bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational excitation and energy redistribution after ultrafast internal conversion in 4-nitroaniline
Authors: Kozich V. , Werncke W. , Dreyer J. , Brzezinka K.-W. , Rini M. , Kummrow A. , Elsaesser T.
Published: July 2002

Nonequilibrium vibrational excitations of para-nitroaniline (PNA, 4-nitroaniline) occurring after internal conversion from the photoexcited charge transfer state are studied by picosecond anti-Stokes Raman scattering. Vibrational excess populations with distinctly different picosecond rise and decay times are found for a number of modes with frequencies between 860 and 1510 cm−1, including the overtone of a non-Raman active mode. A nonthermal distribution of vibrational populations exists up to about 6 ps after photoexcitation. The time-resolved experiments are complemented by steady-state infrared and Raman measurements as well as calculations based on density functional theory, providing a detailed analysis of the steady-state vibrational spectra of PNA and two of its isotopomers. A weakly Raman active vibration at about 1510 cm−1 displays the fastest rise time and a pronounced excess population and—thus—represents the main accepting mode. We suggest that an out-of-plane mode giving rise to the overtone Raman band at this frequency acts both as coupling and accepting mode in the internal conversion process.
Kinetics of reversible diffusion influenced reactions: The self-consistent relaxation time approximation
Authors: Gopich Irina V. , Szabo Attila
Published: July 2002

The simplest general theory of the kinetics of reversible diffusion-influenced reactions that is exact both at short and long times for A+B⇌C and A+B⇌C+D is presented. The formalism is based on an approximate set of reaction-diffusion equations for the pair distribution functions which incorporate the influence of the chemical reaction by using effective rate constants that are determined self-consistently. For small deviations from equilibrium and contact reactivity, the relaxation function is given explicitly in the Laplace domain in terms of the Smoluchowski rate coefficient that describes the corresponding diffusion controlled irreversible reaction. Consequently, the kinetics can be easily obtained for arbitrary diffusion coefficients and equilibrium concentrations.
Three-body hypernetted-chain equation and its numerical solution
Authors: Kim Kang , Munakata Toyonori
Published: July 2002

The HNC (hypernetted-chain) equation is generalized at a three-body level based on the three-body density functional theory and the Percus’ idea. Supplementing with the generalized Ornstein–Zernike relations, we derive the three-body HNC theory for two- and three-body correlation functions. We solved the three-body HNC theory numerically for the case of a one-dimensional fluid and obtained the solution self-consistently. Our results are compared with the (two-body) HNC theory and molecular dynamics simulations. It is found that the three-body HNC theory improves the HNC one from the viewpoint of the radial distribution function.
Scaling for mixtures of hard ions and dipoles in the mean spherical approximation
Authors: Blum L.
Published: July 2002

Using new scaling parameters βi, we derive simple expressions for the excess thermodynamic properties of the mean spherical approximation (MSA) for the ion-dipole mixture. For the MSA and its extensions we have shown that the thermodynamic excess functions are a function of a reduced set of scaling matrices Γχ. We show now that for factorizable interactions like the hard ion–dipole mixture there is a further reduction to a diagonal matrices βχ. The excess thermodynamic properties are simple functions of these new parameters. For the entropy density we get S=−{k/3π}(F[βα])α∈χ, where F is a simple algebraic functional (such as the cube of the modulus of a vector in function space) of the scaling matrices of irreducible representations χ of the closure of the Ornstein–Zernike equation. The new scaling parameters βi are also simply related to the chemical potentials of the components. The analysis also provides a new definition of the Born solvation energy for arbitrary concentrations of electrolytes.
Structural and electrical quality of the high-k dielectric Y2O3 on Si (001): Dependence on growth parameters
Authors: Dimoulas A. , Vellianitis G. , Travlos A. , Ioannou-Sougleridis V. , Nassiopoulou A. G.
Published: July 2002

Thin films of the high-k dielectric Y2O3 are grown on Si (001) substrates by e-beam evaporation in ultrahigh vacuum (UHV), aiming at correlating structural quality with electrical behavior. Films grown at high temperature of ∼450 °C have reproducibly good epitaxial crystalline quality although they exhibit poor electrical behavior. The best electrical properties are measured in films grown at a low to intermediate temperature range around 200 °C, although these films have inferior structural quality, exhibiting texturing or polycrystallinity. A possible explanation for the observed low leakage current (∼10−6 A/cm2 at +1 V) in these films is the presence of a thick (15–20 Å) and uniform interfacial amorphous layer typically formed during growth because of the oxidation of the silicon substrate. This layer is significantly reduced in samples grown at high temperature, while it almost disappears after in situ annealing to 650 °C in UHV, producing sharp interfaces and very good, stoichiometric crystalline Y2O3 epitaxial layers.
Application of lattice Boltzmann method to simulate microchannel flows
Authors: Lim C. Y. , Shu C. , Niu X. D. , Chew Y. T.
Published: July 2002

Microflow has become a popular field of interest due to the advent of microelectromechanical systems. In this work, the lattice Boltzmann method, a particle-based approach, is applied to simulate the two-dimensional isothermal pressure driven microchannel flow. Two boundary treatment schemes are incorporated to investigate their impacts to the entire flow field. We pay particular attention to the pressure and the slip velocity distributions along the channel in our simulation. We also look at the mass flow rate which is constant throughout the channel and the overall average velocity for the pressure-driven flow. In addition, we include a simulation of shear-driven flow in our results for verification. Our numerical results compare well with those obtained analytically and experimentally. From this study, we may conclude that the lattice Boltzmann method is an efficient approach for simulation of microflows.
Excited-state dynamics of alizarin-sensitized TiO2 nanoparticles from resonance Raman spectroscopy
Authors: Shoute Lian C. T. , Loppnow Glen R.
Published: July 2002

Resonance Raman spectra of alizarin-sensitized TiO2 nanoparticles have been obtained at excitation wavelengths throughout the 488-nm charge transfer absorption band. The resonance Raman spectrum of the alizarin-sensitized TiO2 nanoparticle is significantly different from the spectrum of free alizarin, consistent with a chemisorption-type interaction. This interaction is probably chelation of surface titanium ions by the hydroxy groups of alizarin, supported by the observed enhancement of bridging C–O modes at 1326 cm−1. In contrast to resonance Raman intensity analysis of homogeneous electron transfer where vibrations of both the donor and acceptor are observed, self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using the time-dependent wave packet propagation formalism show mode-specific reorganization along alizarin vibrations exclusively; no resonance-enhanced vibrations attributable to the TiO2 moiety are observed. Therefore, the total resonance Raman-derived reorganization energy is only 0.04 eV, significantly smaller than the observed outer-sphere reorganization energy of 0.2 eV for this system and inner-sphere reorganization energies measured for other molecular systems. The discrepancy is ascribed to a significant environmental component to the outer-sphere reorganization energy arising from rapid dephasing of surface TiO2 units involved in adsorption by strongly coupled interior bath vibrations.
Polyelectrolytes confined to spherical cavities
Authors: Pais A. A. C. C. , Miguel M. G. , Linse P. , Lindman B.
Published: July 2002

The effects of confining a polyelectrolyte solution containing a polyion and its counterions in spherical cavities of different sizes have been investigated by Monte Carlo simulations using a simple model system. Polyions of three different linear charge densities and counterions of three different valences have been examined. Structure, energy, and free energy properties of all systems have been determined as a function of the sphere radius. In nearly all cases, the free energy of confining the polyelectrolyte solution increases as the sphere radius is decreased. The free energy cost decreases as the linear charge density of the polyion is reduced and as the counterion valence is increased, although for completely different reasons. A strong consistency among the variations of the different structural and thermodynamic results on the linear charge density and counterion valence was found. The study provides information relevant for experimental systems of polyions confined to, e.g., viruses, vesicles, and zeolite cavities.
Basal plane slip and formation of mixed-tilt boundaries in sublimation-grown hexagonal polytype silicon carbide single crystals
Authors: Ha Seoyong , Skowronski Marek , Vetter William M. , Dudley Michael
Published: July 2002

Optical microscopy, synchrotron white beam x-ray topography (SWBXT), and high resolution x-ray diffraction (HRXRD) were used to study the distribution of basal plane dislocations in bulk 4H silicon carbide crystals grown by the physical vapor transport method. An etch pit array was observed on the silicon face of KOH-etched off-cut wafers. The arrays were aligned parallel to each other and perpendicular to the off-cut direction. The etch pits were oval-shaped, which is characteristic of basal plane dislocations. Corresponding array images have been observed by SWBXT. Based on the characteristic distribution, the etch pit arrays are interpreted as the slip traces of high temperature deformation during the growth process. Thermoelastic stress is proposed as the plausible cause of the deformation. In addition, basal plane dislocation pileups were found in the proximity of polygonized threading edge dislocation arrays. SWBXT and HRXRD were used to study the misorientation related to such dislocation structure. The pileups are interpreted as the origin of the basal-plane-tilt domains in SiC crystals.
Stress, virial, and pressure in the theory of atoms in molecules
Authors: Pendás A. Martı́n
Published: July 2002

The quantum theory of stress is developed within the atoms in molecules (AIM) framework. The complete local stress field is introduced and integrated within atomic basins, and it is shown that the kinetic term gives rise to the atomic virial theorem. The role of the potential part of the stress field in the AIM theory is discussed, and its necessary consideration in order to define atomic pressures presented. These atomic pressures are shown to tend to the thermodynamic limit as the size of the system grows. A link between the AIM theory and the theory of electronic separability has also been found. A set of simple examples illustrates our results.
Collision induced fragmentation of small ionic argon clusters
Authors: Barat M. , Brenot J. C. , Fayeton J. A. , Picard Y. J.
Published: July 2002

The mechanisms of collision induced fragmentation of small Arn+ (n=2–9) clusters are investigated in the 100 eV center-of-mass energy range. The velocity vectors of the fragments are measured in a multicoincidence experiment for two- and three-body fragmentation. The relative role of the two basic dynamics, electronic transitions, and momentum transfer in binary collisions is evaluated. The structure of the clusters deeply influences the type of mechanism. This is clearly the case of Ar3+ for which a specific impulsive process called “diatom” mechanism plays an important part in the fragmentation of one isomer.
Isotropic and anisotropic Raman scattering from molecular liquids measured by spatially masked optical Kerr effect spectroscopy
Authors: Fecko C. J. , Eaves J. D. , Tokmakoff A.
Published: July 2002

Spatially masked optical Kerr effect (SM-OKE) spectroscopy is a nonresonant femtosecond pump–probe technique capable of measuring isotropic contributions to the transient birefringence of molecular liquids. In conjunction with traditional optical-heterodyne-detected optical Kerr effect spectroscopy, polarization-selective SM-OKE measurements are used to experimentally measure the anisotropic and isotropic third-order nonlinear response of CS2, acetonitrile, methanol, and water. These two responses, which allow the intermolecular dynamics to be separated by symmetry, form a complete and independent basis for describing the polarization dependence of nonresonant third-order experiments. The Fourier transform spectral densities of these responses are presented for each liquid and are interpreted in terms of the molecular and interaction-induced contributions to the many-body polarizability. The molecular contributions are suppressed in the isotropic response for all liquids, while the line shape in the interaction-induced portion of the spectra varies with the liquid. For the non-hydrogen-bonding liquids, the isotropic line shape is similar (albeit suppressed) as compared with that of the anisotropic spectrum, but the high-frequency wing of the isotropic spectrum exhibits completely new features in methanol and water. The isotropic water response is especially notable, since it is exceedingly fast and distinct from the anisotropic response.
Finite electron temperature effects in plasma polarimetry and interferometry
Authors: Segre S. E. , Zanza V.
Published: July 2002

The effects of a finite electron temperature on the refractive index N and on the evolution of polarization for radiation propagating in a magnetized plasma are considered. Analytic expressions for N and for the Stokes vector evolution equation are found to lowest order in Te/mec2. For interferometric and polarimetric measurements on typical plasma confinement configurations, the thermal effects are found to be significant. Neglecting them can lead to systematic errors in polarimetry greater than 10% for a temperature of 5 keV and to a systematic (positive) error on electron density from interferometry of 3% for Te=15 keV.
Range of validity of McCumber theory in relating absorption and emission cross sections
Authors: Quimby R. S.
Published: July 2002

The relation between absorption and emission spectra in a rare-earth doped solid is explored analytically and through numerical simulations on a model system. It is found that the widely used theory of McCumber [D. E. McCumber, Phys. Rev. 136, A954 (1964)] is strictly valid only when the linewidth of each individual transition between Stark levels is ≪kBT. When this assumption is relaxed, deviations between actual simulated spectra and McCumber-calculated spectra are found for both homogeneous and inhomogeneous broadening. The McCumber transform introduces a characteristic distortion of each component transition, in which the low-energy side of the spectrum is increased, and the high-energy side is decreased. The average degree of distortion for the model system is calculated to be 5% for a homogeneous width of 50 cm−1 at room temperature, but only 1% for a purely inhomogeneous width of 50 cm−1. For both homogeneous and inhomogeneous spectra, the distortion varies with the linewidth Δν as (Δν)3/2. The dependence of the distortion on temperature is different for the two types of line shapes, however: ∼T2 for homogeneous and ∼T−1 for inhomogeneous broadening. For most rare-earth doped glasses and crystals at room temperature, the average degree of distortion is expected to be on the order of 5% or less, although the actual discrepancy in the spectral wings can be much larger. The effects described here are likely to be of importance when using McCumber’s theory to obtain accurate cross section values in the spectral wings of rare-earth transitions.
Photoluminescence of size-separated silicon nanocrystals: Confirmation of quantum confinement
Authors: Ledoux G. , Gong J. , Huisken F. , Guillois O. , Reynaud C.
Published: July 2002

Silicon nanocrystals with diameters between 2.5 and 8 nm were prepared by pulsed CO2 laser pyrolysis of silane in a gas flow reactor and expanded through a conical nozzle into a high vacuum. Using a fast-spinning molecular-beam chopper, they were size-selectively deposited on dedicated quartz substrates. Finally, the photoluminescence of the silicon nanocrystals and their yield were measured as a function of their size. It was found that the photoluminescence follows very closely the quantum-confinement model. The yield shows a pronounced maximum for sizes between 3 and 4 nm.
Oxygen and ozone oxidation-enhanced field emission of carbon nanotubes
Authors: Kung Sheng-Chin , Hwang Kuo Chu , Lin I. Nan
Published: July 2002

Vertically aligned carbon nanotube (CNT) arrays were grown on p-type silicon wafer using acetylene and iron phthalocyanine as the sources of hydrocarbons and catalysts, respectively. The CNT arrays were treated by chemical reagents, such as oxygen (O2), ozone (O3), bromine, and acids. When treated by O2 and O3, the emission current of the CNT array was increased ∼800% along with a decrease of the onset field emission voltage from 0.8 to 0.6 V/μm. Other chemical treatments, e.g., bromination and acid oxidation, lead to poorer field emission performance. The effects of these chemical processes on the field emission properties of CNT arrays will be discussed.
A linear response approach to second-order electronic transition intensities for multiconfigurational self-consistent field wave functions
Authors: Stålring Jonna , Bernhardsson Anders , Malmqvist Per-Åke
Published: July 2002

A new theoretical approach to two-photon transition intensities at the multiconfigurational self-consistent field (MCSCF) level of theory, is described in detail. The fundamental property of an MCSCF wave function, that it is possible to define the response equations for an excited state, is a prerequisite. The method requires solely first-order multiconfigurational response calculations, because the equations involve the response of both the initial and final state. However, the method is approximate as the coupling between the +ω and −ω parts of the linear response is disregarded. The complete active space state interaction (CASSI) method is applied in the evaluation of the involved matrix elements. To illustrate the performance and the requirements of this method, it was used to determine TP transitions in trans-1,3-butadiene and trans-stilbene.
Experimental studies on the shape and path of small air bubbles rising in clean water
Authors: Wu Mingming , Gharib Morteza
Published: July 2002

This Letter reports experiments on the shape and path of air bubbles (diameter range 0.1–0.2 cm) rising in clean water. We find that bubbles in this diameter range have two steady shapes, a sphere and an ellipsoid, depending on the size of the capillary tube from which they detach. The spherical bubbles move significantly slower than the ellipsoidal ones of equivalent volume. Bubbles with diameter less than about 0.15 cm rise rectilinearly. The larger spherical bubbles follow zigzag paths while the larger ellipsoidal bubbles follow spiral paths.
Reduction of momentum transfer rates by parallel electric fields: A two-fluid demonstration
Authors: Delamere P. A. , Stenbaek-Nielsen H. C. , Otto A.
Published: July 2002

Momentum transfer between an ionized gas cloud moving relative to an ambient magnetized plasma is a general problem in space plasma physics. Obvious examples include the Io–Jupiter interaction, comets, and coronal mass ejections. Active plasma experiments have demonstrated that momentum transfer rates associated with Alfvén wave propagation are poorly understood. Barium injection experiments from the Combined Release and Radiation Effects Satellite (CRRES) have shown that dense ionized clouds are capable of E×B drifting over large distances perpendicular to the magnetic field. The CRRES “skidding” distances were much larger than predicted by magnetohydrodynamic theory and it has been proposed that parallel electric fields were a key component in the skidding phenomenon. A two-fluid code was used to demonstrate the role of parallel electric fields in reducing momentum transfer between two distinct plasma populations. In this study, a dense plasma was initialized moving relative to an ambient plasma and perpendicular to B. Parallel electric fields were introduced via a friction term in the electron momentum equation and the collision frequency was scaled in proportion to the field-aligned current density. The simulation results showed that parallel electric fields decreased the decelerating magnetic tension force on the plasma cloud through a magnetic diffusion/reconnection process.
On slow manifolds of chemically reactive systems
Authors: Singh Sandeep , Powers Joseph M. , Paolucci Samuel
Published: July 2002

This work addresses the construction of slow manifolds for chemically reactive flows. This construction relies on the same decomposition of a local eigensystem that is used in formation of what are known as Intrinsic Low Dimensional Manifolds (ILDMs). We first clarify the accuracy of the standard ILDM approximation to the set of ordinary differential equations which model spatially homogeneous reactive systems. It is shown that the ILDM is actually only an approximation of the more fundamental Slow Invariant Manifold (SIM) for the same system. Subsequently, we give an improved extension of the standard ILDM method to systems where reaction couples with convection and diffusion. Reduced model equations are obtained by equilibrating the fast dynamics of a closely coupled reaction/convection/diffusion system and resolving only the slow dynamics of the same system in order to reduce computational costs, while maintaining a desired level of accuracy. The improvement is realized through formulation of an elliptic system of partial differential equations which describe the infinite-dimensional Approximate Slow Invariant Manifold (ASIM) for the reactive flow system. This is demonstrated on a simple reaction-diffusion system, where we show that the error incurred when using the ASIM is less than that incurred by use of the Maas-Pope Projection (MPP) of the diffusion effects onto the ILDM. This comparison is further done for ozone decomposition in a premixed laminar flame where an error analysis shows a similar trend.
Multiple electron beam propagation and Langmuir wave generation in plasmas
Authors: Li B. , Robinson P. A. , Cairns I. H.
Published: July 2002

The propagation of multiple electron beams in a plasma and the generation of Langmuir waves via a streaming instability is investigated numerically using quasilinear theory. The generation of waves by two equal copropagating beams injected at different times is studied in detail. The two beams are observed merging into one far from the injection points. Meanwhile, waves are enhanced in the vicinity of the mean beam speed of the leading beam, and are suppressed in a localized region after the injection of the trailing beam. Effects of beam injection parameters on the generation of the waves are studied. In particular, for the injection of two beams, the temperature, initial number density, and location of the injected particles are found to be relevant to fine structures in wave levels. It is also observed that the mechanism of beam merging is via interactions between beam particles and associated waves, i.e., fast particles in a trailing beam lose energy to waves generated initially by the leading beam, while slow particles in the leading beam absorb energy from waves driven by the trailing beam, which eventually leads to the elimination of systematic speed differences between the two beams. This mechanism of energy exchange generalizes the version studied in previous works, in which fast particles in a single beam lose energy that is later reabsorbed by slower particles. The characteristics of wave generation for multiple beam injections are found to be similar to the basic case of two beams. Finally, the applicability of this work to type III solar radio storms and shock associated type III-like bursts is commented upon.
Calculations of the electronic structure of strained InAs quantum dots in InP
Authors: Holm Magnus , Pistol Mats-Erik , Pryor Craig
Published: July 2002

We have calculated the electronic structure of InAs quantum dots embedded in InP as a function of size, using strain dependent eight-band k⋅p theory in the envelope function approximation. A realistic three-dimensional shape was used for the simulations and the piezoelectric polarization of the system was included. In order to avoid spurious solutions, an extra term was added to the Hamiltonian. Polarization dependent dipole matrix elements were calculated as well as the exciton binding energies. A comparison between measurements and calculated transition energies shows good agreement.
Influence of dielectric deposition parameters on the In0.2Ga0.8As/GaAs quantum well intermixing by impurity-free vacancy disordering
Authors: Yu Jae Su , Song Jin Dong , Lee Yong Tak , Lim H.
Published: July 2002

We investigated the influence of the deposition parameters of SiOx and SiNx capping layers in the plasma-enhanced chemical vapor deposition on the band gap energy shift of the In0.2Ga0.8As/GaAs multiple quantum well (QW) structures induced by impurity-free vacancy disordering. The investigated deposition parameters were deposition pressure, deposition temperature, and rf power. A blueshift of photoluminescence (PL) peak energy up to 161 meV was observed after rapid thermal annealing at 950 °C for 50 s in the samples capped with SiOx deposited at 1.5 Torr. We observed that the blueshift of the PL peak energy increased greatly with the increase of deposition pressure and slightly with the decrease of deposition temperature. The influence of rf power was found to be negligible. All these dependences were related to the porosity in the dielectric capping layers in the QW intermixing.
Composition dependence of thermoelectric properties of AlPdRe icosahedral quasicrystals
Authors: Kirihara K. , Kimura K.
Published: July 2002

We present the composition dependence of the Seebeck coefficient and the electrical conductivity of AlPdRe icosahedral alloys. As the concentration of transition metal (either Pd or Re) increases, the Seebeck coefficient rapidly increases. The strong composition dependence is related to the pseudogap structure in the electron density of states at the Fermi energy, and to the variation in bonding nature between Al and transition metal. Glass-like transport behavior in thermal conduction is also observed. The dimensionless thermoelectric figure of merit has a maximum value of approximately 0.1 in the temperature range from 600 to 700 K and reveals strong composition dependence.
Effect of donor (Nb) concentration on the bulk electrical resistivity of Nb-doped barium titanate
Authors: Yoon Seok-Hyun , Kim Hwan
Published: July 2002

The behavior of the bulk electrical resistivity of donor-doped BaTiO3 with the increase of donor concentration was investigated. A series of coarse-grained specimens with different donor concentrations were prepared by adjusting the oxygen partial pressure during sintering. Then they were heat treated in air for an extended period in order to reach the equilibrium defect distribution corresponding to each heating temperature, which was followed by fast cooling. The critical donor concentrations, beyond which the bulk resistivity increases abruptly, were experimentally evaluated and they were found to decrease with a decrease in the equilibrium temperature in air. Each defect and electron concentrations as a function of donor concentration were theoretically calculated based on the model that the resistivity anomaly is caused by the partially ionized cation vacancies (VTi‴) at high temperature, which trap electrons, and become fully ionized cation vacancies (VTi⁗) as specimens are cooled to room temperature. The results calculated could explain the experimentally observed behavior of the bulk resistivity versus donor concentration.
From single molecules to aggregates to gels in dilute solution: Self-organization of nanoscale rodlike molecules
Authors: Perahia Dvora , Traiphol Rakchart , Bunz Uwe H. F.
Published: July 2002

A transition from a fluid to a constrained phase, in dilute solutions of a rodlike molecule, poly(2,5-dinonylparaphenylene ethynylene)s (PPE) in toluene has been studied, exploring the dynamics and the structure of the PPE molecules and the solvent in both phases. The transition is characterized by visual changes in the viscosity of the system and in its color, where a transparent liquid transforms into a yellow glassy phase. Nuclear magnetic resonance relaxation measurements indicated that significant restriction of motion of the solvent and of the polymeric molecule take place as the gel-like phase is formed. Small angle neutron scattering studies have shown that in the liquid phase, PPE forms molecular solutions where the molecules are fully extended. Upon transition into the constrained phase, aggregation of PPE molecules into large flat clusters occurs. When the aggregates are too large to freely move in the solution, a transition into a constrained phase takes place. The interaction between the highly conjugated PPE molecules and the solvent results in constraint of the motion of the solvent as well.
High-contrast photoluminescent patterns in lithium fluoride crystals produced by soft x-rays from a laser-plasma source
Authors: Baldacchini G. , Bonfigli F. , Flora F. , Montereali R. M. , Murra D. , Nichelatti E. , Faenov A. , Pikuz T.
Published: July 2002

A technique using soft x-rays and extreme ultraviolet light generated by a laser-plasma source has been investigated for producing low-dimensionality photoluminescent patterns based on active color centers in lithium fluoride (LiF) crystals. Strong visible photoluminescence at room temperature has been observed in LiF crystals from fluorescent patterns obtained by masking the incoming radiation. This technique is able to produce colored patterns with high spatial resolution on large areas and in short exposure times as compared with other coloration methods.
Raman spectroscopic investigations of activated carbon materials
Authors: Shimodaira N. , Masui A.
Published: July 2002

Raman spectra of activated carbon materials have been investigated by a peak-deconvolution technique. It has been found as a result of our fitting trials that four Gaussians and a linear background are necessary and enough to reproduce the spectral data throughout our experiment, with a pair of relatively sharp peaks at about 1600 cm−1 and about 1350 cm−1, namely G1 and D1, and a pair of relatively broad peaks around 1560 cm−1 and 1340 cm−1, namely G2 and D2. From the characteristic behavior of these paired peaks, it has been concluded that the structure of activated carbon materials is schematically classified into two parts. The former part, represented by sharp G1 and G2 peaks, arises from winding short basal planes with bond angle order, while the latter, by broad G2 and D2 peaks, sp2 clusters like a-Cs with bond angle disorder. We have found the peak intensity ratio of G2 to G1, the I(G1)/I(G2) ratio, as a useful parameter expressing the relative content of the disorder part to the order part in the activated carbons, revealing the structural changes induced by anneal or activations. Since the changes of fitting parameters including the I(G2)/I(G1) ratio were completely opposite for H2O and KOH activation, it has been clarified that the part with bond angle disorder is selectively removed or become ordered by H2O activation, whereas induced by KOH activation, though both activation methods yield similar surface area.
He
3
spin exchange cells for magnetic resonance imaging
Authors: Jacob R. E. , Morgan S. W. , Saam B.
Published: July 2002

We present a protocol for the consistent fabrication of glass cells to provide hyperpolarized (HP) He3 for pulmonary magnetic resonance imaging. The method for producing HP He3 is spin-exchange optical pumping. The valved cells must hold of order 1 atm⋅L of gas at up to 15 atm pressure. Because characteristic spin-exchange times are several hours, the longitudinal nuclear relaxation time T1 for He3 must be several tens of hours and robust with respect to repeated refilling and repolarization. Collisions with the cell wall are a significant and often dominant cause of relaxation. Consistent control of wall relaxation through cell fabrication procedures has historically proven difficult. With the help of the discovery of an important mechanism for wall relaxation that involves magnetic surface sites in the glass, and with the further confirmation of the importance of Rb metal to long wall-relaxation times, we have developed a successful protocol for fabrication of He3 spin exchange cells from inexpensive and easily worked borosilicate (Pyrex) glass. The cells are prepared under vacuum using a high-vacuum oil-free turbomolecular pumping station, and they are sealed off under vacuum after ⩾100 mg of distilled Rb metal is driven in. Filling of cells with the requisite He–N23 mixture is done on an entirely separate gas-handling system. Our cells can be refilled and the gas repolarized indefinitely with no significant change in their wall properties. Relaxation data are presented for about 30 cells; the majority of these reach a “40/40” benchmark: T1>40 h, and He3 polarizations reach or exceed 40%. Typical polarization times range from 12 to 20 h; 20% polarization can be achieved in 3–5 h.
The effect of polarization on ultrashort pulsed laser ablation of thin metal films
Authors: Venkatakrishnan K. , Tan B. , Stanley P. , Sivakumar N. R.
Published: July 2002

Ultrashort pulse lasers have proven to have superior advantages over conventional continuous wave and long pulse lasers for ablation of thin metal films. Though several investigations have been carried out to understand the phenomena of ultrashort pulse laser machining, the effect of the beam polarization on ablation of thin metal films has been seldom investigated. In this article, we report our recent observations on how the shape of the machined feature and also the damage threshold of the material varies according to the polarization of the ultrashort pulse laser beam. Based on this we have explained how the polarization of the beam controls the laser cutting rate, kerf width, edge quality, and ablation depth of the ablated feature.
Classical polarizable force fields parametrized from ab initio calculations
Authors: Tabacchi Gloria , Mundy Christopher J. , Hutter Jürg , Parrinello Michele
Published: July 2002

A computationally efficient molecular dynamics implementation of a polarizable force field parametrized from ab initio data is presented. Our formulation, based on a second-order expansion of the energy density, models the density response using Gaussian basis functions derived from density functional linear response theory. Polarization effects are described by the time evolution of the basis function coefficients propagated via an extended Lagrangian formalism. We have devised a general protocol for the parametrization of the force field. We will show that a single parametrization of the model can describe the polarization effects of LiI in the condensed phase.
Determination of transition rate constants of trans–cis isomerization in a poly(malonic ester) containing disperse red 1
Authors: Shin H. D. , Joo W. J. , Oh C. H. , Kim P. S. , Han Y. K.
Published: July 2002

We suggested a simple method for obtaining the quantitative transition rate constants of trans–cis isomerization in the pseudo-stilbene-type of azobenzene molecule. The absorbances of the solution and film of a poly(malonic ester) containing disperse red 1(PDR1) were measured with the pumping beam intensity, and analyzed by the rate equation for the fractions of trans and cis conformers. The rate constants of PDR1 solution and film were successfully determined.
Excited and ionized states of aniline: Symmetry adapted cluster configuration interaction theoretical study
Authors: Honda Yasushi , Hada Masahiko , Ehara Masahiro , Nakatsuji Hiroshi
Published: July 2002

Singlet excited states and ionized states of aniline are studied by the symmetry adapted cluster/configuration interaction method. Absorption bands of states that have mainly π–π* nature are assigned as A″1 (∼1B2), A′1 (∼1A1), A″1 (∼1B2), A′1 (∼1A1), A″1 (∼1B2) in increasing-energy order. An s-Rydberg state is predicted to lie between the first and second valence states, in agreement with recent experimental results. The lowest band has a charge-resonance character with a slight charge-transfer (CT) character (CT is defined as NH2→C6H5); third and fifth valence bands have back-CT (BCT) nature, and second and fourth are local excitations within the benzene ring. The extent of CT of excited states depends on amino group conformation. In the planar form, CT characters of several states were altered; however, spectral shapes are very similar to that of the equilibrium form. On the other hand, amino group twisting altered both the spectrum and nature of excited states. Third and fourth lowest valence states exhibited strong CT character, while fifth to eighth states are of the strong BCT type, implying that the CT nature of excited states of aniline can be changed by amino group twisting. For ionized states, the lowest three states are assigned to A′2 (∼2B1), A″2 (∼2A2), A′2 (∼2B1) in increasing-energy order, all being π-ionizations. The sixth one is also due to π-ionization (∼2B1) and the others are σ-ionizations. Ordering was the same as Koopmans’ case.
Benchmark quantum Monte Carlo calculations
Authors: Grossman Jeffrey C.
Published: July 2002

Fixed node diffusion Monte Carlo (FN-DMC) atomization energies are calculated for a benchmark set of 55 molecules. Using single determinant trial wave functions, comparison with experiment yields an average absolute deviation of 2.9 kcal/mol, placing this simplest form of FN-DMC roughly at the same level of accuracy as the CCSD(T)/aug-cc-pVQZ method. However, unlike perturbative wave function expansion approaches, FN-DMC is applicable to systems containing thousands of valence electrons. For the P2 molecule, a number of possible sources of error are explored in detail. Results show that the main error is due to the fixed-node approximation and that this can be improved significantly with multireference trial wave functions.
A low noise transimpedance amplifier for cryogenically cooled quartz tuning fork force sensors
Authors: Yang C. H. , Chang T. H. , Yang M. J. , Moore W. J.
Published: July 2002

We have designed and built a low noise, broadband transimpedance amplifier that allows for sensing both the amplitude and phase of an oscillating quartz tuning fork for a cryogenic atomic force microscope. The circuit uses a global feedback scheme, where the input stage, located next to the quartz tuning fork at low temperature, is followed by an operational amplifier stage operated at room temperature. At 4.2 K, with a 1 MΩ metal film feedback resistor, the amplifier yields an output noise floor of 2×10−7 V/Hz and a bandwidth of 200 kHz. When it is used with a commercial 32 kHz quartz tuning fork, the calibrated sensitivity at 4.2 K is determined to be 30.8 μV/pm.
Absolute small angle light scattering measurements from weakly scattering systems in a shear flow apparatus
Authors: Weber V. , Schosseler F.
Published: July 2002

We describe an optical detection layout that allows absolute small angle light scattering measurements on samples sheared in a concentric cylinders geometry. The two main features are the use of a focused incident beam and of a spatial filter that minimize the problem of stray light arising from digs and scratches in the optical path. The scattering intensity is imaged directly onto a cooled 12 bits charge-coupled device sensor. We are thus able to measure scattering intensities as low as about 4×10−4 cm−1 (about ten times the intensity scattered from a toluene standard) for scattering wave vector values between 5×10−5 and 3.5×10−4 Å−1. Some preliminary results of experiments on dilute surfactant solutions are given to demonstrate the performance of the instrument.
The modified Rankine balance: A highly efficient, low-cost method to measure low-temperature magnetic susceptibility in rock samples
Authors: Hagermann A. , Schnepp E.
Published: July 2002

In this article, we present a method to measure the relative changes in the magnetic susceptibility of rock samples in the low-temperature range (−200 °C to +20 °C). The method differs from other experimental methods currently used in that it requires—in contrast to ac bridges, the most widely used devices—very little sophisticated laboratory equipment: A high-precision laboratory balance, a Pt-100 thermoelement and a computer with standard input/output interface and analog/digital processing capabilities, as well as a few rare earth magnets, are the only devices needed in addition to standard laboratory equipment. A Dewar container and a few plexiglass panes can either be handcrafted in the workshops of any larger research institute or are commercially available at little cost. The results of our measurements reveal that the temperature-varying magnetic properties of rock samples can be reliably observed.
Exchange-coupling interaction in nanocomposite SrFe12O19/γ-Fe2O3 permanent ferrites
Authors: Liu Xiansong , Zhong Wei , Gu Benxi , Du Youwei
Published: July 2002

Nanocomposite SrFe12O19/γ-Fe2O3 permanent ferrites have been prepared by the sol-gel method and their structure and magnetic properties have been analyzed by x-ray diffraction, transmission electron microscopy, and vibrating sample magnetometer. When thermal treatment temperature is below 800 °C, the samples pressed into a flake exhibit exchange-coupling interaction between the two nanostructured phases of γ-Fe2O3 and SrFe12O19. The flake sample treated at 800 °C produces an isotropic nanocomposite with a intrinsic coercivity of 6015 Oe, a maximum magnetization of 75.6 emu/g and a maximum energy product of 1.87 MGOe as compared with the powder sample of 6400 Oe, 75.9 emu/g, and 1.52 MGOe, respectively. The exchange-coupling interaction and the enhancement of isotropic (BH)max in nanocomposite SrFe12O19/γ-Fe2O3 permanent ferrites have been discussed.
Optical modeling of a-Si:H solar cells deposited on textured glass/SnO2 substrates
Authors: Krč J. , Zeman M. , Smole F. , Topič M.
Published: July 2002

In this article we determine descriptive scattering parameters—haze and angular distribution functions—of scattered light for textured glass/SnO2 Asahi U-type substrates. These scattering parameters are input parameters of our optical model that enables us to analyze multilayer optical systems with rough interfaces. The scalar scattering theory is used to calculate the haze parameters of all internal rough interfaces in the a-Si:H solar cells deposited on the glass/SnO2 substrates. In the equations of the scalar scattering theory the correction functions are introduced in order to match the calculations with the measurements of the haze parameters of the substrates. The angular distribution functions of the substrates are applied to the rough internal interfaces. Using these scattering parameters we investigate the optical behavior of a-Si:H solar cells with different intrinsic layer thicknesses deposited on the textured glass/SnO2 substrates with different roughnesses.
Optimization of a rotary Q-switched Er:YAG laser
Authors: Livingston Frank E. , George Steven M. , Shori Ramesh K.
Published: July 2002

An erbium:yttrium–aluminum–garnet Er:YAG (λ=2.94 μm) rotary Q-switched laser was optimized for long-term stability and reliability, maximum output energy, and TEMoo mode quality. This optimization was achieved employing a close-coupled BaSO4 diffuse reflector pump chamber and a dehumidifying assembly and an ultra-dry-air purge system. The performance and efficiency of the Er:YAG laser were further enhanced by appropriate variations in the coolant temperature, rotational frequency of the Q-switch mirror, and pulse repetition rate. These improvements should facilitate the implementation of rotating mirror Q-switched Er:YAG lasers in various laser photoablation and depth-profiling applications.
Nonlinear harmonic components of the electric polarization of symmetric-top molecules
Authors: Déjardin J.-L.
Published: July 2002

The nonlinear dielectric relaxation of an assembly of polar molecules submitted to the action of a dc field superimposed on an ac electric field is studied theoretically in the framework of the noninertial rotational diffusion model. It is also assumed that the permanent dipole moment vector makes an arbitrary angle with respect to the long molecular axis of each rodlike molecule. By calculating the ensemble averages of two Wigner rotation matrix elements 〈D0,01(Ω)〈(t) and 〈D0,11(Ω)〉(t), it is possible to express the first three harmonic components of the electric polarization varying in ω, 2ω and 3ω in analytic form, together with the steady-state frequency-dependent component. Three-dimensional dispersion and absorption plots are presented in order to show the influence of the diffusion anisotropy parameter Δ. Cole–Cole diagrams of the second and third harmonic components are also presented allowing one to distinguish two relaxation processes along the short and the long molecular axes, respectively.
The morphology of coexisting liquid and frozen phases in porous materials as revealed by exchange of nuclear spin magnetization followed by H1 nuclear magnetic resonance
Authors: Valiullin Rustem , Furó István
Published: July 2002

At low temperatures, liquids imbibed into nanoporous materials form frozen solid cores in the interior of pores. Inbetween the cores and the pore walls, there exists a layer of nonfrozen liquid. As demonstrated here by nuclear magnetic resonance magnetization transfer and cryoporometry experiments, pools of nuclear spins that reside in those two separate domains, core and liquid layer, exchange nuclear Zeeman magnetization. This exchange is detectable in these materials thanks to the high contact area. Based on this phenomenon, we propose a new method for measuring the surface-to-volume ratio and for accessing the pore space morphology in nanoporous materials. By monitoring the magnetization transfer process, information can be obtained: (i) on the geometrical characteristics of the porous space and (ii) on the diffusivity of nuclear magnetization in the frozen phase. The diffusive transport of nuclear magnetization in the frozen core can be contributed by two mechanisms: spatial atomic/molecular diffusion and spin diffusion through nuclear dipole–dipole interaction. For the water–ice system in controlled porous glasses of known morphology, we can separate these two mechanisms to obtain an estimate of the proton diffusion coefficient of 4×10−16 m2/s in ice at 255 K.
The energy landscape for solvent dynamics in electron transfer reactions: A minimalist model
Authors: Tanimura Y. , Leite V. B. P. , Onuchic J. N.
Published: July 2002

Energy fluctuations of a solute molecule embedded in a polar solvent are investigated to depict the energy landscape for solvation dynamics. The system is modeled by a charged molecule surrounded by two layers of solvent dipolar molecules with simple rotational dynamics. Individual solvent molecules are treated as simple dipoles that can point toward or away from the central charge (Ising spins). Single-spin-flip Monte Carlo kinetics simulations are carried out in a two-dimensional lattice for different central charges, radii of outer shell, and temperatures. By analyzing the density of states as a function of energy and temperatures, we have determined the existence of multiple freezing transitions. Each of them can be associated with the freezing of a different layer of the solvent.