Characteristic Line Shape Research Articles

Magic-angle-spinning-induced local ordering in polymer electrolytes and its effects on solid-state diffusion and relaxation NMR measurements.

Magic-angle-spinning (MAS) enhances sensitivity and resolution in solid-state nuclear magnetic resonance (NMR) measurements. MAS is obtained by aerodynamic levitation and drive of a rotor, which results in large centrifugal forces that may affect the physical state of soft materials, such as polymers, and subsequent solid-state NMR measurements. Here, we investigate the effects of MAS on the solid-state NMR measurements of a polymer electrolyte for lithium-ion battery applications, poly(ethylene oxide) (PEO) doped with the lithium salt LiTFSI. We show that MAS induces local chain ordering, which manifests itself as characteristic lineshapes with doublet-like splittings in subsequent solid-state 1 H, 7 Li, and 19 F static NMR spectra characterizing the PEO chains and solvated ions. MAS results in distributions of stresses and hence local chain orientations within the rotor, yielding distributions in the local magnetic susceptibility tensor that give rise to the observed NMR anisotropy and lineshapes. The effects of MAS were investigated on solid-state 7 Li and 19 F pulsed-field-gradient (PFG) diffusion and 7 Li longitudinal relaxation NMR measurements. Activation energies for ion diffusion were affected modestly by MAS. 7 Li longitudinal relaxation rates, which are sensitive to lithium-ion dynamics in the nanosecond regime, were essentially unchanged by MAS. We recommend that NMR researchers studying soft polymeric materials use only the spin rates necessary to achieve the desired enhancements in sensitivity and resolution, as well as acquire static NMR spectra after MAS experiments to reveal any signs of stress-induced local ordering.

Real-time reflectance anisotropy spectroscopy of GaAs homoepitaxy.

Reflectance anisotropy spectroscopy (RAS) is a highly sensitive optical probe for the real-time study of the epitaxial growth of zincblende semiconductors. Here we report on (1) non-equilibrium RAS spectra acquired in real time during the homoepitaxial growth of GaAs, and (2) RAS spectra for GaAs surfaces under equilibrium with several arsenic overpressures. We show that in both cases RAS spectra can be decomposed into two basic components, each with a characteristic line shape. We further show that both dynamic and equilibrium RAS spectra are described by the same pair of basic components. We conclude that the time evolution of non-equilibrium RAS spectra acquired during the epitaxial growth can be described in terms of RAS spectra for equilibrium surfaces. The results reported here should be useful for the interpretation of the physics underlying the rapid time evolution of dynamic RAS spectra during the first monolayer growth. Thus, we show that RAS constitutes a valuable tool for the study of epitaxial growth mechanisms.

Feshbach Resonances in p-Wave Three-Body Recombination within Fermi-Fermi Mixtures of Open-Shell 6Li and Closed-Shell 173Yb Atoms.

We report on the observation of magnetic Feshbach resonances in a Fermi-Fermi mixture of ultracold atoms with extreme mass imbalance and on their unique p-wave dominated three-body recombination processes. Our system consists of open-shell alkali-metal 6Li and closed-shell 173Yb atoms, both spin polarized and held at various temperatures between 1 and 20 μK. We confirm that Feshbach resonances in this system are solely the result of a weak separation-dependent hyperfine coupling between the electronic spin of 6Li and the nuclear spin of 173Yb. Our analysis also shows that three-body recombination rates are controlled by the identical fermion nature of the mixture, even in the presence of s-wave collisions between the two species and with recombination rate coefficients outside the Wigner threshold regime at our lowest temperature. Specifically, a comparison of experimental and theoretical line shapes of the recombination process indicates that the characteristic asymmetric line shape as a function of applied magnetic field and a maximum recombination rate coefficient that is independent of temperature can only be explained by triatomic collisions with nonzero, p-wave total orbital angular momentum. The resonances can be used to form ultracold doublet ground-state molecules and to simulate quantum superfluidity in mass-imbalanced mixtures.

Triangle singularities in B¯0→χc1K−π+ relevant to Z1(4050) and Z2(4250)

$Z_1(4050)$ and $Z_2(4250)$ observed in $\bar{B}^0\to\chi_{c1}K^-\pi^+$ by the Belle Collaboration are candidates of charged charmonium-like states that minimally includes two quarks and two antiquarks. While $Z_1(4050)$ and $Z_2(4250)$ have been interpreted as tetraquark states previously, we propose a completely different scenario based on a kinematical effect called the triangle singularity. We demonstrate that the triangle singularities cause in the $\chi_{c1}\pi^+$ invariant mass distribution resonance-like bumps that fit very well the Belle data. If these bumps are simulated by the $Z_1(4050)$ and $Z_2(4250)$ resonance excitations, the spin-parity of them are predicted to be $1^-$ for $Z_1(4050)$ and $1^+$ or $1^-$ for $Z_2(4250)$. The bump corresponding to $Z_1(4050)$ has a highly asymmetric shape, which the Belle data exactly indicate. We show that the asymmetric shape originates from an interplay between the triangle singularity and the opening of the $X(3872)\pi^+$ channel near the triangle-singularity energy. This characteristic lineshape could be used to discriminate different interpretations of $Z_1(4050)$. An interesting prediction from interpretting $Z_1(4050)$ and $Z_2(4250)$ as the triangle singularities is that similar bumps caused by the same mechanisms possibly appear also in $\bar{B}^0\to J/\psi K^-\pi^+$ data; the already observed $Z_c(4200)$ corresponds to $Z_2(4250)$ of $J^P=1^+$.

Diagram, valence‐to‐core, and hypersatellite Kβ X‐ray transitions in metallic chromium

We report on measurements of the Kβ diagram, valence‐to‐core (VtC), and hypersatellite X‐ray spectra induced in metallic Cr by photon single and double K‐shell ionization. The experiment was carried out at the Stanford Synchrotron Radiation Lightsource using the seven‐crystal Johann‐type hard X‐ray spectrometer of the beamline 6‐2. For the Kβ diagram and VtC transitions, the present study confirms the line shape features observed in previous works, whereas the Khβ hypersatellite transition was found to exhibit a complex spectral line shape and a characteristic low‐energy shoulder. The energy shift of the hypersatellite relative to the parent diagram line was deduced from the measurements and compared with the result of extensive multiconfiguration Dirac–Fock (MCDF) calculations. A very good agreement between experiment and theory was found. The MCDF calculations were also used to compute the theoretical line shape of the hypersatellite. A satisfactory agreement was obtained between the overall shapes of the experimental and theoretical spectra, but deviations were observed on the low‐ and high‐energy flanks of the hypersatellite line. The discrepancies were explained by chemical effects, which were not considered in the MCDF calculations performed for isolated atoms.

Determination of Handedness in a Single Chiral Nanocrystal via Circularly Polarized Luminescence.

The occurrence of biological homochirality is attributed to symmetry-breaking mechanisms which are still debatable. Studies of symmetry breaking require tools for monitoring the population ratios of individual chiral nano-objects, such as molecules, polymers, or nanocrystals. Moreover, mapping their spatial distributions may elucidate on their symmetry-breaking mechanism. While luminescence is preferred for detecting single particle chirality due to its high signal-to-noise ratio, the typical low optical activity of chromophores limits its applicability. Here, we report on handedness determination of single chiral lanthanide-based luminescent nanocrystals with a total photon count of 2 × 104. Due to the large emission dissymmetry, we could determine the handedness of individual particles using only a single circular polarization component of the emission spectrum, without polarization modulation. A machine learning algorithm, trained to several spectral line shape features, enabled us to determine and spatially map the handedness of individual nanocrystals with high accuracy and speed. This technique may become invaluable in studies of symmetry breaking in chiral materials.

Coulomb-Volkov S-matrix theory of ionisation

In this paper we outline the recently derived Coulomb-Volkov (CV) S-matrix theory and present the results of calculations of the energy and the momentum spectra of the electrons emitted from ionisation of Ar atoms by an intense long-wavelength laser field. The results are compared with the corresponding calculations using the usual (plane-wave) SFA S-matrix theory, up to the second order of the series. We introduce a modified stationary phase method with a parametric integration to evaluate the high-dimensional integrals that occur in the second order of the ionisation amplitudes. The final-state Coulomb effect is found to be dominant at the lower energies, where the two theories differ greatly. In contrast, at high energies, the two theories produce qualitatively similar (in the logarithmic scale) distributions. It is found that despite the similarity of the positions of the ‘forward rescattering’ (or ‘soft collision’) peaks shown by the second order CV- and the SFA-amplitude, the line-shapes produced by them are qualitatively different. For example, the CV-spectrum shows a peak at 0.1 Up with a characteristic line-shape that is convex on one side and steeply rising on the other side; this is qualitatively similar to the line-shape of the peak observed experimentally earlier at the same energy. In contrast, the SFA-spectrum shows at the same energy a pointed peak with a line-shape that is concave on both sides. The characteristic line-shape of the low energy LES-peak(s) thus appears to suggest itself as a signature of the final-state Coulomb interaction in strong-field ionisation processes.

Piezoelectric polar nanoregions and relaxation-coupled resonances in relaxor ferroelectrics

\indent It is a generally accepted fact that the unique dielectric properties of relaxor ferroelectrics are related to the formation of polar nanoregion (PNRs). Less well recognized is the corollary that, because they are polar and therefore lack inversion symmetry, PNRs are also piezoelectric at the nanoscale and can therefore behave as nanoresonators. Using the particular relaxor ferroelectric K$_{\tt1-x}$Li$_{\tt x}$TaO$_{\tt 3}$ (KLT), we show that, when electrically excited into oscillation, these piezoelectric nanoresonators can drive macroscopic electro-mechanical resonances. Unexpectedly however, pairs of coupled resonances are observed, with one of the two exhibiting a characteristic Fano-like lineshape. The complex resonance spectra can be described equally well by two alternative but complementary models both involving two resonances coupled through a relaxation: a purely classical one based on two coupled damped harmonic oscillators and a semi-classical based on two discrete excitations coupled to each other through a continuum. Together, they provide complementary perspectives on the underlying physics of the system. Both reproduce the rapid evolution of the resonance spectrum across three wide temperature ranges, including a phase transition range. In the high temperature range, the coupling between modes is due to the collective $\pi$ relaxation of the lithium ions within PNRs and in the phase transition range to "heterophase relaxation" of the surrounding lattice between its high temperature cubic and low temperature tetragonal phases. The coupling is suppressed in the intermediate range of the collective $\pi/2$ relaxation of the lithium ions. Incidentally, the measured dielectric spectra are shown to bear a surprising but justifiable resemblance to the optical spectra of certain atomic vapors that exhibit electromagnetically induced transparency.

Laser frequency stabilization and spectroscopy at 2051 nm using a compact CO2-filled Kagome hollow core fiber gas-cell system.

We describe a compact, all fiber, frequency stabilized diode laser system at 2051 nm using CO2 gas-filled Kagome Hollow Core Fiber (HCF), capable of tuning continuously over four transitions in 12C16O2: R(24), R(26), R(28), and R(30). This laser system has been designed for use in future space-based atmospheric monitoring using differential absorption lidar (DIAL). The fully spliced Kagome HCF gas cell is filled to 2 kPa CO2 partial pressure and we compare the observed CO2 lineshape features with those calculated using HITRAN, to quantify the properties of the CO2-filled fiber cell. In this first demonstration of Kagome HCF used in a fully sealed gas cell configuration for spectroscopy at 2 µm, we characterize the frequency stability of the locked system by beat frequency comparison against a reference laser. Results are presented for the laser locked to the center of the 12C16O2 R(30) transition, with frequency stability of ∼40 kHz or better at 1 s, and a frequency reproducibility at the 0.4-MHz level over a period of > 1 month. For DIAL applications, we also demonstrate two methods of stabilizing the laser frequency ~3 GHz from this line. Furthermore, no pressure degradation was observed during the ~15-month period in which frequency stability measurements were acquired.

Exposing Key Vibrational Contributions to Properties of Organic Molecular Solids with High Signal, Low Frequency Neutron Spectroscopy and Ab Initio Simulations

Stability and response of supramolecular forms is important to many areas in materials science, and contributions from vibrations can be crucial. We have collected the first spectra of organic molecular crystals and polymorphic cocrystals using the next-generation, high-signal VISION spectrometer in the far-infrared (FIR) and mid-infrared (MIR) range. Unambiguously different spectral signatures were found for carbamazepine and two polymorphs of the carbamazepine-saccharin cocrystal, including numerous modes undetectable with optical methods. For FIR-range frequencies, in addition to correct calculation of peak positions, accurate line-shape features were reproducible by simulation using ab initio vibrational calculations. High confidence spectral assignments and thermochemical information for this region are thus expected, and a benchmark for vibrational ab initio calculations is provided, creating a significant addition to methods used in investigation and design of organic materials.

Laboratory Hydrogen-Beta Emission Spectroscopy for Analysis of Astrophysical White Dwarf Spectra

This work communicates a review on Balmer series hydrogen beta line measurements and applications for analysis of white dwarf stars. Laser-induced plasma investigations explore electron density and temperature ranges comparable to white dwarf star signatures such as Sirius B, the companion to the brightest star observable from the earth. Spectral line shape characteristics of the hydrogen beta line include width, peak separation, and central dip-shift, thereby providing three indicators for electron density measurements. The hydrogen alpha line shows two primary line-profile parameters for electron density determination, namely, width and shift. Both Boltzmann plot and line-to-continuum ratios yield temperature. The line-shifts recorded with temporally- and spatially-resolved optical emission spectroscopy of hydrogen plasma in laboratory settings can be larger than gravitational redshifts that occur in absorption spectra from radiating white dwarfs. Published astrophysical spectra display significantly diminished Stark or pressure broadening contributions to red-shifted atomic lines. Gravitational redshifts allow one to assess the ratio of mass and radius of these stars, and, subsequently, the mass from cooling models.

Excitation of O(1S) auroral green line from ambient air in a microwave-induced plasma jet at atmospheric pressure

Strong emission from the auroral green line O(1S) → O(1D) at 557.7 nm was observed in the afterglow of an atmospheric-pressure 2.45 GHz microwave-induced plasma jet. The device used Ar as the working gas for the plasma, and a surrounding N2 gas flow to stabilize the discharge. The characteristic line-shape of the peak at 557.7 nm indicated that the emission was from the O(1S)N2 excimer. Vibrational temperatures of NO and N2, estimated based on the emission spectra of the afterglow, were compared against the intensity of the green line and the concentration of N2 gas flowing in the system. The data points yielding the most intense green lines fall at vibrational temperatures of around 2000 K for both NO and N2. It was observed that the amount of N2 gas flowing in the system positively contributed to the intensity of the green line. Using fluid simulation, the O2 concentration in the afterglow was estimated to be in the range of 1–5.5%.

Evolution of the magnetoresistance lineshape with temperature and electric field across Nb-doped SrTiO3 interface

We report on the temperature and electric field driven evolution of the magnetoresistance lineshape at an interface between Ni/AlOx and Nb-doped SrTiO3. This is manifested as a superposition of the Lorentzian lineshape due to spin accumulation and a parabolic background related to tunneling anisotropic magnetoresistance (TAMR). The characteristic Lorentzian line shape of the spin voltage is retrieved only at low temperatures and large positive applied bias. This is caused by the reduction of the electric field at large positive applied bias which results in a simultaneous reduction of the background TAMR and a sharp enhancement in spin injection. Such mechanisms to tune magnetoresistance are uncommon in conventional semiconductors.

Ultrafast Two-Dimensional Infrared Spectroscopy of a Quasifree Rotor: J Scrambling and Perfectly Anticorrelated Cross Peaks.

Ultrafast two-dimensional infrared (2DIR) spectra of the N_{2}O ν_{3} mode in moderately dense SF_{6} gas exhibit complex line shapes with diagonal and antidiagonal features in contrast to condensed phase vibrational 2DIR spectroscopy. Observed spectra for this quasifree rotor system are well captured by a model that includes all 36 possible rovibrational pathways and treats P (ΔJ=-1) and R (ΔJ=+1) branch resonances as distinct Kubo line shape features. Transition frequency correlation decay is due to J scrambling within one to two gas collisions at each density. Studies of supercritical solvation and relaxation at high pressure and temperature are enabled by this methodology.

The Fusion Pore Lifetime during SNARE Mediated Fusion of Dense Core Vesicles with t-SNARE Containing Supported Membranes can be Modulated by Asymmetric Lipid Distributions

The two leaflets of the plasma membrane contain different lipid compositions. Phosphatidylethanolamine (PE) is predominantly located in the inner cytosolic leaflet and has been proposed to promote membrane deformation and stabilize fusion pores during exocytotic events. Docking and fusion of neuroendocrine dense core vesicles (DCVs) purified from PC12 cells with supported membranes that contain the neuronal SNARE proteins syntaxin 1a and SNAP-25A can be observed by recording the fluorescence of mRuby-tagged neuropeptide Y in a total internal reflection microscope (TIRFM). Single DCV fusion events can be identified by distinct characteristic time-dependent fluorescence line shapes. These line shapes can be simulated by a mathematical model that includes the opening of a stable fusion pore followed by the collapse of the vesicle into the plane of the supported membrane. We prepared supported t-SNARE membranes with different asymmetric PE distributions between the two leaflets and analyzed the life-time of the fusion pore and the fusion probabilities. With PE in the leaflet facing the DCVs, overall fusion was most efficient and the extended fusion pore lifetime enabled notable detection of content release preceding vesicle collapse. All other PE distributions decreased fusion efficiency, altered pore lifetime and reduced content release. With PE exclusively in the opposite leaflet, resolution of pore opening and content release was lost.

Optical Properties of Free and Si(001)‐Adsorbed Pyrimidinic Nucleobases

In this work, we predict and analyze the optical spectra of pyrimidinic uracil‐like nucleobases thymine (THY), uracil (URA), and 5‐fluorouracil (5‐FU) and their reflection anisotropy spectra (RAS) upon adsorption on the silicon (001) surface. First‐principles results based on plane‐wave density functional theory show chemically sensitive features in gas phase optical absorption spectra that redshift/blueshift according to the orbitals involved in the corresponding transition. In the RAS, a characteristic lineshape is found, typical of the energetically favored “dimer bridge” configuration, and remarkably similar for all the investigated Si(001):X systems (X = THY, URA, 5‐FU). We show that molecular tilting and breaking of the glide plane symmetry have a negligible effect on the optical spectra, despite their influence on the surface bandstructure. Contrarily to gas phase spectra, chemically sensitive RAS features only appear above 4.5 eV, and can be recognized as molecular contributions consistent with gas phase optical absorption results whereas substrate effects dominate at lower energies.

Investigating Optical Properties of Atomic Layer Deposited ZnO/TiOx Multi-stacked Thin Films Above Mott Critical Density

The evolution of the optical properties with dopant concentration has been investigated for a series of ZnO/TiOx multi-stacked layers having electron density exceeding the Mott critical limit of insulator-to-metal transition. These films were grown by vertically stacking multiple ZnO/TiOx bilayers on (0001) sapphire substrates using atomic layer deposition. The films in the sparsely doped regime showed room-temperature UV photoluminescence (PL) while being transparent and heavily degenerate in nature. Optical absorption spectra of these films did not exhibit any feature of excitonic resonance, indicating a possible excitonic Mott transition in the metallic limit. The low-temperature PL spectra also support this observation that shows line-shape characteristics typical for band-to-band emission. The sharp cutoff of the PL emission at the high energy edge corresponds to the Fermi level position inside the conduction band. In contrast, the broad low-energy wing is determined by the combined density of states...

The Gauss-Eyring model: A new thermodynamic model for biochemical and microbial inactivation kinetics

A new primary model has been developed, using Gaussian distributed populations and Eyrings rate constant for the transition state, to describe inactivation kinetics of enzymes and micro-organisms subjected to heat and chemical treatment. The inactivation of both enzymes and micro-organisms could be associated with the irreversible transition to an inactivated state, as suggested by the Lumry-Eyring model for protein denaturation and enzyme inactivation. The characteristic inactivation model parameters, standard activation enthalpy and entropy, are directly related to the reference temperature and Z-value commonly used for kinetic analysis in food microbiology. An essential feature of the kinetic model is that its parameters, and hence the transition temperature, are treated as stochastic variables. The characteristic line shape of the primary model is the log-normal distribution. The performance of the model was validated, using literature data for enzyme and microbial inactivation over a wide range of temperature and pH.

Optical properties of organically functionalized silicon surfaces: Uracil-like nucleobases on Si(001)

We predict UV reflectance anisotropy spectra (RAS) of the organically functionalized silicon (001) surface covered by pyrimidinic uracil-like nucleobases. First-principles results based on density functional theory show characteristic spectral features appearing in the UV range between 3 and 7 eV, besides the expected quench in the well-known two-minima RAS signal of clean Si(001). Nucleobase adsorption in the energetically favored ``dimer bridge'' configuration gives rise to a characteristic RAS line shape, common to thymine, uracil, and 5-fluorouracil. We trace back the origin of such spectral features by singling out RAS structures induced by relaxation and passivation effects on the Si surface, and those directly associated with molecular excitations. The former turn out to be the same for the three nucleobases, and are totally unaffected by molecular tilting. The sign and position of the latter RAS peaks at higher energy exhibit a moderate nucleobase dependence, and can be fully rationalized in terms of the molecular orbitals involved. The present theoretical results call for a RAS experimental study in the UV region extending up to $\ensuremath{\simeq}6--7$ eV.

Application of parahydrogen for mechanistic investigations of heterogeneous catalytic processes

Parahydrogen-induced polarization technique (PHIP), based on the pairwise addition of molecular hydrogen to a substrate, was successfully applied to obtain novel information on the mechanisms of heterogeneous catalytic hydrogenation, hydrodesulfurization, and oligomerization processes. In particular, the PHIP effects were observed upon hydrogenation with parahydrogen catalyzed by the immobilized neutral complexes of rhodium and iridium, which confirms the similarity in the mechanisms of homogeneous and heterogeneous hydrogenation for such systems. In the study of acetylene oligomerization, a significant NMR signal enhancement was revealed for a number of C4 oligomers, with the enhancement levels by far exceeding that observed in hydrogenation of carbon-carbon triple bonds. The mechanistic features of heterogeneous hydrogenation of a number of six-membered cyclic hydrocarbons over supported metal catalysts were investigated, and their hydrogenation scheme based on the pairwise addition of molecular hydrogen was proposed. Furthermore, the PHIP technique revealed that heterogeneous hydrodesulfurization of thiophene mainly proceeds via hydrogenation followed by a C—S bond cleavage. A significant enhancement of sensitivity in combination with characteristic line shapes of NMR signals make the PHIP method a unique and highly informative tool for the investigation of heterogeneous catalytic processes.