Angle-dependent Spectra Research Articles

Hydrogen storage mechanism of multiwalled carbon nanotube bundles studied by X-ray absorption spectra

Multiwalled carbon nanotube bundles have been investigated with the near-edge X-ray absorption fine structure (NEXAFS) spectra. In the angle-dependent NEXAFS spectra of multiwalled carbon nanotube bundles, a change of the intensity of the C—H σ* with the synchrotron radiation light incident angle has been observed. The presence of the C—H bond at room temperature and ambient pressure may be related to the defects in the carbon nanotubes. Neither the C—C π* nor the C—C σ* is similar to the C—H σ*, which strongly suggests that the hydrogen atoms are adsorbed on the defects on the interior surface of the wall of carbon nanotubes at room temperature and ambient pressure. These results indicate that NEXAFS spectrum is a powerful tool for the investigation of multiwalled carbon nanotube bundles.

Investigation of local ordering and electronic structure in Si- and hydrogen-doped tetrahedral amorphous carbon thin films

This work investigates the CK-edge near-edge x-ray absorption fine structure (NEXAFS) and x-ray reflectivity spectra of Si- and hydrogen-incorporated tetrahedral amorphous carbon (ta-C:Si:H) films. The CK-edge NEXAFS spectra indicate that the sp2∕sp3 hybridization ratio increases with the amount of tetramethylsilane vapor precursor introduced during deposition. This suggests that Si addition enhances sp2- and reduces sp3-bonding configurations. The increase in sp2 sites correlates well with the decrease in hardness, mass density, and thermal stability. The comparison of angle-dependent NEXAFS spectra of nondoped and Si-doped ta-C films at the K edge reveals significant dopant-induced local ordering in πCC* and σC–H* orbitals. In contrast to the highly oriented pyrolytic graphite, the πCC* orbitals lie parallel to the surface.

The direct determination of X-ray diffraction data from specific depths

A direct method for determining powder diffraction data from a range of depths is described, where the linear absorption coefficient may vary with depth. A series of traditional data collections with varying angles of incidence are required, and the X-ray diffraction data arising from specific depths will be calculated by the transformation of these measured, angle-dependent spectra. These may then be analysed using any conventional method in order to gain information about characteristics of the sample in question at specific depths. Regularisation techniques have been used to solve the governing Fredholm integral equation to determine the depth-dependent diffractograms. The method has been validated by the use of simulated data having known model profiles, and has also been applied to experimental data from polycrystalline thin film samples.

Synchrotron radiation photoabsorption and photoemission spectroscopy for thermal-induced reoriented Si polymer

The effect of laser annealing on electronic structures and molecular orientation for poly(dimethylsilane) {PDMS, [Si(CH 3) 2] n } has been studied by synchrotron radiation photoemission and photoabsorption spectroscopy. Prior to annealing, PDMS powder was mounted on the basal plane of highly oriented pyrolytic graphite. Both Si 1s X-ray photoemission spectroscopy and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at Si 1s edge show that electronic structures have been modified due to annealing. Furthermore, the angle-dependent NEXAFS spectra clearly indicate that the annealed products maintain a specific orientation. Interestingly, no such kind of orientation is present in as-received PDMS powder as no angle-dependency is observed before annealing.

Determination of depth-dependent diffraction data: a new approach

A direct method for determining powder diffraction data at specific depths from angle-dependent diffraction data is described. The method is non-destructive and only traditional data collections, where the angle of incidence is varied, are required. These angle-dependent spectra are transformed to give diffraction data arising from different depths, which may then be exploited using any conventional method. This is a novel approach as traditional methods are forced to tolerate the inherent depth averaging of grazing-angle diffraction, or only examine specific structural characteristics. In order to obtain depth-dependent X-ray diffraction data, a Fredholm integral equation of the first kind is solved using regularization techniques. The method has been validated by the generation of pseudo-experimental data having known depth profiles and solving the Fredholm integral equation to recover the solution. The method has also been applied to experimental data from a number of thin film systems.

Terahertz Signal Generation in 1-D Photonic Crystals

Theoretical and numerical results are presented to assure that a tunable, narrow-band, coherent THz radiation source can be based on parametric down-conversion in aphotonic crystal. Our proposal is based on down-conversion mixing and a local-field enhancement mechanism that is available by tuning each of the two driving laserfields either to band-edge or to a defect mode in the band gap. The frequency of the down-converted signal can be tuned by intersecting two non co-linear laser sources. The polarizations are degenerate at normal incidence and have sub-THz down-conversion maximum. For aspecific sample geometry we show that by changing the angle of incidence of one tunable laser to 30 degrees the THz frequency is about11.5 THz for p-polarization and 3.5THz for s-polarization, since the angle-dependent transmission spectrum is different for p- and spolarizations.The peak conversion efficiency for both polarizations is enhanced by over two orders of magnitude. Finally we also introduce some preliminary experimental results which agree with the numerical results we present here.

Insulating surface layer on single crystal K $\mathsf{_{3}}\mathsf{C}\mathsf{_{60}}$

Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K3C60 is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved.

Magnetic-Field-Induced Orientation of Photosynthetic Reaction Centers, As Revealed by Time-Resolved D-Band Electron Paramagnetic Resonance of Spin-Correlated Radical Pairs. II. Field Dependence of the Alignment

The magnetic-field-induced orientation of photosynthetic reaction centers has been studied by time-resolved D-band (130 GHz) electron paramagnetic resonance (EPR) of the secondary radical pair, , in plant photosystem I (PSI). Experiments have been performed for fresh and lyophilized whole cells of the deuterated cyanobacterium S. lividus. A computer fit of the angle-dependent D-band spectra, measured for two different sample orientations, provides the order parameter SZ‘Z‘ of the symmetry axis, Z‘, of the susceptibility tensor, relative to the magnetic field. The positive sign of this order parameter indicates that membrane proteins are the major source for the anisotropy of the diamagnetic susceptibility, ΔχV. A value for ΔχV has been extracted from the magnetic-field dependence of SZ‘Z‘. The value of ΔχV = 5.7 × 10-27 m3 is in good agreement with an estimate for the susceptibility anisotropy of a cyanobacterial cell. This demonstrates that whole cells are aligned in the magneto-orientation process. The combination of high-field EPR of a magnetically oriented sample with the analysis of quantum beat oscillations allows determination of the three-dimensional structure of in the photosynthetic membrane.

Stereochemistry and EPR investigation of a chiral molecular magnet

An electron paramagnetic resonance investigation of an indexed single crystal of the chiral molecular magnet ( R)- 3MLNN·Mn(hfac) 2 which is comprised of a chiral phenyl α-nitronyl nitroxide and a manganese(II) ion is presented. The crystal of the compound, which consists of ferrimagnetic chains of radical and metal ion and orders ferromagnetically below 4 K, shows temperature and angle-dependent spectra which may give clues as to the nature of the anisotropy in the system and the types of interaction at play.

Optical Determination of the Lattice Constants of Colloidal Crystals without Use of the Refractive Index

Angle-dependent reflectance spectra were measured for charge-stabilized colloidal crystals of polystyrene particles dispersed in water. We found that diffraction peaks due to the two-dimensional surface lattice of the colloidal crystal were observable in the spectra, in addition to normal three-dimensional Bragg peaks. On the basis of the nature of the two-dimensional diffraction, we show that the lattice constant can be determined without the knowledge of the refractive index of the crystal, unlike the case of using three-dimensional Bragg diffraction. The lattice constant depends only on the diffraction angle and the wavelength of light and therefore can be determined with high precision. The accurate determination of the lattice constant would provide a basis for quantitative evaluation and design of colloidal crystals for applications in the optical regime, such as photonic crystals.

Evidence for braggoriton excitations in opal photonic crystals infiltrated with highly polarizable dyes

We studied angle-dependent reflectivity spectra of opal photonic crystals infiltrated with cyanine dye aggregates, which are highly polarizable media with very large Rabi frequency. We show that, at resonance condition between the exciton-polariton of the dye aggregate and the Bragg gap, the Bragg stop band decomposes into two reflectivity bands with a semitransparent spectral range in between, that is due to propagation of braggoritons inside the gap.

Photonic band structure and emissive characteristics of MEH-PPV textured microcavities

Abstract A detailed comparison is made between the experimentally determined photonic band structure and the measured angle-dependent emission spectra of textured, metal-clad microcavities containing the light-emitting polymer MEH-PPV. Transmittivity measurements were made over a range of optical frequency and in-plane wavevector in order to obtain a detailed map of the dispersion of the waveguide modes supported by the textured cavities. Emission spectra were also recorded, for a large number of sample orientations, and are compared with the transmittivity maps. It is found that the band structure strongly influences the wavelength of the peaks in the emission spectra, and that the periodic texture leads to the generation of Bragg-scattered emission. The positions of these spectral peaks in both frequency and in-plane wavevector are found to agree very well with the positions of peak transmittivity of the cavities. The emission spectra also show clear evidence of the photonic band gaps formed by the periodic texture. The importance of these findings for light-emissive devices such as the light emitting diode is discussed.

Photonic band structure and emissive characteristics of MEH-PPV textured microcavities

A detailed comparison is made between the experimentally determined photonic band structure and the measured angle-dependent emission spectra of textured, metal-clad microcavities containing the light-emitting polymer MEH-PPV. Transmittivity measurements were made over a range of optical frequency and in-plane wavevector in order to obtain a detailed map of the dispersion of the waveguide modes supported by the textured cavities. Emission spectra were also recorded, for a large number of sample orientations, and are compared with the transmittivity maps. It is found that the band structure strongly influences the wavelength of the peaks in the emission spectra, and that the periodic texture leads to the generation of Bragg-scattered emission. The positions of these spectral peaks in both frequency and in-plane wavevector are found to agree very well with the positions of peak transmittivity of the cavities. The emission spectra also show clear evidence of the photonic band gaps formed by the periodic texture. The importance of these findings for light-emissive devices such as the light emitting diode is discussed.

Angle-dependent photoluminescence spectra of hydrogenated amorphous silicon thin films

Multiple sharp peaks were observed in the visible photoluminescence spectra of amorphous silicon thin films, prepared by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition on oxidized silicon substrates. The angular dependence of the photoluminescence, measured by a home-built fiber-optics device, revealed that the origin of these sharp features was due to Fabry–Pérot cavity interference effects. The interference is enhanced by deposition on thermally grown oxide layers with relatively smooth surfaces. We also consider how thin-film interference effects can add to the already existing confusion regarding the photoluminescence (PL) mechanism of porous and other luminescent forms of silicon and propose angle-dependent PL spectroscopy as a remedy for identifying spectral features due to interference effects.

Theoretical investigation of the autoionization process in molecular collision complexes: He*(2 3S)+Li(2 2S)→He+Li++e−

A complete ab initio treatment is applied to the autoionization process in the He*(2s 3S)+Li(2s 2S) collisional complex. Feshbach projection based on orbital occupancy, implemented in a multireference configuration interaction (MRCI) code, defines the resonance state and provides the entrance channel potential curve as well as all pertinent information on the resonance–continuum coupling. The l-dependent coupling elements in local approximation are obtained by projecting a compact one-electron function, named Penning molecular orbital (PMO), onto the wave function of the ejected electron with proper energy. The continuum wave function is obtained by coupled channel calculations in the static-exchange approximation. A converged set of seven complex coupling matrix elements, used in the nuclear dynamics calculation based on a complex Numerov algorithm, fully describes the electron angular momentum transfer. The calculated angle-dependent spectra, as well as the total, angle-, and energy-integrated ionization cross sections agree well with available experimental data.

Characterization of the Surface to Thiol Bonding in Self-Assembled Monolayer Films of C12H25SH on InP(100) by Angle-Resolved X-ray Photoelectron Spectroscopy

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) was used to characterize the bonding of alkanethiols to n-InP surfaces and to measure the monolayer thickness. A careful comparison of the angle-dependent spectra for three different sample preparations (oxidized InP(100), HF-etched InP(100), and HF-etched InP(100) with a self-assembled monolayer of C12H25SH) shows that the sulfur binds to In atoms on the surface. Comparison of the angle-dependence of the intensity ratios for the In 3d5/2 core level of InP with the self-assembled monolayer (SAM) to that of sputtered InP provides a thickness for the SAM of 14 ± 4 Å, corresponding to alkane chains at a tilt angle of 44 ± 16° from the surface normal.

Surface and bulk4fphotoemission spectra ofCeIr2

Resonance photoemission spectroscopy was performed on ${\mathrm{CeIr}}_{2}$ at the Ce $4\stackrel{\ensuremath{\rightarrow}}{d}4f$ and $3\stackrel{\ensuremath{\rightarrow}}{d}4f$ core thresholds. We also obtained angle-dependent $4\stackrel{\ensuremath{\rightarrow}}{d}4f$ resonant spectra. Surface and bulk $\mathrm{Ce}\ensuremath{-}4f$ spectra were derived using differences both in the angle-dependent spectra and in the resonant spectra at the two different thresholds. The surface and bulk spectra are different depending on the derivation methods, and the spectra derived from the angle dependence are more consistent with theoretical spectra using the Gunnarsson-Sch\"onhammer method of the Anderson impurity model, which implies that the two resonant spectra may reflect different $4f$-removal spectra.

Inelastic background subtraction from a set of angle-dependent XPS spectra using PCA and polynomial approximation

The contribution describes an application of the window principal component analysis (WPCA) to determine precisely the energy subregions in a set of angle-dependent XPS spectra where only the inelastic background exists. Localisation of the subregions enables to approximate the background with polinomial and to subtract the approximated background from the spectra. The reconstructed shapes of the lines obtained in this way were evaluated using several criteria. Application of the procedure to sets of simulated spectra proved that the optimal degrees of approximating polynomials ranged between the 2 and the 5°, depending on the signal-to-noise ratio (SNR) and on the structure of the spectra.

X-ray photoelectron diffraction of ABO3-type perovskites: site determination for dopants

Abstract We present a set of polar-angle-dependent X-ray photoelectron spectra (XPS) obtained from in-situ-cleaved KNbO 3 . The angle-dependent variation of intensity can be explained by the diffraction of the photoemitted electron by the surrounding atoms. This anisotropy in intensity may be used to determine the lattice site of the emitting atom. Measurements were carried out on KTa 1− x Nb x O 3 single crystals ( x =0.92). In spite of the low concentration of Ta, the atomic position of this element could be deduced from a comparison of the angle-dependent spectra, giving a hint that both tantalum and niobium occupy the same lattice site. Angle-dependent measurements were carried out on Ba 0.77 Ca 0.23 TiO 3 single crystals in order to determine the atomic position of the calcium atoms. Extended comparison of the measurements leads to the assumption that calcium is located on the barium site only.

New interpretation of ESR of the S = ½ broken valence bond in Zn-doped NENP

The unique singlet ground state of an S = 1 one-dimensional antiferromagnet is studied using ESR measurements on NENP, Ni(C 2H 8N 2) 2NO 2(ClO 4) doped with nonmagnetic Zn 2+ impurities. A new interpretation is proposed to analyze angle-dependent ESR spectra by introducing a model in which the observed signals are due to the S = ½ broken valence bond subjected to the field-induced staggered internal fields possibly originating from the underlying antiferromagnetic polarization.