bluuebunny/crossref_metadata_2025 huggingface dataset:
A method to produce reverse-mode polymer-dispersed liquid-crystal shutters
Authors: Nicoletta F. P. , De Filpo G. , Lanzo J. , Chidichimo G.
Published: July 2002

Reverse-mode operation shutters have been achieved by combining the techniques of the traditional means of making polymer-dispersed liquid crystals and nematic curvilinear aligned phases. Nematic microemulsions, obtained by a thermally induced phase separation, have been photopolymerized in an external force field. After the polymerization, films show 85% transmittance in the OFF state, while it decreases to less than 1% when an electric field of about 2 V μm−1 at 1 kHz is applied. The rise-time values, about 2 ms, are in the same range as those obtained with normal-mode polymer-dispersed liquid-crystal films. On the contrary, decay time shows longer values.
Enhanced tunnel magnetoresistance in Fe-doped Al2O3 barriers
Authors: Jansen R. , Moodera J. S.
Published: July 2002

It is shown that the tunnel magnetoresistance of a Co/Al2O3/Ni80Fe20 magnetic tunnel junction is enhanced by δ doping of the oxide barrier with Fe. Enhancements by a factor of up to 1.25 are observed for Fe layers less than a monolayer thick, positioned halfway in the Al2O3 insulator. The effect exists not only at low temperature, but persists up to room temperature, albeit slightly weakened. Also, the enhancement remains present under the application of voltages as large as 0.6 V. Possible explanations are discussed.
On the long pulse operation of an x-ray preionized, gas discharge pumped ArF excimer laser
Authors: Feenstra L. , Bastiaens H. M. J. , Peters P. J. M. , Witteman W. J.
Published: July 2002

Long laser pulses of up to 120 ns full width at half maximum have been achieved in a discharge pumped ArF laser, using x-ray preionization and prepulse-mainpulse excitation using a magnetic switch. For the realization of these long pulse lengths a fast current rise time in the first stage of the discharge is a crucial factor. The laser performance depends strongly on the discharge stability, and thus on the partial pressure of the fluorine in the laser gas mixture.
Room-temperature gain at 1.3 μm in PbS-doped glasses
Authors: Wundke K. , Auxier J. , Schülzgen A. , Peyghambarian N. , Borrelli N. F.
Published: July 2002

We report on room-temperature optical gain at the ground exciton transition of PbS quantum-dot-doped glasses while optical pumping into the next-higher exciton resonance. The material gain in the quantum dots is as large as 80 cm−1. The dot-size selective excitation provides tunability of the optical gain. This is demonstrated by tuning the gain from 1317 to 1352 nm by changing the pump wavelength from 900 to 980 nm.
Effect of current crowding on vacancy diffusion and void formation in electromigration
Authors: Tu K. N. , Yeh C. C. , Liu C. Y. , Chen Chih
Published: July 2002

In multilevel interconnects, current crowding occurs whenever the current changes direction, such as when passing through a via. We propose that in current crowding, the current-density gradient can exert a driving force strong enough to cause excess vacancies (point defects) to migrate from high to low current-density regions. This leads to void formation in the latter. This is a key feature of electromigration-induced damage in very large scale integrated interconnects.
Nonlinear optical absorption in undoped and cerium-doped BaTiO3 thin films using Z-scan technique
Authors: Zhang W. F. , Huang Y. B. , Zhang M. S. , Liu Z. G.
Published: July 2002

BaTiO 3 and BaTiO3:Ce thin films with good surface morphology and tetragonal perovskite structure were fabricated on quartz substrates by pulsed-laser deposition technique at a deposition temperature of 750 °C. The fundamental optical constants (the band gaps, linear refractive indices and absorption coefficients) of the films were obtained through optical transmittance measurements. The nonlinear optical properties of the films were investigated using Z-scan method. The films display large nonlinear absorption effects. The two-photon absorption coefficients of the BaTiO3 and BaTiO3:Ce films were determined to be 51.7 and 59.3 cm/GW, respectively. All the results show that undoped and cerium-doped BaTiO3 ferroelectric thin films have potential applications in absorbing-type optical devices.
Photoluminescence of CdSe nanocrystallites embedded in BaTiO3 matrix
Authors: Zhou Ji , Li Longtu , Gui Zhilun , Buddhudu S. , Zhou Yan
Published: July 2002

This letter reports the photoluminescence properties of CdSe quantum dots (QDs) embedded in a ferroelectric BaTiO3 matrix. The main emission from the samples has been assigned to the band–edge transition of QDs. With an increase in the heat treatment temperature, the emission peak has been shifted to a longer wavelength. This shift has been attributed due to the dielectric environment effects of the matrix on the electronic structure of the QDs. The dependences of the peak position on the heat-treatment temperature and dot size are found in agreement with the theoretical simulation results reported previously.
Ferroelectricity in a pure BiFeO3 ceramic
Authors: Kumar M. Mahesh , Palkar V. R. , Srinivas K. , Suryanarayana S. V.
Published: July 2002

The difficulties in synthesizing phase pure BiFeO3 are well known. In this letter we are reporting the optimized synthesis conditions for obtaining phase pure BiFeO3 ceramic. The oxide mixing technique followed by leaching with dilute nitric acid has been used for the synthesis. X-ray diffraction pattern indicated that the sample is phase pure. Scanning electron microscopy along with energy dispersive x-ray fluorescence analysis confirmed the chemical homogeneity of the sample. No segregation of the impurity phase in the matrix was detected. Moreover, Bi/Fe atomic ratio is observed to be ∼1. The ferroelectric transition of the sample at 836 °C has been detected by differential thermal analysis.
Detection of single photons using a field-effect transistor gated by a layer of quantum dots
Authors: Shields A. J. , O’Sullivan M. P. , Farrer I. , Ritchie D. A. , Hogg R. A. , Leadbeater M. L. , Norman C. E. , Pepper M.
Published: July 2002

We demonstrate that the conductance of a field-effect transistor (FET) gated by a layer of nanometer-sized quantum dots is sensitive to the absorption of single photons. Rather than relying upon an avalanche process, as in conventional semiconductor single-photon detectors, the gain in this device derives from the fact that the conductivity of the FET channel is very sensitive to the photoexcited charge trapped in the dots. This phenomenon may allow a type of three-terminal single-photon detector to be developed based upon FET technology.
Dependence of crystallographic tilt and defect distribution on mask material in epitaxial lateral overgrown GaN layers
Authors: Tomiya S. , Funato K. , Asatsuma T. , Hino T. , Kijima S. , Asano T. , Ikeda M.
Published: July 2002

We have investigated the dependence of crystallographic tilt and defect distribution on mask material in metalorganic chemical vapor deposition grown GaN layers formed utilizing an epitaxial lateral overgrowth (ELO) technique using x-ray diffraction and transmission electron microscopy. Crystallographic tilt in the ELO GaN layer was suppressed by changing the mask material from electron beam (EB)-evaporated SiO2 to plasma enhanced chemical vapor deposition (PECVD) grown SiO2 and PECVD SiNx. Defect distribution also changes in accordance with mask materials. By depositing a thin PECVD SiNx layer on the PECVD SiO2 mask, the crystalline quality of the ELO layer changes from that used with the SiO2 mask to that used with the SiNx mask. These results suggest that the interface between the ELO GaN layer and the mask has a significant effect on crystallographic tilt and defect distribution.
Near-field acoustic densimeter and viscosimeter
Authors: Patois R. , Vairac P. , Cretin B.
Published: July 2002

We have developed a vibrating microsensor allowing simultaneous measurement of density and viscosity in liquids. A small-sized spherical oscillating probe is immersed in the liquid and is connected to a microcantilever driven in flexural vibration by a piezoelectric transducer. Cantilever resonance parameters (frequency and damping) are used to measure the probe–fluid interaction. The developed analytical model enables us to compute fluid characteristics from measured data by solving the inverse problem. Measurements performed for various fluids are compared to literature data. Investigation depth of the sensor is determined by studying the effect of the presence of an immersed wall in the vicinity of the probe.
Moderate phonon dispersion shown by the temperature dependence of fundamental band gaps of various elemental and binary semiconductors including wide-band gap materials
Authors: Pässler R.
Published: July 2002

A recently devised analytical four-parameter model describing the temperature dependence of fundamental band gaps Eg(T) and/or exciton peak positions Egx(T) in semiconductors is used for estimating the material-specific magnitudes of phonon dispersion. A set of four fundamental parameters, the magnitudes of which are essentially independent of various details of the analytical model used, is established by direct connections with the experimentally detectable low-order moments of the electron–phonon spectral function. Numerical fittings of experimental E(T) data sets are performed for a variety of group IV, III–V, and II–VI materials showing a moderate degree of phonon dispersion. The resulting sets of basic parameters are listed. For all materials in question we find that the limiting (T→∞) slopes of the E(T) curves are confined to magnitudes between about 0.2 and 0.9 meV/K. The effective phonon temperatures amount to about 2/3 of the corresponding Debye temperatures. The simultaneous knowledge of both the effective phonon temperature and the associated dispersion coefficient enables a rough estimation of the relative weights of the contributions of various parts of the phonon spectrum to the T→∞ slope of the measured E(T) dependence. From the parameter sets determined recently for zinc chalcogenides we can assume the dominating contributions to the limiting slopes of their E(T) curves are due to short-wavelength longitudinal acoustical phonons. This conclusion is confirmed by a comparison with alternative fittings using a three-oscillator model.
HONO in solid Kr: Site-selective trans↔cis isomerization with narrow-band infrared radiation
Authors: Khriachtchev Leonid , Lundell Jan , Isoniemi Esa , Räsänen Markku
Published: July 2002

Narrow-band resonant IR radiation promotes trans↔cis isomerization of HONO in a Kr matrix when the O–H stretching or N=O first overtone modes are excited. The isomerization process is selective with respect to matrix sites, and the HONO concentrations in various matrix sites are unchanged, demonstrating the stability of the host cage under the resonant IR irradiation. The isomerization process is localized at the vibrationally excited molecule and possesses a high quantum yield, possibly approaching 1 for the cis to trans reaction. The present experimental results on IR-induced isomerization of HONO agree with the known molecular dynamics simulations. Our anharmonic ab initio calculations describe well the vibrational properties of both HONO isomers, including a number of observed overtone and combination bands. Especially remarkable in our opinion is the very good numerical agreement between the calculated and experimental IR absorption cross sections found for all the fundamental modes.
In situ transmission electron microscopy study of dislocations in a polycrystalline Cu thin film constrained by a substrate
Authors: Dehm Gerhard , Arzt Eduard
Published: July 2002

Dislocation plasticity in a thin Cu film on a SiNx/SiOx coated silicon wafer was observed by transmission electron microscopy (TEM) during thermal cycling of a cross-sectional specimen. While the in situ TEM study revealed jerky dislocation motion at low homologous temperatures, continuous dislocation glide occurred at elevated temperatures. Furthermore, dislocations were pulled into the Cu/SiNx interface, where dislocation contrast disappeared. It is assumed from these preliminary observations that the limited mobility of dislocations at low homologous temperatures rather than interfacial dislocation segments may be responsible for the high yield stress of the Cu film.
Direct-dynamics approach to catalytic effects: The tautomerization of 3-hydroxyisoquinoline as a test case
Authors: Fernández-Ramos Antonio , Smedarchina Zorka , Zgierski Marek Z.
Published: July 2002

The mechanism of tautomerization of 3-hydroxyisoquinoline (3HIQ) in its first excited singlet state is studied theoretically for the isolated molecule and the 1:1 complexes with water (3HIQ/H2O) and acetic acid (3HIQ/AA). It is found that the proton transfer is a tunneling process which is strongly mediated by the motion of the heavier atoms involved in the hydrogen bond bridges. Therefore it is argued that quantitative assessment of the tremendous catalytic effect of complexation observed experimentally is possible only through the evaluation of multidimensional tunneling rate constants. These are addressed using a direct dynamics approach based on the multidimensional instanton model. The potential energy surface, which governs the tautomerization dynamics, is generated from ab initio calculations at CIS/6-31G* and CASSCF(8,8)/6-31G* levels of theory. It is formulated in terms of the normal modes of the transition state and consists of 33, 57, and 72 degrees of freedom for 3HIQ, 3HIQ/H2O, and 3HIQ/AA, respectively. The catalytic effect of complexation is discussed as an interplay between the static component, reflected in the change of geometries and relative stabilities of the three stationary points, and the dynamic one, resulting from the effects of coupling of the tunneling motion to the skeletal modes. Since the coupling parameters reported in the present study are typical for proton transfer along hydrogen bridges, the relative weight of these effects in the overall acceleration of the reaction will be larger in complexes with smaller reduction of the barrier height upon complexation.
Thermal stability of Pt Schottky contacts to 4H–SiC
Authors: Shalish I. , de Oliveira C. E. M. , Shapira Yoram , Burstein L. , Eizenberg M.
Published: July 2002

Depth profiles by x-ray photoelectron spectroscopy have been used in conjunction with current–voltage measurements to study the thermal stability of a 50-nm-thick Pt contact to n-4H–SiC substrate. A reaction between the Pt and the SiC substrate is observed at temperatures of 600 °C and above. Annealing below that temperature improves the ideality and the uniformity of the Schottky characteristics, while annealing above this temperature degrades the electrical performance and uniformity. Thermodynamic stability is not reached even after annealing for 1 h at 900 °C. A local improvement of the characteristics at 800 °C is correlated with the formation of a second graphite film in the Pt–SiC reaction.
Triplet state solvation dynamics: Basics and applications
Authors: Richert Ranko
Published: July 2002

Applying solvation dynamics experiments to viscous liquids or glassy materials near their glass transition involves long lived triplet probes, whose time dependent phosphorescence signals depend upon the local dipolar orientational dynamics, mechanical responses, and polarities. The current understanding of experimental results regarding steady state and time dependent optical line shapes and positions is reviewed with emphasis on the relation to the macroscopic dielectric properties. Several applications are discussed in detail, where advantage is taken of the spatially local instead of ensemble averaging character of this technique. These examples include studies of dynamical heterogeneity, rotational solute/solvent coupling, secondary relaxations in the glassy state, as well as confinement and interfacial effects.
Ion-nitriding induced plastic deformation in austenitic stainless steel
Authors: Grigull S. , Parascandola S.
Published: July 2002

Lattice parameter measurements in ion-nitrided surface layers of austenitic stainless steel have been performed using x-ray microbeam diffraction on samples with different nitrogen contents as a function of depth and grain orientation, respectively. The lattice expansion observed in the layers as a result of nitriding is anisotropic due to the presence of elastic strains, but the austenite (fcc) structure is largely retained. Parallel to the layer–substrate interface the expanded lattice is highly relaxed which is, along with significant changes in the surface morphology, indicative of plastic deformation in the nitrided zone.
Modeling the splash of a droplet impacting a solid surface
Authors: Bussmann M. , Chandra S. , Mostaghimi J.
Published: July 2002

A numerical model is used to simulate the fingering and splashing of a droplet impacting a solid surface. A methodology is presented for perturbing the velocity of fluid near the solid surface at a time shortly after impact. Simulation results are presented of the impact of molten tin, water, and heptane droplets, and compared with photographs of corresponding impacts. Agreement between simulation and experiment is good for a wide range of behaviors. An expression for a splashing threshold predicts the behavior of the molten tin. The results of water and especially heptane, however, suggest that the contact angle plays an important role, and that the expression may be applicable only to impacts characterized by a relatively low value of the Ohnesorge number. Various experimental data of the number of fingers about an impacting droplet agree well with predictions of a previously published correlation derived from application of Rayleigh–Taylor instability theory.
Analytical calculation of nuclear magnetic resonance indirect spin–spin coupling constants at the generalized gradient approximation and hybrid levels of density-functional theory
Authors: Helgaker Trygve , Watson Mark , Handy Nicholas C.
Published: July 2002

A fully analytical implementation of the nuclear magnetic resonance (NMR) indirect nuclear spin–spin coupling constants at the density-functional theory (DFT) level is presented. The implementation involves all four contributions of the nonrelativistic Ramsey theory: The dia- and para-magnetic spin–orbit contributions as well as the paramagnetic Fermi-contact and spin–dipole contributions. Three different exchange-correlation functionals—LDA (local density approximation), BLYP (Becke–Lee–Yang–Parr), and B3LYP (hybrid BLYP)—are tested by comparison with experiment and high-level ab initio calculations for a series of molecules containing first-row elements. All three levels of theory represent a significant improvement on restrictred Hartree–Fock (RHF) theory in the sense that the RHF instability problems are absent in DFT. Also, there is a steady improvement in the quality of the calculated spin–spin couplings in the sequence LDA, BLYP, and B3LYP. For the first-row molecules investigated by us, the hybrid B3LYP functional performs particularly well, with errors similar to those observed at the best ab initio levels of theory.
An externally controlled, nanosecond-pulsed, Xe lamp using a high voltage semiconductor switch
Authors: Iwata Tetsuo , Tanaka Toshiki , Komatsu Toshiyuki , Araki Tsutomu
Published: July 2002

We have constructed a nanosecond pulsed Xe lamp which is controllable by a small-sized, high-voltage semiconductor switch that is driven by a transistor-transistor-logic level external signal. The Xe lamp used here is commercially available and it is usually operated in a direct-current (dc) mode. The aim of the external control is to solve a discharge difficulty in a free-running Xe lamp that we have developed previously. The problem is that the free-running discharge Xe lamp suddenly switches in its operation mode from a pulsed spark-discharge mode to an undesirable, weak, dc arc-discharge mode. The pulse width of emitted light from the pulsed Xe lamp was 3.8 ns and the repetition frequency was 0.5–1 kHz with a peak power 100 W. In order to obtain an accurate timing signal required for a fluorescence-lifetime measurement system, a current probe circuit was added in the control circuit. Details of the lamp and the results of fundamental performance tests are shown.
Formation and evolution of charged domain walls in congruent lithium niobate
Authors: Shur V. Ya. , Rumyantsev E. L. , Nikolaeva E. V. , Shishkin E. I.
Published: July 2002

We present experimental evidence of the formation of stable charged domain walls (CDWs) in congruent lithium niobate during switching. CDW evolution under the action of field pulses was in situ visualized. CDW boundary motion velocity is about 60 μm/s at 20 kV/mm. Relief of CDW strongly depends on applied field. Dielectric response in the presence of CDW demonstrates the pronounced frequency dependence in the range 50–150 °C. We propose the mechanism of CDW self-maintained propagation governed by self-consistent electrostatic interaction between the wall’s steps.
Stabilization and destabilization of channel flow by location of viscosity-stratified fluid layer
Authors: Ranganathan Balaji T. , Govindarajan Rama
Published: July 2002

The stability of the channel flow of two fluids of different viscosities with a mixed layer in between is demonstrated to be qualitatively different from both interface dominated flows and stratified flows. More important, this flow displays unexpected changes in stability when the mixed layer overlaps the critical layer of the disturbance: this feature can be exploited for flow control. When these layers are distinct, the flow is mildly destabilized when the less viscous fluid is in the outer region. When the layers overlap, however, there is an order of magnitude stabilization of the flow. The reverse occurs when the more viscous fluid is in the outer region. This behavior may be explained by the balance of stresses in the critical layer.
Preparation, crystal structure, magnetic and magnetothermal properties of (GdxR5−x)Si4, where R=Pr and Tb, alloys
Authors: Spichkin Y. I. , Pecharsky V. K. , Gschneidner K. A.
Published: July 2002

The series of alloys (GdxTb5−x)Si4 (x=0,2.5,3,3.5,4,4.5,5) and (GdxPr5−x)Si4 (x=4,4.25,4.5) have been prepared. Their room temperature crystal structure has been established, and dc magnetization and ac magnetic susceptibility have been measured in the temperature range from 4.2 to 550 K and in dc magnetic fields up to 50 kOe. The magnetocaloric effect has been calculated from the magnetization data and, for one of the alloys, from the heat capacity data. The magnetocaloric effect values from the two different measurements are in excellent agreement. The (GdxTb5−x)Si4 alloys exhibit magnetocaloric effects in low magnetic fields (<20 kOe) comparable to that of Gd metal and, therefore, they represent a new class of promising magnetic refrigerant materials.
Photoabsorption of dioxasilyrane and silanone groups at the surface of silica
Authors: Raghavachari Krishnan , Pacchioni Gianfranco
Published: July 2002

The lowest singlet→singlet and singlet→triplet transitions of two point defects observed at the surface of amorphous silica, the dioxasilyrane group, =Si(O2), and the silanone group, =Si=O, have been computed by means of first-principles quantum chemical techniques using cluster models of various sizes. The lowest allowed transitions of the dioxasilyrane ring are found at ≈3 and ≈5 eV; the first excitation in silanone is computed at 5.7 eV. These values are in quantitative agreement with the observed bands and allow the unambiguous assignment of the observed optical transitions in the surface of activated silica to the =Si(O2) and =Si=O point defects.
A unified treatment of ultrafast optical heterodyne detected and Z-scan spectroscopies
Authors: Gardecki J. A. , Yu G. , Constantine S. , Peng J. , Zhou Y. , Ziegler L. D.
Published: July 2002

An analysis of femtosecond optical heterodyne detected (OHD) birefringence and dichroism is presented within the framework of the Z-scan technique previously developed for the measurement of nonlinear refractive indices. In particular, the phase of the effective local oscillator field, in the language of OHD spectroscopy, is shown to depend on the sample position (z) and the spatially detected portion of the signal pulse in the far field for nonresonant pump–probe responses due to focused Gaussian beams. Consequently, this treatment shows how the contributions of OHD birefringence and dichroism, and the corresponding homodyne signals, can best be separated in this simple two-beam experimental configuration and provides a description of the previously reported technique called position-sensitive Kerr lens spectroscopy. The nonresonant third-order polarization electronic response of fused silica illustrates this apertured OHD description. Spatial and frequency filtering results in ultrafast Z-scan responses that are dominated by dichroic and birefringent responses, respectively. An analysis of apertured OHD spectroscopy which incorporates a description of the optical phase fronts of the signal fields is required in order to exploit this relatively simple experimental technique for quantitative determinations of polarization-specific electronic responses and Raman nuclear spectral densities.
Laser ablation of solid substrates in water and ambient air
Authors: Zhu S. , Lu Y. F. , Hong M. H. , Chen X. Y.
Published: July 2002

Laser ablation of solid substrates in ambient air and under water is investigated. It is found that the laser ablation rate is highly enhanced by the water film. A wide-band microphone is used to detect the audible acoustic wave generated during laser ablation. Peak-to-peak amplitude of the acoustic wave recorded in water confinement regime (WCR) is greater than that recorded in ambient. It is assumed that the plasma generated in WCR induces a much stronger pressure. This high-pressure, high-temperature plasma results in a much higher ablation rate. Theoretical calculation is also carried out to verify this assumption. By proper calibration, acoustic wave detection can be used as a real-time monitoring of the laser ablation.
Simultaneous interaction of methyl radicals and atomic hydrogen with amorphous hydrogenated carbon films
Authors: von Keudell A. , Schwarz-Selinger T. , Jacob W.
Published: July 2002

The simultaneous interaction of methyl radicals (CH3) and atomic hydrogen (H) with the surface of amorphous hydrogenated carbon (a-C:H) films is investigated by using quantified radical beam sources. The growth and/or erosion of the films during the interaction of the H and CH3 radical beam with the surface is monitored by means of in situ real-time ellipsometry at a substrate temperature of 320 K. Interaction with the CH3 beam alone results in a slow growth rate corresponding to a sticking coefficient for CH3 of ∼10−4. Simultaneous interaction of an atomic hydrogen beam and the CH3 radical beam with the surface results in a sticking coefficient for CH3 of ∼10−2. A microscopic modeling of this synergistic growth yields a cross section of 3.8 Å2 for CH3 adsorption at a dangling bond, created by abstraction of surface bonded hydrogen due to impinging atomic hydrogen. The cross section for the abstraction of surface bonded hydrogen by impinging CH3 radicals is 1.5×10−3 Å2.
Control of amorphous solid water morphology using molecular beams. I. Experimental results
Authors: Kimmel Greg A. , Stevenson K. P. , Dohnálek Z. , Smith R. Scott , Kay Bruce D.
Published: July 2002

The adsorption of N2 was used to investigate the porosity/morphology of thin films of amorphous solid water. Molecular beams were used to vapor deposit amorphous solid water films on a Pt(111) crystal at a variety of incident growth angles. The amount of N2 adsorbed by the amorphous solid water depends very sensitively on the growth angle and thermal history of the film. For normal and nearly normal incidence growth, the water films are relatively dense and smooth and adsorb only a small amount of N2. For larger growth angles, the films are porous and adsorb large quantities of N2 with apparent surface areas as high as ∼2700 m2/g. The physical and chemical properties of amorphous solid water are of interest because of its presence in astrophysical environments. The observations have important implications for laboratory studies which use vapor deposited amorphous solid water films as analogs for astrophysical icy bodies such as comets.
Application of acoustic resonators in photoacoustic trace gas analysis and metrology
Authors: Miklós András , Hess Peter , Bozóki Zoltán
Published: July 2002

The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics is considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes is discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology are covered in detail. The characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification, are discussed. Setup properties and noise features are considered in detail. This review is intended to give newcomers the information needed to design and construct state-of-the-art photoacoustic detectors for specific purposes such as trace gas analysis, spectroscopy, and metrology.
Theoretical calculation of the optical properties of Y3Fe5O12
Authors: Ching W. Y. , Gu Zong-quan , Xu Yong-Nian
Published: July 2002

The electronic structure and the optical properties of Y3Fe5O12 crystal are calculated using the local spin density approximation+U approach. The intra-atomic correlation effect of the Fe 3d electrons is shown to be important in describing the insulating nature of the crystal. With a choice of the parameters U=3.5 eV and J=0.8 eV in the model Hamiltonian, a gap of 2.66 eV is obtained and a significant lowering of the occupied potion of the Fe 3d states is observed. The calculated optical absorption in the range 3–6 eV is from the bulk O 2p to the Fe 3d band states. The calculated spin magnetic moments are in good agreement with experiment.
Density functional theory of inhomogeneous fluid mixture based on bridge function
Authors: Choudhury Niharendu , Ghosh Swapan K.
Published: July 2002

A simple density functional theory is proposed for an inhomogeneous fluid mixture by approximating its one-particle correlation function in terms of the second-order direct correlation functions and the bridge function of the corresponding homogeneous system. The theory is applied to predict the structure of a binary hard sphere mixture as well as Lennard-Jones fluid mixture near a hard wall, and the calculated density profiles for both the components are shown to agree quite well with the corresponding computer simulation results for both the systems. This theory for an inhomogeneous fluid mixture is further applied to homogeneous hard sphere mixture as well as Lennard-Jones fluid mixture and the calculated radial distribution functions are found to compare quite well with the same obtained through integral equation theory of fluid mixture.
Midinfrared absorption and photocurrent spectroscopy of InAs/GaAs self-assembled quantum dots
Authors: Sauvage S. , Boucaud P. , Brunhes T. , Immer V. , Finkman E. , Gérard J.-M.
Published: July 2002

We report on a comparison between the midinfrared absorption and the photocurrent response of n-doped InAs/GaAs self-assembled quantum dots. The absorption, resonant at 160 meV, is polarized along the z growth axis of the dots. The photocurrent is dominated by a z-polarized resonance around 220 meV (5.6 μm wavelength). A weaker component of the photocurrent is observed for an in-plane polarized excitation. The photoresponse can be measured for a 0 V applied bias. The photoresponsivity is investigated as a function of the applied bias. The responsivity and the dark current exhibit an asymmetric profile versus the external bias. This asymmetry is correlated to the structural asymmetry of the quantum dot layers.
Wave packet methods for charge exchange processes in ion-atom collisions
Authors: Baloı̈tcha E. , Desouter-Lecomte M. , Bacchus-Montabonel M.-C. , Vaeck N.
Published: July 2002

The efficiency of different time-independent and time-dependent wave packet methods to calculate charge-exchange cross sections is discussed. The time-independent spectral projection method is based on the Chebyshev expansion of the resolvent function and represents an interesting alternative to the usual Fourier method which involves a time propagation. On the other hand, the flux operator method still requires propagation in time but uses the properties of absorbing potentials in order to calculate the flux operator matrix elements. We show the necessity of introducing the appropriate Hankel–Riccati functions when the full Hamiltonian contains a centrifugal term in 1/R2 in order to reduce the computational time. The collisional system Si4++He is studied as a test case.
Infrared reflectance of GaN films grown on Si(001) substrates
Authors: Zhang Xiong , Hou Yong-Tian , Feng Zhe-Chuan , Chen Jin-Li
Published: July 2002

GaN thin films on Si(001) substrates are studied by infrared reflectance (IRR) spectroscopy at room temperature (RT). Variations in the IRR spectral line shape with the microstructure of GaN/Si(011) film are quantitatively explained in terms of a three-component effective medium model. In this model, the nominally undoped GaN film is considered to consist of three elementary components, i.e., single crystalline GaN grains, pores (voids), and inter-granulated materials (amorphous GaN clusters). Such a polycrystalline nature of the GaN/Si(001) films was confirmed by scanning electron microscopy measurements. It was demonstrated that based on the proposed three-component effective medium model, excellent overall simulation of the RT-IRR spectra can be achieved, and the fine structures of the GaN reststrahlen band in the measured RT-IRR spectra can also be interpreted very well. Furthermore, the volume fraction for each component in the GaN/Si(001) film was accurately determined by fitting the experimental RT-IRR spectra with the theoretical simulation. These results indicate that IRR spectroscopy can offer a sensitive and convenient tool to probe the microstructure of GaN films grown on silicon.
Salt effects on multiple-point adsorption of target molecules by heteropolymer gel
Authors: Watanabe Tsuyoshi , Ito Kenji , Alvarez-Lorenzo Carmen , Grosberg Alexander Yu. , Tanaka Toyoichi
Published: July 2002

The effect of salt concentration on the affinity of a thermosensitive heteropolymer gel for various multiple-point adsorbing target molecules was examined. The gel has positively charged adsorber monomers which can interact electrostatically with negatively charged groups on the target molecules. Adsorption of the targets was found to depend strongly on the salt concentration. The adsorption affinity decreased by several orders of magnitude with a slight increase of the salt concentration, showing that, quite unlike the trivial Donnan potential effect, the negative ions of the salt compete with target molecules for the adsorption sites. We have found the power law relationship between the affinity for the target and the concentration of the coexistent salt for gels in the collapsed state.
Adsorption of a semiflexible polymer onto interfaces and surfaces
Authors: Stepanow Semjon
Published: July 2002

We consider the adsorption of a semiflexible polymer chain onto interfaces and surfaces by using the differential equation of the distribution function G(R,L) of the end-to-end distance R, which is associated with the moment expansion of the latter. We present the results of the approximative treatment consisting of taking into account the second and fourth moments in the differential equation for G(R,L). The essential features of adsorption of the semiflexible polymer are (i) the existence of a new local length scale, which results in two-exponential decay of the monomer density of adsorbed polymer; (ii) the binding of the semiflexible polymer is weaker than that for the flexible one for both interface and wall. The approximative theory presented is restricted to the regime of weak adsorption, where the effect of the rodlike behavior of the polymer on small scales is weak.
Origins of solidification when a simple molecular fluid is confined between two plates
Authors: Demirel A. Levent , Granick Steve
Published: July 2002

A simple globular-shaped liquid (octamethylcyclotetrasiloxane, OMCTS) was placed between two rigid mica plates at variable spacings comparable to the size of this molecule and the linear shear viscoelasticity of the confined interfacial film was measured. Strong monotonic increase of the shear relaxation time, elastic modulus, and effective viscosity were observed as the spacing was decreased below about 10 molecular dimensions. The frequency dependence of the viscoelastic spectra measured at different film thicknesses appeared to scale with reduced variables. The data are inconsistent with the abrupt first-order transition, from bulk fluid to solid with decreasing film thickness, whose possibility has been hypothesized, and suggest a glasslike transition instead.
Simulation of Ã 1B1→X̃ 1A1 CF2 single vibronic level emissions: Including anharmonic and Duschinsky effects
Authors: Chau Foo-Tim , Dyke John M. , Lee Edmond P. F. , Mok Daniel K. W.
Published: July 2002

CASSCF/MRCI/aug-cc-pVQZ(no g) and RCCSD(T)/aug-cc-pVQZ potential energy functions were reported for the Ã 1B1 and X̃ 1A1 states of CF2, respectively. Vibrational wave functions of the symmetric stretching and bending modes of the two states of CF2 were obtained in variational calculations, employing Watson’s Hamiltonian for a nonlinear molecule and anharmonic vibrational wave functions expressed as linear combinations of harmonic basis functions. Franck–Condon factors (FCFs) were computed for Ã 1B1→X̃ 1A1 CF2 single vibronic level (SVL) emissions and the SVL emission spectra were simulated with the computed FCFs. When compared with the observed spectra, the simulated spectra obtained in the present investigation, which include allowance for anharmonicity and the Duschinsky effect, were found to be significantly superior to those reported previously, based on the harmonic oscillator model. Using the iterative Franck–Condon analysis procedure, with the geometry of the X̃ 1A1 state fixed at the recently determined experimental equilibrium geometry, the geometry of the Ã 1B1 state of CF2, which gave the best match between simulated and observed spectra, was found to be re(CF)=1.317 Å and θe(FCF)=121.25 °.
Effects of conformational isomerism on the desorption kinetics of n-alkanes from graphite
Authors: Paserba Kris R. , Gellman Andrew J.
Published: July 2002

The dynamics of oligomer desorption from surfaces have been studied by measuring the desorption kinetics of a set of n-alkanes from the surface of single crystalline graphite. Desorption rates were measured using a set of 21 monodispersed n-alkanes (CNH2N+2,5⩽N⩽60) each adsorbed at coverages in the range <0.1 to >1 monolayers. Desorption is observed to be a first-order process with a desorption barrier (ΔEdes‡) that is independent of coverage. The pre-exponential of the desorption rate constant is independent of the oligomer chain length and has a value of ν=1019.6±0.5 s−1. We also find that ΔEdes‡ has a nonlinear dependence on chain length and takes the empirical form ΔEdes‡=a+bNγ, with the exponent having a value of γ=0.50±0.01. More interestingly, we have proposed a mechanism for the desorption process and a model for the energetics and the entropy of the oligomers on the surface that provide an extremely good quantitative fit to the observed chain length dependence of ΔEdes‡. ΔEdes‡ is given by the difference in energy between the gas phase n-alkane and the conformation of the adsorbed n-alkane with the minimum free energy at the desorption temperature. These results reveal that conformational isomerism plays a significant role in determining the desorption kinetics of oligomers from surfaces.
Group velocity and propagation losses measurement in a single-line photonic-crystal waveguide on InP membranes
Authors: Letartre X. , Seassal C. , Grillet C. , Rojo-Romeo P. , Viktorovitch P. , Le Vassor d’Yerville M. , Cassagne D. , Jouanin C.
Published: July 2002

Single-line photonic-crystal waveguides are investigated. Photoluminescence experiments and three-dimensional calculation are performed and allow a clear identification of the guided modes. The propagation properties of the latter (group velocity, losses) are extracted from photoluminescence spectra obtained on closed waveguides which act as linear cavities.
Ab initio prediction of the vibrational-rotational energy levels of hydrogen peroxide and its isotopomers
Authors: Koput Jacek , Carter Stuart , Handy Nicholas C.
Published: July 2002

The vibrational-rotational energy levels of the hydrogen peroxide isotopomers H2O2, D2O2, HOOD, and H218O2 have been predicted by the variational method using a high-quality ab initio six-dimensional potential energy surface. The calculated energy levels are found to be in good agreement with available experimental data. The predicted spectroscopic constants for various isotopomers can be useful in a future analysis of the rotational-torsional spectra and experimental determination of the equilibrium structure of hydrogen peroxide.
Surface hydroxyl formation on vacuum-annealed TiO2(110)
Authors: Fujino Toshiaki , Katayama Mitsuhiro , Inudzuka Katsuhiko , Okuno Tomohisa , Oura Kenjiro , Hirao Takashi
Published: July 2002

The change in surface composition and structure of a rutile TiO2(110) surface during thermal annealing in an ultrahigh vacuum was studied by coaxial impact–collision ion scattering spectroscopy and time-of-flight elastic recoil detection analysis. When the clean TiO2(110) surface with a 1×1 bridging-oxygen-rows structure was obtained by annealing at 730 °C, about one monolayer of hydrogen atoms still resided on the surface. These hydrogen atoms were assigned to surface hydroxyls as an ingredient of the TiO2(110)1×1 structure, which was formed in the self-restoration process of surface oxygen vacancy defects by dissociative adsorption of water molecules during thermal annealing.
Formation kinetics of fractal nanofiber networks in organogels
Authors: Liu X. Y. , Sawant Prashant D.
Published: July 2002

The fractal structure of nanofiber networks formed in organogels is identified, using an in situ rheological method, together with a coupled supercritical fluid CO2 extraction/scanning electron microscopy technique. The rheological method allows us to have in-line measurements of the fractal growth of the nanofiber networks. In comparison with conventional light scattering techniques, the results obtained from this technique show much less scatter in data. Unlike conventional fractal aggregates, the growth of this type of fractal pattern is governed by a noncrystallographic branching mechanism, which occurs via self-mismatch nucleation and growth.
Properties of ferroelectric PbTiO3 thin films
Authors: Kighelman Zian , Damjanovic Dragan , Cantoni Marco , Setter Nava
Published: July 2002

PbTiO 3 thin films were prepared from alkoxide solution precursors and crystallized onto Pt/TiO2/SiO2/Si substrates. Microscopy observations revealed that the complexity of the domain walls structure decreased with the grain size. Dielectric, electrostrictive, and piezoelectric properties of the films were characterized in details. A shift of the temperature of the dielectric permittivity maximum due to stresses in the plane of films has been observed. Electrostrictive M and Q coefficients were estimated by measuring strain as a function of the ac electric field amplitude. The d33 vs Edc loops are rectangular with a maximum weak field piezoelectric d33 coefficient equal to 65 pm/V. The dielectric permittivity and piezoelectric nonlinearities can be explained by taking into account domain-walls contributions. Dielectric and piezoelectric aging was investigated. It was found that both coefficients follow logarithmic time dependence, with comparable rates. The aging behavior in the PT films is thus qualitatively closer to that in ceramics than in thin films of lead zirconate titanate.
A rapid quenched-flow device for the study of homogeneous polymerization kinetics
Authors: White Curtis B. , Rosaaen Kimberly A. , Landis Clark R.
Published: July 2002

We have designed and constructed a device, the rapid flow mixer (RFM), for investigating the kinetics of metallocene-catalyzed alkene polymerization reactions. The RFM operates over a wide range of reaction conditions (temperatures from −40 to 150 °C and pressures up to 500 psi) and has wetted components constructed from inert materials (stainless steel, Kalrez™, Teflon™). The closed system design and interface to an inert atmosphere glove box facilitates the handling of air and moisture sensitive reagents. Total reaction volumes per run may range from 750 μL to 15 mL. The drive assembly is interfaced to a personal computer that provides user control of reaction times and flow rates. Total reaction times span a range of ∼10 ms to >10 min.
Minority-carrier kinetics in heavily doped GaAs:C studied by transient photoluminescence
Authors: Maaßdorf A. , Gramlich S. , Richter E. , Brunner F. , Weyers M. , Tränkle G. , Tomm J. W. , Mazur Y. I. , Nickel D. , Malyarchuk V. , Günther T. , Lienau Ch. , Bärwolff A. , Elsaesser T.
Published: July 2002

Room-temperature photoluminescence decay time measurements in heavily doped GaAs:C-layers designed as base layers for heterojunction bipolar transistors are reported. These measurements provide access to nonequilibrium minority carrier lifetimes that determine the current gains of those devices. By systematically studying transient luminescence spectra over a wide range of excitation densities between 1013 and 1018 cm−3, we demonstrate the importance of carrier trapping processes at low excitation densities. Optimized excitation conditions that achieve trap saturation but also avoid stimulated emission are found for densities of (1–3)×1017 cm−3/pulse. Detection is limited to a spectral window well above the energy gap (beyond 1.5 eV). Values for both Auger and radiative recombination coefficients are given.
Ion–electron velocity vector correlations in dissociative photoionization of simple molecules using electrostatic lenses
Authors: Lebech M. , Houver J. C. , Dowek D.
Published: July 2002

A new development of electrostatic lenses for ion and/or electron trajectory focusing has been implemented in a double velocity spectrometer which combines time of flight resolved coincidence and imaging techniques using fast position sensitive detectors. Ions and electrons produced by photoionization are extracted from the interaction region by a unique uniform electrostatic field, while the electrostatic lenses create a nonuniform field outside the extraction region. The space focusing reducing the effect of the finite dimensions of the interaction region on one side, and the global bending of the ion and/or electron trajectories preserving the time of flight information on the other side, enable a significant improvement of the resolution of the three measured velocity components for each particle. Such a device is particularly well adapted for the study of vector correlations in dissociative photoionization of simple molecules in the vacuum ultraviolet photon energy range. The gain achieved in the energy resolution of the processes by ion–electron kinetic energy correlation, as well as in the polar and azimuthal angle resolution in the I(θe,φe) molecular frame photoelectron angular distributions, is illustrated on the example of dissociative photoionization of the NO molecule.