Unusual Line Shapes Research Articles

Postcollision-interaction effects in multistep Auger transitions following Ar 1s photoionization

Postcollision-interaction (PCI) effects involving multistep decay processes following Ar $1s$ photoionization has been studied by Auger electron spectroscopy. The experiment focused on $LMM$ Auger electrons measured in small photon energy steps across the Ar $1s$ photoionization threshold. Decay pathways that we studied include (1) the ${\mathrm{Ar}}^{+*}2{p}^{--1}\ensuremath{\rightarrow}{\mathrm{Ar}}^{2+}3{p}^{--2}$ LMM\ensuremath{\alpha} Auger process due to a single $L$ hole created by $KL$ fluorescence, (2) the ${\mathrm{Ar}}^{2+*}2{p}^{--2}\ensuremath{\rightarrow}{\mathrm{Ar}}^{3+*}2{p}^{--1}3{p}^{--2}$ ${LMM}_{1}$ Auger process following double $L$ shell hole states produced by a $KLL$ Auger processes, and (3) the subsequent ${\mathrm{Ar}}^{3+*}2{p}^{--1}3{p}^{--2}\ensuremath{\rightarrow}{\mathrm{Ar}}^{4+}3{p}^{--4}$ ${LMM}_{2}$ Auger transitions. Particularly pronounced PCI shifts and unusual line shapes compared to the ordinary one-step PCI process were found in the spectra of Auger processes following a $KLL$ Auger first step. The experimental results were compared with calculations based on the semiclassical approach to PCI. Good agreement was found between the calculated and experimental PCI shifts. The result opens possibilities for further studies of the multielectron dynamics between Auger electrons mediated through the photoelectron in these and similar systems.

A combined electronic structure and molecular dynamics approach to computing the OH vibrational feature of strongly hydrogen-bonded carboxylic acids.

Medium and strong hydrogen bonds give rise to vibrational features that can span several hundreds of wavenumbers and have unusual line shapes. For example, dimers consisting of carboxylic acids hydrogen-bonded to nitrogen-containing aromatic bases exhibit a vibrational feature that spans over 900 cm-1 and contains two very broad peaks. In this report, we demonstrate how this feature can be reproduced using a combined molecular dynamics (MD) and electronic structure "spectral map" approach, which has been very successful in modeling the vibrational spectrum of water in different environments. In this approach, spectral maps are created, relating the transition frequencies and probabilities to the electric field along the OH bond, which are obtained from the density functional theory calculations of snapshots taken from a classical MD simulation. This map was used to compute the spectral properties of thousands of geometries of the pyridine-acetic acid dimer sampled by a MD simulation, which were used to compute the overall spectral feature. It was found that this approach reproduced the experimental spectrum better than the previous dimer stretch approaches (which were based on describing the dimer geometries harmonically) through a more accurate sampling of dimer geometries. The broadness of these vibrational features largely originates from the range of geometries present in the condensed phase, while the unusual line shape is caused by strong Fermi resonances.

Solid-State NMR Investigations of MgCl2 Catalyst Support

MgCl2 is a vital component of Ziegler–Natta catalysts for olefin polymerization. Here we synthesized anhydrous MgCl2 using different drying protocols and exploited 1H NMR to quantify the proton content. We report on our study of neat and ball-milled MgCl2 samples by means of 25Mg and 35Cl solid-state NMR. DFT calculations of the quadrupole tensor aid in analysis of the spectra. The results show that, due to the morphology of the neat particles, a preferred orientation is induced which manifests itself in unusual powder line shapes. Ball milling reduces particle size, which subsequently leads to a small distribution of quadrupole parameters for the bulk. Surface sites, highly relevant for catalysis, are not directly observed, due to their broad lines of low intensity.

Raman Scattering in Carbon Nanosystems: Solving Polyacetylene.

Polyacetylene has been a paradigm conjugated organic conductor since well before other conjugated carbon systems such as nanotubes and graphene became front and center. It is widely acknowledged that Raman spectroscopy of these systems is extremely important to characterize them and understand their internal quantum behavior. Here we show, for the first time, what information the Raman spectrum of polyacetylene contains, by solving the 35-year-old mystery of its spectrum. Our methods have immediate and clear implications for other conjugated carbon systems. By relaxing the nearly universal approximation of ignoring the nuclear coordinate dependence of the transition moment (Condon approximation), we find the reasons for its unusual spectroscopic features. When the Kramers–Heisenberg–Dirac Raman scattering theory is fully applied, incorporating this nuclear coordinate dependence, and also the energy and momentum dependence of the electronic and phonon band structure, then unusual line shapes, growth, and dispersion of the bands are explained and very well matched by theory.

Nonlinear atomic spectroscopy inside a random porous medium

We have studied the pump-probe spectroscopy of rubidium vapor confined to the micrometric interstices of a random porous glass. Due to the propagation in the highly scattering medium, the light fields are randomized inside the sample with significant consequences for the atomic spectra. A two-frequency modulation technique was used to isolate the atomic response proportional to the product of the intensities of the pump and probe fields. Unusual line shapes were observed which include relatively narrow structures that are present in spite of the Doppler broadening due to the atomic velocity distribution. A simple theoretical modeling of the light-atom interaction that assumes statistical isotropy of the diffuse light field and mutual temporal incoherence of the pump and probe fields is presented. Using a single adjustable parameter to account for the atomic confinement, the model successfully describes the diversity of the observed spectral line shapes.

Observation of splitting of EPR spectral lines without any concomitant splitting in energy levels.

Splitting of spectral lines is generally associated with corresponding splitting of appropriate energy levels. However, here we report our observation of the splittings of hyperfine lines in the time-resolved EPR spectrum of a stable free radical that participates in the quenching of an excited molecule. The observed splitting does not arise from any splitting of the energy levels of the radical, nor due to Torrey oscillations but is a culmination of a detailed interplay of photophysical and magnetic resonance dynamics of the quenching process. In particular, sequential quenching of an excited singlet and triplet states by the free radical, generation of opposite electron spin polarization, and time-dependent EPR line width evolving according to the Bloch equations contribute to the appearance of unusual lineshapes of the observed EPR spectrum. This effect is sufficiently general and should always be observable in the time-resolved EPR experiments on free radicals involved in photophysical quenching of excited molecules. We also point out that this effect cannot be seen in Fourier transform EPR spectroscopy.

Observation of unusual absorption and scattering cross-section line shapes of individual optical double-wire antennas

We measure directly, simultaneously, and quantitatively the absorption and scattering cross-section spectra of individual pairs of vertically stacked gold antennas resonating in the 0.9–1.6 μm wavelength regime. We find unusually different line shapes for absorption and scattering. The origin of this observation can be traced back to the interference of the scattering amplitudes of the two underlying and spectrally overlapping effective orthogonal modes of the optical-antenna pair. We speculate that such unusual line shapes should also occur in many other coupled-antenna systems.

Anatomy of the self-energy

The general problem of representing the Greens function $G(k,z)$ in terms of self-energy in field theories lacking Wick's theorem is considered. A simple construction shows that a Dyson-like representation with a self-energy $\ensuremath{\Sigma}(k,z)$ is always possible, provided we start with a spectral representation for $G(k,z)$ for finite-sized systems and take the thermodynamic limit. The self-energy itself can then be iteratively expressed in terms of another higher order self-energy, capturing the spirit of Mori's formulation. We further discuss alternative and more general forms of $G(k,z)$ that are possible. In particular, a recent theory, by the author, of extremely correlated Fermi liquids at density $n$, for Gutzwiller projected noncanonical Fermi operators, obtains a new form of the Greens function: $G(k,z)=[(1\ensuremath{-}\frac{n}{2})+\ensuremath{\Psi}(k,z)]/[z\ensuremath{-}{\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{E}}_{k}\ensuremath{-}\ensuremath{\Phi}(k,z)],$ with a pair of self-energies $\ensuremath{\Phi}(z)$ and $\ensuremath{\Psi}(z)$. Its relationship with the Dyson form is explored. A simple version of the two-self-energies model was shown recently to successfully fit several data sets of photoemission line shapes in cuprates. We provide details of the unusual spectral line shapes that arise in this model, with the characteristic skewed shape depending upon a single parameter. The energy distribution curve (EDC) and momentum distribution curve (MDC) line shapes are shown to be skewed in opposite directions, and provide a testable prediction of the theory.

Breakup of Quasiparticles in Thin-Film Quantum Wells

Quantum well states in thin films are commonly described in terms of a quasiparticle confined in a quantum box, but this single-particle picture can fail dramatically near a substrate band edge, as shown by this angle-resolved photoemission study. Atomically uniform Ag films are prepared on Ge(111) to facilitate accurate line shape and dispersion relation measurements. A quantum well peak is observed to split into two peaks near the Ge valence band edge. The unusual line shapes are shown to be due to many-body interactions and are quantitatively explained by a Green's function calculation.

Exciton spin coherence and electromagnetically induced transparency in the transient optical response of GaAs quantum wells

We report experimental studies of electromagnetically induced transparency (EIT) arising from exciton spin coherence in the transient optical response of GaAs quantum wells. The exciton spin coherence, which is a direct result of Coulomb correlations between excitons with opposite spins, is induced via bound or unbound two-exciton states. Theoretical analyses based on a three-level model illustrate the manifestation of EIT in a transient regime and provide qualitative guidance for understanding the transient behaviors observed in the EIT experiments. Additional studies of the effects of exciton energy renormalization on optical Stark splitting also provide insights on how exciton many-body interactions can lead to unusual time-dependent asymmetric EIT line shapes.

Photoemission spectroscopy of correlated transition-metal compounds in the charge-transfer regime

An overview is given on the effects of strong d-d Coulomb repulsion on the electronic structure of charge-transfer (CT)-type 3d transition-metal (TM) compounds, especially of 3d TM chalcogenides. Mean-field effects determine the ground-state properties of the system including magnetic and orbital ordering whereas correlation effects are necessary to correctly describe excitation properties such as photoemission spectral weight distribution. Because of the strong d-d repulsion even in the metallic state of CT-type compounds, the overall photoemission spectral line shapes remain relatively unchanged across metal-insulator transitions. In the vicinity of the Fermi level, high-resolution photoemission spectra exhibit unusual line shapes and spectral weight transfer, which implies strong renormalization of quasi-particles due to electron correlation.

Ultrafast coherent transients due to exciton-continuum scattering in bulk GaAs.

Recent two-beam four-wave-mixing experiments on GaAs using pulses of 15 fs in duration have exhibited unusual line shapes that simultaneously show narrow spectral lines with a width of 1 meV and decays in the time-delay domain on a scale of 10 fs. This observation appears to violate time-energy uncertainty and has not been explained so far. Here we present three-beam four-wave-mixing experiments under identical conditions. The data are compared with a model of exciton-continuum coupling that explains the signatures of the three-beam data as well as the two-beam data.

Steplike Lineshape of Low Temperature Photoreflectance Spectra of InAlAs

AbstractWe have performed low temperature photoreflectance spectra on several MBE-InAlAs layers lattice matched to InP. Unusual lineshapes of PR spectra are observed at temperatures below 40K, characterized by a step at the InAlAs band gap energy. This step is shown to be related to a strong modification of the photo luminescence background of the photoreflectance spectra. This modification is attributed to an effect of carrier localization due to clustering effects in the InAlAs layers

Bismuth Ruthenium Oxides. Neutron Diffraction and Photoelectron Spectroscopic Study of Bi2Ru2O7 and Bi3Ru3O11

The structures of Bi3Ru3O11, a = 9.3050(2)Ǻ, Pn3, and Bi2Ru2O7, a = 10.2957(1)Ǻ, Fd3m, have been redetermined by Rietveld analysis of powder neutron diffraction data. For both compounds there is no evidence for appreciable oxygen non- stoichiometry . X-Ray photoelectron spectra of both compounds show unusual line shapes. In the case of Bi3Ru3O11 this is a result of the mixed valency and surface oxidation. In Bi2Ru2O7 final state effects appear to be important.

Influence of Growth Conditions on the Modulation Mechanism of Photoreflectance Spectra of Single InGaAs/InAlAs Quantum Wells

AbstractUsing Photoreflectance (PR) measurements, we have investigated In0.53Ga0.47As single quantum wells (SQW) with In0.52Al0.48As barriers grown by MBE on InP substrates. Unusual lineshapes of PR spectra are observed for the fundamental transition in some of the SQW. This phenomenon is shown to be independent on the widths of both the SQW (5nm or 10nm) and the surface barrier layer (between 65nm and 300nm). PR spectra are recorded at different temperatures and in different samples, as well as with a secondary pump laser beam. From these measurements, it is concluded that interface defects exist in the SQW grown at 525°C without growth interruption. Such defects are clearly evidenced in room temperature PR experiments and confirmed by PL measurements.

Interference effects in the degenerate-wave-mixing spectroscopy of alexandrite.

The degenerate-multiwave-mixing spectroscopy of alexandrite at 77 K using a pulsed dye laser in resonance with the ${\mathit{R}}_{1\mathit{m}}$ line was studied in this work. Unusual asymmetric line shapes that can be explained by an interference effect between resonant and nonresonant contributions of the media susceptibility were observed. The resonant effect is caused by the saturation of the two-level system susceptibility and the nonresonant contribution comes from the linear polarizability difference between excited and ground states. The present work shows an observation of this kind of interference phenomena in degenerate-wave-mixing experiments. Furthermore, we note that it is an observation of saturation line splitting in degenerate wave mixing in a narrow line in solids.

Vibrational line shapes at surfaces.

This work deals with the vibrational line shape of a molecule adsorbed on a substrate surface due to anharmonicity in the molecule-substrate bond. In particular, we consider the line shape of the mode, which consists predominantly of the whole molecule vibrating normal to the surface in the top position, at a frequency \ensuremath{\Omega} which is substantially higher than other normal modes of relevance. It is shown that other modes vibrating perpendicular to the surface have negligible effect on the line shape, thus generalizing a result of B. Persson and Ryberg [Phys. Rev. B 40, 10273 (1989)], which had been only shown to apply to the case of large substrate to adsorbate mass ratio. We then derive a set of equations for the effect of the modes vibrating parallel to the surface which become the exact solution for interaction Hamiltonians of the type ${\mathit{H}}_{\mathrm{int}}$=K(${\mathit{x}}^{2}$+${\mathit{y}}^{2}$)${\mathit{z}}^{2}$ in the limit of large \ensuremath{\Omega} for arbitrary K, where (x,y,z) is the excursion of the molecule relative to the surface atom to which it is bound. These equations are solved exactly for the special case of a projected density of parallel vibrational states which is a Lorentzian of width much smaller than its central frequency. All previous solutions of the dephasing problem of which we are aware are special cases of our exact solution, which we apply to existing experimental data, for example, CO on Pt(111), as well as to make predictions of where some of the new features of the unusual line shapes that we predict might be seen.

Neutron scattering study of the martensitic transformation in a Ni-Al beta -phase alloy.

A high-resolution neutron scattering study of ${\mathrm{Ni}}_{62.5}$${\mathrm{Al}}_{37.5}$ shows that the martensitic transformation is associated with a homogeneous distortion and a nearly seven-fold modulation of the cubic lattice which is predicted by an observed phonon anomaly above ${T}_{M}$. In the low-temperature 7R phase a nonuniform spacing of superlattice reflections and unusual line shapes are observed which are believed to arise from stacking faults.

Subpicosecond uv kinetic spectroscopy: Photolysis of thallium halide vapors

The formation of Tl atoms resulting from subpicosecond UV (308, 248.5 nm) photolysis of TlCl and TlI vapors is probed via subpicosecond UV kinetic absorption spectroscopy. Unusual line shapes are observed for a period lasting roughly 1 ps, from the moment the atomic transition first appears, to the point at which the final, asymptotic, line shape is attained. These spectra are explained in terms of a model based upon the transient behavior of the polarization induced by the probe continuum pulse as it interacts with a growing population of two-level atoms whose resonance frequencies shift in time.

Doppler-broadened Hα line shapes in A dc low-pressure discharge for TiN deposition

The presented results concern the unusual Hα line shapes observed in a dc discharge. The lines which are highly broadened by Doppler effect lead to the translational energy distribution of the H ( n=3) atoms. Moreover they can serve as diagnostic tools to obtain some indications about the discharge conditions and the phenomena in the electrode sheaths such as the electron energies and collisions of particles with the electrodes.