 represents the relative intensity of the energy dissipation as compared with other channels and it attains a universal value in the turbulent regime.

Ultrafast relaxation of hot minority carriers in p-GaAs

The dynamics of hot minority carriers in p-GaAs is calculated for doping concentrations in the range of 1.5×1017 cm−3 to 1.5×1018 cm−3. It is shown that the electron-hole interaction increases the rate of dissipation of the excess energy of the minority carriers in the early stages of the process. However, this channel for energy dissipation becomes weaker as the cooling of the minority carriers proceeds, an effect more noticeable in the case of high doping levels. When the electron-hole interaction is disregarded, the dissipation rate is always smaller for low doping concentrations.

Fundamental mechanisms of surface photochemistry

The fundamental mechanisms of photochemistry of molecules adsorbed on solid surfaces are revealed from results obtained by a combination of optical techniques, surface probes, and gas phase analysis. While photon-induced processes similar to those in the gas and liquid phase are observed for the adsorbed molecules, the presence of the substrate introduces important channels for energy exchange, dissipation, and adsorbate photochemistry.

Photoacoustic X-ray absorption fine-structure spectroscopy of the NiK-edge

The X-ray absorption in a 5 μm thick Ni foil for energies from 8.2 keV to 8.8 keV was measured by the photoacoustic method using a gas-microphone-cell. Results are presented for both photoacoustic amplitude and phase as a function of incident photon energy. The absorption fine structure above the NiK-edge is clearly resolved. The data are compared with a simple theory for the signal generation, and discrepancies are explained by discussing the various energy dissipation channels following the X-ray absorption.