Showing papers on "Relaxation (NMR) published in 2006"

Use of the Adam‐Gibbs Equation in the Analysis of Structural Relaxation

Abstract: Narayanaswamy's model of structural relaxation has been shown to provide an excellent description of the behavior of a variety of glasses. In the standard formulation, the relaxation time, τ is represented by the Arrhenius equation, with the activation energy partitioned between the temperature and Active temperature. That form for τ is successful, but lacks theoretical justification. In this paper, the Adam-Gibbs equation is shown to describe accurately both τ and the viscosity of NBS 710 (alkali lime silicate) glass. This equation is expected to be accurate over a wider range of temperature and Active temperature than the Arrhenius equation.

Decoherence of Flux Qubits due to 1 / f Flux Noise

Abstract: We have investigated decoherence in Josephson-junction flux qubits. Based on the measurements of decoherence at various bias conditions, we discriminate contributions of different noise sources. We present a Gaussian decay function extracted from the echo signal as evidence of dephasing due to 1/f flux noise whose spectral density is evaluated to be about (10(-6)Phi0)2/Hz at 1 Hz. We also demonstrate that, at an optimal bias condition where the noise sources are well decoupled, the coherence observed in the echo measurement is limited mainly by energy relaxation of the qubit.

Testing the core/shell model of nanoconfined water in reverse micelles using linear and nonlinear IR spectroscopy

Abstract: A core/shell model has often been used to describe water confined to the interior of reverse micelles. The validity of this model for water encapsulated in AOT/isooctane reverse micelles ranging in diameter from 1.7 to 28 nm (w0 = 2−60) and bulk water is investigated using four experimental observables: the hydroxyl stretch absorption spectra, vibrational population relaxation times, orientational relaxation rates, and spectral diffusion dynamics. The time dependent observables are measured with ultrafast infrared spectrally resolved pump−probe and vibrational echo spectroscopies. Major progressive changes appear in all observables as the system moves from bulk water to the smallest water nanopool, w0 = 2. The dynamics are readily distinguishable for reverse micelle sizes smaller than 7 nm in diameter (w0 = 20) compared to the response of bulk water. The results also demonstrate that the size dependent absorption spectra and population relaxation times can be quantitatively predicted using a core−shell m...

High magnetic field water and metabolite proton T1 and T2 relaxation in rat brain in vivo.

Abstract: Comprehensive and quantitative measurements of T1 and T2 relaxation times of water, metabolites, and macromolecules in rat brain under similar experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T) are presented. Water relaxation showed a highly significant increase (T1) and decrease (T2) with increasing field strength for all nine analyzed brain structures. Similar but less pronounced effects were observed for all metabolites. Macromolecules displayed field-independent T2 relaxation and a strong increase of T1 with field strength. Among other features, these data show that while spectral resolution continues to increase with field strength, the absolute signal-to-noise ratio (SNR) in T1/T2-based anatomical MRI quickly levels off beyond approximately 7 T and may actually decrease at higher magnetic fields.

Water-Soluble GdF3 and GdF3/LaF3 NanoparticlesPhysical Characterization and NMR Relaxation Properties

Abstract: Nanoparticles consisting of either a solid core of GdF3 or an 80/20 mixture of GdF3 and LaF3 have been prepared for use as NMR and MRI relaxation agents. To obtain high aqueous solubilities, the particles were coated with either citrate (cit) groups (in the case of GdF3 nanoparticles), giving the nanoparticle a negatively charged surface, or 2-aminoethyl phosphate (AEP) groups (in the case of GdF3/LaF3 = 80/20), giving the nanoparticle a positively charged surface at physiological pH. In the presence of the 80/20 GdF3/LaF3:AEP, the paramagnetic contribution to the water spin−lattice relaxation rate was observed to be 7.5 s-1 at a nanoparticle concentration of 9.0 nM (0.78 mg/mL, 25 °C, 600 MHz 1H Larmor frequency). Similarly, paramagnetic rates of 10.5 s-1 were observed for water using the GdF3:cit nanoparticles at a nanoparticle concentration of 0.55 nM (0.77 mg/mL, 25 °C, 600 MHz 1H Larmor frequency). Relaxivity measurements confirmed the potential of the particles for applications as contrast agents at...

Anisotropic Interactions of a Single Spin and Dark-Spin Spectroscopy in Diamond

Abstract: Experiments on single nitrogen–vacancy (N–V) centres in diamond, which include electron spin resonance1, Rabi oscillations2, single-shot spin readout3 and two-qubit operations with a nearby13C nuclear spin4, show the potential of this spin system for solid-state quantum information processing. Moreover, N–V centre ensembles can have spin-coherence times exceeding 50 μs at room temperature5. We have developed an angle-resolved magneto-photoluminescence microscope apparatus to investigate the anisotropic electron-spin interactions of single N–V centres at room temperature. We observe negative peaks in the photoluminescence as a function of both magnetic-field magnitude and angle that are explained by coherent spin precession and anisotropic relaxation at spin-level anti-crossings. In addition, precise field alignment unmasks the resonant coupling to neighbouring ‘dark’ nitrogen spins, otherwise undetected by photoluminescence. These results demonstrate the capability of our spectroscopic technique for measuring small numbers of dark spins by means of a single bright spin under ambient conditions.

Ultrafast Vibrational Dynamics at Water Interfaces

Abstract: Time-resolved sum-frequency vibrational spectroscopy permits the study of hitherto neglected ultrafast vibrational dynamics of neat water interfaces. Measurements on interfacial bonded OH stretch modes revealed relaxation behavior on sub-picosecond time scales in close resemblance to that of bulk water. Vibrational excitation is followed by spectral diffusion, vibrational relaxation, and thermalization in the hydrogen-bonding network. Dephasing of the excitation occurs in ≤100 femtoseconds. Population relaxation of the dangling OH stretch was found to have a time constant of 1.3 picoseconds, the same as that for excitation transfer between hydrogen-bonded and unbonded OH stretches of water molecules surrounded by acetone.

Spin precession and real-time dynamics in the Kondo model:Time-dependent numerical renormalization-group study

Abstract: A detailed derivation of the recently proposed time-dependent numerical renormalization-group (TD-NRG) approach to nonequilibrium dynamics in quantum-impurity systems is presented. We demonstrate that the method is suitable for fermionic as well as bosonic baths. Comparisons with exact analytical results for the charge relaxation in the resonant-level model and for dephasing in the spin-boson model establish the accuracy of the method. The real-time dynamics of a single spin coupled to each type of bath is investigated. We use the TD-NRG to calculate the spin relaxation and spin precession of a single Kondo impurity. The short- and long-time dynamics are studied as a function of temperature in the ferromagnetic and antiferromagnetic regimes. The short-time dynamics agrees very well with analytical results obtained at second order in the exchange coupling $J$. In the ferromagnetic regime, the transient spin decay is described by the scaling variable $x=2{\ensuremath{\rho}}_{F}\ensuremath{\mid}J(T)\ensuremath{\mid}Tt$. In the antiferromagnetic regime, the long-time decay is governed for $Tl{T}_{K}$ by the Kondo time scale $1∕{T}_{K}$. Here ${\ensuremath{\rho}}_{F}$ is the conduction-electron density of states, ${T}_{K}$ is the Kondo temperature, and $J(T)$ is the effective exchange coupling at temperature $T$. Results for spin precession are obtained by rotating the external magnetic field from the $x$ axis to the $z$ axis.

An S = 6 Cyanide-Bridged Octanuclear FeIII4NiII4 Complex that Exhibits Slow Relaxation of the Magnetization

Abstract: The synthesis and structural and magnetic characterization of an S = 6 cyanide-bridged octanuclear FeIII4NiII4 (1) complex is described. Ac susceptibility and μ-SQUID measurements suggest that fast magnetization relaxation is present in zero-field due to quantum tunneling of the ground spin state (QTM) while application of small magnetic fields induces slow relaxation of the magnetization.

Hard-core bosons on optical superlattices: Dynamics and relaxation in the superfluid and insulating regimes

Abstract: We study the ground-state properties and nonequilibrium dynamics of hard-core bosons confined in one-dimensional lattices in the presence of an additional periodic potential (superlattice) and a harmonic trap. The dynamics is analyzed after a sudden switch-on or switch-off of the superlattice potential, which can bring the system into insulating or superfluid phases, respectively. A collapse and revival of the zero-momentum peak can be seen in the first case. We study in detail the relaxation of these integrable systems towards equilibrium. We show how after relaxation time averages of physical observables, like the momentum distribution function, can be predicted by means of a generalization of the Gibbs distribution.

Fundamentals of protein NMR spectroscopy

Abstract: List of Figures. List of Tables. 1. Introduction To NMR Spectroscopy. 2. Acquiring One-Dimensional Spectra. 3. Processing One-Dimensional Spectra. 4. Quantum Mechanical Description Of NMR. 5. Quantum Mechanical Description Of A One Pulse Experiment. 6. The Density Matrix. 7. Scalar Coupling. 8. Density Matrix And Product Operator of Coupled Spins. 9. Two Dimensional J-Correlations. 10. Heteronuclear J-Correlation Spectroscopy. 11. Coherence Editing. 12. Quadrature Detection In two Dimensional NMR Spectroscopy. 13. Exchange Processes. 14. Nuclear Spin Relaxation. 15. Resonance Assignments: Homonuclear Methods. 16. Resonance Assignments: Heteronuclear Methods. 17. Practical Aspects of N-Dimensional Data Acquisition And Processing. 18. Dipolar Coupling. 19. Protein Structure Determination. Appendices: A Fourier Transforms. B Complex Variables. C Solving Simultaneous Differential Equations: Laplace Transforms. Index.

Thermodynamic interpretation of the scaling of the dynamics of supercooled liquids.

Abstract: The recently discovered scaling law for the relaxation times, τ(T,υ)=I(Tυγ), where T is temperature and υ the specific volume, is derived by a revision of the entropy model of the glass transition dynamics originally proposed by Avramov [J. Non-Cryst. Solids 262, 258 (2000)]. In this modification the entropy is calculated by an alternative route. The resulting expression for the variation of the relaxation time with T and υ is shown to accurately fit experimental data for several glass-forming liquids and polymers over an extended range encompassing the dynamic crossover. From this analysis, which is valid for any model in which the relaxation time is a function of the entropy, we find that the scaling exponent γ can be identified with the Gruneisen constant.

Exchange bias associated with phase separation in the perovskite cobaltite La 1 − x Sr x CoO 3

Abstract: We report the observation of exchange bias phenomena in the hole-doped perovskite cobaltites ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ ($x=0.12$, 0.15, 0.18, and 0.30) in which a spontaneous phase separation occurs. When the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ samples are cooled in a static magnetic field through a freezing temperature, the magnetization hysteresis loops exhibit both horizontal and vertical shifts. We also observed training effect of the exchange bias, which can be interpreted by a spin configurational relaxation model. Moreover, exchange bias in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ is strongly dependent on the measuring field and the cooling field due to the influence of magnetic field on the relative proportion of the coexisting phases. These results suggest that the intrinsic phase inhomogeneity in a spontaneously phase-separated system may induce an interfacial exchange anisotropy.

Single‐Chain Magnet Behavior in an Alternated One‐Dimensional Assembly of a MnIII Schiff‐Base Complex and a TCNQ Radical

Abstract: An alternated 1:1 chain compound of a MnIII salen derivative and the TCNQ monoradical was synthesized: [Mn(5-TMAMsaltmen)(TCNQ)](ClO4)2 (1) (TCNQ=tetracyano-p-quinodimethane; 5-TMAMsaltmen=N,N′-(1,1,2,2-tetramethylethylene) bis(5-trimethylammoniomethylsalicylideneiminato)). Compound 1 has a zigzag chain structure packed with adjacent chains with an interchain Mn⋅⋅⋅Mn distance of over 8 A. As compound 1 contains no crystallization solvent, the void spaces between chains are occupied only by ClO4− counter ions. Compound 1 has a structure reminiscent of what has been observed in the family of MnIII(porphyrin)-TCNE or -TCNQ compounds reported previously by Miller and co-workers and we demonstrate herein its unique single-chain magnet behavior among this family of compounds. The direct current (dc) magnetic measurements established the one-dimensional nature of compound 1 with an antiferromagnetic exchange coupling, J/kB≈−96 K, between the MnIII ion and TCNQ radical and with an activated correlation length (Δξ=26.5 K) at low temperatures (50–15 K). The slow relaxation of the magnetization was shown in compound 1 by the field hysteresis of the magnetization observed below 3.5 K (with a coercive field up to 14 kOe at 1.8 K). Single-crystal magnetization measurements demonstrated the uniaxial symmetry of this compound and allowed an estimation of the anisotropy field, Ha≈97 kOe. The absence of magnetic ordered phase or spin-glass behavior was established by heat-capacity calorimetry measurements that exhibit no abnormality of Cp between 0.5 K and 10 K. The study of the magnetization relaxation by combined ac (alternating current) and dc techniques showed that compound 1 possesses a single relaxation time (τ). As the consequence of the finite size of the chain, the temperature dependence of τ presents two activated regimes above and below 4.5 K with τ01=2.1×10−10 s, Δτ1=94.1 K and τ02=6.8×10−8 s and Δτ2=67.7 K, respectively. The detailed analysis of these dynamics properties together with the correlation length, allows an unambiguous demonstration of the single-chain magnet behavior in 1.

The effect of field parameters, nanoparticle properties and immobilization on the specific heating power in magnetic particle hyperthermia

Abstract: Magnetic nanoparticles (MNP) are intended for utilization in cancer therapy as they produce damaging heat in the presence of AC magnetic fields. In order to reach the required temperature with minimum particle concentration in tissue the specific heating power (SHP) of MNP should be as high as possible. The aim was to clarify the influence of magnetic field parameters and nanoparticle properties on the SHP. As usual ferrofluids exhibit broad size distributions, a magnetic fractionation of a commercial iron oxide nanoparticle suspension was performed in order to obtain particles with varying properties. The fractions obtained were characterized by means of atomic force microscopy and magnetometry, among other techniques. Frequency spectra of the susceptibility show clear peaks at low frequencies related to the Brown relaxation. This effect vanishes after particle immobilization. Theoretical spectra considering experimentally determined size distributions are in agreement with experimental data. The SHP derived from AC susceptometry is in accordance with that directly determined by calorimetry. A maximum SHP of 160 W g−1 (400 kHz, 8 kA m−1) was detected for the largest particles, showing a behaviour in the transitional regime between superparamagnetic and stable ferromagnetic.

Observation of exchange of micropore water in cement pastes by two-dimensional T(2)-T(2) nuclear magnetic resonance relaxometry.

Abstract: The first detailed analysis of the two-dimensional (2D) NMR T(2)-T(2) exchange experiment with a period of magnetization storage between the two T(2) relaxation encoding periods (T(2)-store-T(2)) is presented. It is shown that this experiment has certain advantages over the T(1)-T(2) variant for the quantization of chemical exchange. New T(2)-store-T(2) 2D 1H NMR spectra of the pore water within white cement paste are presented. Based on these spectra, the exchange rate of water between the two smallest porosity reservoirs is estimated for the first time. It is found to be of the order of 5 ms{-1}. Further, a careful estimate of the pore sizes of these reservoirs is made. They are found to be of the order of 1.4 nm and 10-30 nm , respectively. A discussion of the results is developed in terms of possible calcium silicate hydrate products. A water diffusion coefficient inferred from the exchange rate and the cement particle size is found to compare favorably with the results of molecular-dynamics simulations to be found in the literature.

The structure of low-index surfaces of β-Ga2O3

Abstract: The physical and electronic structure of the (1 0 0), (0 1 0), (0 0 1) and ( 1 0 1 ¯ ) faces of β-Ga 2 O 3 are addressed using ab initio theory. Restricted Hartree–Fock calculations, with large-core Ga and O pseudopotentials, are done to optimize the structure of first the bulk and then of slabs “cut” in the required orientations. The slab unit cells are fully relaxed during optimization, and the displacements of all atoms from the ideally-terminated positions are obtained as functions of depth into the bulk. For the relaxed slabs, single-point density functional theory calculations using the B3LYP functional and all-electron basis sets are performed to obtain surface energies, ionic charges and bond overlap populations. All surfaces exhibit a decrease in surface energy upon relaxation, and the local bonding at the surface is analyzed by comparing nearest-neighbor bond lengths and overlap populations with those in the bulk. The ( 1 0 1 ¯ ) surface, which exhibits a high energy when ideally terminated, undergoes large displacements and changes in bonding during relaxation leading to a substantial lowering of the surface energy. The band structure is also obtained for the lowest-energy surface, which is one of the possible non-polar terminations of the (1 0 0). The results provide insight into the growth and structure of β-Ga 2 O 3 nanoribbons.

Dynamics in Polymer−Silicate Nanocomposites As Studied by Dielectric Relaxation Spectroscopy and Dynamic Mechanical Spectroscopy

Abstract: Nanocomposites of organically modified clay nanoparticles and a polyisoprene (PI) matrix were prepared by solution-mediated intercalation, and their dynamics were investigated over a broad range of frequency and temperature by dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS). The principal goal was to address the effect of geometric confinement and elucidate how the dynamics vary as a function of the type and concentration of clay and the molecular weight of PI. Dielectric spectra of nanocomposites with low-molecular-weight PI reveal no effect of clay loading on the average relaxation time for segmental and normal mode relaxation, but dc conductivity and interfacial polarization are affected. In nanocomposites with high molecular weight PI (in the entangled regime), however, a clear effect of clay loading on the average relaxation time for the normal mode process is observed. Most interestingly, it is found that the normal mode becomes faster with increasing clay content,...

Ultrafast Dynamics in DNA: “Fraying” at the End of the Helix

Abstract: The dynamics of the electric fields in the interior of DNA are measured by using oligonucleotides in which a native base pair is replaced by a dye molecule (coumarin 102) whose emission spectrum is sensitive to the local electric field. Time-resolved measurements of the emission spectrum have been extended to a six decade time range (40 fs to 40 ns) by combining results from time-correlated photon counting, fluorescence up-conversion, and transient absorption. Recent results showed that when the reporter is placed in the center of the oligonucleotide, the dynamics are very broadly distributed over this entire time range and do not show specific time constants associated with individual processes (Andreatta, D.; et al. J. Am. Chem. Soc. 2005, 127, 7270). This paper examines an oligonucleotide with the reporter near its end. The broadly distributed relaxation seen before remains with little attenuation. In addition, a new relaxation with a well-defined relaxation time of 5 ps appears. This process is assign...

High proton relaxivity for gadolinium oxide nanoparticles.

Abstract: Objective: Nanosized materials of gadolinium oxide can provide high-contrast enhancement in magnetic resonance imaging (MRI). The objective of the present study was to investigate proton relaxation ...

Role of impact ionization in multiple exciton generation in PbSe nanocrystals

Abstract: In the impact ionization process, a hot carrier relaxes by generating an exciton. We present tight binding calculations showing that the rate of this process in PbSe nanocrystals has a strong energy dependence and that the relaxation energy increases linearly with the carrier excess energy. The impact ionization can be extremely fast $(\ensuremath{\sim}\mathrm{fs})$ but it is not enhanced by the confinement in contrast to the usual belief. It explains the multiple exciton generation observed experimentally in these dots except at high photon energy where more complex many-particle phenomena are involved.

Theoretical model of intravascular paramagnetic tracers effect on tissue relaxation.

Abstract: The concentration of MRI tracers cannot be measured directly by MRI and is commonly evaluated indirectly using their relaxation effect. This study develops a comprehensive theoretical model to describe the transverse relaxation in perfused tissue caused by intravascular tracers. The model takes into account a number of individual compartments. The signal dephasing is simulated in a semianalytical way by embedding Monte Carlo simulations in the framework of analytical theory. This approach yields a tool for fast, realistic simulation of the change in the transverse relaxation. The results indicate that the relaxivity of intravascular contrast agents depends significantly on the host tissue. This agrees with experimental data by Johnson et al. (Magn Reson Med 2000;44:909). In particular, the present results suggest a several-fold increase in the relaxivity of Gd-based contrast agents in brain tissue compared with bulk blood. The enhancement of relaxation in tissue is due to the contrast in magnetic susceptibility between blood vessels and parenchyma induced by the presence of paramagnetic tracer. Beyond the perfusion measurements, the results can be applied to quantitation of functional MRI and to vessel size imaging.

Ultrafast relaxation and exciton–exciton annihilation in PTCDA thin films at high excitation densities

Abstract: Femtosecond pump–probe spectroscopy is applied to thin films of the quasi-one-dimensional organic semiconductor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). We present transient absorption spectra over a broad spectral range. Ultrafast intraband relaxation in the S1 manifold towards the border of the Brillouin zone is shown to depend on temperature and excitation density. The intraband relaxation time is of the order of 100 fs. At high excitation densities (>1019 cm−3), the major de-excitation mechanism for the relaxed excitons is exciton–exciton annihilation. The experimental decay dynamics can be explained very well by two alternative annihilation models: one-dimensional diffusion limited bimolecular recombination or single-step long range Forster-type annihilation. In contrast, a three-dimensional diffusion limited annihilation model is significantly inferior. For all three models, we extract annihilation rates, diffusion constants, diffusion lengths, and Forster radii for room and liquid Helium temperature.

Structure and Dynamics of the Aqueous Liquid−Vapor Interface: A Comprehensive Particle-Based Simulation Study⊥

Abstract: This research addresses a comprehensive particle-based simulation study of the structural, dynamic, and electronic properties of the liquid−vapor interface of water utilizing both ab initio (based on density functional theory) and empirical (fixed charge and polarizable) models. Numerous properties such as interfacial width, hydrogen bond populations, dipole moments, and correlation times will be characterized with identical schemes to draw useful conclusions on the strengths and weakness of the proposed models for interfacial water. Our findings indicate that all models considered in this study yield similar results for the radial distribution functions, hydrogen bond populations, and orientational relaxation times. Significant differences in the models appear when examining both the dipole moments and surface relaxation near the aqueous liquid−vapor interface. Here, the ab initio interaction potential predicts a significant decrease in the molecular dipole moment and expansion in the oxygen−oxygen dista...

Dynamics in atomistic simulations of phospholipid membranes: Nuclear magnetic resonance relaxation rates and lateral diffusion

Abstract: It is shown that a long, near microsecond, atomistic simulation can shed some light upon the dynamical processes occurring in a lipid bilayer. The analysis focuses on reorientational dynamics of the chains and lateral diffusion of lipids. It is shown that the reorientational correlation functions exhibits an algebraic decay (rather than exponential) for several orders of magnitude in time. The calculated nuclear magnetic resonance relaxation rates agree with experiments for carbons at the C7 position while there are some differences for C3. Lateral diffusion can be divided into two stages. In a first stage occurring at short times, t<5 ns, the center of mass of the lipid moves due to conformational changes of the chains while the headgroup position remains relatively fixed. In this stage, the center of mass can move up to approximately 0.8 nm. The fitted short-time diffusion coefficient is D(1)=13 x 10(-7) cm(2) s(-1) On a longer time scale, the diffusion coefficient becomes D(2)=0.79 x 10(-7) cm(2) s(-1).

Two-dimensional optical three-pulse photon echo spectroscopy. II. Signatures of coherent electronic motion and exciton population transfer in dimer two-dimensional spectra.

Abstract: Using the nonperturbative approach to the calculation of nonlinear optical spectra developed in a foregoing paper [Mancal et al., J. Chem. Phys. 124, 234504 (2006), preceding paper], calculations of two-dimensional electronic spectra of an excitonically coupled dimer model system are presented. The dissipative exciton transfer dynamics is treated within the Redfield theory and energetic disorder within the molecular ensemble is taken into account. The manner in which the two-dimensional spectra reveal electronic couplings in the aggregate system and the evolution of the spectra in time is studied in detail. Changes in the intensity and shape of the peaks in the two-dimensional relaxation spectra are related to the coherent and dissipative dynamics of the system. It is shown that coherent electronic motion, an electronic analog of a vibrational wave packet, can manifest itself in two-dimensional optical spectra of molecular aggregate systems as a periodic modulation of both the diagonal and off-diagonal peaks.

Ultrafast Electron Relaxation Dynamics in Coupled Metal Nanoparticles in Aggregates

Abstract: We report the effect of aggregation in gold nanoparticles on their ultrafast electron-phonon relaxation dynamics measured by femtosecond transient absorption pump-probe spectroscopy UV-visible extinction and transient absorption of the solution-stable aggregates of gold nanoparticles show a broad absorption in the 550-700-nm region in addition to the isolated gold nanoparticle plasmon resonance This broad red-shifted absorption can be attributed to contributions from gold nanoparticle aggregates with different sizes and/or different fractal structures The electron-phonon relaxation, reflected as a fast decay component of the transient bleach, is found to depend on the probe wavelength, suggesting that each wavelength interrogates one particular subset of the aggregates As the probe wavelength is changed from 520 to 635 nm across the broad aggregate absorption, the rate of electron-phonon relaxation increases The observed trend in the hot electron lifetimes can be explained on the basis of an increased overlap of the electron oscillation frequency with the phonon spectrum and enhanced interfacial electron scattering, with increasing extent of aggregation The experimental results strongly suggest the presence of intercolloid electronic coupling within the nanoparticle aggregates, besides the well-known dipolar plasmon coupling

The State of Strain in Single GaN Nanocolumns As Derived from Micro-Photoluminescence Measurements

Abstract: In the present paper, studies on the state of strain in single and ensembles of nanocolumns investigated by photoluminescence spectroscopy will be presented. The GaN nanocolumns were either grown in a bottom-up approach or prepared in a top-down approach by etching compact GaN layers grown on Si(111) and sapphire (0001) substrates. Experimental evidence for strain relaxation of the nanocolumns was found. The difference and development of the strain value for different nanocolumns could be verified by spatially resolved micro-photoluminescence on single nanocolumns separated from their substrate. A common D0X spectral position at 3.473 eV was found for all separated single GaN nanocolumns independent of the substrate or processing technique used, as expected for a relaxed system.

Electron spin relaxation of N@C60 in CS2

Abstract: We examine the temperature dependence of the electron spin relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S⩾1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C60, indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25ms at 170K.