Lorentzian line shape due to an inhomogeneous size distribution without relaxation

In this paper, it is demonstrated that a Lorentzian line shape is predicted for a resonance interacting with a bath of equally spaced levels, even in the sparse, low density of states limit, if one performs an inhomogeneous average over the position of the bath states relative to the bright state. The implication for the spectroscopy of molecules in helium nanodroplets and possibly other samples with a significant size distribution is that coupling of excitations to phonons can lead to Lorentzian shaped transitions entirely from inhomogeneous broadening with no population relaxation in the sample.     

Temperature dependence of the Breit–Wigner–Fano Raman line in single-wall carbon nanotube bundles

The G band in Raman spectra of single-wall carbon nanotube (SWNT) bundles is studied between 3 and 500 K. The G band is best fit with five Lorentzian lines and one Breit–Wigner–Fano (BWF) line, indicating coupling of phonons to the electronic continuum of metallic SWNTs. It is found that the line width of the BWF line decreases with increasing temperature. This temperature-dependent behavior is contrary to that of the Lorentzian lines, where the line width increases with increasing temperature. The coupling constant 1/q of the BWF line is also found to decrease with increasing temperature. These temperature-dependent behaviors of BWF line provide evidence that it is the bundling effect of SWNTs that greatly enhances the BWF coupling.     

Infrared-infrared double resonance spectroscopy of cyanoacetylene in helium nanodroplets

Infrared-infrared double resonance spectroscopy is used as a probe of the vibrational dynamics of cyanoacetylene in helium droplets. The nu1 C-H stretching vibration of cyanoacetylene is excited by an infrared laser and subsequent vibrational relaxation results in the evaporation of approximately 660 helium atoms from the droplet. A second probe laser is then used to excite the same C-H stretching vibration downstream of the pump, corresponding to a time delay of approximately 175 micros. The hole burned by the pump laser is narrower than the single resonance spectrum, owing to the fact that the latter is inhomogeneously broadened by the droplet size distribution. The line width of the hole is characteristic of another broadening source that depends strongly on droplet size.     

Line shape studies of a state coupled to a random background including large fluctuations of the couplings

Line shape functions of a model system are analyzed, describing an oscillator carrying state coupled to background states randomly distributed in energy and with random coupling constants. Depending on the energy distribution functions or the nature of the coupling distribution, different line shape functions, such as the Lorentzian, the Fano, or that related to the nonexponential decay of the Forster type are recovered as limiting cases. Conditions for the range of applicability of a specially introduced mean square coupling approximation are derived. It is shown that the appearance of a Lorentzian line shape does not imply directly a homogeneous decay mechanism and that, on the other hand, commonly accepted conditions for the so-called statistical limit, expressed in terms of an average density and an average coupling, do not necessarily lead to a Lorentzian line shape. This is illustrated analytically through a model with randomly distributed transition dipolar couplings. Other applications relate to spectral diffusion in proteins and to bridged charge transfer.     

ESR line width and line shape dependence of Overhauser‐enhanced magnetic resonance imaging

Electron spin resonance and Overhauser‐enhanced magnetic resonance imaging studies were carried out for various concentrations of ¹⁴N‐labeled 3‐carbamoyl‐2,2,5,5‐tetramethyl‐pyrrolidine‐1‐oxyl in pure water. Overhauser‐enhancement factor attains maxima in the range of 2.5–3 mm concentration. The leakage factor showed an asymptotic increase with increasing agent concentration. The coupling parameter showed the interaction between the electron and nuclear spins to be mainly dipolar in origin. The electron spin resonance parameters, such as the line width, line shape and g‐factor, were determined. The line width analysis confirms that the line broadening is proportional to the agent concentration, and also the agent concentration is optimized in the range of 2.5–3 mm . The line shape analysis shows that the observed electron spin resonance line shape is a Voigt line shape, in which the Lorentzian component is dominant. The contribution of Lorentzian component was estimated using the winsim package. The Lorentzian component of the resonance line attains maxima in the range of 2.5–3 mm concentration. Therefore, this study reveals that the agent concentration, line width and Lorentzian component are the important factors in determining the Overhauser‐enhancement factor. Hence, the agent concentration was optimized as 2.5–3 mm for in vivo/in vitro electron spin resonance imaging and Overhauser‐enhanced magnetic resonance imaging phantom studies. Copyright © 2016 John Wiley & Sons, Ltd.     

Stochastic diffusion interactions and coarsening in a system of droplets growing from a supersaturated gas mixture

In this work we study diffusion interactions among liquid droplets growing in stochastic population by condensation from supersaturated binary gas mixture. During the postnucleation transient regime collective growth of liquid droplets competing for the available water vapor decreases local supersaturation leading to the increase of critical radius and the onset of coarsening process. In coarsening regime the growth of larger droplets is prevailing noticeably broadening the droplet size-distribution function when the condensation process becomes more intensive than the supersaturation yield. Modifications in the kinetic equation are discussed and formulated for a stochastic population of liquid droplets when diffusional interactions among droplets become noteworthy. The kinetic equation for the droplet size-distribution function is solved together with field equations for the mass fraction of disperse liquid phase, mass fraction of water vapor component of moist air, and temperature during diffusion-dominated regime of droplet coarsening. The droplet size and mass distributions are found as functions of the liquid volume fraction, showing considerable broadening of droplet spectra. It is demonstrated that the effect of latent heat of condensation considerably changes coarsening process. The coarsening rate constant, the droplet density (number of droplets per unit volume), the screening length, the mean droplet size, and mass are determined as functions of the temperature, pressure, and liquid volume fraction.     

Resonant coupling of bound excitons with LO phonons in ZnO: excitonic polaron states and Fano interference

We report on a photoluminescence observation of robust excitonic polarons due to resonant coupling of exciton and longitudinal optical (LO) phonon as well as Fano-type interference in high quality ZnO crystal. At low enough temperatures, resonant coupling of excitons and LO phonons leads to not only traditional Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror symmetric with respect to the ZPL, strongly suggesting that they are from different coupling states of exciton and phonons. Besides these spectral features showing the quasiparticle properties of exciton-phonon coupling system, the first-order SL is found to exhibit characteristic Fano lineshape, caused by quantum interference between the LO components of excitonic polarons and the continuous phonon bath. These findings lead to a new insight into fundamental effects of exciton-phonon interactions.     

Vibrational dephasing and hydrodynamic effects on vibrational relaxation rates in acetophenone: Raman bandshape analysis

The isotropic component of Raman band for C=O stretching mode of acetophenone in solution was analyzed by estimating the correlation coefficient with reference to Lorentzian lineshape. In the intermediate region of solute/solvent concentration there is a sharp decrease in the correlation coefficient and there appears to be a transition from non-Lorentzian to Lorentzian lineshape. The vibrational relaxation rates have been estimated from the isotropic component of Raman band in different solvents. The rate is shown to be dependent on several macroscopic as well as microscopic properties of the solute-solvent system and intermolecular interactions. The hydrodynamic and dispersion forces appear to play a major role in determining the vibrational relaxation rate and the broadening of the bands.     

Correlated Rayleigh Scattering Spectroscopy and Scanning Electron Microscopy Studies of Au-Ag Bimetallic Nanoboxes and Nanocages

The optical properties of hollow nanoparticles (Au-Ag nanoboxes and nanocages) were investigated by recording Rayleigh scattering spectra of single particles, whose morphology and composition had been analyzed by scanning electron microscopy (SEM). This was achieved by depositing the particles on optically transparent substrates with registration marks, which are compatible with SEM imaging. Fitting the experimental spectra to a Lorentzian function yields the frequencies and homogeneous line widths of the plasmon resonance for the particles. The resonances are extremely broad, with dephasing times of 2-5 fs. Analysis of the line width data using the dimensions determined by SEM shows that the broadening is due to a combination of electron-surface scattering and radiation damping. The sensitivity of the plasmon resonance to the dielectric constant of the environment was also investigated by adding a drop of water to the substrate. The nanoboxes show similar dielectric sensitivities compared to other metal nanoparticle systems. A significant increase in the line width was also observed for the nanoboxes in water compared with air. This was attributed to increased radiation damping in the environment with a higher dielectric constant. Both the red shift and the increase in line width are reversible.     

Two-compartment radial diffusive exchange analysis of the NMR lineshape of (129)Xe dissolved in a perfluorooctyl bromide emulsion

Hyperpolarized (129)Xe (xenon) gas dissolved in a perfluorooctyl bromide (PFOB) emulsion stabilized with egg yolk phospholipid (EYP) is a possible contrast agent for quantitative blood flow measurements using magnetic resonance imaging. The NMR line shape of xenon dissolved in PFOB emulsion depends strongly on the exchange of spins between PFOB and water. The exchange in this system depends on three factors: the geometrical factors (i.e., droplet size and surrounding water volume), the permeability of the EYP monolayer surrounding the droplet, and the diffusion coefficients of xenon in the two media. A theoretical model which predicts the line shape of xenon in the emulsion based on the Bloch-Torrey equations is presented. Fitting the full width at half maximum (FWHM) of the theoretical line shapes with the FWHM of the experimental spectra obtained from emulsions with different water dilutions allows estimation of the volume-weighted average diameter of the PFOB droplets (3.5+/-0.8) microm and the permeability of the EYP membrane surrounding the droplet (58+/-14) microm / s.     

Oximetry of oxygen supersaturated solutions using nitroxides as EPR probe

The broadening of the three hyperfine EPR nitroxide lines in oxygen supersaturated solutions was examined. The solutions were supersaturated with oxygen either by thermal decomposition of 1,4-peroxy-1,4-dimethylnaphthalene or by pressurizing molecular oxygen above the sample solution. The linear relationship between the Lorentzian component of the line width and the O(2) concentration was proven to hold even when the hyperfine splitting is unresolved. In this region of line broadening, the requirement for spectral simulation is discussed.     

Preparation and characterization of a new lipid nano-emulsion containing two cosurfactants, sodium palmitate for droplet size reduction and sucrose palmitate for stability enhancement

A new lipid nano-emulsion (LNE) was prepared from soybean oil and phosphatidylcholine (PC) employing two cosurfactants, sodium palmitate (PA) for reduced droplet size and sucrose palmitate (SP) for stability enhancement. The mean droplet size of LNEs prepared at a PA/PC (w/w) ratio of larger than 1/10 was found to be ca. 50 nm by dynamic light scattering and atomic force microscopy. However, during the 12-month storage, the PA/PC (1/10)-LNE showed an increase in mean droplet size and broadening of the droplet size distribution due to coalescence of the LNE particles. In a saline solution, the coalescence proceeded very rapidly, i.e., the mean droplet size increased to more than 150 nm within 0.5 h. To suppress the coalescence of LNE particles, four sucrose fatty acid esters of different chain lengths were examined as candidate cosurfactants. The results showed that PA/SP/PC (1/4/10)-LNE could maintain a mean droplet size around 50 nm for 12 months. In a saline solution, the mean droplet size could be maintained within 100 nm even after 24 h. Slight formation of flocculation in the LNEs depending on the storage period was suggested by measurement of the 31P nuclear magnetic resonance line width of the LNEs.     

Laser Linewidth and Spectral Resolution in Infrared Scanning Sum Frequency Generation Vibrational Spectroscopy System

Sum frequency generation vibrational spectroscopy (SFG-VS) is a robust technique for interfacial investigation at molecular level. The performance of SFG-VS mostly depends on the spectral resolution of the SFG system. In this research, a simplified function was deduced to calculate the spectral resolution of picosecond SFG system and the lineshape of SFG spectra based on the Guassian shaped functions of IR beam and visible beam. The function indicates that the lineshpe of SFG spectra from nonresonant samples can be calculated by the Guassian widths of both IR beam and visible beam. And the Voigt lineshape of SFG spectra from vibrational resonant samples can be calculated by the Homogeneous broadening (Lorentzian width) and Inhomogeneous broadening (Guassian width) of vibrational modes, as well as the Guassian widths of both IR beam and visible beam. Such functions were also applied to verify the spectral resolution of the polarization-resolved and frequency-resolved picosecond SFG-VS system which was developed by our group recently. It is shown that the linewidths of IR beams that generated from current laser system are about 1.5 cm^-1. The calculated spectral resolution of current picosecond IR scanning SFG-VS system is about 4.6 cm^-1, which is consist with he spctral resolution shown in the spectra of cholesterol monolayer (3.5-5 cm^-1).     

Synthesis of Polyacrylic/Silica Nanocomposite Latexes using Static Mixer

Summary: Static mixers (Sulzer Chemtech; SMX) were used to prepare silica/ MMA-co-BA miniemulsions that were polymerized to produce nanocomposite latexes. Acceptable conditions for the formulation of polymerizable nanodroplets were found and subsequently used to produce silica/poly(MMA-co-BA) nanocomposites. The droplet size distribution of the resulting miniemulsions was narrow enough that it could be successfully polymerized. It was found that the droplet size depends on the silica content and increases with increasing the silica concentration. It was also shown that there is a relationship between the droplet size and the viscosity of the dispersed phase. The majority of droplets were nucleated upon polymerization when less than 15% silica was used. However, when the silica content exceeded 15%, the ratio of the number of particles in the final latex to the number of droplets (Np/Nd) increased to value much higher than 1 indicating the occurrence of homogeneous nucleation.     

Homogenization Efficiency of Two Immiscible Fluids in Static Mixer Using Droplet Tracking Technique

Comprehensive understanding of the mechanism of two-phase flow agitation is essential to control the mixing performance in chemical processes. The aim of the present study is to understand mixing behavior of two phase flow emulsification process in details by utilizing a three-dimensional computational fluid dynamics (CFD) scheme and predicting the flow characteristics of O/W emulsion in a Kenics static mixer (KSM) operating as an in line continuous homogenizer. The overall study is carried out in three steps: (a) a turbulent flow analysis, to obtain an overall characteristic of the emulsion resulting in CFD model and (b) comparing theoretical data of model with those of experimental studies in order to validate the CFD approach; (c) a droplet tracking step, to extensively study the distribution of marked droplets during the mixing procedure. To achieve this goal, the individual droplets being numerically labeled and visually colored regarding their droplet size; a quantitatively scrutiny of mixing for the droplet distribution was introduced. As a result, the droplet tracking using CFD has successfully evaluated the mixing performance and is proposed as a practical numerical scheme for predicting the KSM behavior.     

Line tension effects for liquid droplets on circular surface domains

We study the morphologies of single liquid droplets wetting a substrate in the presence of the line tension of the three-phase contact line. On a homogeneous substrate, the line tension leads to a discontinuous unbinding of the droplet if its volume is decreased below a critical value. For a droplet wetting a structured surface with a circular domain, a line tension contrast gives rise to discontinuous depinning transitions of the contact line from the domain boundary as the droplet volume is varied. We calculate the corresponding free energy bifurcation diagram analytically for axisymmetric droplet shapes. Numerical minimization of the droplet free energy shows that line tension contrasts can stabilize nonaxisymmetric droplet shapes, thus modifying the bifurcation diagram. These latter shapes should be accessible to experiments and can be used to reveal the presence of a line tension contrast.     

Argon solvent effects on optical properties of silver metal clusters

Argon gas at a high pressure (～80 bar) has been expanded using a miniaturized pulsed valve at room temperature, producing a supersonic beam of cold, large argon droplets. Atoms of silver are subsequently embedded into the droplet using the pick-up technique. The resulting Ag(n)Ar(droplet) distribution was analyzed using multiphoton laser ionization time-of-flight mass spectrometry. Besides bare metal clusters, snowballs of silver monomers and dimers encapsulated in up to 50 argon atoms have been observed. The influence of the solvent on the optical absorption of the solute was studied for embedded Ag(8) using resonant two-photon ionization in the ultraviolet. A redshift and broadening of the Ag(8)Ar(droplet) optical spectrum compared to that measured in pure [Federmann et al., Eur. Phys. J. D 1999, 9, 11] and Ar-doped helium droplets [Diederich et al., J. Chem. Phys.2002, 116, 3263] was observed, which is attributed to the interaction with the larger Ar matrix environment.     

A simple asymmetric lineshape for fitting infrared absorption spectra

Almost exclusively, lineshape functions used to model infrared (IR) absorption peaks are based on symmetric frequency distributions. However, in complex systems such as large biological macromolecules in aqueous solutions, the distribution of vibrational frequencies may in fact be asymmetric. In this communication, we show that asymmetry can be introduced to the standard symmetric Lorentzian and Gaussian lineshapes using a simple, easy to implement method. Our technique involves replacing the static width parameter of the symmetric profiles with a smoothly varying function that is wave number dependent. In this way, the width varies across the IR band resulting in an asymmetric peak. In our model, the width varies sigmoidally with wave number. As a demonstration, we fit experimental spectra of adenosine 5′-monophosphate (AMP).     

Preparation of Poly(vinyl alcohol) Microspheres Based on Droplet Microfluidic Technology

A new method for preparing poly (vinyl alcohol) (PVA) microspheres was developed by using droplet microfluidic technology. In the microfluidic chip, a large number of uniform, monodispersed PVA droplets were prepared quickly and continuously by using droplet formation technology, and the droplet preparation speed reached 7 per second. The size of the PVA droplets could be controlled by changing the injection flow rate of the two-phase fluid and the width of microfluidic channel. Then the PVA microspheres were formed by physical crosslinking. This method has high preparation efficiency and good monodispersity of the obtained microspheres. Moreover, the process does not require the incorporation of chemical crosslinking agents, avoiding interference with the inclusion material, and is well suited for applications such as drug carrier.     

Structure of microemulsion-ABA triblock copolymer networks

Structural equilibrium properties of transient networks formed by microemulsion droplets and ABA triblock copolymers in solution have been studied by Monte Carlo simulation. The droplets were represented by soft spheres, and the polymers were represented by junctions connected by harmonic bonds with an angular potential regulating the intrinsic chain stiffness. The interaction parameters were selected such that the end A-blocks were localized inside the droplets and the middle B-block in the continuous phase. The influence of (i) the polymer concentration, (ii) the polymer stiffness, and (iii) the contour length of the middle B-block on the formation and the structure of the microemulsion-polymer network were investigated using polymer end-to-end separation probability distribution functions, droplet radial distribution functions, droplet-droplet nearest-neighbor probability distribution functions, and network connectivity indicators. An increase of the polymer-droplet number ratio had a strong impact on the network formation. Under typical conditions and at an intermediate polymer-droplet number ratio, (i) the fraction of polymers forming bridges between droplets increased from essentially zero to unity and (ii) the fraction of polymers that were forming loops decreased as the ratio of the polymer end-to-end separation and the surface-to-surface separation between neighboring droplets for a hypothetical homogeneous droplet distribution was increased from 0.5 to 2. For long and flexible polymers, a mesoscopic segregation triggered by a depletion attraction between droplets appeared, and, furthermore, for sufficiently stiff chains, only bridge conformations occurred. The percolation probability could be represented as a function of the average droplet cluster size only, across all systems.