bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-dimensional ferromagnetic correlations above TC in the naturally layered CMR manganite La2−2xSr1+2xMn2O7 (x=0.3–0.4) (invited)
Authors: Argyriou D. N. , Kelley T. M. , Mitchell J. F. , Robinson R. A. , Osborn R. , Rosenkranz S. , Sheldon R. I. , Jorgensen J. D.
Published: July 2002

Neutron diffuse scattering in the form of rod-like features has been observed in single crystals of the layered CMR material La2−2xSr1+2xMn2O7 (x=0.4,0.36), consistent with the presence of 2D ferromagnetic spin correlations. These diffuse features are observed over a wide temperature region. However, their coherence length does not appear to diverge at TC, although there is evidence of the development of three-dimensional correlations around ferromagnetic reflections of the 3D-ordered magnetic structure close to TC. Quasi-elastic neutron scattering on a ceramic sample of x=0.3 shows that the lifetime of these ferromagnetic correlations increases at T→TC. They exhibit a spin-diffusion constant above TC of ∼5 meV Å2, much lower than that reported for La2/3Ca1/3MnO3. We discuss the relationship of these magnetic correlations to models of the ferromagnetic transition in CMR compounds.
Influence of excitation frequency, temperature, and hydrogen dilution on the stability of plasma enhanced chemical vapor deposited a-Si:H
Authors: Platz R. , Wagner S. , Hof C. , Shah A. , Wieder S. , Rech B.
Published: July 2002

The first comparative study of dc, rf, and very high frequency (VHF) excitation for the plasma enhanced chemical vapor deposition of intrinsic layers of hydrogenated amorphous silicon (a-Si:H) is presented. The effects of hydrogen dilution on film stability are emphasized. Growth rates at comparable plasma power are presented for substrate temperatures between 100 and 300 °C and for various H2 dilution ratios. The optical band gap, H content, and electronic transport properties in the light-soaked state were measured. H2 dilution strongly reduces the growth rate for all techniques. The growth rate for the highest H2 dilution ratio is higher for VHF (∼4 Å/s) than for dc (∼3 Å/s) or rf (0.5–1 Å/s) excitation. In all three cases, increasing the substrate temperature reduces the optical gap and the H content CH. Raising the substrate temperature slightly enhances stability. H2 dilution increases the optical gap for all three techniques. The H content of rf- and VHF-deposited samples increases with increasing H2 dilution ratio, while in dc deposition it produces an initial drop of the H content, followed by an increase. In all three cases, H2 dilution improves the electronic transport properties of the material by roughly a factor of 2. The gain in stability is most pronounced for relatively small dilution; in the case of dc deposition, too strong a dilution even has an adverse effect on stability. We rule out a universal relation between hydrogen content of the a-Si:H films and their stability against light soaking. Enhanced stability may be obtained for films which incorporate either more or less hydrogen than standard a-Si:H.
Hole injection from diamond into conducting polymer
Authors: Tada Kazuya , Sonoda Tooru , Yokota Yoshihiro , Kobashi Koji , Yoshino Katsumi
Published: July 2002

Current–voltage characteristic and electroluminescence (EL) have been observed in a heterojunction device using a conducting polymer, poly(2-methoxy-5-dodecyloxy1,4-phenylenevinylene) (MDDO-PPV), and a diamond film prepared by microwave plasma chemical vapor deposition with Al/MDDO-PPV/diamond/Si structure. These results can be interpreted in terms of the hole injection from diamond into MDDO-PPV and occurrence of radiative recombination in MDDO-PPV. The electronic energy structure evaluated from electrochemical and photoelectrochemical measurements suggests that the diamond film is suitable for hole injection in the conducting polymers, being consistent with the current–rectifying characteristics of the EL device. The observed photocurrent spectra suggest the important roles of surface and/or interface effects of the diamond films.
Intergrain magnetic coupling and microstructure in CoPtCr, CoPtCrTa, and CoPtCrB alloys
Authors: Kubota Yukiko , Folks Liesl , Marinero Ernesto E.
Published: July 2002

We have investigated intergrain magnetic interactions in three types of Co-based alloys of interest for longitudinal magnetic recording: CoPtCr, CoPtCrTa, and CoPtCrB. The goal is to quantify how intergranular exchange interactions in CoPtCr alloys are modified by doping said alloys with additives such as Ta and B. These 20-nm-thick alloys were grown under identical deposition conditions on AlMg/NiP substrates and a comprehensive study of their microstructural and magnetic properties was conducted to evaluate the degree of intergranular exchange coupling and the manner how the coupling depends on the corresponding microstructural properties. We find that the ternary alloy exhibits the strongest intergrain coupling. In the case of the quaternary alloys, the degree of exchange interactions is virtually the same. Consequently, any significant difference pertaining recording performance in these quaternary alloys must be dominated by the inherent differences in grain microstructure and the spatial distribution of the alloy constituent elements.
Ion–ion upconversion excitation of the 4f5d configuration in Pr:Y3Al5O12 Experiments and Förster theory-based rate equation model
Authors: Anker David S. , Merkle Larry D.
Published: July 2002

We observe visible pumping of the ultraviolet 4f5d emission of Pr3+ in Y3Al5O12 by ion–ion upconversion. This process contributes significantly to the 4f5d population, indicating that it should be taken into account (along with excited state absorption) when Pr hosts are evaluated for possible upconversion pumped laser operation in the ultraviolet. In addition, we find that a single, time-independent energy transfer parameter cannot explain the observed 4f5d decay wave forms, but that the familiar Förster theory for range-dependent strength of energy transfer can be adapted to explain the main features of our data. Förster theory leads to a formula which the authors believe can be used to assess the potential for upconversion-based lasing on specific transitions.
Optical gain measurements based on fundamental properties and comparison with many-body theory
Authors: Keating T. , Park S. H. , Minch J. , Jin X. , Chuang S. L. , Tanbun-Ek T.
Published: July 2002

We present high accuracy measurements of gain, loss, and transparency energy in long-wavelength semiconductors based on a hybrid approach using the fundamental relationship between the gain and the spontaneous emission spectra. Independent measurements of optical gain, transparency energy, and loss show the accuracy and validity of this technique. These results are compared with those obtained by the non-Markovian gain model with many-body effects under the spontaneous emission transformation method. It is found that the hybrid approach for the gain spectrum alleviates many of the problems related to the poor signal to noise ratio in the amplified-spontaneous emission near and below the band edge. The theoretical spectra compare well with the measured spectra for both the transverse electric and transverse magnetic polarizations.
Optical measurement of the director relaxation time in a periodically reoriented nematic liquid crystal
Authors: Papadopoulos P. L. , Zenginoglou H. M. , Kosmopoulos J. A.
Published: July 2002

The director field of an electrohydrodynamically excited nematic liquid crystal layer exhibits a spatially periodic reorientation, provided the ac voltage applied to the layer is sufficiently close to its threshold value. This fact, combined with the strong optical anisotropy which, as a rule, characterizes any nematic phase, causes the nematic layer to act as a diffraction grating when illuminated by a monochromatic light beam. The time dependence of the intensities of diffraction fringes so formed are a manifestation of the motion of the nematic director. In the present article we propose an experimental method for the determination of the decay time of the nematic director under conditions where the electric field across the nematic layer is temporarily zero. The method is based on a linearized theory of the electrohydrodynamic instability of nematic liquid crystals, as well as, on an optical theory of the diffraction grating action of a periodically reoriented nematic layer. We applied the proposed method in a case where one can compute the respective theoretical value of the relaxation time of the nematic director. The agreement of the experimental result with what one obtains on the grounds of the electrohydrodynamic threshold theory, is found to be fairly satisfactory. Thus, taking advantage of the relatively simple dependence of the measured relaxation time on (a) the experimentally measurable distortion wave number of the electrohydrodynamically excited nematic layer, and (b) the elastic constants and the viscosity coefficients of the nematic material, we get the possibility to obtain further experimental information about the mentioned material parameters, by experimental investigation of the dependence of the director relaxation time on the distortion wave number.
Forces between cavities and dislocations and their influence on semiconductor microstructures
Authors: Myers S. M. , Follstaedt D. M.
Published: July 2002

An approximate continuum method for computing the energy of interaction between cavities and strain fields in complex configurations is described and tested by comparison with results for simple, exactly solvable cases. The method is then used to examine semiquantitatively the effective forces between cavities and screw and edge dislocations, taking into account the effects of surface tension and pressurized gas within the cavity. The discussion encompasses not only local interactions involving individual cavities, but also the combined forces acting upon dislocations in the vicinity of multiple cavities and simultaneously within range of external-surface image forces. The results are used to interpret a range of observed microstructures in semiconductors and to assess the possible exploitation of cavity–dislocation binding for dislocation control in Si–Ge heteroepitaxial structures.
Nonlinear timing shift in high frequency magnetic recording determined with time resolved Kerr microscopy
Authors: Back C. H. , Taratorin A. , Heidmann J.
Published: July 2002

As we move to higher data rates and higher areal densities, nonlinear effects start to determine the data patterns written on a hard disk. In this context it is necessary to develop methods that can help to determine the origin of nonlinear effects, e.g., that can help to distinguish between nonlinear effects caused by areal density from transient effects caused by high data rates. In this article we concentrate on the latter and show data for nonlinear timing shift measured directly on the air bearing surface of recording heads by means of time resolved magneto-optical Kerr microscopy. We compare the data to a simple t/(t+τ) model for the head field rise time.
Laser ablation deposition of metal oxides/nitrides films at room temperature
Authors: Misra A. , Thareja R. K.
Published: July 2002

We report on the role of laser ablated plume size for deposition of aluminum films in oxygen and nitrogen ambient atmosphere. The films were deposited at room temperature and were characterized using x-ray diffraction, scanning electron microscopy, and micro-Raman. The formation of aluminum nitride is attributed to breakdown of nitrogen gas due to high shock temperature and subsequent mixing with aluminum plasma in the shocked region.
Photovoltage effects under sinusoidal illumination
Authors: Weiser K. , Martinez F.
Published: July 2002

A theory is presented for open-circuit voltage (PV) and short-circuit current (SI) effects when a p-n junction diode is illuminated through a slit with a sinusoidal pattern moving across it. The amplitude of PV oscillations is strongly reduced due to annihilation of photocarriers diffusing to the metal contacts on the surface so as to keep the voltage beneath the contacts constant. From the dependence of the oscillation amplitude on the wave vector of the illumination pattern, the lateral diffusion length of the photocarriers can be deduced. The short-circuit currents under sinusoidal and uniform illumination are equal. A combination of PV and SI measurements should reveal anisotropy in the diffusivities of photocarriers in the plane of the diode and perpendicular to it. Recent results on quantum well diodes are discussed.
Growth of particles in cluster-size range in low pressure and low power SiH4 rf discharges
Authors: Fukuzawa Tsuyoshi , Kushima Shinji , Matsuoka Yasuhiro , Shiratani Masaharu , Watanabe Yukio
Published: July 2002

Growth processes of particles in a cluster-size range below a few nm in size in low pressure and low power SiH4 rf discharges are studied using the new method, in which the threshold photoemission method is coupled with the microwave interferometry, for measurements of their size and density. The density of particles is above 1010 cm−3 and much exceeds that of positive ions, the result of which shows that most of them are neutral. The particles grow mainly around the plasma/sheath boundary near the powered electrode and their size growth rate is 3.4–4.4 nm/s, being much higher than a film growth rate of 0.064–0.12 nm/s. These features strongly indicate that their growth is due to deposition of polymerized species, originated from short lifetime SiH2 radicals, on them, while coagulation between particles becomes appreciable after a time when their density reaches about 1011 cm−3. Moreover, the pulse modulation of rf discharge is found to be effective in reducing the density of cluster-size particles. The reduction can be explained by a model taking account of diffusion of the polymerized species through the radical production region, where the particles nucleate and grow.
Van der Waals and capacitive forces in atomic force microscopies
Authors: Saint Jean M. , Hudlet S. , Guthmann C. , Berger J.
Published: July 2002

In this article we show that in the atomic force microscopy experiments performed on a metallic surface, there is always a long range electrostatic force in addition to the van der Waals forces. This capacitive force is due to the contact potential between the tip and the surface and exists even without external applied potential. We have calculated this capacitive force for a real geometry of the tip–sample system and compared it to the van der Waals force calculated for the same geometry. We conclude that the electrostatic force is always dominant for a tip–surface distance larger than half of the tip radius of curvature.
Monte Carlo investigation of transition and compensation temperatures of Fe/Tb multilayers
Authors: Veiller L. , Ledue D. , Teillet J.
Published: July 2002

A Monte Carlo investigation of the A thickness (A≡Fe) dependence of the transition and compensation temperatures of a simple cubic Heisenberg A/B bilayer is carried out. Our model, which includes a few mixed planes of the A1−xBx type that represent the disordered interfaces, basically consists of several coupled magnetic parts with different bulk transition temperatures. Numerical results are compared with the experimental data for amorphous Fe/Tb multilayers. The simulated Fe thickness (tFe) dependence of TC, which reproduces the decrease of TC as tFe increases, is consistent with the experimental one. Since our simulations indicate that the magnetic ordering is driven by the disordered interfaces, the decrease of TC is explained by the fact that the interfaces are more and more decoupled as tFe increases. The experimental thermal behavior of each sublattice magnetization can be reproduced with a Tb-magnetic moment of 6μB which seems to be reasonable for amorphous layers, whereas a value of 9μB is too large. Accordingly, a compensation point has been observed for a Fe thickness range that agrees with the experimental one. The simulated and experimental Fe thickness variations of Tcomp are in reasonable agreement.
Transport properties of high-Tc planar Josephson junctions fabricated by nanolithography and ion implantation
Authors: Katz A. S. , Woods S. I. , Dynes R. C.
Published: July 2002

We have developed a process for fabrication of planar high-Tc Josephson junctions using nanolithography and a 200 keV ion implanter. Conduction occurs in the ab plane and has no metallurgical interface. Devices may be tuned to operate at temperatures between 1 K and the Tc of the undamaged superconducting material by varying the length of the weak link or by changing the amount of ion damage. We have examined the normal state and superconducting properties of these films and find behavior consistent with a de Gennes dirty limit proximity effect model. Current–voltage curves, Ic(T) and R(T) data suggest a temperature dependent superconducting-normal metal interface that we have incorporated into the proximity effect model.
Scanning tunneling microscopy investigation of single electron tunneling in Co–Al–O and Cu–Al–O granular films
Authors: Takanashi K. , Mitani S. , Chiba J. , Fujimori H.
Published: July 2002

We have investigated single electron tunneling in Co–Al–O and Cu–Al–O granular films using scanning tunneling microscopy (STM). Topographic images show well-defined granular structures where nanometer-sized metal granules are embedded in insulating matrix. The Coulomb staircases in the current–voltage (I–V) curves are clearly observed even at room temperature in both films. For the Co–Al–O film, furthermore, negative differential conductance appears in the Coulomb staircase.
Magnetic and structural properties of SmCo7−xCux alloys
Authors: Al-Omari I. A. , Yeshurun Y. , Zhou J. , Sellmyer D. J.
Published: July 2002

We report the structural and magnetic properties of SmCo7−xCux, where x=0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.7. X-ray diffraction shows that these alloys from the disordered hexagonal TbCu7-type structure. For large values of x(x⩾0.8) the hexagonal TbCu7-type structure cannot be formed. X-ray diffraction on magnetically aligned samples show that these samples have uniaxial anisotropy. The lattice parameters (a and c) are dependent on the Cu concentration, and the unit cell volume is found to increase with x. The saturation magnetization decreases with x at both room temperature and 25 K. The Curie temperature increases with x for small values of x while it decreases with x for large values of x. A maximum value of TC=852 °C is found in these alloys.
Enhanced room temperature magnetoresistance response in textured La0.7Sr0.3MnO3 strips made by pulsed laser deposition
Authors: Cadieu F. J. , Chen Li , Li Biao , Theodoropoulos T.
Published: July 2002

Magnetoresistive La0.7Sr0.3MnO3 polycrystalline films have been deposited by high laser pulse energy pulsed laser deposition that exhibit an extreme variability in the temperature of the resistance maximum, Tp, as a function of the deposition conditions. Films have been deposited that exhibit the resistance maximum at temperatures ranging from 155 to 360 K as a function of laser pulse energy and shadowing conditions. High laser pulse energies, shadowing to eliminate film particulates, and a specific laser pulse rate range, were necessary to synthesize films with the temperature of the resistance maximum elevated to room temperature. The room temperature low field magnetoresistive response of samples with Tp approximately room temperature was highly anisotropic with respect to applied magnetic fields in plane and perpendicular to the plane. The (110) textured films exhibited an enhanced low field magnetoresistance compared to films with appreciable (111) texture.
Modeling the magnetoimpedance in anisotropic wires
Authors: Ménard D. , Melo L. G. C. , Britel M. R. , Ciureanu P. , Yelon A. , Rouabhi M. , Cochrane R. W.
Published: July 2002

We have developed a theory of giant magnetoimpedance (GMI) in ideal anisotropic magnetic wires, which is valid over a broad field and frequency range. The emphasis is put on the moderate frequency GMI response in the low field region, where the wire is not saturated. The model agrees with experimental data on amorphous CoFeSiB wires, over broad frequency and field ranges, but does not correspond to an experiment at low field.
Observation of quantum-confined Stark shifts in SiGe/Si type-I multiple quantum wells
Authors: Li Cheng , Yang Qinqing , Wang Hongjie , Wei Hongzheng , Yu Jinzhong , Wang Qiming
Published: July 2002

Quantum-confined Stark shifts in SiGe/Si type-I multiple quantum wells are suggested by the bias dependence of the photocurrent spectra of p-i-n photodiodes. Both Stark redshift and blueshift have been observed for the same sample in the different ranges of electric fields applied to the quantum wells. The turnaround point corresponds to a certain electric field (named “critical” field). This phenomenon was generally predicted by Austin in 1985 [Phys. Rev. B 31, 5569 (1985)] and calculated in detail for SiGe quantum structure by Kim recently [Thin Solid Films 321, 215 (1998)]. The critical electric field obtained from the photocurrent spectra is in reasonable agreement with the theoretical prediction.
The equilibrium solvation structure for the solvent-separated hydrophobic bond
Authors: Zichi Dominic A. , Rossky Peter J.
Published: July 2002

We present a detailed geometrical analysis of the solvent in an aqueous solution containing two nonpolar atomic solutes which are constrained to an interatomic distance corresponding to a solvent-separated free energy minimum. The results are obtained from a molecular dynamics simulation using ST2 water. The interpretation of structural features attributed to the hydrophobic effect are facilitated by a time averaging procedure which removes the high frequency librational excitation from the underlying liquid structure. Solvent reordering within the hydration shell surrounding the solutes is clearly revealed by octahedral resolution of orientational correlations and is consistent with the widespread view of ‘‘clathrate-like’’ structure promotion induced by apolar moieties. Of particular interest, the relatively lengthy simulation reveals a rich variety of cage-like solvation shell structures which encompass the solvent separated solute particles. The librational freedom of the shell molecules is briefly examined; the simulation reveals a decrease in librational amplitudes for the shell solvent.
Corresponding states analysis for viscosity and nuclear magnetic resonance data in the glass transition regime
Authors: Rössler Ernst
Published: July 2002

The viscosity as a function of the temperature has been reanalyzed within a concept of corresponding states for organic glass-forming liquids including the viscous and fluid regime. A universal behavior is found for the viscosity in the range of 103 and up to 1014 P when the absolute temperature is reduced by the glass transition temperature Tg for each liquid. The master plot is significantly improved when minor corrections of Tg are introduced. Below 103 P, the liquids are characterized by individual properties. However, a power-law behavior, i.e. η=B+A(T−Tc)−γ, is found above a critical temperature Tc as predicted by the recent mode coupling theory with Tc =1.20 Tg for all studied systems. The exponent γ is found to be almost constant, i.e., γ=2.84±0.40, whereas A and B vary for the different systems. Hence, two different dynamical regimes above and below 103 P—separated by Tc —exist for a supercooled liquid which have to be described independently. In contrast to the cross-over regime near Tc , the curvature of η(T) above 103 P is rather small and several analytical descriptions have been checked to describe the master curve, but still not enough data are available to discriminate unambiguously between different approaches. The corresponding state analysis also holds for rotational correlation times τ in the range 10−12 s≤τ≤1 s as revealed by nuclear magnetic resonance (NMR) for three recently investigated organic liquids. In addition, the characteristic features of the nonexponential motional correlation function which have been used to describe the NMR results are discussed.
Chemically induced hyperthermal surface ionization
Authors: Danon Albert , Amirav Aviv
Published: July 2002

Hyperthermal beams of cyclohexane and carbon tetrachloride were scatte from a 2% W−Th filament. Chlorine negative ions generated by chemically induced hyperthermal surface ionization were monitored by quadrupole mass spectrometer. The surface temperature of the filament was 2400K. (AIP)
Rotational spectra, dipole moment, and structure of the SiF4–NH3 dimer
Authors: Ruoff R. S. , Emilsson T. , Jaman A. I. , Germann T. C. , Gutowsky H. S.
Published: July 2002

Rotational spectra of several isotopic species of the SiF4–NH3 dimer were obtained with the Mark II Flygare–Balle FT-MW spectrometer. This is the first determination of the gas phase structure of a penta-coordinated silicon. The spectra indicate a symmetric top, trigonal bipyramid SiF3N structure with the lone electron pair of the NH3 pointed at one face of the SiF4 and the three equatorial F’s splayed significantly away from the N. The ∠Fax–Si–Feq is about 12° less than tetrahedral. The Si–N distance is determined to be 2.090 Å. The experimental results are in excellent agreement with the SiF4 deformation predicted by the ab initio calculations of Rossi and Jasinski [Chem. Phys. Lett. 169, 399 (1990)]. The measured dipole moment is 5.61 D, an enormous enhancement compared to the sum of the monomer moments, 1.47 D. The increase of 4.14 D is due largely to the geometric distortion of the tetrahedral SiF4 molecule upon dimer formation, demonstrating that the Si–F bond is much more ionic than covalent. If the charge is simply partitioned between Si and F so as to obtain a moment of 4.14 D, the Si has a charge of +3.52 and each F, −0.88. This conclusion is consistent with recent applications of the atoms-in-molecules charge partition methodology developed by Bader and co-workers.
Indirect nuclear spin–spin coupling constants from multiconfiguration linear response theory
Authors: Vahtras Olav , Ågren Hans , Jo/rgensen Poul , Jo/rgen Aa. Jensen Hans , Padkjær So/ren B. , Helgaker Trygve
Published: July 2002

We have used multiconfiguration self-consistent-field theory to determine indirect nuclear spin–spin coupling constants. The Fermi contact, spin dipole, and paramagnetic spin–orbit contributions are evaluated as multiconfiguration linear response functions at zero frequency and the diamagnetic spin–orbit contribution as an average value of the multiconfiguration wave function. Sample calculations on HD and CH4 demonstrate that most of the correlation contributions can be recovered in relatively small complete active space (CAS) reference state calculations.
Chaos and fractals in the turning point search problem
Authors: Chatterjee Reema , Sathyamurthy N.
Published: July 2002

Occurrence of chaos and fractals in the solution to the problem of turning points for a rotating vibrator is pointed out.
An adiabatic dynamical simulation study of the Zundel polarization of strongly H-bonded complexes in solution
Authors: Borgis D. , Tarjus G. , Azzouz H.
Published: July 2002

We study, by an adiabatic dynamical simulation technique, a mixed classical-quantum model for strongly H-bonded complexes in polar solvents. The solvent influence on the adiabatic proton dynamics is interpreted in terms of a protonic polarization effect, usually referred to as the Zundel polarization. The relation to the solvent-induced proton transfer and the consequences on the broadening of the infrared absorption spectrum are discussed. We show that for increasing solvent-complex coupling, the system passes from an ‘‘oscillatory’’ to a ‘‘reactive’’ behavior, whereas the Zundel polarization passes from a familiar electronic-like regime to a saturated regime. In the latter case, a large band broadening, comparable to experimental observations, is obtained in the calculated infrared spectrum.
Viscosity of self-assembled fluids
Authors: Mundy C. J. , Levin Y. , Dawson K. A.
Published: July 2002

We consider a Hamiltonian for a complex fluid. The linear excess bulk viscosity due to a spatially varying order parameter is investigated. We calculate the dependence of the excess viscosity Δη, on the fundamental parameters d and ξ, everywhere within the disordered phase (d being the domain size of water or oil and ξ being the correlation length between domains). It is found that there is a crossover region in the isotropic disordered phase of this model. More specifically, in the structured region of the isotropic disordered phase Δη grows as ξ3 in the vicinity of disordered-lamellar transition. This is quite distinct from the behavior of the excess viscosity in the unstructured region where Δη grows as ξ in the vicinity of the order-disorder transition.
Self-consistent-field theory for hard-sphere chains close to hard walls
Authors: Björling Mikael , Linse Per
Published: July 2002

A continuum theory for confined hard-sphere polymers is presented. Starting from fundamental relations and applying defined approximations, a constitutive relation for the conformation probability (analogous to the result in mean-field lattice theories) is developed. The main problem of hard-sphere correlations is attacked by two approximate methods: First, using the Carnahan–Starling equation of state and local volume fractions (CS). Second, by an extension of the lattice theory to spherical components with unequal volumes (LATT). The agreement with Monte Carlo simulations is good for both approximations at low densities, but becomes only qualitative at the higher concentrations. The CS approximation seems to be favored over the LATT approach at the higher concentrations when correlation becomes more important. Both free and grafted chains are treated. The influence of chain length, grafting density, solvent concentration, solvent chain length, and surface curvature on the segment distribution is investigated.
Thermodynamic perturbations of molecular systems
Authors: Demetrius Lloyd
Published: July 2002

This note invokes thermodynamic perturbation theory to provide explicit expressions for the changes in free energy and entropy as a result of small variations in the structure of complex molecular systems. We use these perturbation methods to establish, under certain kinds of variations, correlations between the changes in free energy and the changes in entropy. The analytic results described in this note are intended to complement the procedures being implemented in J. Mol. Biol. 224, 771 (1992) to experimentally assess the effect of mutations on the stability of certain proteins.
Diode laser probing of CO2 and CO vibrational excitation produced by collisions with high energy electrons from 193 nm excimer laser photolysis of iodine
Authors: Hewitt Scott A. , Zhu Lei , Flynn George W.
Published: July 2002

Time domain diode laser absorption spectroscopy has been used to measure vibrational, rotational, and translational excitation of CO2 and CO following excimer laser photolysis of iodine in a low pressure mixture of CO2 and I2 or CO and I2. Nascent rotational population distributions have been measured in a number of low-lying CO2 vibrational levels, including 0001, 1000, 0200, 0220, and 0002 as well as the v=1 level of CO. In addition, measurements of CO2 translational excitation have been obtained for the majority of the rovibrational states probed. Significant vibrational excitation of CO2 has been observed with almost no increase in rotational and translational energy of the molecule. These results are consistent with the production of vibrationally excited CO2 via collisions with hot electrons which arise from multiphoton ionization of I2. Direct detection of the electrons has been accomplished using a time-resolved magnetic induction technique.
Two-photon spectroscopy of the F 1Πg and f 3Πg states of molecular fluorine
Authors: Faris Gregory W. , Dyer Mark J. , Huestis David L. , Bischel William K.
Published: July 2002

We report what is to our knowledge the first two-photon excitation spectroscopy of molecular fluorine. The F 1Πg and f 3Πg states are excited with two photons in the range of 206–212 nm. Detection is through vacuum ultraviolet (VUV) fluorescence or ionization. Measurement of the fluorescence spectrum with a VUV spectrometer indicates that the VUV fluorescence occurs on the 157 nm F2 laser transition frequency. Using our spectra and the data of Porter [J. Chem. Phys. 48, 2071 (1968)] we report improved spectroscopic constants for vibrational levels in the F, f, and I 1Σu+ states.
Degenerate ground states of simple slip-link systems
Authors: Sommer Jens-Uwe
Published: July 2002

The free energy of polymer chains connected by slip links has been investigated in this work. For the problem of two network chains connected by a single slip link a degeneracy of the free energy surface above the slip-link configuration space was obtained. The degeneracy depends on the lateral separation d of the chains. In the case of d≥dc, where dc is some critical distance, the degeneracy vanishes. The reason for this nontrivial free energy behavior is the concurrence of two entropic forces. One is the usual force against stretching of the chains parts and the other is the attempt by system to reduce the influence of the constraint caused by the slip link. For chain configurations with more than one slip link, a tendency for the slip links to form clusters to get a higher amount of entropy can be shown.
Molecular dynamics simulation of liquid–solid phase transition of cyclohexane. I
Authors: Brodka A. , Zerda T. W.
Published: July 2002

Molecular dynamics simulations for the six-center Lennard-Jones model of C6H12 are reported. The potential parameters have been adjusted to fit cyclohexane properties along the saturated vapor pressure curve. The behavior of cyclohexane molecules in the bulk liquid and in the plastic crystal state is studied. Satisfactory agreements between experimental data and calculated heat capacities, diffusion coefficients, rotational and angular velocity relaxation times, as well as radial distribution functions, are reported.
Rotationally resolved photoelectron spectra in resonance enhanced multiphoton ionization of SiF
Authors: Wang Kwanghsi , McKoy V.
Published: July 2002

Results of calculations of rotationally resolved photoelectron spectra for resonance enhanced multiphoton ionization (REMPI) of SiF via the B 2Σ+ (4sσ), C″ 2Σ+ (4pσ), and C′ 2Π (4pπ) Rydberg states are reported. In addition to the expected ΔN=even peaks, unusually strong ΔN=±1 transitions are predicted for photoionization of the B 2Σ+ state. These unusual transitions are due to even angular momentum components of the photoelectron matrix element and arise from the formation of Cooper minima in the ionization channels and strong l mixing in the electronic continuum induced by the nonspherical molecular ion potential. Unexpected ΔN=0,±2 transitions, due to odd wave contributions to the photoelectron matrix element, are also predicted for photoionization of the C″ 2Σ+ state. Asymmetrical ion distributions with respect to ΔN=0 are also predicted for the C′ 2Π state. Cooper minima are predicted to occur in the l=2 wave of the kπ photoelectron channel for the B state and in the l=4 wave of the kσ and kπ channels for the C″ state. Photoelectron angular distributions provide further insight into the photoionization dynamics.
Characterization of excited electronic states of naphthalene by resonance Raman and hyper-Raman scattering
Authors: Bonang C. C. , Cameron S. M.
Published: July 2002

The first resonance Raman and hyper-Raman scattering from naphthalene are reported. Fourth harmonic of a mode-locked Nd:YAG laser is used to resonantly excite the 1B1u+ transition, producing Raman spectra that confirm the dominance of the vibronically active ν28 (b3g) mode and the Franck–Condon active ag modes, ν5 and ν3. A synchronously pumped stilbene dye laser and its second harmonic are employed as the excitation sources for hyper-Raman and Raman scattering from the overlapping 1B2u+ and 1Ag− states. The Raman spectra indicate that the equilibrium geometry of naphthalene is distorted primarily along ν5, ν8, and ν7 normal coordinates upon excitation to 1B2u+. The hyper-Raman spectrum shows that ν25 (b2u) is the mode principally responsible for vibronic coupling between the 1Ag− and 1B2u+ states. The results demonstrate the advantageous features of resonance hyper-Raman scattering for the case of overlapping one- and two-photon allowed transitions. Calculations based on simple molecular orbital configurations are shown to qualitatively agree with the experimental results.
Coherent scattering of the hydrogen dimer from a LiF crystal
Authors: Tepper Gary , Miller David
Published: July 2002

A molecular beam of hydrogen dimers is scattered from a LiF(001) crystal at 48 meV. About 5% of the dimers are found to scatter elastically and appear in well-defined coherent first-order diffraction peaks. Debye–Waller experiments are used to estimate the amount of thermal phonon interaction and, together with elastic scattering, accounts for about 20% of the scattering. The remaining 80% are believed to be fragmented and lost due to exchange of translational-internal energy within the dimer bond in an elastic collision with the surface. Para enriched hydrogen beams are used to show that para dimers, despite having a larger binding energy, are more likely to fragment during surface scattering than ortho dimers. The survival of trimer and higher clusters is at least an order of magnitude less than the dimer so that the diffractive scattering provides a filter to form well-defined pure dimer beams.
Potential energy surface and vibrational analysis along the stretching vibrations of XeHXe+ ion
Authors: Nieminen J. , Kauppi E. , Lundell J. , Kunttu H.
Published: July 2002

An analytical potential energy surface (PES) along the stretching coordinates of a linear XeHXe+ ion is presented. Ab initio calculations within the effective core potential approach are used as input for the PES. The present vibrational analysis indicates extensive mixing of the zeroth-order harmonic oscillator vibrational states, and a rather complete collapse of the normal mode picture already near the bottom of the potential well. At higher vibrational energies, and elongated Xe–Xe distances, development of a double minimum in the PES is observed. The simulated absorption spectrum consists of a strong vibrational progression near 1000–1700 cm−1, and is in qualitative agreement with the previous matrix isolation data. The intensity distribution of the vibrational progression is mostly due to the potential terms rather than nonlinear contributions in the Taylor series expansion of the electric dipole moment. Due to the highly anharmonic potential, and subsequent breakdown of selection rules, the emission spectrum is predicted to be particularly rich in the 850–1500 cm−1 spectral region. Several of the vibrational transitions possess transition dipoles exceeding 1 D.
Methyl rotational potentials and transferable pair potentials in toluene
Authors: Prager M. , Monkenbusch M. , Ibberson R. M. , David W. I. F. , Cavagnat D.
Published: July 2002

The crystal structure of α-phase toluene at 5 K has been refined from high resolution neutron powder diffraction data and the charge distribution in the molecule calculated ab initio. From these data, methyl rotational potentials of the two inequivalent methyl groups have been calculated for various parametrizations of transferable pair potentials and compared with potentials derived from tunneling and librational transitions. The results demonstrate that the inclusion of Coulomb interactions is necessary to reproduce the equilibrium orientations of the methyl groups. However, none of the model pair potentials yield the very strong sixfold contribution of the rotational potential of methyl group 1. The best approximation to the experimentally determined values is obtained with the pair potential parameters of Kitaigorodskii [Acta Crystallogr. 18, 585 (1965)]. However, by the inclusion of existing results on p-xylene, the average agreement obtained with the parametrization of Claverie [Acta Crystallogr. Part A 31, 448 (1975)] is equally good. Overall, it was found that general improvements can be made if the repulsive parts of the model pair potentials are softened. The presence of two inequivalent methyl groups in toluene which are influenced in different ways by the various contributions to the interaction provides a rigorous test of the modeling techniques.
Auger electron spectroscopy of molecules: Theory for angular and spin correlations with photoelectrons
Authors: Chandra N. , Chakraborty M.
Published: July 2002

This paper introduces a theory to interpret future experiments to simultaneously observe angular distribution of spin-resolved Auger and photoelectrons from a molecule belonging to one of the 32 point groups. The Auger electrons are emitted in the decay of the vacancy created by photoionization. We show that the desired correlation can be completely characterized by 12 parameters which are coefficients of trigonometric functions of the spherical angles of spin quantization directions of two outgoing electrons. The expressions for the parameters themselves, although reduced to the simplest possible forms by using the symmetry properties of the molecular point group to maximum advantage, depend upon bipolar harmonics involving the propagation directions of the Auger and photoelectrons. The angular and spin correlation function thus obtained is completely general and can be readily specialized to any experimental geometry used to observe the ejected electrons. In particular, it is found that for a linear experimental arrangement with spins oriented longitudinally to the respective propagation vectors of the two electrons moving out in opposite directions, the parameters become geometry independent with their number reducing to three, which are now coefficients of the first three Legendre polynomials. Correlation between the spin-quantization directions of Auger and photoelectrons is, on the other hand, described by six parameters which do not depend upon the experimental arrangement. Directional correlation between the two outgoing electrons has also been studied without observing their spins. These angular and/or spin correlation functions are shown to take particularly simpler forms for Auger and photoelectrons emitted from linear molecules. We have applied the procedures developed in this paper to study directional correlation between photoelectrons from 2a1 orbital in a Td molecule and the Auger electrons emitted in the decay of the consequent vacancy.
The importance of high impact parameter interactions in the collision induced dissociation of protonated water clusters by argon using a Wien velocity filter
Authors: Cowen Kenneth A. , Plastridge Bob , Wood Deron A. , Coe James V.
Published: July 2002

High impact parameter, i.e., glancing collisions, are of particular interest in ion beam experiments because ions experiencing such collisions remain available in the ion beam for further experiments. The collisional activation and dissociation processes for glancing collisions of protonated water clusters and argon have been studied with a new and simple, single-stage technique to detect fragment ions using a Wien velocity filter. The technique is specific with regard to the mass of a fragment ion and the mass of the parent from which it originates. A relation is derived and experimentally verified which governs the operation of the device. Absolute values of the attenuation cross section with argon of 11(6), 25(8), 36(11), 47(17), and 66(10) Å have been determined for H3O+, H5O2+, H7O3+, H9O4+, and H11O5+, respectively, at beam energies in the range of 300–1000 eV. Absolute values of the glancing collision induced dissociation cross section of 0.62(4), 4.3(1), and 9.1(6) Å2 have been determined for the detectable fragment ions of H5O2+, H7O3+, and H9O4+, respectively. Branching ratios upon activation by glancing collisions of 0.91:0.09:0 and 0.78:0.18:0.04:0 have been determined for successive loss of waters by H7O3+ and H9O4+, respectively. The one water loss channel predominates. A multiple collision analysis was performed which characterizes the fraction of parent ions which suffer a glancing collision without dissociating or being knocked out of the beam. Our results suggest that the ion beam which emerges from a collision cell can harbor a surprisingly large fraction of parent ions that have obtained a large amount of internal excitation, perhaps ∼0.7 eV per ion suffering a glancing collision.
Surface segregation in polymer blends due to stiffness disparity
Authors: Yethiraj Arun , Kumar Sanat , Hariharan Arvind , Schweizer Kenneth S.
Published: July 2002

The entropy-driven surface segregation of polymer blends is investigated via computer simulations and integral equation theory. The model system is composed of a binary blend at a hard wall where one of the components of the blend is stiffer than the other. It is found that, at meltlike densities relevant to experiments, both simulations and microscopic theory predict the segments of the stiffer chains segregate to the surface.
Delayed appearance of the liquid-condensed phase in 1-octadecanol films on levitated waterdrops
Authors: Frost A. E. , Seaver Mark , Rubel Glenn O.
Published: July 2002

We have observed dramatic delays in the formation of the liquid-condensed phase of 1-octadecanol thin films on the surface of levitated water drops as the drops evaporate into a flowing gas stream. When the aqueous substrate starts with more than 1.0×10−4 M Eu3+ (EDTA), the liquid-condensed phase of 1-octadecanol appears, as expected, at a surface concentration equivalent to one monolayer. As we reduce the initial Eu3+ (EDTA) content, higher and higher surface concentrations of 1-octadecanol are reached prior to the appearance of the liquid-condensed phase. At initial concentrations of Eu3+ (EDTA) below ∼4.0×10−7 M, the average surface concentration at the liquid-condensed phase change exceeds ten monolayers. Light-scattering data attest to the presence of small, <400 nm radius, 1-octadecanol particles initially dispersed throughout the substrate. We demonstrate that these particles are collected by the shrinking drop surface with minimal diffusion of the particles to the surface.
Temperature evolution of the electronic band structure of the undoped and doped regioregular analog of poly(3-alkylthiophenes): A spectroscopic and theoretical study
Authors: Barta P. , Dannetun P. , Stafström S. , Zagórska M. , Proń A.
Published: July 2002

The undoped and ferric chloride p-doped ‘‘head-to-head’’ ‘‘tail-to-tail’’ analog of poly(3-decylthiophene) obtained from substituted bithiophenes, i.e., poly(4,4’didecyl-2,2’bithiophene)-PDDBT has been investigated. The samples were studied by means of ultraviolet photoelectron spectroscopy, optical absorption measurements, and polymer bands structure calculations. Experiments were carried out at different temperatures between 100 °C and −180 °C. The results indicate that PDDBT is nonplanar in the undoped state. No temperature effects have been observed in the π-electron properties. In the doped state, however, lowering of the temperature results in an increase of the system planarity that modifies the electronic band structure. This effect has been shown to be fully reversible with temperature.