Characteristic Line Shape Research Articles

Optical line-shape and the time-domain photon echo measurement in semiconductor quantum dots

It is shown theoretically that, because of the characteristic line-shape of the electronic spectral density, the experimental photon-echo signal, giving the dephasing of the electronic state of a quantum dot, should be modulated in time quasi-periodically. This suggestion is demonstrated numerically for the case of InAs quantum dots for several temperatures of the sample. A comparison with the recent experiments on CdSe nanocrystals is presented. The theoretical argumentation is based on the recent theoretical calculation of the homogeneous optical line-shape based on the multiple electron–LO–phonon scattering in quantum dots.

Small Volume Flow Probe for Automated Direct-Injection NMR Analysis: Design and Performance

A detailed characterization of an NMR flow probe for use in direct-injection sample analysis is presented. A 600-MHz, indirect detection NMR flow probe with a 120-μl active volume is evaluated in two configurations: first as a stand-alone small volume probe for the analysis of static, nonflowing solutions, and second as a component in an integrated liquids-handling system used for high-throughput NMR analysis. In the stand-alone mode, 1H lineshape, sensitivity, radiofrequency (RF) homogeneity, and heat transfer characteristics are measured and compared to conventional-format NMR probes of related design. Commonly used descriptive terminology for the hardware, sample regions, and RF coils are reviewed or defined, and test procedures developed for flow probes are described. The flow probe displayed general performance that is competitive with standard probes. Key advantages of the flow probe include high molar sensitivity, ease of use in an automation setup, and superior reproducibility of magnetic field homogeneity which enables the practical implementation of 1D T2-edited analysis of protein–ligand interactions.

2H MAS NMR of strongly dipolar coupled deuterium pairs in transition metal dihydrides: extracting dipolar coupling and quadrupolar tensor orientations from the lineshape of spinning sidebands

A selectively D2 labeled transition metal dihydride, Os(D2)(Cl)2(CO)(PiPr3)2, was synthesized and studied by 2H magic angle spinning (MAS) NMR spectroscopy. It was found that the interference between the quadrupolar and homonuclear dipolar interaction results in a characteristic lineshape of the MAS sidebands. The basic properties of the interference of homonuclear dipolar and quadrupolar coupling on the 2H NMR lineshape were elucidated, using average Hamiltonian theory, and exact simulations of the experiments were achieved by stepwise integration of the equation of motion of the density matrix. These simulations show that it is possible to determine the size of the dipolar interaction and thus the D–D distance from the lineshape of the sidebands.

Two-Dimensional Short-Range Magnetic Order in the Tetragonal Spinel Li2Mn2O4

Metastable, tetragonal Li2Mn2O4, prepared by chemical insertion of Li into the cubic spinel, LiMn2O4, was studied by dc magnetic susceptibility and neutron diffraction. The susceptibility data show a very broad maximum at about 120 K, a sharp maximum at about 50 K, and no evidence for Curie−Weiss behavior even up to 600 K. This is consistent with the dominance of short-range, antiferromagnetic correlations attributed to the geometrical frustration inherent in the Mn3+ sublattice which is a slightly distorted pyrochlore lattice, a three-dimensional array of corner-sharing tetrahedra. The temperature dependence of two diffuse peaks in the neutron diffraction data correlate well with the 50 K anomaly but show, remarkably, an asymmetric Warren line-shape characteristic of two-dimensional correlations with no sign of long-range order down to 1.6 K. The magnetic reflections can be indexed as (2 0) and (1 3) of a √3 × √3 magnetic structure arising from a Kagomé layer. The structure and properties of tetragonal Li2Mn2O4 are compared with the thermodynamically stable orthorhombic form.

ENDOR Studies of Pyruvate:Ferredoxin Oxidoreductase Reaction Intermediates

Electron−nuclear double resonance (ENDOR) studies of radical intermediates formed by the oxidative decarboxylation of pyruvate by pyruvate:ferredoxin oxidoreductase were carried out to characterize their electronic structure and elucidate aspects of the recently proposed catalytic mechanism (Menon, S.; Ragsdale, S. W. Biochemistry 1997, 36, 8484−8494). The EPR spectrum of the PFOR/pyruvate adduct at 4 K displays a narrow resonance centered at g = 2.008 that has been attributed to a hydroxyethyl thiamine pyrophosphate (HE-TPP) radical. This spectral feature is superimposed on a broad, complex line shape characteristic of magnetically coupled [Fe4S4] clusters. The ENDOR spectrum at g = 2.008 reveals a broad peak with a complex line shape that can be analyzed, assuming that it arises from a composite of two axially symmetric proton hyperfine couplings. The principle coupling values for these two hyperfine tensors were: A∥(1) = 18.9 MHz, A⊥(1) = 12.6 MHz; and A∥(2) = 20.3 MHz, A⊥(2) = 14.9 MHz. The assignment of these features to the methyl protons of the pyruvate substrate was made using isotopic substitution. The temperature independence of these 1H ENDOR line shapes from 4 to 200 K indicates that the methyl group of pyruvate undergoes rapid rotation even at 4 K. The ENDOR spectrum at g = 2.008 also shows a pair of derivative peaks centered about the 31P Larmor frequency that are assigned to a weak hyperfine coupling with the phosphorus nuclei of the TPP cofactor. Two models for the electronic structure of the radical intermediate are discussed. A σ radical model which postulates a pyruvate-derived acetyl-type radical where little unpaired spin density resides on the TPP cofactor, and a π radical model that calls for more extensive delocalization of the unpaired electron spin over the HE-TPP framework. Both models require association of the radical center with the pyrophosphate group of TPP to interpret the observed 31P hyperfine coupling.

The Valence Electronic Structure of N-Doped P-Sexiphenyl

ABSTRACTThin films of p-sexiphenyl (6P) were doped with increasing amounts of potassium in situ, and the change in the valence electronic structure of 6P upon the alkali metal deposition was followed with ultraviolet photoelectron spectroscopy. We observe the evolution of new intragap emissions, which are attributed to the formation of bipolarons, even for very low doping concentrations. The low binding energy intra-gap emission exhibits a pronounced asymmetric lineshape, in contrast to the findings when cesium is used as dopant. In order to investigate whether this lineshape is due to different emissive electronic species in the bulk and on the surface of the 6P film the take-off angle for the photoelectrons was varied. As no change in the lineshape is found when going from normal to near-grazing emission we can exclude that charged 6P molecules in the bulk and on the surface yield different valence electronic spectra. Therefore, the characteristic lineshape of the low binding energy emission is proposed to be related to the interaction of the doped organic molecule with the different counterions.

Sensitivity of Electrically Active Defect Spectra to Processing Conditions in Mev Heavy Ion Implanted Silicon

ABSTRACTWe have carried out electrical characterization of defects in heavily damaged silicon, where damage is created by MeV heavy ions at doses near but below amorphization threshold and samples are subjected to different annealing conditions. Defect characterization is carried out using combination of deep level transient spectroscopy (DLTS) and isothermal time analyzed spectroscopy (TATS). The defect spectrum is observed to be sensitive to I) furnace annealing between 400–600°C, ii) low temperature oven annealing at 160°C, and iii) forward injection current. The observed changes in spectra are indicators of extreme sensitivity of dominant defects to relaxation of the disordered medium, and recombination enhanced reactions. Surprisingly the defect spectra is usually dominated by a single peak with unbroadened lineshape characteristic of discrete energy level in the bandgap, though electrical signatures keep varying for different processing conditions. In the light of these measurements, we discuss the nature of stability and metastability of the defects believed to be due to intrinsic defect clusters.

Infrared spectra evaluation of syndiotactic polystyrene by dynamic infrared linear dichroism spectra and static difference spectra

Uniaxially drawn syndiotactic polystyrene (sPS) in various modifications has been analyzed by static difference polarization spectra and DIRLD (dynamic infrared linear dichroism) spectra during mechanical stretching. Both, derivative bipolar bands and monopolar bands, are observed as a hint to submolecular changes induced by the applied external strain. The lineshape features in the dynamic in-phase spectra are described on the basis of frequency shifts and absorption amplitude variations during the stretching cycle. The conformationally sensitive vibrational modes are most sensitive to the external perturbation. The different responses of the conformationally insensitive vibrational modes to the external perturbation are discussed.

Dynamic rheo-optical characterization of uniaxially oriented polyamide 11

Dynamic mechanical analysis, coupled with polarized step-scan FTIR transmission spectroscopy, has been used to monitor the submolecular motional behavior of uniaxially oriented polyamide 11. The dynamic in-phase spectra depend upon the morphology of the samples as well as on the polarization direction of the infrared radiation. The lineshape features of the dynamic in-phase spectra and their relationship to sample deformation are analyzed on the basis of changes of the internal coordinates, the reorientation movement of several functional groups, and the thickness change of the film during the stretching cycle. Dynamic infrared spectra are helpful for deconvolution of overlapping bands on the basis of their different responses to the external perturbation, which sometimes cannot be resolved well by derivative spectroscopy or curve-fitting analysis. The lineshape features have been used to follow microstructural changes after isothermal heat treatment. Near the N-H stretching frequency, two bands at 3270 cm -1 and 3200 cm -1 are resolved and analyzed in terms of Fermi resonance between the N-H stretching fundamental mode and the overtone and combination modes of the amide I and II vibrations. The dynamic response of the N-H stretching mode correlates with the modulation of hydrogen bond strength in uniaxially oriented PA-11. After thermal treatment at the highest temperature (190°C), the dynamic response in this region is mainly caused by the modulation of crystals. In amide I region, three bands at 1680 cm -1 , 1648 cm -1 , and 1638 cm -1 are separated and assigned to hydrogen bond-free, hydrogen-bonded amorphous, and hydrogen-bonded crystalline regions, respectively. The dynamic responses of the hydrogen-bonded regions are more sensitive to external perturbation. Two components are found in the amide II region, and the band at 3080 cm -1 is assigned to the overtone resonance of the component with perpendicular polarization.

An analytical model for collisional effects on spectral line shape from the Doppler to the collision regime

A spectral line shape model accounting for both the collisional confinement narrowing of the Doppler distribution and the inhomogeneous effects due to the radiator speed dependence of the collisional broadening and shifting parameters is proposed. The velocity changes are assumed to be induced as well by hard as soft collisions. Doppler-collision correlations and speed-class exchanges are taken into account. A comparison with the previous models used for the Doppler regime and for the collisional one is done. The present model also applies for the intermediate regime where the two above mechanisms are simultaneously efficient. The spectral line shape characteristics are exemplified through the H2–Ar and C2H2–Xe prototype molecular systems.

An electron spin resonance study of the soy bean phosphatidylcholine-based reversed micelles

The ESR spectra of l-palmitoyl-2-stearoyl-(n-doxyl)-glycero-3-phosphocholine spin label positional isomers (n = 5, 7, 10, 12 and 16) have been studied in soy bean phosphatidylcholine (SPC)-based microemulsions with various volume fractions of disperse phase over the wide temperature range. The maximum hyperfine splitting 2A max and the order parametersS were taken as indices of the rotational mobility and the motion spatial restrictions of the labeled lipid chain segments. It is found that the temperaturesT tr at which sharp enhancements of2A max andS occur depend on concentration and size of the reversed micelles in solutions. To explain this, a plausible model, taking into account capability of the SPC molecule hydrocarbon chains to change a tilt angle with respect to the surface of a polar head group as temperature varies, is proposed. The estimations of the correlation times τsl obtained from the lineshape characteristics of the ESR spectra provided the possibility to suggest that these correlation times characterize the reorientations of the SPC chain axis about the normal to the surface of a polar head group of a reversed micelle.

Enhanced boson peak in Pr-doped Ge-S-I glasses with varying composition and Pr-content

Raman scattering from Ge-rich, undoped and Pr-doped Ge-S-I glasses is reported over the spectral range 5-450 cm −1 . In the region of molecular vibrations, apart from the bands of the Ge-S binary glassy system, an additional band at 222 cm −1 has been observed and assigned to I vibrations in terminating Ge-I bonds. A well-resolved, strong Boson peak has also been observed at ∼26 cm −1 in all samples, the intensity of which increases with increasing I-content. The strength of this peak is associated with the large atomic mass and polarizability of I, and its frequency is connected with the medium range order of the glass. Doping with Pr 3+ results in further enhancement of the Boson peak, indicating that Pr ions are incorporated mainly into regions of the glass responsible for the collective modes producing this peak. Measured lineshape characteristics on both sides of the Boson peak are in satisfactory agreement with the predicted ones by the soft potential model.

Combined X-ray Diffraction and 15N CPMAS NMR Study of Molecular Structure and Proton Order/Disorder Phenomena in Cyclic N,N‘-Bisarylformamidine Dimers

Crystal structures of a series of five symmetrically substituted N,N-bisarylformamidines ArNHCHNAr with Ar = X-C6H4, X = p-OCH3 (IV), p-CH3 (V), p-F (VI), p-NO2 (VII), and m-Br (VIII) have been determined by single-crystal X-ray diffraction (XRD) and complete the series studied previously where X = H (I), p-Br (II), and p-Cl (III). In addition, the results of variable-temperature 15N CPMAS NMR experiments performed on 15N-labeled I, II, and IV are reported. All compounds form cyclic dimers linked by two NH···N hydrogen bonds which can form two different tautomers, a and b, interconverting by fast double proton transfers. The NMR experiments indicate three types of amidines characterized by different magnitudes of the equilibrium constants Kab of the tautomerism. In dimers of type such as V−VIII, we find Kab ≪ 1 (i.e., only a single tautomer in the temperature range between 100 and 300 K). In this case, the hydrogen-bonded protons are ordered and can be localized by XRD. Furthermore, the C···N bond lengths and torsional and valence angles involving the two aryl groups of an amidine unit are different. For dimers such as II and III, characteristic temperature dependent 15N CPMAS NMR line shape changes are observed indicating that Kab = 1 within the margin of error. Rate constants of the tautomerism can in this case be obtained by line shape analysis. For this degeneracy to occur, the aryl group conformations at both amidine nitrogen atoms must be similar. XRD then observes disordered hydrogen-bond protons and, in principle, also disordered nitrogen atoms. However, in practice, the disorder of the latter is not resolved leading to the observation of equalized C···N bond lengths. Finally, dimers (I, IV) represent an intermediate case with Kab < 1, which could be labeled as “dynamic partial order”. The implications of the molecular structure and the hydrogen bond and proton transfer characteristics are discussed.

Collision-induced first overtone band of H2 in H2–Kr and H2–Xe mixtures

Enhancement of the collision-induced absorption (CIA) spectra of the first overtone infrared band of H2 in the H2–Kr and H2–Xe binary mixtures were recorded at 295 K for base densities of H2 in the range 30–57 amagat and for partial densities in the range 50–250 amagat for each of Kr and Xe. The observed spectra consist of single transitions O2(J), Q2(J), and S2(J) corresponding to the selection rule ΔJ=−2, 0, and 2, respectively. Binary, ternary, and quarternary absorption coefficients of the band arising from collisions of the type H2–X, H2–X–X, H2–H2–X,H2–X–X–X, H2–H2–X–X, and H2–H2–H2–X, where X represents Kr or Xe, have been determined from the integrated absorption coefficients of the band. The spectra are interpreted in terms of the quadrupolar induction mechanism and no evidence of the contribution from the isotropic overlap induction mechanism is found unlike in the CIA spectra of the fundamental band of H2 [see S. P. Reddy, in Phenomena Induced by Intermolecular Interactions, edited by G. Birnbaum (Plenum, New York, 1985), p. 129]. Analysis of the absorption profiles of the observed spectra was carried out using the Birnbaum–Cohen line-shape function [G. Birnbaum and E. R. Cohen, Can. J. Phys. 54, 593 (1976)] for individual components of the band and characteristic line shape parameters were determined from the analysis.

EELS analysis of redox in glasses for plutonium immobilization

The chemical and structural environments of f-electron elements in glasses are the origin of many of the important optical, electronic, and magnetic properties of materials incorporating these elements. Thus, the oxidation state and chemical coordination of lanthanides and actinides in host materials constitute an important design consideration in optically active glasses, magnetic materials, perovskite superconductors, and nuclear waste materials. We have made use of the characteristic line shapes of cerium to determine its oxidation state in alkali borosilicate glasses that are being developed for immobilization of plutonium. Cerium, it should be noted, is often used as a “surrogate” element for plutonium in materials design because of its similar ionic size (for Pu in the + 3 and + 4 states) and preferred chemical coordination. The solubility of the plutonium (or cerium) in a waste glass will likely be determined by its redox state in the glass. By examining several compositions of prototype immobilization glass using electron energy loss spectroscopy (EELS), we found that the redox state of cerium doped to 7 wt% could be varied by a suitable choice of alkali elements in the glass formula. Preliminary results on plutonium-doped glasses confirm the design strategy employed, leading to 5 wt% (or more) plutonium being truly dissolved in the glass.

Autonomous stochastic resonance in bursting neurons

Noise-induced firing is studied in two major classes of bursting neuron models in the absence of periodic input. In the biologically relevant subthreshold regime where no deterministic firing occurs, additive noise induces spiking limit cycles. This noise makes the output firing patterns sensitive to the characteristics of autonomous subthreshold oscillations, which can change in response to various physicochemical stimuli. The nonmonotonic behavior with increasing noise of the phase locking between spikes and subthreshold oscillations, measured using spectral signal-to-noise ratios and line shape characteristics, are a manifestation of autonomous stochastic resonance in these systems. The type of bifurcation giving rise to bursting activity determines the behavior with noise of the mean firing frequency, interspike interval histogram, spike train power spectrum, and phase locking. In particular, it is shown that a saddle-node bifurcation is not required to see stochastic resonance (SR) without periodic input when there exists a stable deterministic subthreshold oscillation. This paper also studies SR in a detailed ionic neuron model, an approach that leads to tests of hypotheses regarding the nature of noise in real neurons.

Zero-field splitting of the lowest excited triplet state in thiophene oligomers. An experimental and theoretical investigation

The lowest photoexcited triplet state of thiophene oligomers ( nT) with n = 2 – 8 has been investigated by pulsed EPR spectroscopy. The characteristic EPR lineshape is analyzed by performing spectral simulations. The determined D values decrease with increasing chain length showing a linear dependence on the inverse chain length. Furthermore conformational effects have been observed. We compare the results with theoretical calculations of the zero-field splitting parameters. The calculations are based on a semiempirical ’one electron per site’ model including electron-electron interactions. The computed D values are in excellent agreement with the EPR data. The chain length dependence of D could be rationalized by computing electron spin densities and bond length distortions, which clearly indicate that the triplet excitation reaches a finite extention over about four thiophene rings.

Monitoring magnetohydrodynamics boundary layers using emission line shape features

A strong correlation has been found between the center dip slope of self-reversed atomic potassium emission lines in a coal-e red magnetohydrodynamics e ow and the thickness of the cool boundary layer surrounding the hot core e ow. The boundary-layer proe le was determined indirectly from line shape e tting of simultaneous time-resolved multiwavelength emission and absorption spectra. For line shape e tting, a power law model with an effective boundary-layer width is proposed for modeling the radiative transfer across a turbulent boundary layer, rather than an inverse power law model that is used for a turbulent velocity boundary layer. A simple relationship can be written between the center dip slope and the effective boundary layer width that will allow real-time monitoring of the turbulent e ow.

Dynamic Infrared Linear Dichroic Spectra of Prestretched Polypropylene

DIRLD (dynamic infrared linear dichroic) spectra of prestretched isotactic polypropylene have been recorded. The line shape features in the in-phase spectra are described on the basis of frequency shifts and absorption amplitude variations during the stretching cycle. Details such as the dichroism of the bands, direction of the shift, and changes in the absorption intensities must be considered to explain signs and shapes of the DIRLD bands. Evidence for chain reorientations under stress has not been found. The general principle of frequency shifts and changes in absorption intensities as the origin of DIRLD bands is demonstrated by spectral simulations.

Triplet-level-crossing emission resonances in backward degenerate four-wave mixing

Resonant degenerate backward four-wave mixing in a Doppler-broadened Λ-type three-level system is investigated using two intense cross-polarized grating waves. In this saturation setup, line shape behaviour changes drastically and well-resolved triplet structures may be observed by varying the Larmor frequency associated with the spacing of the two lower levels. Line shape features are analysed in the optical Bloch formalism with resort to dressed-atom energy level diagrams and extra side peaks appearing in addition to a very strong Zeeman-coherence resonance are shown to come from the destructive interference between two- and three-photon processes.