Model Of Energy Relaxation Research Articles

Air-Stable Bulk Halide Single-Crystal Scintillator Cs3Cu2I5 by Melt Growth: Intrinsic and Tl Doped with High Light Yield.

Ternary metal halides with large exciton binding energy have recently gained considerable attention in the optoelectronic field due to their high photoluminescence quantum yield and large Stokes shift. Here, efficient scintillators are designed based on these advantageous properties. For the first time, bulk Cs3Cu2I5 is grown using a melt method other than the intensively reported solution growth, and behaved as an intrinsic scintillator, emitting bright blue (∼450 nm) light under X-ray and γ-ray irradiation. Successful Tl doping at Cs sites tune the emission band over the entire visible range (400-700 nm) due to the synergetic effects of self-trapped excitons (STEs) and Tl centers. Notably, after doping with 1% Tl+, the scintillation light yield of Cs3Cu2I5 increases by nearly three times to 51 000 ± 2000 ph/MeV (Cs-137, 662 keV). Cs3Cu2I5:Tl shows a higher energy resolution of 4.5% at 662 keV than that of NaI:Tl and an excellent nonproportionality (<3%) in the γ-ray energy range of 60-1275 keV. A model of energy relaxation in Cs3Cu2I5:Tl scintillators is proposed and discussed. In particular, it is the first Cu-based halide scintillator that has air stability, good stopping power, and the ability to grow large bulk single crystals for practical application. This work provides a strategy for tuning and broadening the spectral range of STE emitters, and bridges the lead-free halide derivatives with scintillators.

Multiple temperature model of nonlinear coupled constitutive relations for hypersonic diatomic gas flows

The rotational energy of diatomic gases would be activated by the process of intermolecular collisions in high-temperature hypersonic flows. In this paper, a multi-temperature nonlinear coupled constitutive model has been proposed for simulating the transfer of energy between translational and rotational motions in hypersonic non-equilibrium flows. In this model, the nonlinear coupled constitutive equations are modified by introducing a rotational energy relaxation model and a changeable viscosity ratio related to local temperature. To confirm its accuracy, the new model is applied to investigate steady shock wave structures and high-speed gas flows around a cylinder and across a flat plate. The computational results are compared with the multi-temperature Navier–Stokes (NS) equations, the direct simulation Monte Carlo (DSMC) solutions, and the experiment data. The final results show the new model would reproduce the NS results at low Knudsen numbers but behave quite differently from the NS results as the non-equilibrium degree is enhanced. The new model is in better agreement with the DSMC solutions and the experimental data than the NS solutions in the far-from-equilibrium regions, which demonstrates the potential of the new relaxation model in the simulation of hypersonic non-equilibrium flows.

Fluorescence anisotropy of indole molecules under two-photon excitation in the spectral range of 485–510 nm

Decay of polarized fluorescence in indole dissolved in propylene glycol under two-photon excitation by femtosecond laser pulses in the wavelength range of 485–510 nm has been studied. It is shown that under the experimental conditions used the fluorescence decay signal can be well described by a single excited state lifetime τf and a single rotation diffusion time τrot. By processing the data obtained, the times τf and τrot as well as anisotropy parameter r 0 characterizing the symmetry of two-photon excitation of indole molecules have been determined. Decreasing of the anisotropy parameter r0 down to zero under two-photon excitation energy higher than 5.1 eV has been observed. Interpretation of the obtained results have been done on the basis of ab initio quantum-mechanical computations. A model of energy relaxation under the condition of twophoton excitation of indole in a polar solvent has been discussed.

Development of a two-dimensional binning model for N2–N relaxation in hypersonic shock conditions

A high fidelity internal energy relaxation model for N2–N suitable for use in direct simulation Monte Carlo (DSMC) modeling of chemically reacting flows is proposed. A novel two-dimensional binning approach with variable bin energy resolutions in the rotational and vibrational modes is developed for treating the internal mode of N2. Both bin-to-bin and state-specific relaxation cross sections are obtained using the molecular dynamics/quasi-classical trajectory (MD/QCT) method with two potential energy surfaces as well as the state-specific database of Jaffe et al. The MD/QCT simulations of inelastic energy exchange between N2 and N show that there is a strong forward-preferential scattering behavior at high collision velocities. The 99 bin model is used in homogeneous DSMC relaxation simulations and is found to be able to recover the state-specific master equation results of Panesi et al. when the Jaffe state-specific cross sections are used. Rotational relaxation energy profiles and relaxation times obtained using the ReaxFF and Jaffe potential energy surfaces (PESs) are in general agreement but there are larger differences between the vibrational relaxation times. These differences become smaller as the translational temperature increases because the difference in the PES energy barrier becomes less important.

Kinetic Model of Energy Relaxation in CsI:A (A = Tl and In) Scintillators

A model of energy relaxation in alkali halide scintillators doped with Tl-like activators is presented. Interaction between thermalized charge carriers, their diffusion, and capture by traps are considered. The model of energy relaxation suggested in the work includes essential electron excited states in alkali halides doped with Tl-like activators. Self-trapping of holes occurs in alkali halides at LNT, giving rise to creation of self-trapped excitons (STEs). Thallium-like activator impurity can act both as an electron or a hole trap. Once both of the charge carriers are trapped by the dopant, activator recombination channel comes to action. The model is verified using CsI classical scintillation crystals doped with thallium and indium ions in a range of concentrations from 10–4 to 10–1 mol %. Temperature dependences of the STE and the activator-induced emission yield are measured as a function of the activator concentration under continuous X-ray excitation. A system of rate equations is used to simulat...

General multi-group macroscopic modeling for thermo-chemical non-equilibrium gas mixtures.

This paper opens a new door to macroscopic modeling for thermal and chemical non-equilibrium. In a game-changing approach, we discard conventional theories and practices stemming from the separation of internal energy modes and the Landau-Teller relaxation equation. Instead, we solve the fundamental microscopic equations in their moment forms but seek only optimum representations for the microscopic state distribution function that provides converged and time accurate solutions for certain macroscopic quantities at all times. The modeling makes no ad hoc assumptions or simplifications at the microscopic level and includes all possible collisional and radiative processes; it therefore retains all non-equilibrium fluid physics. We formulate the thermal and chemical non-equilibrium macroscopic equations and rate coefficients in a coupled and unified fashion for gases undergoing completely general transitions. All collisional partners can have internal structures and can change their internal energy states after transitions. The model is based on the reconstruction of the state distribution function. The internal energy space is subdivided into multiple groups in order to better describe non-equilibrium state distributions. The logarithm of the distribution function in each group is expressed as a power series in internal energy based on the maximum entropy principle. The method of weighted residuals is applied to the microscopic equations to obtain macroscopic moment equations and rate coefficients succinctly to any order. The model's accuracy depends only on the assumed expression of the state distribution function and the number of groups used and can be self-checked for accuracy and convergence. We show that the macroscopic internal energy transfer, similar to mass and momentum transfers, occurs through nonlinear collisional processes and is not a simple relaxation process described by, e.g., the Landau-Teller equation. Unlike the classical vibrational energy relaxation model, which can only be applied to molecules, the new model is applicable to atoms, molecules, ions, and their mixtures. Numerical examples and model validations are carried out with two gas mixtures using the maximum entropy linear model: one mixture consists of nitrogen molecules undergoing internal excitation and dissociation and the other consists of nitrogen atoms undergoing internal excitation and ionization. Results show that the original hundreds to thousands of microscopic equations can be reduced to two macroscopic equations with almost perfect agreement for the total number density and total internal energy using only one or two groups. We also obtain good prediction of the microscopic state populations using 5-10 groups in the macroscopic equations.

STELLAR ENERGY RELAXATION AROUND A MASSIVE BLACK HOLE

[abridged] Energy relaxation around a massive black hole (MBH) is key to establishing the dynamical state of galactic nuclei, and the nature of close stellar interactions with the MBH. The standard description of relaxation as diffusion provides a perturbative 2nd-order solution in the weak two-body interaction limit. We run N-body simulations and find that this solution fails to describe the non-Gaussian relaxation on short timescale, which is strongly influenced by extreme events even in the weak limit, and is thus difficult to characterize and measure. We derive a non-perturbative solution for relaxation as an anomalous diffusion process, and develop a robust estimation technique to measure it in simulations. These enable us to analyze and model our numerical results, and validate in detail, for the first time, this model of energy relaxation around an MBH on all timescales. We derive the relation between the energy diffusion time, t_E, and the time for a small perturbation to return to steady state, t_r, in a relaxed, single mass cusp around a MBH. We constrain the contribution of strong encounters, measure that of the weakest encounters, determine the value of the Coulomb logarithm, and provide a robust analytical estimate for t_E in a finite nuclear stellar cusp. We find that t_r ~ 10t_E ~(5/32)Q^2P_h/N_h log Q, where Q=M_bh/M_* is the MBH to star mass ratio, the orbital period P_h and number of stars N_h are evaluated at the energy scale corresponding to the MBH's sphere of influence, E_h=sigma_inf^2, where sigma_inf is the velocity dispersion far from the MBH. We conclude, using the observed cosmic M_bh/sigma correlation, that cusps around lower-mass MBHs (M_bh<10^7 Mo), which evolved passively over a Hubble time, should be relaxed. We consider the effects of anomalous energy diffusion on orbital perturbations of stars observed near the Galactic MBH.

Effects of Continuum Breakdown on the Surface Properties of a Hypersonic Sphere

This study investigates the effects of continuum breakdown on the surface aerothermodynamic properties (pressure, shear, heat transfer rate) of a sphere in Mach 10, 25, and 45 flows of nitrogen gas in regimes varying from continuum to rarefied gas. A rotational energy relaxation model is employed in the computational fluid dynamics code and is tested to confirm its accuracy. As the global Knudsen number is increased, from continuum flow to a rarefied gas, the amount of continuum breakdown seen in the flow and on the surface is increased. This increase in continuum breakdown affects the surface properties, such that an increase in the differences between computational fluid dynamics and direct simulation Monte Carlo method is observed. As the Mach number is increased, the amount of continuum breakdown observed in the flow is increased, but the gradient length local Knudsen number stays approximately constant. Even though the amount of continuum breakdown has increased, the difference between computational fluid dynamics and direct simulation Monte Carlo method remains relatively constant. The last part of this study compares the results of the sphere with that of the analogous cylinder case. At the same global Knudsen number, the differences in the surface properties between computational fluid dynamics and direct simulation Monte Carlo method increase when the simulation is run axisymmetrically.

The stationary non-Poissonian collision model of energy relaxation and stochastic motion in condensed phase processes

The effect of energy dissipation on transport and activated rate processes in condensed phase is analyzed in detail within the non-Poissonian collision model (NPCM). The NPCM is a generalized variant of the collision model (CM) describing the instantaneous change of the velocity of probe particles induced by random collisions with particles of a medium. Unlike the conventional CM, the NPCM assumes the non-Poissonian collision statistics. In this work we concentrate on the stationary variant of the NPCM (SNPCM), which differs from the nonstationary NPCM (NNPCM) discussed in previous studies by the proper treatment of the collision statistics ensuring the time homogeneity of the process. The SNPCM is shown to be free of inconsistencies inherent in the NNPCM. In particular, the SNPCM reproduces the physically natural relations between the average parameters (the average displacement and velocity, correlation functions, etc.) well known in the transport theory. The SNPCM describes properly the specific features of the processes under study, for example, the kinetic cage effect predicted earlier. Within the SNPCM the analytical expressions for the rate of passage over a parabolic barrier, valid in the intermediate-to-strong friction limit, are derived for some particular values of the parameters of the model. The expressions obtained are analyzed in detail.

Experimental results and Monte Carlo simulations for a gadolinium atomic beam produced with a focused electron gun

The parameters of a beam of gadolinium atoms produced with focused 11 kV electrons were measured and their values compared with those obtained by Monte Carlo simulations. The net mass flow rate, the velocity and its standard deviation, the angular distribution and the population density of the ground and first four metastable states were measured by weight-loss, time-of-flight, deposition and laser resonance absorption methods. An atomic beam velocity of with a standard deviation of were obtained for a net flow rate of . The measured angular distributions are fitted by an equation of the form , where a and n are the distribution parameters and the angle with the vertical line. The atomic excitation temperature derived from the state distributions decreases with the flow rate to about 850 K for the highest flow rate. The simulations computed with the test-particle Monte Carlo (PTMC) code, including an electronic energy relaxation model, are in good agreement with the experimental values.

Relaxation of a hot-electron-two-mode-phonon system in highly excited CdS1-xSex crystals.

An investigation of the electron-hole-plasma effective-temperature relaxation in highly excited ${\mathrm{CdS}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Se}}_{\mathit{x}}$ mixed crystals is presented. The slow (\ensuremath{\sim}100-ps) relaxation stage, attributed to the depopulation of the fragments (decay products) of the initially produced nonequilibrium LO phonons, is examined with variation of the alloy composition. The relevant relaxation time dependence on x exhibiting a remarkable drop at small CdSe mole fractions is analyzed in terms of a two-route energy relaxation model considering hot-carrier plasma and two generations of nonequilibrium phonons each originating from both pure constituents of the alloy. The disorder-enhanced cross relaxation between two sublattices of the alloy is inferred to account for the experimental results.

The effect of stimulated emission on the cooling rate of hot carriers in GaAs

The effect of a short carrier lifetime due to stimulated emission on the cooling rate of nearly resonantly excited carriers in bulk GaAs has been investigated using picosecond time-resolved photoluminescence. It has been found that intense photoexcitation of carriers results in very short carrier density decay times of about 20 ps, as opposed to common carrier lifetimes of about 1 ns observed for low excitation levels. Furthermore a considerable reduction of the cooling rate has been found. The rapid density decay and the significant cooling reduction can be well described by an energy relaxation model, only when stimulated recombination is taken into account in addition to the usual carrier-phonon interactions. Within this model the short carrier lifetime considerably reduces the carrier cooling in nice agreement with the experimental results.

Picosecond photoluminescence intensity correlation measurements of hot carriers in GaAs/Al xGa 1− xAs quantum wells

We have measured the energy relaxation of hot carriers in doped and undoped GaAs/Al x Ga 1− x As quantum wells by detecting time-resolved hot luminescence using a two-pulse intensity correlation technique. The results are compared with time-dependent intensity correlation functions calculated using a model of energy relaxation by optical phonon interactions which includes non-equilibrium phonon effects. The excellent agreement between calculated and experimental correlation functions shows the great potential of this method in measuring ultrafast relaxation processes.

Energy relaxation of lower-dimensional hot carriers studied with picosecond photoluminescence.

To study the energy relaxation of lower-dimensional hot carriers, picosecond time- and energy-resolved photoluminescence measurements have been carried out on bulk GaAs and GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum-well structures in the presence of magnetic fields up to B=20 T. For GaAs the results show that the energy-relaxation rate reduces with increasing strength of the magnetic field. This cooling behavior is adequately described by a model for energy relaxation containing the magnetic-field-dependent kinetics of the coupled carrier--nonequilibrium-LO-phonon system. For the quantum-well structures, an increasing magnetic field normal to the quasi-two-dimensional layers reduces the carrier cooling up to B=8 T, while at higher field strengths an enhancement in cooling is observed up to B=20 T. We suggest this effect to be due to a reduction in energy relaxation rate by LO-phonon emission, so that at B&gt;8 T carrier cooling is taken over by acoustic-phonon emission, which increases with magnetic field.

Picosecond coherent Raman measurements of optical-phonon relaxation inLaF3Ce3+

An investigation of dephasing rates of some optical phonons in ${\mathrm{LaF}}_{3}$: ${\mathrm{Ce}}^{3+}$ (2 at.% ${\mathrm{Ce}}^{3+}$) crystals by picosecond time-resolved coherent anti-Stokes Raman spectroscopy is presented. At low temperature the lifetime of the 78-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon is 380 ps, while those of five other phonons of &gt;200 ${\mathrm{cm}}^{\ensuremath{-}1}$ energy are in the range $2&lt;{T}_{1}&lt;10$ ps. The decay of the 78-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon is due to anharmonic spontaneous emission of two 39-${\mathrm{cm}}^{\ensuremath{-}1}$ optical phonons with a cubic anharmonic matrix element $〈{{B}^{(3)}}_{78 39 39}^{0 k \ensuremath{-}k}〉=0.15\ifmmode\pm\else\textpm\fi{}0.05$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The decay of the other phonons studied likely occurs by an analogous process. The matrix elements for these phonons appear to be in the range 0.2-0.6 ${\mathrm{cm}}^{\ensuremath{-}1}$. The dephasing rate of the 78-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon is followed as a function of temperature to ~100 K, and the rates are well fitted by an energy relaxation model involving emission to 39-${\mathrm{cm}}^{\ensuremath{-}1}$ phonons and absorption of ~40 and ~118-${\mathrm{cm}}^{\ensuremath{-}1}$ thermally excited phonons. The matrix elements for absorption processes are (0.3\ifmmode\pm\else\textpm\fi{}0.1) and (0.5\ifmmode\pm\else\textpm\fi{}0.2) ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively. The rate of absorption of thermally excited 78-${\mathrm{cm}}^{\ensuremath{-}1}$ phonons is 10% of the rate of the other up-conversion processes, and it is suggested that this is due to a type of summetry selection rule for phonon-phonon scattering arising from small distortions away from a bimolecular unit cell to a hexamolecular cell, which creates two distinct types of phonons.

A picosecond CARS study of vibron dynamics in molecular crystals: Temperature dependence of homogeneous and inhomogeneous linewidths

An extensive picosecond CARS and spontaneous Raman investigation of temperature dependent dephasing of the totally symmetric vibrons in naphthalene (h8N), perdeuteronaphthalene (d8N), anthracene (h10A), and perdeuteroanthracene (d10A) crystals is presented. The low temperature picosecond decays, ranging from 128 to less than 10 ps, are due to vibrational relaxation by phonon emission. Unlike optical phonons in these crystals, vibron lifetimes of h8N and h10A do not decrease monotonically with frequency, a pattern which is observed in other molecular systems. The low temperature decay rate is proportional to the square of a cubic anharmonic matrix element and a density of difference states function which has a contribution from phonons and lower frequency vibrons. This function is evaluated for each substance and together with measured lifetimes yields matrix elements which are roughly equal for each vibron. We conclude that vibrational relaxation rates are controlled primarily by the density of final states. Relaxation in the deuterated crystals is faster than predicted by this model due to residual isotopic impurities and purely intramolecular vibrational relaxation. Three h8N and one h10A vibrons are studied over an extended temperature range. The homogeneous linewidth increases as Tn with n=2.0–2.4. The observed increase is consistent with the above energy relaxation model up to ∼80 K. From ∼80 K to the melting point, this temperature dependence is consistent with a pure dephasing process involving energy exchange with several thermally populated low frequency vibrons. From ∼250 K to the melting point inhomogeneous broadening is observed. It is suggested that the inhomogeneity is due to slowly (≳10 ps) varying density fluctuations which modulate intermolecular attractive interactions. In this respect, molecular crystals near the melting point resemble some molecular liquids.

Damping of magnetic domain wall motion by manganic ions in manganese ferrite

Published values of the magnetic domain wall mobility and ferromagnetic resonance linewidth for manganese rich manganese ferrite are shown to be consistent with one another on the basis of an atomic model for energy relaxation via manganic ions. The discrepancy which existed according to the previous phenomenological discussion is therebye resolved.