Similar Line Shapes Research Articles

Tunnelling transmission resonances through a zero-dimensional structure

We have performed low-temperature conductance measurements on a quantum dot. For zero magnetic field we find transmission resonances in the regions between successive ballistic quantized conductance plateaus. The results of the conductance measurements are compared with single-particle transmission coefficient calculations for the estimated two-dimensional device potential. In accordance with the experimental results, the calculation predicts transmission resonances of similar lineshape and energy spacing. We also include, in a simple but realistic model, the effect of electron - electron interaction.

Line-shape analysis of differential transmission spectra in the coherent regime

We show that for a resonant optical excitation of a semiconductor with laser pulses shorter than the intrinsic dephasing time, the shape of the differential transmission spectrum can deviate strongly from the distribution function of the nonequilibrium electron-hole pairs. The spectrum shows bleaching below and induced absorption above the central laser frequency. For pulses with durations \ensuremath{\gtrsim} 100 fs, this effect has been explained previously as a dynamic Fermi edge singularity. It is shown here that one can understand this effect as a nonequilibrium Coulomb enhancement. This explanation predicts a similar line shape for even shorter pulses (e.g., 30-fs duration), where one would not expect to see a dynamic Fermi edge singularity due to the smoothness of the induced carrier distribution.

Non-ohmic behavior of the high-field magnetoresistances in AlGaAs/GaAs heterostructures

In this communication are reported the measurements and analyses of the dependence of the high-field magnetoresistivities (ρxx and ρxy) on the current density in AlGaAs/GaAs heterojunctions. The strong current dependence of spin-up and spin-down levels can be observed both for ρxx and for ρxy. It was found that at low temperatures and in strong magnetic fields a quantity R=B(dρxy/dB), with B the magnetic field, exhibits similar line shape and non-ohmic behavior to those of ρxx, which dramatically confirms the unresolved proportionality ρxx∼R. In high magnetic fields R shows a stronger oscillation structure, especially for spin levels, than that observed in ρxx.

Circularly polarized luminescence from terbium(III) as a probe of metal ion binding in calcium-binding proteins.

A number of different experimental techniques have been used to probe the details of structural changes on the binding of Ca(II) to the large number of known calcium-binding proteins. The use of luminescent lanthanide(III) ions, especially terbium(III) and europium(III), as substitutional replacement for calcium(II), has led to a number of useful experiments from which important details concerning the metal ion coordination sites have been obtained. This work is concerned with the measurement of the circularly polarized luminescence (CPL) from the 5D4----7F5 transition of Tb(III) bound to the calcium binding sites of bovine trypsin, bovine brain calmodulin, and frog muscle parvalbumin. It is demonstrated that it is possible to make these polarization measurements from very dilute solutions (less than 20 microM) and monitor structural changes as equivalents of Tb(III) are added. It is shown that the two proteins that belong to the class of "EF-hand" structures (calmodulin and parvalbumin) possess quite similar CPL line shapes, whereas Tb(III) bound to trypsin has a much different band structure. CPL results following competitive and consecutive binding of Ca(II) and Tb(III) bound to calmodulin are also reported and yield information concerning known differences between the sequence of binding of these two species.

Structure and ferromagnetic resonance of Tb/FeCo multilayer thin films

Multilayer thin films of Tb and Fe95Co5 were prepared by thermal evaporation from two sources. The films were studied by x-ray diffraction, Auger electron spectroscopy, and ferromagnetic resonance. The low-angle x-ray diffraction and Auger analysis showed that a multilayer structure was achieved for films with a modulation wavelength Λ≥31.5Å. FMR results were consistent with the structural data. The films with Λ≤31.5Å had similar HKeff, line width, and line shape to those of a coevaporated alloy with the same composition. The films with Λ>31.5 Å had an HKeff greater than or equal to that of an Fe95Co5 film. A film with HKeff greater than that of the Fe95Co5 film had multiple resonances. Interfacial stress due to incoherent matching of the Tb and Fe95Co5 layers is thought to give rise to differences in HKeff in the various layers.

Charge transfer, polarization, and relaxation effects on the Auger line shapes of Si.

The Auger line shapes of Si are quantitatively interpreted noting particularly the core-hole screening effects as exhibited through charge transfer, polarization, and atomic relaxation. The ${\mathrm{KL}}_{1}$V, ${\mathrm{KL}}_{2}$,3V, and ${L}_{1}$${L}_{2}$,3V line shapes reflect a core-hole-screened density of states (DOS) consistent with the core hole in the final state of these processes. A DOS appropriate for the screened core hole is obtained by distorting the theoretical DOS for the ground state utilizing the Green's function for a tight-binding Hamiltonian and a central-cell potential. Comparison of the ${L}_{2}$,3VV and KVV line shapes reveal large differences. These differences are discussed in the context of surface effects, intrinsic and extrinsic plasmon losses, and final-state shakeoff. The ${L}_{2}$,3VV and KVV line shapes also suggest some distortion effects due to final-state hole correlation. The ${\mathrm{KL}}_{2}$,3${\mathrm{L}}_{2}$,3 line shape is interpreted in the context of similar line shapes for Na, Mg, Al, and P; all show plasmon losses and, except for P, initial-state shakeoff contributions.

A Fourier transform method of decomposing overlapping spectral peaks of similar line shape into individual components

In various spectroscopies composite curves are often encountered which are linear superpositions of overlapping, chemically shifted spectral peaks. Separation of such a composite into its individual components allows the identification of the chemical complex by its chemical shift, and the determination of the amount of the element in the complex from the strength of the spectral component. This paper describes a Fourier transform method for the decomposition of such a composite spectrum which is applicable under two conditions which are often satisfied in practice: all the chemically shifted peaks of the element of interest must have very similar line shapes, and one of these peaks must be observable in isolation to provide a reference line shape. Even in the event of line broadening this technique should give good starting parameters for the number and positions of the components. Applications to XPS and Auger spectroscopy are discussed.

The bonding state of carbon segregated to α-iron surfaces and on iron carbide surfaces studied by electron spectroscopy

Segregated carbon on the Fe(100) surface has been studied by means of X-rayand ultraviolet photoelectron (XPS, UPS), Auger electron (AES) and electron energy loss spectroscopy (ELS). For comparison, the surfaces of polycrystalline graphite and of iron carbides stabilized by chromium or manganese additions have been investigated. On the iron surface, carbon exists as a chemisorbed state or graphitic multilayer. The two states exhibit different energy positions in XPS, and are different in energy positions and lineshapes in AES and ELS. During the transition of graphitic carbon to chemisorbed carbon on Fe(100) a novel coverage-dependent Auger feature is reported. The spectra of graphitic carbon on the iron surface always coincide with those of solid graphite. The carbon Auger transitions of chemisorbed carbon and of iron carbides exhibit very similar lineshapes, but the energy positions of both states differ in AES as well as XPS.

Further Observational Studies of the High Velocity Molecular Source (“Plateau”) in Orion

Sensitive measurements have been made of the profiles of various molecular lines in the direction of the BN/KL infrared complex in Orion. Similar line shapes are found for the high velocity wings (“plateau”) of most lines observed. The position of the high velocity source (HVS) was found in SO and HCO+ relative to the SiO maser. A search for HV emission in other sources was unsuccessful.

Elastic and inelastic contributions to the auger electron appearance potential spectrum of titanium

The energy distribution of electrons contributing to the L-shell Auger electron appearance potential spectrum of a polycrystalline titanium surface has been measured. The Auger electron appearance potential spectrum is obtained by differentiating the total secondary electron yield of an electron bombarded sample as a function of incident electron energy. At the threshold for scattering from a core level the secondary yield increases. Most of the electrons contributing to this increase have energies below 30 eV, and result from secondary processes following Auger recombination of the core hole. The elastic yield decreases at the threshold, however, due to opening a new channel for inelastic scattering. A comparison of the elastic yield spectrum (DAPS), the total yield spectrum (AEAPS) and the soft X-ray yield spectrum (SXAPS), shows very similar line shapes, but differences in the relative strengths of the lines.

Intersubband spectroscopy by photoconductivity and absorption in inversion layers onp−Si(100)

Subband spectroscopy in an inversion layer on $p\ensuremath{-}\mathrm{Si}(100)$ is done simultaneously in photoconductivity and absorption. The resonances observed are investigated in their dependence on inversion electron density ${N}_{s}$ and depletion charge ${N}_{\mathrm{dep}}$ and are interpreted as transitions from the ground subband to higher subbands of the two valleys with heavy mass perpendicular to the surface. Absorption and photoconductivity give identical resonance positions and similar lineshape. The dependence of the subband splitting on ${N}_{s}$ and ${N}_{\mathrm{dep}}$ is found to agree with the measurements of Kneschaurek et al. and the most recent theory by Ando. Agreement with the photoconductivity results of Wheeler and Goldberg and the hot-electron-induced emission experiment of Gornik and Tsui can only be achieved if the latter results are assumed to correspond to a much lower ${N}_{\mathrm{dep}}$ than derived from the substrate resistivity at 300 K. In contrast to the results of Wheeler and Goldberg the amplitude of the photoconductivity signal exhibits a strong temperature dependence between 1.7 and 6 K.

Thermodynamic and EPR characteristics of two ferredoxin-type iron-sulfur centers in the succinate-ubiquinone reductase segment of the respiratory chain.

Two distinct ferredosin-type iron-sulfur centers (designated as Centers S-1 and S-2) are present in the soulble succinate dehydrogenase in approximately equivalent concentrations to that of bound flavin. Both Centers S-1 and S-2 exhibit electron paramagnetic resonance absorbance in the reduced state at the same magnetic field (gz = 2.03, gy = 1.93, and gx = 1.91) with similar line shape. Center S-2 is reducible only chemically with dithionite and remains oxidized under physiological conditions. Thus, its functional role is unknown; however, thermodynamic and EPR characterization of this iron-sulfur center has revealed important molecular events related to this dehydrogenase. The midpoint potentials of Centers S-1 and S-2 determined in the soluble succinate dehydrogenase preparations are -5 +/- 15 mV and -400 +/- 15 mV, respectively, while corresponding midpoint potentials determined in particulate preparations, such as succinate-cytochrome c reductase or succinate-ubiquinone reductase, are 0 +/- 15 mV and -260 +/- 15 mV. Reconstitution of soluble succinate dehydrogenase with the cytochrome b-c1 complex is accompanied by a reversion of the Center S-I midpoint from -400 +/- 15 mV to -250 +/- 15 mV with a concomitant restoration of antimycin A-sensitive succinate-cytochrome c reductase activity. There observations indicate that, during the reconstitution process, Center S-I is restored to its original molecular environment. In the reconstitutively active succinate dehydrogenase, the relaxation time of Center S-2 is much shorter than that of S-1, thus Center S-2 spectra are well discernible only below 20 K (at 1 milliwatt of power), while the resonance absorbance of Center S-1 is detectable at higher temperatures and readily saturates below 15 K. Over a wide temperature range the power saturation of Center S-1 resonance absorbance is relieved by Center S-2 in the paramagnetic state, and the Center S-2 central resonance absorbance is broadened by Center S-1 spins, due to a spin-spin interaction between these centers. These observations indicate an adjacent location of these centers in the enzyme molecule. In reconstitutively inactive enzymes, subtle modification of the enzyme structure appears to shift the temperature dependence of Center S-2 relaxation to the higher temperature. Thus the EPR signals of Center S-2 are also detectable at higher temperature. In this system a splitting of the central peak of the Center S-2 spectrum due to spin-spin interaction was observed at extremely low temperatures, while this was not observed in reconstitutively active enzymes or in paritculate preparations. This spin-spin interaction phenomena of inactive enzymes disappeared upon chemical reactivation with concomitant appearance of the reconstitutive activity. These observations provide a close correlation between the molecular integrity of the enzyme and its physiological function.

Exciton dynamics in 1,4-dibromonaphthalene from an analysis of asymmetric absorption lineshapes

From an analysis of the triplet exciton absorption lineshape in 1,4-dibromonaphthalene we have been able to obtain information on the decay of the initially prepared k ≈ 0 state into other exciton quasi-momentum states. Of particular interest is the asymmetric lineshape and corresponding non-exponential decay, resulting from the position of the k ≈ 0 level at one edge of the band. Similar lineshapes are found in the FeS2 Raman spectrum.

Linewidths and line shapes in solid state ESCA studies: Electric field gradient broadening of Sn 3d lines

ESCA Sn 3d linewidths of nonconducting organometallic tin solids, prepared by in situ evaporation, are within about 10% of the values for analogous gas phase measurements. The similar line shapes (≈ 35% gaussian, 65% lorentzian) observed for solid state and gas phase spectra show that effects which give increasing gaussian components, such as charge broadening and phonon broadening, are very small in our solid state spectra. The general correlation between ESCA Sn 3d linewidths and Mössbauer quadrupole splittings indicates that electric field gradient splitting of the Sn 3d lines approaches ≈ 0.4 eV.