Two-frequency spin echo in molecular crystals

The two-frequency pulse response of a multilevel system in NQR is investigated. Additional spin echo signals are shown to appear. The application of the two-frequency spin echo method to some of the crystals is demonstrated. The method is of great value for the investigation of local fields in crystals.     

Spin-lattice relaxation of radicals in molecular crystals

The mechanism of spin-lattice relaxation of radicals in magnetically diluted molecular crystals caused by delocalization of the unpaired electron is considered.The authors thank I. V. Aleksandrov for a number of helpful remarks during discussions of this investigation.     

Anisotropy of the triplet spin—lattice relaxation in molecular crystals at low temperatures and high magnetic fields

The anisotropy of the triplet relaxation rates in molecular crystals at low temperatures (T = 1.6 K) and very high magnetic fields (B = 5.2–10 T) is explained by a model, which assumes time dependent matrix elements of the electronic dipole tensor and of the electronic g-tensor. The time dependencies may be due, for instance, to librations of the molecule or to changes in the electronic configuration; they arise from a direct process. This fact is used in order to reduce the number of parameters. The relaxation rates are given as functions of the direction cosines of the magnetic field and of eight parameters which are determined in a least-squares fit for the system quinoxaline in perdeuterated naphthalene. Some uncertainties concerning the numerical values of the parameters would be reduced by measurements with the magnetic field directed along the principal axes of the electronic dipole tensor.     

A proton spin relaxation study of ferroelectric liquid crystals

Here we present and analyse N.M.R. measurements of the Larmor frequency dependence (dispersion) of the longitudinal proton spin relaxation time, T ₁(v), for two chiral ferroelectric mesogens (Merck IS-1912 and DOBAMBC) in the isotropic, smectic A and smectic C* phases, making use of fast field cycling techniques. Although in the low frequency range the relaxation times of IS-1912 are much shorter than those of DOBAMBC, the form of the dispersion profiles is not basically different for the two materials. This reveals contributions by smectic order fluctuations, self-diffusion and molecular rotations. The order fluctuation term, which means relaxation by collective molecular reorientations, is clearly seen by characteristic dispersion profiles in the kHz regime (T ₁ ～ v ¹ or T ₁ ～ v ^1/2), which disappear in the isotropic phase. Our results do not indicate significant dissimilarities between the main relaxation processes in the S_C and S*_C mesophases.     

Photoionization and recombination delayed fluorescence in anthracene single crystals

The delayed fluorescence (DF), the action DF spectrum, the incident light intensity dependence of the DF and the DF decay of an anthracene single crystal at 77°K are analyzed. It is proposed that the delayed fluorescence originates from electron-hole recombinations after photogeneration of charge carriers by the near UV light illumination.     

Two-stage model for molecular orientational relaxation in liquid crystals

Abstract

The molecular reorientation in anisotropic fluids is considered as twostage process—fast single molecule rotation in the volume restricted by close neighbours and slow collective relaxation of the local surrounding. The theoretical results are applied to explain the discrepancy between the rotational diffusion coefficients D_⊥ ^r obtained by infrared bandshape analysis and polarized fluorescence techniques.     

Two-stage model for molecular orientational relaxation in liquid crystals

The molecular reorientation in anisotropic fluids is considered as twostage process—fast single molecule rotation in the volume restricted by close neighbours and slow collective relaxation of the local surrounding. The theoretical results are applied to explain the discrepancy between the rotational diffusion coefficients D_⊥^r obtained by infrared bandshape analysis and polarized fluorescence techniques.     

Ammonium ions in alkali metal halide crystals: tunnelling and spin relaxation

We have used low energy inelastic neutron scattering spectroscopy to examine the tunnelling spectroscopy of the ammonium ion in the (NH4)0.02Rb(x)K(0.98-x)I system. The concentration of different species were varied as x increased, this was followed systematically and the first consistent assignment scheme for these features is given. Differences were also found for the relaxation rate of the spin temperature inversions that could be generated in these species. At a critical concentration--about x = 0.04 mole fraction--the relaxation rates of the species changed dramatically.     

On the molecular interpretation of the dielectric relaxation of nematic liquid crystals

This paper presents the results of studies of the dielectric relaxation of nematic 6CHBT obtained for different values of the angle between the directions of the macroscopic orientation of the sample (director n) and the probing electric field E. Analysis of the evolution of the relaxation spectrum from epsilon*(omega) (E n) to epsilon*(omega) (E n) allows one to explain the hitherto existing inconsistency in the molecular interpretation of the spectra. A model of the molecular dynamics in the oriented nematics is proposed.     

Magnetic-field induced biaxiality in nematic liquid crystals. Consequences for nuclear spin relaxation

When a uniaxial nematic liquid crystal is subjected to a magnetic field making a non-zero angle with the C_∞ axis, the uniaxial symmetry is broken. The principal effect is a field-induced biaxiality in the long-wavelength region of the director fluctuation spectrum. Whereas the induced biaxiality has little effect on the mean square director fluctuation amplitudes 〈n²_x〉 and 〈n²_y〉, which are dominated by short-wavelength modes, it can profoundly affect the nuclear spin relaxation behaviour, which is sensitive to long-wavelength modes. Motivated by the increasing number of nuclear spin relaxation studies of director fluctuations in thermotropic, amphiphilic, and polymeric nematic liquid crystals, we present here a theoretical analysis of the effects of field-induced biaxiality on nuclear spin relaxation.     

Rotational dynamics in supercooled water from nuclear spin relaxation and molecular simulations

Structural dynamics in liquid water slow down dramatically in the supercooled regime. To shed further light on the origin of this super-Arrhenius temperature dependence, we report high-precision (17)O and (2)H NMR relaxation data for H(2)O and D(2)O, respectively, down to 37 K below the equilibrium freezing point. With the aid of molecular dynamics (MD) simulations, we provide a detailed analysis of the rotational motions probed by the NMR experiments. The NMR-derived rotational correlation time τ(R) is the integral of a time correlation function (TCF) that, after a subpicosecond librational decay, can be described as a sum of two exponentials. Using a coarse-graining algorithm to map the MD trajectory on a continuous-time random walk (CTRW) in angular space, we show that the slowest TCF component can be attributed to large-angle molecular jumps. The mean jump angle is ～48° at all temperatures and the waiting time distribution is non-exponential, implying dynamical heterogeneity. We have previously used an analogous CTRW model to analyze quasielastic neutron scattering data from supercooled water. Although the translational and rotational waiting times are of similar magnitude, most translational jumps are not synchronized with a rotational jump of the same molecule. The rotational waiting time has a stronger temperature dependence than the translation one, consistent with the strong increase of the experimentally derived product τ(R)?D(T) at low temperatures. The present CTRW jump model is related to, but differs in essential ways from the extended jump model proposed by Laage and co-workers. Our analysis traces the super-Arrhenius temperature dependence of τ(R) to the rotational waiting time. We present arguments against interpreting this temperature dependence in terms of mode-coupling theory or in terms of mixture models of water structure.     

The role of relaxation in the nuclear spin conversion process

The nuclear spin conversion rate depends on the collisions, which break the coherence created by magnetic intramolecular interactions between pairs of quasi degenerate levels belonging to the different spin isomers. The collisions act similarly to break the coherence created by a radiation field between two levels inducing pressure broadening of molecular transitions. Collisional relaxation rates have been extensively studied in this last situation using semi-classical approach and rectilign trajectory for collisional path.Taking advantage of the analogy, the present paper shows that calculations can be efficiently adapted for the collisional relaxation terms present in the ‘quantum relaxation’ model of nuclear spin conversion.For ¹³CH₃F, numerous experimental measurements of spin conversion rates in the presence of an electric field have allowed to derive directly relaxation rates. Our calculation appears to agree satisfactorily with these experimental values. For ¹²CH₃F, calculated relaxations rates are also given for the pairs involved in nuclear spin conversion.     

Microstructure and dynamics in lyotropic liquid crystals. Principles and applications of nuclear spin relaxation

Abstract

Nuclear spin relaxation of quadrupolar nuclei provides access to a wide range of properties of lyotropic liquid crystals, ranging from the molecular ordering and dynamics at the interface to the macroscopic viscoelastic behaviour. We emphasize here the unique capability of the spin relaxation method to provide detailed geometric and dynamic information relating to the microstructure of lyotropic liquid crystals, i.e. the metric, curvature, and fluctuations of the dividing interface that separates polar and non-polar regions. This information is conveyed to the spin system via the translational diffusion of surfactants or counterions over the interface. The general principles of the spin relaxation method, as applied to lyotropic liquid crystals, are described, with emphasis on the model-independent information content of the relaxation observables and on the relation to microstructure. Specific results for lamellar, hexagonal, cubic, and nematic phases are also described.     

Quenching of the delayed fluorescence by charge carriers in crystals of anthracene—TCNB CT complex

The decrease in delayed fluorescence intensity in anthracene—tetracyanobenzene upon application of an electric field is investigated. It is shown, that photogenerated free carriers are responsible for the observed quenching and that the entity quenched is an intermediate state between a triplet-pair state and a fluorescent singlet state. A model is presented that identifies this state as a separated charge pair. This model can explain the main feature of the observations, namely, the large quenching cross section, which is of the order of cross sections typical of carrier—carrier interactions.     

Search for ultra slow molecular motion in liquid crystals by spin thermometry

The existence of a slow dynamic mode, which relaxes the nuclear Zeeman energy at low fields, has been detected in liquid crystals by several nuclear magnetic resonance studies. To data, through, this slow mode has not been uniquely determined. The cross relaxation in the liquid crystal PAA and the dimer-exchange in the alkoxy-benzoic acid homologous series, interfere with the analysis of the low frequency relaxation. In p-(pentyl) phenyl-p-pentyloxy benzoate, which is not a dimer and cross relaxation between protons and other nuclei is not possible, the spin relaxation was analysed at low and high fields. By measuring T₁, T_1ρ, T_1D and applying spin thermometry, the existence of a slow mode was uniquely determined in this liquid crystal.     

Microstructure and dynamics in lyotropic liquid crystals. Principles and applications of nuclear spin relaxation

Nuclear spin relaxation of quadrupolar nuclei provides access to a wide range of properties of lyotropic liquid crystals, ranging from the molecular ordering and dynamics at the interface to the macroscopic viscoelastic behaviour. We emphasize here the unique capability of the spin relaxation method to provide detailed geometric and dynamic information relating to the microstructure of lyotropic liquid crystals, i.e. the metric, curvature, and fluctuations of the dividing interface that separates polar and non-polar regions. This information is conveyed to the spin system via the translational diffusion of surfactants or counterions over the interface. The general principles of the spin relaxation method, as applied to lyotropic liquid crystals, are described, with emphasis on the model-independent information content of the relaxation observables and on the relation to microstructure. Specific results for lamellar, hexagonal, cubic, and nematic phases are also described.     

The Stern-Gerlach experiment and the effects of spin relaxation

The classical Stern-Gerlach experiment is analyzed with an emphasis on the spin dynamics. The central question asked is whether there occurs a relaxation of the spin angular momentum during the time the particle passes through the Stern-Gerlach magnet. We examine in particular the transverse relaxation, involving angular momentum exchange between the spin of the particles and the spins of the magnet. A method is presented describing relaxation effects at an individual particle level. This leads to a stochastic equation of motion for the spins. This is coupled to a classical equation of motion for the particle translation. The experimental situation is then modeled through simulations of individual trajectories using two sets of parameter choices and three different sets of initial conditions. The two main conclusions are: (A) if the coupling between the magnet and the spin is solely described by the Zeeman interaction with the average magnetic field the simulations show a clear disagreement with the experimental observation of Stern and Gerlach. (B) If one, on the other hand, also allows for a T(2) relaxation time shorter than the passage time one can obtain a practically quantitative agreement with the experimental observations. These conclusions are at variance with the standard textbook explanation of the Stern-Gerlach experiment.     

Phenomenological model of spin crossover in molecular crystals as derived from atom-atom potentials

The method of atom-atom potentials, previously applied to the analysis of pure molecular crystals formed by either low-spin (LS) or high-spin (HS) forms (spin isomers) of Fe(II) coordination compounds (Sinitskiy et al., Phys. Chem. Chem. Phys., 2009, 11, 10983), is used to estimate the lattice enthalpies of mixed crystals containing different fractions of the spin isomers. The crystals under study were formed by LS and HS isomers of Fe(phen)(2)(NCS)(2) (phen = 1,10-phenanthroline), Fe(btz)(2)(NCS)(2) (btz = 5,5',6,6'-tetrahydro-4H,4'H-2,2'-bi-1,3-thiazine), and Fe(bpz)(2)(bipy) (bpz = dihydrobis(1-pyrazolil)borate, and bipy = 2,2'-bipyridine). For the first time the phenomenological parameters Γ pertinent to the Slichter-Drickamer model (SDM) of several materials were independently derived from the microscopic model of the crystals with use of atom-atom potentials of intermolecular interaction. The accuracy of the SDM was checked against the numerical data on the enthalpies of mixed crystals. Fair semiquantitative agreement with the experimental dependence of the HS fraction on temperature was achieved with use of these values. Prediction of trends in Γ values as a function of chemical composition and geometry of the crystals is possible with the proposed approach, which opens a way to rational design of spin crossover materials with desired properties.