A soluble random-matrix model for relaxation in quantum systems

We study the relaxation of a degenerate two-level system interacting with a heat bath, assuming a random-matrix model for the system-bath interaction. For times larger than the duration of a collision and smaller than the Poincaré recurrence time, the survival probability of still finding the system at timet in the same state in which it was prepared att=0 is exactly calculated.

Dynamics of polar solvation: Route to single exponential relaxation via translational diffusion

A microscopic theoretical calculation of time-dependent solvation energy shows that the solvation of an ion or a dipole is dominated by a single relaxation time if the translational contribution to relaxation is significant.

Proton NMR investigations of ferroelectricity in ammonium sulphate

Results of cw wide-line proton magnetic resonance investigations on ammonium sulphate and rubidium ammonium sulphate are presented. The pressure and temperature dependence of some of the properties of ammonium sulphate are explained stressing the importance of the role of the ammonium ions.

Proton-bifurcated C-H center dot center dot center dot(O,O) hydrogen bonds in 2,3-dichloro-6-nitrobenzylaminium chloride

In the crystal structure of the title salt, C7H7Cl2N2O2+ center dot Cl-, the chloride anions participate in extensive hydrogen bonding with the aminium cations and indirectly link the molecules through multiple N+-H center dot center dot center dot Cl- salt bridges. There are two independent molecules in the asymmetric unit, related by a pseudo-inversion center. The direct intermolecular coupling is established by C-H center dot center dot center dot O, C-H center dot center dot center dot Cl and C-Cl center dot center dot center dot Cl- interactions. A rare three-center (donor bifurcated) C-H center dot center dot center dot (O,O) hydrogen bond is observed between the methylene and nitro groups, with a side-on intramolecular component of closed-ring type and a head-on intermolecular component.

Influence of cross-correlation between dipolar and chemical shift anisotropy relaxation mechanisms upon the transverse relaxation rates of 15N in macromolecules

Heteronuclear multiple-quantum coherence relaxation rate are calculated for the individual transitions of the S spin in an AIS nuclear spin system assuming that the heteronucleus (S spin) has relaxation contributions from both intramolecular dipole-dipole and chemical shift anisotropy relaxation. The individual multiplet components of the heteronuclear zero- and double-quantum coherences are shown to have different transverse relaxation rates. The cross-correlation between the two relaxation mechanisms is shown to be the dominant cause of the calculated differential line broadening. Experimental data are presented using as an example a uniformly 15N labelled sample of human epidermal growth factor.

Sodium-alginate-based proton-exchange membranes as electrolytes for DMFCs

Novel mixed-matrix membranes prepared by blending sodium alginate (NaAlg) with polyvinyl alcohol (PVA) and certain heteropolyacids (HPAs), such as phosphomolybdic acid (PMoA), phosphotungstic acid (PWA) and silicotungstic acid (SWA), followed by ex-situ cross-linking with glutaraldehyde (GA) to achieve the desired mechanical and chemical stability, are reported for use as electrolytes in direct methanol fuel cells (DMFCs). NaAlg-PVA-HPA mixed matrices possess a polymeric network with micro-domains that restrict methanol cross-over. The mixed-matrix membranes are characterised for their mechanical and thermal properties. Methanol cross-over rates across NaAlg-PVA and NaAlg-PVA-HPA mixed-matrix membranes are studied by measuring the mass balance of methanol using a density meter. The DMFC using NaAlg-PVA-SWA exhibits a peak power-density of 68 mW cm(-2) at a load current-density of 225 mA cm(-2), while operating at 343 K. The rheological properties of NaAlg and NaAlg-PVA-SWA viscous solutions are studied and their behaviour validated by a non-Newtonian power-law.

Molecular hydrodynamic theory of non‐Markovian collective orientational relaxation in dense dipolar liquids

A microscopic study of the non‐Markovian (or memory) effects on the collective orientational relaxation in a dense dipolar liquid is carried out by using an extended hydrodynamic approach which provides a reliable description of the dynamical processes occuring at the molecular length scales. Detailed calculations of the wave‐vector dependent orientational correlation functions are presented. The memory effects are found to play an important role; the non‐Markovian results differ considerably from that of the Markovian theory. In particular, a slow long‐time decay of the longitudinal orientational correlation function is observed for dense liquids which becomes weaker in the presence of a sizeable translational contribution to the collective orientational relaxation. This slow decay can be attributed to the intermolecular correlations at the molecular length scales. The longitudinal component of the orientational correlation function becomes oscillatory in the underdamped limit of momenta relaxations and the frequency dependence of the friction reduce the frictional resistance on the collective excitations (commonly known as dipolarons) to make them long lived. The theory predicts that these dipolarons can, therefore, be important in chemical relaxation processes, in contradiction to the claims of some earlier theoretical studies.

Molecular theory of dielectric relaxation in a dense binary dipolar liquid

A molecular theory of dielectric relaxation in a dense binary dipolar liquid is presented. The theory takes into account the effects of intra- and interspecies intermolecular interactions. It is shown that the relaxation is, in general, nonexponential. In certain limits, we recover the biexponential form traditionally used to analyze the experimental data of dielectric relaxation in a binary mixture. However, the relaxation times are widely different from the prediction of the noninteracting rotational diffusion model of Debye for a binary system. Detailed numerical evaluation of the frequency-dependent dielectric function epsilon-(omega) is carried out by using the known analytic solution of the mean spherical approximation (MSA) model for the two-particle direct correlation function for a polar mixture. A microscopic expression for both wave vector (k) and frequency (omega) dependent dielectric function, epsilon-(k,omega), of a binary mixture is also presented. The theoretical predictions on epsilon-(omega) (= epsilon-(k = 0, omega)) have been compared with the available experimental results. In particular, the present theory offers a molecular explanation of the phenomenon of fusing of the two relaxation channels of the neat liquids, observed by Schallamach many years ago.

Relaxation dominated by inertia: Solvation dynamics of a small ion in a dipolar solvent

It is shown from an analytical theory that the solvation dynamics of a small ion can be controlled largely by the inertial response of the dipolar solvent when the liquid is in the underdamped limit. It is also shown that this inertial response arises primarily from the long wavelength (with wavevector k≃0) processes which have a collective excitation-like behaviour. The long time decay is dominated by the processes occurring at molecular lengthscales. The theoretical results are in good agreement with recent computer simulation results.

Sensitivity of the simulated precipitation to changes in convective relaxation time scale

The paper describes the sensitivity of the simulated precipitation to changes in convective relaxation time scale (TAU) of Zhang and McFarlane (ZM) cumulus parameterization, in NCAR-Community Atmosphere Model version 3 (CAM3). In the default configuration of the model, the prescribed value of TAU, a characteristic time scale with which convective available potential energy (CAPE) is removed at an exponential rate by convection, is assumed to be 1 h. However, some recent observational findings suggest that, it is larger by around one order of magnitude. In order to explore the sensitivity of the model simulation to TAU, two model frameworks have been used, namely, aqua-planet and actual-planet configurations. Numerical integrations have been carried out by using different values of TAU, and its effect on simulated precipitation has been analyzed. The aqua-planet simulations reveal that when TAU increases, rate of deep convective precipitation (DCP) decreases and this leads to an accumulation of convective instability in the atmosphere. Consequently, the moisture content in the lower-and mid-troposphere increases. On the other hand, the shallow convective precipitation (SCP) and large-scale precipitation (LSP) intensify, predominantly the SCP, and thus capping the accumulation of convective instability in the atmosphere. The total precipitation (TP) remains approximately constant, but the proportion of the three components changes significantly, which in turn alters the vertical distribution of total precipitation production. The vertical structure of moist heating changes from a vertically extended profile to a bottom heavy profile, with the increase of TAU. Altitude of the maximum vertical velocity shifts from upper troposphere to lower troposphere. Similar response was seen in the actual-planet simulations. With an increase in TAU from 1 h to 8 h, there was a significant improvement in the simulation of the seasonal mean precipitation. The fraction of deep convective precipitation was in much better agreement with satellite observations.

Time-Resolved Resonance Raman Studies on Proton-Induced Electron-Transfer Reaction from Triplet Excited State of 2-Methoxynaphthalene to Decafluorobenzophenone

In this paper time-resolved resonance Raman (TR3) spectra of intermediates generated by proton induced electron-transfer reaction between triplet 2-methoxynaphthalene ((ROMe)-R-3) and decafluorobenzophenone (DFBP) are presented The TR3 vibrational spectra and structure of 2-methoxynaphthalene cation radical (ROMe+) have been analyzed by density functional theory (DFT) calculation It is observed that the structure of naphthalene ring of ROMe+ deviates from the structure of cation radical of naphthalene

Dielectric relaxation in bismuth layer-structured BaBi4Ti4O15 ferroelectric ceramics

The dielectric properties of BaBi4Ti4O15 ceramics were investigated as a function of frequency (10(2)-10(6) Hz) at various temperatures (30 degrees C-470 degrees C), covering the phase transition temperature. Two different conduction mechanisms were obtained by fitting the complex impedance data to Cole-Cole equation. The grain and grain boundary resistivities were found to follow the Arrhenius law associated with activation energies: E-g similar to 1.12 eV below T-m and E-g similar to 0.70 eV above T-m for the grain conduction; and E-gb similar to 0.93 eV below T-m and E-gb similar to 0.71 eV above T-m for the grain boundary conduction. Relaxation times extracted using imaginary part of complex impedance Z `'(omega) and modulus M `'(omega) were also found to follow the Arrhenius law and showed an anomaly around the phase transition temperature. The frequency dependence of conductivity was interpreted in terms of the jump relaxation model and was fitted to the double power law. (C) 2010 Elsevier B. V. All rights reserved.

Relationship between microscopic and macroscopic orientational relaxation times in polar liquids

Microscopic relations between single-particle orientational relaxation time (T, ) , dielectric relaxation time ( T ~ )a,n d many-body orientational relaxation time ( T ~o)f a dipolar liquid are derived. We show that both T~ and T~ are influenced significantly by many-body effects. In the present theory, these many-body effects enter through the anisotropic part of the two-particle direct correlation function of the polar liquid. We use mean-spherical approximation (MSA) for dipolar hard spheres for explicit numerical evaluation of the relaxation times. We find that, although the dipolar correlation function is biexponential, the frequency-dependent dielectric constant is of simple Debye form, with T~ equal to the transverse polarization relaxation time. The microscopic T~ falls in between Debye and Onsager-Glarum expressions at large values of the static dielectric constant.