Vibrational phase relaxation along the critical isochore of nitrogen: the role of local density fluctuations in the rate enhancement

Vibrational dephasing of the nitrogen molecule is known to show highly interesting anomalies near its gas-liquid critical point. Here we present theoretical and computational studies of the Raman linewidth of nitrogen along the critical isochore. The linewidth is found to have a lambda-shaped temperature dependence near the critical point. As observed in experimental studies, the calculated line shape becomes Gaussian as the critical temperature (T(c)) is approached. Both the present simulation and a mode coupling theory analysis show that the slow decay of the enhanced density fluctuations near the critical point, probed at the subpicosecond time scales by vibrational frequency modulation, along with an enhanced vibration-rotation coupling, are the main causes of the observed anomalies.     

Theory of muon spin relaxation by critical fluctuations in antiferromagnetic salts

Relaxation rates for positive muons implanted in antiferromagnetic salts are predicted on the basis of calculations performed with a coupled-mode theory of critical fluctuations in isotropic and uniaxial magnets. Anisotropy suppresses fluctuations transmitted by the isotropic (hyperfine) relaxation mechanism. The dipolar mechanism leads to a relaxation rate which increases on lowering the temperature to the critical point; the predicted form, proportional to the critical correlation length, yields the same temperature dependence as for the NMR linewidth. For a magnet with isotropic spin interactions, both contributions (hyperfine and dipolar) to the relaxation rate diverge with a temperature dependence given by the square-root of the correlation length.     

飞秒时间分辨拉曼光谱用于研究β-胡萝卜素单重激发态内转换和振动弛豫过程

搭建了飞秒时间分辨受激拉曼光谱(FSRS)装置,并用于研究全反式β-胡萝卜素单重电子激发态超快内转换和振动弛豫过程.基于三脉冲“抽运-探测”方案搭建的时间分辨受激拉曼光谱装置同时实现了150fs的时间分辨率和23.7cm^-1的光谱分辨率,光谱检测范围为300—4000cm^-1.对全反式β-胡萝卜素电子激发态的飞秒时间分辨拉曼光谱研究表明,β-胡萝卜素被激发到S₂态后,经由寿命约为0.3ps的中间态S_X态实 关键词： 飞秒时间分辨拉曼光谱 β-胡萝卜素 激发态内转换 振动弛豫     

A stochastic theory of non-ideal behaviour

An approximate theory is proposed for the thermal relaxation of condensed binary [A, B] systems. During the relaxation the composition varies, or is kept constant. (The former case corresponds to an Ising lattice, the latter to a binary alloy, for example.) A variation of the composition is described with the help of a stochastic exchange A?B (as in the Glauber Model), but the microscopic transition probabilities are replaced by smoothed probabilities which depend on the system's composition and internal energy, as proposed in Part I. The variation of the internal energy is estimated with the help of a dynamic counter-part of the quasi-chemical approximation. The equations can be integrated numerically, describing the time dependence of the system's composition (if varying), internal energy, entropy and of intensive parameters corresponding to transient ‘quasi-equilibria’. The theory is applied to an Ising lattice undergoing heating or cooling through the critical temperature and compared with Monte Carlo simulation of identical processes. The agreement is judged to be very good, especially since it involves no parameter fitting. The relaxation of an [A, B] system of fixed composition, undergoing cooling to inside the co-existence curve is also studied. Three alternative lines for the evolution of internal energy are estimated: (i) unstable, with no decomposition into two phases; (ii) metastable, with spinodal decomposition and (iii) stable, with binodal decomposition. Comparison with a computer simulation of the process indicates that the initial relaxation (up to order of 100 trial exchanges per particle, depending on the system's size) fits the metastable line, the initial stage being followed by an extremely slow crossover to the stable line.     

Raman imaging and stress quantification in self‐assembled graphene oxide fiber ‘Latin Letters’

To probe the intrinsic stress distribution in terms of spatial Raman shift (ω) and change in the phonon linewidth (Γ), here we analyze self‐assembled graphene oxide fibers (GOF) ‘Latin letters’ by confocal Raman spectroscopy. The self‐assembly of GOF ‘Latin letters’ has been explained through surface tension, π–π stacking, van der Waals interaction at the air–water interface and by systematic time‐dependent investigation using field emission scanning electron microscopy analysis. Intrinsic residual stress due to structural joints and bending is playing a distinct role affecting the E_2g mode (G band) at and away from the physical interface of GOF segments with broadening of phonon linewidth, indicating prominent phonon softening. Linescan across an interface of the GOF ‘letters’ reveals Raman shift to lower wavenumber in all cases but more so in ‘Z’ fiber exhibiting a broader region. Furthermore, intrinsic stress homogeneity is observed for ‘G’ fiber distributed throughout its curvature with negligible shift corresponding to E_2g mode vibration. This article demonstrates the significance of morphology in stress distribution across the self‐assembled and ‘smart‐integrable’ GOF ‘Latin letters’. Copyright © 2016 John Wiley & Sons, Ltd.     

Photoluminescence linewidth broadening due to alloy/thickness fluctuation of CdxZn1 − xSe/ZnSe triple quantum wells

Photoluminescence (PL) linewidth broadening of Cd_xZn_{1 − x}Se/ZnSe triple quantum wells, grown on GaAs substrates by molecular beam epitaxy (MBE), has been investigated. Various quantum well (QW) samples have been prepared with different QW thickness and composition (Cd-composition). Measured and calculated PL linewidth are compared. Both composition and thickness fluctuations are considered for the calculation with the parameters such as the volume of exciton, nominal thickness and composition of QWs. Surface roughness measured by atomic force microscopy (AFM) is used to estimate the interface roughness. Results show that when Cd-composition increases additional linewidth broadening due to Zn/Cd interdiffusion is enhanced.     

Quantum mechanical computational studies of chemical reactions : III. Collinear A + BC reaction with some model potential energy surfaces

The effects of the location of the energy barrier and of changing vibrational frequency along the reaction path on the reaction dynamics of a collinear A + BC → AB + C reaction were studied using a series of ‘diagnostic’ model potential energy surfaces. In accord with the classical trajectory study, vibration was found markedly more effective than translation in promoting reaction on a surface with a barrier located at the exit valley, and vice versa on a surface with a barrier placed in the entry valley. Considerable vibrational population inversion in the reaction product was found for an ‘early-down-hill’-type surface with a barrier placed in the entry valley. The energy release was exceedingly speecific, particularly so for the F + H₂/F + D₂ collinear collision on a realistic surface. Resonances in reactive molecular collisions were evident whenever the static effect of changing vibrational frequency along the reaction path gave rise to the potential wells necessary to support these quasibound states.     

Vibrational relaxation in liquids

A new theory is presented for vibrational energy relaxation in a liquid. It is shown that a vibrationally excited probe molecule relaxes through interaction with the density fluctuations in the surrounding solvent fluid. This interaction occurs through a potential V(k), which is expressed in terms of the intermolecular force between the excited probe molecule and the surrounding fluid molecules. By assuming spherically symmetric solvent particles the T ₁ energy relaxation time for direct V-T processes is related to the translational dynamic structure factor for the fluid S(k, ω_v), evaluated at the vibrational resonance frequency. It is shown that this is described by gas-like particle motions on a very short distance scale corresponding to k vectors lying well beyond the first or second peaks of the fluid structure factor S(k). Such motions can be pictured as high-frequency, short-distance distortions of the local equilibrium configuration of the solvent particles around the probe. T ₁ ^-1 is found to be proportional to ρ_e T ^1/2 ω_v ^-3. The V-V energy exchange relaxation time is also calculated. This is found to be proportional to S(k, ω′) evaluated at a frequency ω′, corresponding to the vibrational energy missmatch. An energy gap law for the V-V process is derived.     

High-order vibrational relaxation: a nonperturbative theory

A nonperturbative theory of the multiphonon relaxation of a localized vibrational mode, caused by a high-order anharmonic interaction with the nearest atoms of the crystal lattice, is proposed. It relates the rate of the process to the positive frequency part of the time-dependent non-stationary displacement correlation function of atoms. The nonlinear integral equation for this function is derived and solved numerically. We have found that the rate exhibits a critical behavior: it sharply increases near a specific (critical) value(s) of the interaction; the corresponding dependence is characterized by the critical index k - 1, where k is the number of the created phonons. Received 2 May 2002 Published online 31 July 2002     

Proton spin relaxation in 2-chloroacrylontirile by double resonance

Nuclear magnetic double resonance experiments were performed on the strongly coupled two-proton system (AB) in 2-chloroacrylontirile to study the spin relaxation processes. The single resonance parameters of the AB spectrum are (operating frequency 100 MHz): |vA - vB|=9.85±0.1 Hz and J _AB=2.8±0.1 Hz. Frequency-sweep double resonance spectra were obtained by irradiating each of the four transitions at different strengths of irradiation in the range 0.015 Hz to 2.35 Hz. These spectra were analysed by using the ‘Bloch approximation’ in the rotating frame for the ‘high’ irradiation strengths and the ‘simple line’ approximation for the ‘low’ irradiation strengths. The analysis showed that the proton relaxation in this molecule can be described by interaction with isotropic random fields of nearly equal strengths at the two nuclei with no appreciable correlation between them. Internal dipole-dipole interaction is not a significant mechanism (at room temperature). It is shown that for analysing ‘low’ irradiation spectra the choice of certain functions of intensity changes would lead to a marked distinction between the different relaxation mechanisms.     

Absence of critical exponents in ferroelectrics: experiments of Hilczer and theory of Levanyuk and Sigov

The pioneering breakthrough of Bozena Hilczer and her coworkers showed that the so-called critical phenomena in many ferroelectric crystals are entirely produced by defects. She studied gadolinium aluminum sulphate hexahydrate (GASH), lithium ammonium sulphate, lithium hydrazinium sulphate, PZT, barium titanate, and especially TGS, via neutron irradiation, X-ray irradiation, and e-beam electron irradiation, which showed that the divergences in specific heat or dielectric constant near the Curie temperature T_c are not caused by ‘critical’ fluctuations but by static defects, which can be annealed out and subsequently reproduced by irradiation. This work is rarely cited (modern physicists often feel that literature searches are optional), leading to frequent rediscovery and generally spurious claims of true ‘critical’ phenomena near T_c.     

Effect of fluctuating inertial forces on the critical dynamics of a pure fluid

A calculation is carried out of the effect of imposed inertial forces on the critical dynamics of a pure fluid near the liquid-vapor critical point. The mechanism for the relaxation of the critical fluctuations of the order parameter is the Brownian motion in the long wavelength hydrodynamic shear modes. The additional fluctuating inertial forces add noise to these modes, leading to an increase inγ_q, the order parameter relaxation rate for a fluctuation of wave number q. A general formula is derived for Δ γ_q/γ_q, the fractional increase in relaxation rate, in terms of the acceleration power spectral density. The predicted strong dependences on temperature, angle, and q provide crucial tests for the theory.     

Linewidth inversion in electron resonance spectra

The electron resonance spectrum of the cation of vanadyl octaethylporphyrin exhibits an unusual linewidth variation with the outer spectral lines sharper than the inner components. This observation corresponds to an inversion of the line broadening normally observed for vanadium (IV) complexes. The linewidth inversion is found to result from the anisotropy in the vanadium hyperfine interaction and the zero-field splitting which, when coupled with the molecular rotation, provides a powerful relaxation process. It is possible to provide a quantitative account of the observed linewidth variation if the anisotropy in the g tensor for the electron on vanadium is also included in the theory.     

Physical cluster mechanics: Statistical thermodynamics and nucleation theory for monatomic systems

We extend previous computations of mechanical stability of atomic microclusters to the realm of statistical thermodynamics, obtaining thermodynamic functions for small, solid-like Van der Waals clusters of less than some 100 atoms possessing non-crystalline structures of ‘polytetrahedral’ type. These are shown to be almost invariably at a thermodynamic advantage over alternative lattice structures of the same number of atoms, at least for the Lennard-Jones potential in the harmonic-oscillator/rigid-rotor approximation. The dependence of thermodynamic functions upon cluster size appears to be essentially monotonic in the number of internal degrees of freedom; although there are certain exceptional structures, particularly with icosahedral symmetry, there proves to be little evidence for the occurrence of ‘magic numbers’ for stability at any temperature and within the size-range considered. Particular attention is given to the heat capacity of model systems in relation to their vibrational spectra. The Debye T ³ law appears reasonably well obeyed at low temperatures with no evidence for the existence of either ‘soft modes’ or distinct surface contributions.

The results for the free energy of formation of minimal clusters ΔG _f are then applied to the computation of nucleation rates in terms of the Becker-Döring-Volmer-Zeldovitch quasi-equilibrium theory. Gibbsian behaviour in the form of a maximum in the curve of ΔG _f versus size is observed with a critical nuclear size at realistic temperatures and pressures of the order of that predicted by macroscopic liquid-drop theories. These figures and those derived for nucleation rate and critical supersaturation appear remarkably insensitive to the details of the model used, in particular to the distinction between ‘microcrystalline’ and ‘amorphous’ atomistic models.

The general status of atomistic nucleation theory is critically examined in the light of these and similar results.     

Model for vibrational relaxation: Pure-dephasing and depopulation

The three dominant mechanisms giving major contributions to vibrational relaxation in molecular systems are (a) pure dephasing, (b) depopulation (or energy relaxation), and (c) resonant transfer. Here (c) is not considered but the effects due to thesimultaneous occurrence of (a) and (b) are treated within a stochastic model. In dealing with (a), the vibrational frequency is assumed to undergo random uncorrelated ‘jump’, due to fluctuations in the environment of the active molecule between a continuous set of values. The ensuing results are somewhat different from those of the commonly used Kubo model of vibrational dephasing, especially at long times and appear to be better suited in interpreting certain experimental data. The model is next extended to include the simultaneous occurrence of (b). The calculation leads to two important conclusions: (i) the lineshape is not just the convolution of those due to (a) and (b), and (ii) the lineshape is asymmetric, if the intermolecular interactions are not isotropic.     

The evolution of the nanoscale dissipative structures in a distribution of defects within the isothermally irradiated f.c.c. crystal

A kinetic model for the influence of external noises such as fluctuations of the vacancies’ generation rate and inhomogeneity of irradiated f.c.c. crystal on the formation of nanoscale modulated dissipative structure in a spatial distribution of vacancies is considered. The generation rate of vacancies all over the sites and a density of their dislocation-type sinks are modelled as independent random uniform stationary fields and with certain defined parameters of fluctuation correlations – spatial and temporal ones. Such stochastic fields can induce a spatial redistribution of vacancies that can lead to their density stationary uniform field or stochastic one. By the average value and correlation functions of these fluctuations, the conditions are determined for interacting fluctuations of the vacancies’ density, under which this homogeneous random field becomes unstable in relation to the stochastic field with a spatially periodic mean distribution of vacancies’ density. For instance, with f.c.c. nickel as a model of the irradiated functional material, the temperature dependence of spatial period d(T) of the modulated dissipative structure of vacancies’ subsystem in f.c.c. crystal is numerically forecasted and analysed, taking into account the total (‘electrochemical’?+?‘strain-induced’) interaction between vacancies. Such d(T)-dependence is also determined by the kinetic characteristics of vacancies’ redistribution.     

The use of fast field cycling to evaluate the time domain relaxation of starches from tropical fruit seeds

ABSTRACT

A proton NMR relaxation study of the molecular dynamics in flour samples of ‘Eugenia uniflora’ (‘Pitanga’), ’Citrus reticulate’ (‘Tangerine’), ‘Prunus persica’ (‘Peach’), ‘Vitis vinifera’ (‘Grape’), and ‘Cucumis melo’ (‘Honeydew melon’) seeds is presented. The spin–lattice relaxation time was obtained over a broad frequency range from 100?kHz to 100?MHz using both conventional and fast field-cycling NMR techniques. This relaxometry study made possible a comparison between the molecular dynamics behaviour of starch from different fruit seeds and from potatoes. I was possible to conclude that the spin–lattice dispersion presents slightly differences for all samples, in particular at low frequencies. All relaxation dispersions could be well interpreted in terms of power-law relaxation models and domains with different power-law relaxation exponents. For the Peach seeds flour the relaxation dispersion at low frequencies was very similar to that observed for potato’s starch incorporated with 5% organoclay Viscogel B8 nanoparticles.     

Raman scattering from spin fluctuations and phonons in the heavy-fermion superconductor UPt3

Quasielastic scattering from spin fluctuations has been observed in UPt₃ by Raman spectroscopy. The experiments for wave vectors q≈0 show a nearly temperature independent linewidth for 5 K ⩽ T ⩽ 300 K Complementary to neutron scattering results this establishes the q independence of the spin relaxation rate, indicating the localized nature of the spin fluctuations. A Raman-active phonon near 79 cm^-1 (10 meV) shows a drastic increase in line-width with decreasing temperature, demonstrating strong electron-phonon coupling.     

Electron paramagnetic resonance study of Cu-Co interactions in Ba2Co(HCOO)64 H2O

Electron paramagnetic resonance studies of Cu²⁺ doped in single crystals of dibarium cobalt formate tetrahydrate are carried out. Unlike in other cobalt salts, resolved hyperfine spectra are observed at room temperature itself. At 77K, the hyperfine linewidth shows an angular and nuclear spin dependence. Further, the linewidth increases as the temperature is lowered. The various possibilities of Cu-Co couplings like spin-flip and cross relaxation etc. are discussed. The spin-Hamiltonian parameters change conspicuously as the temperature is lowered which is ascribed to a vibrational mixing of the electronic levels in the ground state of Cu²⁺.     

An LCAO-MO treatment of the vacancy in diamond

A simple molecular orbital treatment which has previously been applied successfully to the substitutional nitrogen donor and boron acceptor in diamond is here applied to the vacancy in diamond. Localized levels associated with the vacancy are found in the gap and an outward relaxation of the four neighbors surrounding the vacancy is predicted. Jahn-Teller effects are found to give stabilization energies of ～0.5 eV. A critical comparison to the ‘defect molecule’ approach of Coulson and Kearsley and Yamaguchi is included.