Peak Line Shape Research Articles

Resonant Raman scattering of double wall carbon nanotubes prepared by chemical vapor deposition method

Resonant Raman spectra of double wall carbon nanotubes (DWCNTs), with diameters from 0.4 to 3.0 nm, were investigated with several laser excitations. The peak position and line shape of Raman bands were shown to be strongly dependent on the laser energies. With different excitations, the diameter and chirality of the DWCNTs can be discussed in detail. We show that tubes (the inner or outer layers of DWCNTs) with all kinds of chiralities could be synthesized, and a DWCNT can have any combination of chiralities of the inner and outer tubes.

Exciton–phonon interaction in epitaxial CdSe/ZnSe nanostructures

AbstractThe low‐temperature optical properties of submonolayer CdSe/ZnSe nanostructures have been investigated with nominal layer thickness of 0.15 ML and 0.58 ML CdSe. In photoluminescence excitation spectroscopy (PLE) we observe more than 10 equidistant peaks separated by the ZnSe LO‐phonon energy even at energies high above the ZnSe‐band edge. The peak heights and line shapes are extremely sensitive to the monolayer coverage, have strongly different intensities in emission and PLE and deviate from the Poissonian intensity distribution expected within the concept of a constant, material specific Huang–Rhys parameter S. A bound polaron model is proposed to explain the optical properties of quantum structures for which the approach of a pure electronic quantum confinement is not appropriate. Intrinsic coherence of joint exciton–phonon modes is very promising, e.g. for application in quantum information processing or coherent population transfer.

Analysis of correlation patterns in hyperfine sublevel correlation spectroscopy ofS=1/2,I=3/2 systems

The properties of two-dimensional four-pulse electron spin echo envelope modulation spectra of nuclei with nuclear spinI=3/2 in disordered systems are discussed by means of calculated lithium hyperfine sublevel correlation spectra. Since a case of small coupling is treated, the multifrequency transition components appear only in the first quadrant of the two-dimensional spectra. The influence of the hyperfine and nuclear quadrupole interaction parameters on the cross peak ridges in the two-dimensional powder pattern is analyzed. Furthermore the possibility to determine the magnitude of the dipolar hyperfine and the nuclear quadrupole coupling from the cross peak line shapes is investigated. In this context the potential of those spectral features representing the so-called multiquantum transitions for the interpretation of the spectra is demonstrated.

Surface-topography dependence of line shapes in electron spectra due to decay of autoionizing states produced in inelastic ion-surface collisions

The line shapes of peaks in electron spectra due to decay of autoionizing states of Ne, formed in collisions of Ne with Al solid targets, were studied as a function of surface roughness. Strong variations in these line shapes were observed. These were modeled by taking into account the level shifts of the excited and ionic states of the atom in front of the surface and various resonant and Auger capture and loss processes. It is shown that rather complex line shapes may be expected. The electron spectrum generally possesses a peak at energies in the vicinity of that corresponding to autoionization of a free atom, as well as a high-energy tail and a secondary maximum. The intensity of the secondary maximum depends on how close to the surface most excited atoms decay and hence depends on surface flatness.

Spontaneous quantum dot formation at In xGa 1− xAs ∣ In yGa 1− yAs interfaces

We have performed reflectivity and photoluminescence measurements on a set of In x Ga 1− x As ∣ In y Ga 1− y As ∣ GaAs(001) stepped/asymmetric quantum wells with an intentionally abrupt change of indium composition ( x=0.149, y=0.064). These measurements have been compared against extensive spectroscopic data gathered on a set of symmetric quantum wells with similar structural parameters. For both symmetric and asymmetric quantum well samples, reflectivity spectra agree well with theoretical calculations both in terms of peak energy positions and line shapes. The photoluminescence spectra from the asymmetric quantum well samples are consistent with the hypothesis of three-dimensional exciton confinement induced by local and random potential fluctuations at the In-alloy ∣ In-alloy interface. A brief examination of the possible formation mechanisms leading to the formation of such ‘natural quantum dots’ at this poorly studied interface will be presented.

Chemical effects in Auger electron spectra of aluminium

The bonding and electronic structure of surface atoms is most often studied by XPS, where the energy shift of a photoelectron peak can be ascribed to a change in the chemical environment. Because three atomic levels are involved in the Auger emission process, the use of Auger spectroscopy to obtain this information from peak positions and lineshape is more difficult. However, new progress in signal analysis techniques makes the extraction of chemical information intrinsically present in the Auger spectra possible. The advantage of AES over XPS is its higher lateral resolution, allowing study of the surface distribution of the elements (Auger mapping). This, combined with modern computational methods, opens up new opportunities in surface analysis. In order to try to correlate the changes in the Auger peak energies with the changes in the chemical environment, we performed a full experimental and theoretical study of the Auger KLL lines of aluminium in several compounds (Al 2 O 3 , AlN, AlB 2 and AlF 3 ) by AES and x-ray-induced AES, the peak position for AlB 2 being reported for the first time. The inner-shell KLL peaks of Al show large energy shifts, depending on the chemical environment. The experimental results are compared to ab initio calculations based on a new cluster model including chemical, structural and relaxation effects.

NMR spectral quantitation by principal component analysis.

The use of principal component analysis (PCA) for simultaneous spectral quantitation of a single resonant peak across a series of spectra has gained popularity among the NMR community. The approach is fast, requires no assumptions regarding the peak lineshape and provides quantitation even for peaks with very low signal-to-noise ratio. PCA produces estimates of all peak parameters: area, frequency, phase and linewidth. If desired, these estimates can be used to correct the original data so that the peak in all spectra has the same lineshape. This ability makes PCA useful not only for direct peak quantitation, but also for processing spectral data prior to application of pattern recognition/classification techniques. This article briefly reviews the theoretical basis of PCA for spectral quantitation, addresses issues of data processing prior to PCA, describes suitable and unsuitable datasets for PCA applications and summarizes the developments and the limitations of the method.

Small-Angle X-ray Scattering from the Lamellar Phase Formed in a Nonionic Surfactant (C16E7)−Water System. Analysis of Peak Position and Line Shape

The structure of the lamellar phase formed in the heptaethylene glycol n-hexadecyl ether (C16E7)−water system has been studied using small-angle X-ray scattering (SAXS). The concentration dependence of the repeat distance d follows the power law d ∝ φhc-s where φhc is the volume fraction of hydrophobic layer. The exponent s obtained is unity as expected at 75 °C where the “Lα” phase extends to the lower concentration range. As the temperature decreases below about 70 °C, however, s decreases rapidly. Below about 55 °C, s becomes constant again and takes values of about 2/3. From the line shape analysis of SAXS in the “Lα” phase, half-thickness of the hydrophobic layer δhc and the thickness of the hydrophilic layer have been obtained. It has been shown that the anomalous behaviors of the exponent s cannot be explained by the variation of δhc with concentration and temperature. In other words, the structure of the “Lα” phase varies with concentration and temperature. Assuming that the bilayer sheets have water-filled defects, we have made a “map” showing how the fraction of the defects fw depends on concentration and temperature. Strong correlation has been found among the fw value, the appearance of the broad component superimposed on the first-order reflection, and the quadrupolar splitting of 2H2O. Relations between the phase behaviors and the structures of the L1, “Lα”, and V1 phases have been discussed based on these results.

Effects of temperature and other experimental variables on single molecule vibrational spectroscopy with the scanning tunneling microscope

Inelastic electron tunneling spectroscopy (IETS) was performed on single molecules with a variable temperature scanning tunneling microscope. The peak intensity, width, position, and line shape of single molecule vibrational spectra were studied as a function of temperature, modulation bias, bias polarity, and tip position for the (C–H,C–D) stretching vibration of acetylene (C2H2,C2D2) on Cu(001). The temperature broadening of vibrational peaks was found to be a consequence of Fermi smearing as in macroscopic IETS. The modulation broadening of vibrational peaks assumed the expected form for IETS. Extrapolation of the peak width to zero temperature and modulation suggested an intrinsic width of ∼4 meV due primarily to instrumental broadening. The inelastic tunneling cross section at negative bias was reduced by a factor of 1.7 for the C–H stretch mode. Low energy modes of other molecules did not show such a reduction. There was no evidence of a tip-induced Stark shift in the peak positions. The spatial variation of the inelastic signal was measured to determine the junction stability necessary for the acquisition of single molecule vibrational spectra.

Asymmetric peak line shape of infrared dielectric function spectra for thermally grown silicon dioxide films

Measured infrared dielectric function spectra ranging from 300 to 1400 cm−1 for thermally grown silicon dioxide films were studied. The dielectric function spectra were calculated by requiring the calculated spectra to fit to the actual spectra. The peak line shape of the dielectric function can be described by a Gaussian distribution, but not by a Lorentzian line shape. In detail, frequencies of half height for the imaginary part of the dielectric function of high- and low-frequency edges are not actually symmetrical. According to interpretations based on a central and noncentral force network model, the asymmetric distribution of the dielectric function arises from the symmetrical distribution of the bond angles of a random network of the SiO4 tetrahedra. For that reason, we developed a dielectric function model which can describe an asymmetrical Gaussian line shape. This asymmetric Gaussian model minimizes the number of parameters needed, which are the one center position and low- and high-Gaussian distribution widths, compared with the assignment method for multiple purely Gaussian peaks.

Photoemission spectroscopy of platinum overlayers on silicon dioxide films

Soft x-ray photoelectron spectroscopy (SXPS) has been used to study ultrathin Pt films on silicon dioxide as model supported-catalyst materials. Using monochromatic synchrotron radiation. Pt 4f and Si 2p core level photoelectron peaks were measured as a function of platinum coverage in the range 0–10 ML. The bulk silicon and silicon dioxide film Si 2p peaks each show a binding energy decrease within the first ML of dosing. However, the effect is stronger for the silicon dioxide Si 2p peak, indicating an increased screening of the 2p electrons by the metal overlayer. We also observe a monotonic increase of the work function of the ultrathin film Pt/SiO2 system with coverage from 4.52 initially to 5.58 eV at ∼10 ML. The Pt 4f7/2 core level binding energy decreased from ∼72.2 to ∼70.9 eV between 0 and ∼10 ML coverage. This binding energy shift at low dose (⩽1 ML), the coverage dependence of the Pt line shape and intensity, and the large saturation coverage for the work function are each consistent with two-dimensional cluster-island growth. The metallic nature of the Pt overlayer with dose is quantitatively verified by nonlinear, least-squares numerical fitting of the Pt 4f SXPS peak line shapes with Gaussian-broadened Doniach–Sunjic functions.

Grain-Growth Kinetics of Nanocrystalline Iron Studied In Situ by Synchrotron Real-Time X-ray Diffraction

Pulsed electrodeposition (PED) is used to prepare nanocrystalline iron with an average grain size of 19 nm and thermal stability up to 550 K. At 663 K ≤ T ≤ 783 K the kinetics of grain growth, with respect to size and size distribution, is studied in situ by means of real-time synchrotron X-ray diffraction. The Bragg peak line shapes of the large number of diffractograms are analyzed using a Warren/Averbach procedure improved with respect to reliability and efficiency. We observe two regimes of grain growth: at less elevated temperatures grain growth is smooth and moderate up to limiting size values between 50 and 100 nm, depending on temperature. The initially rather narrow width of the size distribution increases slightly, and the activation energy of grain growth, about 100 kJ/mol, corresponds to the literature value for grain boundary self-diffusion in nanocrystalline Fe. At higher temperatures the grains grow first rapidly and then slowly up to limiting values between 200 and 400 nm, depending on te...

Growth and characterization of high-quality GaInAs/AlInAs triple wells

The growth and characterization of GaInAs/AlInAs triple quantum well structures are reported. The structures are characterized with high-resolution X-ray diffraction combined with high-resolution simulation based on the dynamical theory of X-ray diffraction and with photoluminescence. The good agreement in both peak position and intensity of the high-resolution X-ray diffraction spectra with the high-resolution simulation reveals the precise control of the epilayer thickness and the good quality of interfaces. The photoluminescence spectra are analyzed concerning the peak position, interface roughness, and lineshape. These three aspects correlate well with each other, and the interface roughness is determined by fluctuations of 1 monolayer occurring every 3 effective exciton radii in the lateral plane.

Structural and electronic properties of hydrogen-absorbed alkali-C 60 compounds

The effect of hydrogen absorption on the structural and electronic properties of K 3C 60 superconductor was investigated by means of powder X-ray diffraction, magnetic susceptibility, and solid state NMR. No lattice expansion as well as no structural change were observed in powder X-ray diffraction by hydrogen absorption. Superconductivity was preserved and the superconducting transition temperature was not affected by hydrogen absorption. No distinct relationship was observed between the shielding fraction and the amount of absorbed hydrogen. Peak position and line shape of 13C NMR spectrum did not change by hydrogen absorption. Peak position of 1H NMR spectra suggested that hydrogen was almost neutral in the crystal. No significant difference was observed between K 3C 60 and hydrogeniabsorbed sample in 23Na NMR spectra. These facts suggest that absorbed hydrogen does not occupy octahedral or tetrahedral sites in the host f.c.c. lattice. Nuclear Curie-like behavior was observed in the temperature dependence of the 1H NMR intensity.

Second-order central peak in the Raman spectra of disordered ferroelectrics

A theory of the central peak line shape in Raman scattering in ferroelectrics above ${T}_{c}$ is developed. Off-center ions are considered as a possible source of the local symmetry breaking. We argue that in order to properly describe the features of the central peak line shape above ${T}_{c}$ one should take into account the second-order invariants in the modulation of the electronic polarizability by polarization fluctuations. The theory is in excellent agreement with the Raman-scattering experiment in ${\mathrm{KTa}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ [J. P. Sokoloff, L. L. Chase, and L. A. Boatner, Phys. Rev. B 41, 2398 (1990)].

SUSCEPTIBILITY AND VERTEX CORRECTIONS FOR A SQUARE FERMI SURFACE

We investigate the response of an electron system which exhibits ideal nesting features. Using the standard Matsubara formalism we derive analytic expressions for the imaginary and real parts of the bare particle–hole susceptibility. The imaginary part has sharp peaks whose maxima at the nesting momenta approximately scale with (ω/T). The peak lineshapes resemble neutron scattering data on chromium and some copper oxide superconductors. The real part of the bare susceptibility at the nesting vectors diverges logarithmically at low temperatures. Analytic formulas for the first vertex correction to the susceptibility are derived for a Hubbard interaction and its momentum and temperature variations are calculated numerically. This term detracts substantially from the ordinary RPA terms for intermediate values of the Coulomb repulsion. Exact cancellation of a certain class of diagrams at half filling is shown to result from particle–hole symmetry. We discuss the consequences of these results for spin fluctuation theories of high temperature superconductors and spin density wave instabilities.

Effect of a pulse entrapping on weak localization of waves in a resonant random medium

We consider theoretically a new physical effect in coherent backscattering enhancement (CBE) of electromagnetic or acoustic non-stationary waves from a discrete random medium under condition of Mie resonant scattering. The effect manifests itself as an angle-cone broadening of a short pulsed signal CBE from the resonant random medium, compared with the case of a non-resonant random medium. The cone broadening is associated with a pulse-entrapping effect when the pulse, while propagating within the resonant random medium, spends most of the time being ‘entrapped’ inside scatterers. A theory for the predicted effect is based on, first, the well known relation between the contributions of the ladder and cyclical diagrams to the time spectral density of the wave electric field coherence function and, second, a recently derived radiative transfer equation with three Lorentzian kernels of delay describing a pulse entrapping in an ensemble of resonant point-like scatterers. Using the generalized Chandrasekhar H-function, we obtain an exact analytic expression for the non-stationary albedo of the semi-infinite resonant random medium, taking into account the phenomena of a pulse CBE and entrapping. A simple analytic asymptotics is found for the albedo of the later part of the scattered pulse. This asymptotics shows quantitatively how the entrapping affects the peak amplitude and peak line shape of the CBE of a short pulse.

Ageing of Starch Based Systems as Observed with FT-IR and Solid State NMR Spectroscopy

The retrogradation and physical ageing of model starch systems with respect to their glass transition temperatures Tg have been investigated by Fourier transform infrared Spectroscopy and solid state NMR spectroscopy. Diffuse reflectance Fourier transform infrared (DRIFT) spectra demonstrate the commencing retrogradation of starch materials stored above their Tg by changes in peak lineshapes and intensities in the characteristic area between 995 cm−1 and 1020 cm−1. Solid state NMR proton relaxation times in the rotating frame (proton T1ρ) show a characteristic course in relation to the storage conditions (time, humidity), for which a distinction is made between physical ageing which occurs below the Tg, and recrystallisation (retrogradation) which takes place above Tg. The proton T1ρ's of materials stored below Tg increase asymptotically in time due to physical ageing, whereas the proton T1ρ's of materials stored above Tg increase until a moisture content is reached that rises them above Tg, decrease due to further water absorption and then increase due to recrystallisation (retrogradation).

1H MR spectroscopy monitoring of changes in choline peak area and line shape after Gd-contrast administration

Fifteen percent loss in the peak area of choline containing compounds (Cho) was recently observed in 1H MR spectra of contrast-enhancing tumor at 5–10 min after Gd-contrast administration [Magn. Reson. Med. 37:222–225, 1997]. In this study, chemical shift imaging (CSI, 1500/135 ms PRESS) was used to assess the spectral changes in 47 Gd-enhancing glial brain tumors and metastatic brain tumors measured at 0–5, 5–10, and/or 10–15 min after administration of Gd-contrast. Percent Cho peak area losses measured at these times, 3 ± 3, 12 ± 2, and 14 ± 3 SEM, respectively, coincided with trends of line narrowing and up-field shift of the Cho peak. Significant changes in creatine and N-acetyl acetate signals were not observed. It is concluded that the Gd-induced loss of tumor Cho signal measured after 5 min, typically required for post contrast-MRI and the positioning of the CSI volume on tumor, shows little further change with time, if any.

Ab InitioCalculation of Excitonic Effects in the Optical Spectra of Semiconductors

An ab initio approach to the calculation of excitonic effects in the optical absorption spectra of semiconductors and insulators is formulated. It starts from a quasiparticle bandstructure calculation and is based on the relevant Bethe--Salpeter equation. An application to bulk silicon shows a substantial improvement with respect to previous calculations in the description of the experimental spectrum, for both peak positions and lineshape.