bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic properties of very thin single and multilayer NiFeCo and CoFe films deposited by sputtering
Authors: Wang D. , Daughton J. M. , Bussmann K. , Prinz G. A.
Published: July 2002

As the size of the giant magneto resistive (GMR) devices continues to decrease into submicrometer regime, the demagnetizing field from the magnetic layers will increase to very high values, prohibiting switching with a reasonably small current. The most feasible way to reduce it is to make the magnetic layers thinner. Single layers of NiFeCo and CoFe films as well as multilayers consisting of these with various underlayers have been deposited by rf diode sputtering in a magnetic field. NiFeCo becomes nonmagnetic at 15, 10, and 6 Å when using Ta, Si3N4, and Cu underlayers, respectively. The magnetization of NiFeCo films sandwiched with Cu decreases as the films become thinner, indicating that about one atomic layer loses its magnetic moment at each interface with Cu. The induced magnetic anisotropy is a strong function of the NiFeCo film thickness, changing from 7 Oe for 20 Å to 17 Oe for 100 Å. CoFe films lose very little magnetic moment at the interfaces with Cu. Multilayers of [NiFeCo/Cu/CoFe/Cu] with different Cu underlayer thickness have been made and the magnetic and GMR properties show a strong dependence on the Cu buffer layer thickness due to different interface roughnesses. With proper designing of the interfaces, these very thin magnetic films have high potential for ultrahigh-density magnetic memory and other applications.
Influence of Si addition on thermal stability and soft magnetic properties for Fe–Al–Ga–P–C–B glassy alloys
Authors: Mizushima T. , Makino A. , Inoue A.
Published: July 2002

The thermal stability of the supercooled liquid region (ΔTx), defined by the difference between crystallization temperature (Tx) and glass transition temperature (Tg), and soft magnetic properties were investigated for Fe70Al5Ga2P12.65−xC5.75B4.6Six(x=0–4) and Fe77Al2.14Ga0.86P11−xC5B4Six(x=0–3) glassy alloys. The thermal stability, glass forming ability and effective permeability (μe) at 1 kHz are improved with the replacement of P by 1–3 at. % Si for Fe70Al5Ga2P12.65−xC5.75B4.6Six and by 1–2.6 at. % Si for Fe77Al2.14Ga0.86P11−xC5B4Six. The ΔTx and the maximum thickness for glass formation (tmax) reach maximum values of 60 K and 280 μm, respectively, for Fe70Al5Ga2P12.65−xC5.75B4.6Six and 34 K and 220 μm, respectively, for Fe77Al2.14Ga0.86P11−xC5B4Six at Si(at. %)/(Si(at. %)+P(at. %))=0.24. Core losses for Fe77Al2.14Ga0.86P8.4C5B4Si2.6 glassy alloy is much lower than that for amorphous Fe–Si–B alloy at the sheet thickness more than 70 μm. Therefore, it can be said that the Fe–Al–Ga–P–C–B–Si glassy alloys are useful for inductive applications because of their bulky shape and good soft magnetic properties.
Features and technology of ferroelectric electron emission
Authors: Riege H. , Boscolo I. , Handerek J. , Herleb U.
Published: July 2002

Spontaneous electrical polarization of ferroelectric materials can be changed either by reversal or by phase transition from a ferroelectric into a nonpolar state or vice versa. If spontaneous polarization changes are induced at a submicrosecond time scale, strong uncompensated surface charge densities and related fields are generated, which may lead to the intense self-emission of electrons from the negatively charged free surface areas of the ferroelectric cathode. The nature of this self-emission differs essentially from other methods of ferroelectric electron emission and from conventional electron emission in that the latter methods are only achieved by extracting electrons with externally applied electric fields. When electron guns are constructed with ferroelectric cathodes, new design criteria have to be taken into account. The intensity, the energy, the temporal and spatial distribution and the repetition rate of the emitted electron beams can be adjusted within wide limits. The advantages of ferroelectric cathodes and the technological difficulties arising during their production, preparation, and operation are identified and discussed and solutions to the problems are proposed. Experiences with a few applications of ferroelectric electron emission are reported and suggestions for further applications are made.
ON-state reliability of amorphous-silicon antifuses
Authors: Vasudevan N. , Fair R. B. , Massoud H. Z. , Zhao T. , Look K. , Karpovich Y. , Hart M. J.
Published: July 2002

The ON state of metal-to-metal amorphous-silicon antifuses suffers from two reliability concerns: switch-off and dc-stress failure. The switch-off current and dc-stress lifetime are strongly dependent on the temperature of the conducting filament and hence, on the programming current and ambient temperature. Numerical simulations of the filament temperature in the ON state were carried out to explain the experimental characteristics obtained in this work such as the dependence of the switch off and dc-stress failures on ambient temperature, stress current, and programming current. The temperature in the conducting filament is found to increase as the square of the stress current. The temperature and power dissipation at switch off are found to be independent of the programming current. The temperature at switch off is determined to be approximately 1500 °C. The ON-state device lifetime decreases exponentially with increasing stress current and ambient temperature. Numerical simulations of the temperature in the ON state successfully explain the experimentally observed increase in switch-off currents with programming current and the exponential decrease in device lifetime with increasing programming currents, stress currents, and ambient temperature.
Bulk limited conduction in electroluminescent polymer devices
Authors: Campbell A. J. , Weaver M. S. , Lidzey D. G. , Bradley D. D. C.
Published: July 2002

The current–voltage (J–V) characteristics of ITO/polymer film/Al or Au structures of poly(phenylene vinylene) (PPV) and a dialkoxy PPV copolymer have been recorded for a range of different film thickness d and temperatures T. At high applied bias all the characteristics can be fitted over a given range to a power law J=KVm, where m increases with decreasing T, log(K) is proportional to m, and K is proportional to d−α m, where α∼2 (ITO/polymer film/Al devices) and ∼1 (ITO/polymer film/Au devices). Different single carrier space charge limited conduction theories have been used to try and explain this behavior. The analytical theory in which the carrier density is decreased by an exponential trap distribution lying below effectively isoelectronic transport states is in good agreement, but cannot explain the thickness dependence of the ITO/polymer film/Au devices and can be criticized as being physically unreasonable. A numerical analysis in which the mobility has the field and temperature dependence found for hopping transport in disordered systems is also in good agreement, but can only fit a small range of J and cannot explain the magnitude of K, the temperature dependence of m or the abrupt change in slope in the J–V characteristics with increasing bias. Mixed models are equally good but cannot explain the deviations from experiment. We consider that further experimental studies of carrier mobilities and the nature of the traps present in such materials is required to distinguish between these models and resolve the nature of bulk limited conduction in conjugated polymers.
Plasma sheath model and ion energy distribution for all radio frequencies
Authors: Panagopoulos Theodoros , Economou Demetre J.
Published: July 2002

The spatiotemporal structure of the sheath and the ion energy distribution (IED) at the electrode of a collisionless electropositive glow discharge were studied with a model that is valid for arbitrary radio frequencies (rf). The model is based on the work of P. A. Miller and M. E. Riley [J. Appl. Phys. 82, 3689 (1997)] and uses an effective electric field to which the heavy ions respond. Given the plasma density and electron temperature at the sheath edge, and the waveform of either the potential or total current across the sheath, the spatial and/or temporal profiles of the following quantities were obtained: sheath thickness and capacitance, electron and ion densities, potential, and individual components of the current. An analytic expression for the energy split of the IED function was also obtained. The product ωτi of applied radian frequency ω and ion transit time τi is a critical parameter for describing the sheath dynamics.
Te doping of GaInP: Ordering and step structure
Authors: Lee S. H. , Fetzer C. Y. , Stringfellow G. B. , Lee D. H. , Seong T. Y.
Published: July 2002

The donor Te has been added to GaInP during organometallic vapor phase epitaxial growth using the precursor diethyltelluride. In agreement with previous studies, the addition of high Te concentrations leads to the elimination of the CuPt ordering observed in undoped layers. The degree of order is estimated from the low temperature photoluminescence peak energy to decrease from 0.5 at Te concentrations of <2×1017 cm−3 to 0 for Te concentrations of >6×1017 cm−3. This is verified by transmission electron diffraction studies, which show the elimination of the 1/2{111} superlattice spots at high Te doping levels. A remarkable change in the surface structure is found to accompany this decrease in ordering: The surfaces become much smoother. Step bunching is observed to disappear for the vicinal GaAs substrates, misoriented from (001) by 3° in the B direction, and three-dimensional island (or mound) formation is eliminated for the singular (001) substrates. A qualitative model is presented explaining this behavior based on the effect of Te on the step structure and the bonding at step edges, both of which affect the adatom sticking at steps.
Magnetic and acoustic tapping mode microscopy of liquid phase phospholipid bilayers and DNA molecules
Authors: Revenko Irène , Proksch Roger
Published: July 2002

We have constructed a fluid cell for an atomic force microscope that operates in tapping mode using either an oscillating piezo or magnetic drive. This fluid cell allows direct comparison of the image quality using the two drive mechanisms over identical areas of a sample without fluid or cantilever exchange. We found that the magnetically driven cantilever’s tuning curve was very similar to the thermal noise power spectrum, allowing an accurate determination of the cantilever resonance frequency. This is in contrast to the piezo driven tuning curve, which contained a number of peaks that appeared to be a convolution of the true cantilever resonance with the complicated acoustic spectrum of the fluid cell. We imaged two biologically relevant samples: DNA molecules and liquid phase phospholipid bilayers. For both samples, we found that the image quality, as measured by feature height, lateral resolution, and image stability, was independent of the drive method. This suggests that, despite the apparent differences in the frequency response, the physical motion of the cantilever tip, when it is driven near its resonance frequency, is the same for both driving mechanisms.
Voltage shift phenomena introduced by reverse-bias application in multilayer organic light emitting diodes
Authors: Zou Dechun , Tsutsui Tetsuo
Published: July 2002

The effects of reverse-bias on the current–voltage (I–V) and luminance–voltage (L–V) properties of organic light-emitting diodes (OLEDs) were systematically measured. Shifts toward lower voltage both in I–V and L–V curves were observed for the OLEDs treated by reverse-bias application. The voltage-shift phenomena were discussed based on the effective drive voltage change induced by reverse bias application and the voltage-shift process was analyzed by using a relaxation model. Good consistency between experimental results and model calculations was obtained. It was found that the voltage shift process has several time constants. For example, three time constants (t01=17.9 s, t02=507 s, t03=7169 s) exist in the voltage shift process for ITO/TPD/Alq3/Mg:Ag diodes. The voltage shifts are assumed to be closely related to the movement of ionic impurities and rotations of permanent dipoles in organic layers.
On the thermoelectric magnetic field of spherical and cylindrical inclusions
Authors: Nagy Peter B. , Nayfeh Adnan H.
Published: July 2002

It was recently discovered that inclusions and other types of inhomogeneities can be nondestructively detected by thermoelectric measurements in an entirely noncontact way by using high-sensitivity superconducting quantum interference device magnetometers to sense the weak thermoelectric currents around the affected region when the specimen is subjected to directional heating or cooling. In this article we present theoretical models capable of predicting the magnetic field produced by thermoelectric currents around spherical and cylindrical inclusions under external thermal excitation. We investigated how the magnetic signal to be detected depends on (i) the relevant physical properties of the host and the inclusion, (ii) the size of the inclusion, (iii) the depth of the inclusion below the surface of the specimen, (iv) the polarization of the magnetometer, (v) the lift-off distance of the magnetometer from the specimen, and the (vi) direction and (vii) strength of the external heating or cooling applied to the specimen. The analytical models presented are numerically evaluated to illustrate the strength and polarization of the magnetic field for different lift-off distances and inclusion depths.
Background oxygen effects on pulsed laser deposited Na0.5K0.5NbO3 films: From superparaelectric state to ferroelectricity
Authors: Cho Choong-Rae , Grishin Alex
Published: July 2002

Ambient oxygen pressure in a pulsed laser deposition process has been observed to have a critical influence on the compositional, crystalline, and electrical properties of Na0.5K0.5NbO3 (NKN) thin films grown onto polycrystalline Pt80Ir20 and SiO2 (native oxide)/Si(111) substrates. Films prepared at high oxygen pressure (∼400 mTorr) were found to be single phase and highly c-axis oriented. X-ray diffraction θ–2θ scans and rocking curve data show a strong effect of NKN film self-assembling along the [001] direction regardless of the substrate texture. The high dielectric permittivity of 550, low dissipation factor of less than 3%, and high remanent polarization of 12 μC/cm2 indicate the high ferroelectric quality of the fabricated film. The role of the high-energy component of the erosion products has been proven to be crucial to film performance. On the other hand, films grown at low oxygen pressure (∼10 mTorr) have been found to be mixed phases of ferroelectric NKN and paraelectric potassium niobates. These films have shown superparaelectric behavior: 5% tunability at an electric field of 100 kV/cm, losses as low as 0.3%, and excellent stability to temperature and frequency changes.
Micromagnetic study of Ni2MnGa under applied field (invited)
Authors: Pan Qi , James R. D.
Published: July 2002

We present an experimental study of the domain and twin structure in the martensitic phase of the ferromagnetic shape memory alloy Ni2MnGa using magnetic force microscopy. After cooling through the austenite–martensite transformation under no field, a rectangular specimen with {100} faces consistently adopted a structure of fine-scale (101) twins with surface relief, due to demagnetization effects. After cooling under a [010] field of 2 kOe, a completely different twin structure consisting of (110) parallel bands with no surface relief was observed. Under increasing fields between 2 and 8.5 kOe, the following sequence of domain structures were observed: fir tree patterns in twin bands, fir tree patterns localized at twin boundaries, single domains with thick walls (approx. 0.5 μm) coincident with twin boundaries. The domain walls exhibit unusually high contrast, and there is evidence for a wall substructure with irregularly spaced nodes. These observations are consistent with a theoretical micromagnetic study (R. D. James, Q. Pan, R. Tickle, R. Kohn, and M. Wuttig, preprint) in which approximately homogeneous rotation of magnetization occurs in alternate bands at large fields; this rotation reduces the driving force on twin boundaries before the maximum strain is achieved in some tests.
Annealing of Ni80Fe20/Cu and Co/Cu multilayers
Authors: van Loyen Ludwig , Elefant Dieter , Tietjen Detlev , Schneider Claus M. , Hecker Michael , Thomas Jürgen
Published: July 2002

Ni 80 Fe 20 /Cu and Co/Cu multilayers with Cu layer thicknesses corresponding to the first and second antiferromagnetic (afm) coupling maximum were prepared by dc-magnetron sputtering and investigated with respect to their annealing behavior. The as-deposited films showed giant magnetoresistance (GMR) up to 50% at room temperature. The annealing behavior of the multilayers belonging to the same Cu layer thickness is found to be very similar up to 200 °C. For both kind of multilayers with a Cu layer thickness of about 1 nm (first afm maximum) the GMR degrades upon annealing in excess of 140 °C. However, with a Cu layer thickness of about 2 nm (second afm maximum) the multilayers are stable with respect to their GMR properties even after prolonged exposure up to 200 °C. Moreover, in some cases even an improvement of GMR upon the annealing procedure may be found.
Sm–Fe–C nanocomposite magnets prepared from powders of Sm, Fe, and graphite by mechanical alloying
Authors: Geng Dian-yu , Zhang Zhi-dong , Cui Bao-zhi , Zhao Xin-guo , Liu Wei , Guo Zhi-jun
Published: July 2002

The effects of further addition, of carbon on structure and magnetic properties of the Sm20Fe70C10 alloy synthesized by mechanical alloying have been investigated. The main phase is Sm2Fe17Cx or Sm2Fe14C, depending on the composition of starting materials. The Sm2Fe14C phase tends to form when more carbon is added. Remilling and reannealing at a low temperature (600 °C) improve the magnetic properties of the alloys. The ratio of remanence to saturation magnetization as high as 0.79 indicates the existence of enhanced exchange coupling between the hard Sm2Fe17Cx and soft Sm2Fe14C/α-Fe magnetic phases in the Sm20Fe70C15 alloy.
Optics of Si/SiO2 superlattices: Application to Raman scattering and photoluminescence measurements
Authors: Khriachtchev Leonid , Novikov Sergei , Kilpelä Olli
Published: July 2002

A quantitative model of a Si/SiO2 superlattice (SL) is developed and applied to Raman and photoluminescence (PL) measurements. By analyzing the experimental reflection spectra of Si/SiO2 SLs on Si and Al substrates, we obtained optical parameters of amorphous Si layers with thickness below 4 nm. Both refractive index and extinction coefficient are found to decrease with Si-layer thickness, and this behavior reflects interaction of the Si network and the oxide surrounding. Interference-induced modification of Raman scattering and PL is quantitatively studied for Si/SiO2 SLs on Si and Al substrates, and the developed optical model describes well all observed features. PL spectra of the Si/SiO2 SLs are found to change under Ar+-laser irradiation, and this effect of laser annealing becomes stronger for thinner Si layers. For 1-nm-thick Si layers, a prolonged laser exposure decreases the PL intensity at 550 nm by a factor of 10 and red-shifts its maximum by about 50 nm, which indicates essential reorganization of the network under laser irradiation.
Transport properties of Be- and Si-doped AlSb
Authors: Bennett Brian R. , Moore W. J. , Yang M. J. , Shanabrook B. V.
Published: July 2002

Thick epitaxial layers of AlSb(Si) and AlSb(Be) were grown by molecular beam epitaxy and characterized by variable-temperature Hall/van der Pauw measurements. Si is shown to be predominantly an acceptor in AlSb, with an energy level 33±4 meV above the top of the valence band. Be is also an acceptor, with an energy level 38±4 meV above the top of the valence band. Be is a robust doping source for p-AlSb for carrier densities ranging from 1015 to 1019 cm−3. Background impurity levels in AlSb can be assessed by measuring the transport properties of lightly doped AlSb(Be) layers.
Pressure dependence of the absolute rate constant for the reaction Cl+C2H2 from 210–361 K
Authors: Brunning J. , Stief L. J.
Published: July 2002

The pressure dependence of the absolute rate constant for the reaction Cl+C2H2 →M ClC2H2 has been examined at four temperatures ranging from 210–361 K using the flash photolysis-resonance fluorescence (FP-RF) techniques. The data has been fitted to the Troe expression to determine the values of the low pressure, three body rate coefficient k0, and the high pressure limiting bimolecular rate coefficient k∞ at each temperature. The temperature dependence of k0 is given by (3.15±0.30)×10−21 T−3.50±0.05 cm6 s−1 and for k∞ by (1.52±0.15)×10−4 T−2.63±0.05 cm3 s−1. The implication of these results for upper atmospheric chemistry is briefly discussed.
The open chain or chemically bonded structure of H2O4: The hydroperoxyl radical dimer
Authors: Fitzgerald George , Lee Timothy J. , Schaefer Henry F. , Bartlett Rodney J.
Published: July 2002

The straight chain isomer H–O–O–O–O–H of H2O4 is of considerable current interest in combustion and atmospheric chemistry. Ab initio quantum mechanical methods have been used to study the geometrical structure, energetics, and vibrational frequencies of this species. Double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets have been used in this theoretical study, the latter designated O(9s5p1d/4s2p1d), H(4s1p/2s1p). These basis sets have been employed in conjunction with self–consistent field (SCF) and configuration interaction (CI) methods, including variationally up to 470 935 configurations. For the straight chain isomer, stationary points of symmetry C2h , Ci , and C1 have been identified, and correspond to Hessian indices 3,1, and 0, respectively. The equilibrium geometry, having no elements of symmetry at all, is relatively unique. The highest level of theory (unlinked cluster corrected DZ+P CI) predicts the straight chain structure of H2O4 to lie slightly lower in total energy than the cyclic two-hydrogen bond isomer.
Photoelectron spectroscopy and inferred femtosecond intramolecular dynamics of C2H+2 and C2D+2
Authors: Reutt J. E. , Wang L. S. , Pollard J. E. , Trevor D. J. , Lee Y. T. , Shirley D. A.
Published: July 2002

The 584 Å photoelectron spectra of rotationally cold C2H2 and C2D2 were obtained with improved resolution, permitting the first three electronic states of the ions to be characterized in greater detail. Temperature-dependent studies led to a definitive assignment of the low intensity features in the X̃ 2∏u state, yielding ν4=837±12 cm−1 for C2H+2 and ν4=702±12 cm−1 for C2D+2. The ν5 origin of the Renner–Teller multiplet was identified. In the case of C2D+2, a Fermi resonance with this multiplet contributed intensity to the ν1 mode, facilitating its evaluation at 2572±16 cm−1. The C2h geometry of the Ã 2Ag state was determined from the two previously unobserved bending progressions, assigned to ν4 and ν5B, and evaluated at 492±12 and 605±12 cm−1 for C2H+2 and 339±12 and 516±12 cm−1 for C2D+2, respectively. A more extensive vibrational progression than previously evident, comprised of irregular spectral features indicative of nonadiabatic effects, was observed for the B 2∑+u state. Autocorrelation functions were derived from the spectra for all three electronic states, and the two electronically excited states exhibit an ultrafast decay on a 10−14 s time scale. The Ã 2Ag state decays within one period of bending vibration, while the B̃ 2∑+u state survives only 14 fs, corresponding to a single period of symmetric stretching motion.
Transient electric birefringence of dilute rigid-body suspensions at low field strengths
Authors: Wegener William A.
Published: July 2002

Transient electric birefringence expressions are derived in the Kerr limit of low field strengths for a dilute monodisperse suspension of arbitrarily shaped rigid bodies. A body is characterized by its diffusion tensors, its optical anisotropy tensor, its permanent and instantaneously induced electric dipoles, and its electric charge; no special relation is assumed to hold between these quantities. We specifically allow for charged macromolecules with screw-like shapes which will rotate as well as translate under the exertion of an electric field force. We consider the rise of the birefringence as the field is turned on, its decay following field turn off, and its behavior following reversal of field direction. The birefringence expressions generally involve eight relaxation times during rise and reversal, with only five of these present during decay. The expressions for reversal and decay do not assume prior steady-state alignment; the preexponential amplitudes involved explicitly depend on the duration of the preceding buildup. By varying this duration, different birefringence modes involved in reversal and decay can be selectively weighted according to their relaxation times. Certain cases of symmetry are considered which reduce the number of distinct contributing terms. As an example, we examine the behavior of a long bent rod.
The classical and nonclassical forms of protonated acetylene, C2H+3. Structures, vibrational frequencies, and infrared intensities from explicitly correlated wave functions
Authors: Lee Timothy J. , Schaefer Henry F.
Published: July 2002

The vibrational frequencies and IR intensities of C2H+3 and C2D+3 have been predicted using the methods of ab initio molecular quantum mechanics. Specifically, configuration interaction (CI) wave functions have been constructed from double zeta plus polarization (DZ+P) basis sets. In addition, a much larger basis set, triple zeta plus diffuse plus double polarization, has been used at the self-consistent-field (SCF) level of theory. The results are qualitatively similar to those (at much lower levels of theory) reported earlier by Raine and Schaefer. At the highest level of theory, CI including all single, double, and triple excitations (213 780 configurations for the classical structure), the nonclassical structure is the lower by only 0.7 kcal/mol.
High-order correction to the Trotter expansion for use in computer simulation
Authors: Li Xiao-Ping , Broughton Jeremy Q.
Published: July 2002

A convenient form of the generalized Trotter formula for path-integral simulations has been investigated for various systems and proves to be much better than the primitive. The hydrogen atom, which is pathological for the primitive algorithm, is solved by the new method but the results are not completely satisfactory.
Analysis of the visible absorption and SERS excitation spectra of silver sols
Authors: Fornasiero Daniel , Grieser Franz
Published: July 2002

The absorption spectra of silver sols in various stages of particle aggregation, as well as the surface-enhanced Raman scattering (SERS) excitation of citrate adsorbed on the colloid particles have been deconvoluted into a set of Gaussian bands. The absorption wavelength variation upon aggregation of the silver sols can be satisfactorily explained using an exciton model and a linear arrangement of the aggregated silver particles. Evidence is presented which shows SERS is only observed from excitation into the absorption bands associated with the aggregated colloid particles. A linear relationship was found to exist between the intensities of the aggregate absorption band and the SERS signal.
Sulfur and selenium helices: Structure and electronic properties
Authors: Springborg M. , Jones R. O.
Published: July 2002

Density functional calculations of the energy surfaces of infinite helical chains of selenium have been performed for a wide range of geometries using the parameter-free local density approximation. The results are compared with our earlier calculations for the sulfur helix S∞ for which the calculated ground state geometry is in good agreement with experiment. The energy surfaces of both molecules are flat over large regions of configuration space—consistent with the extreme flexibility of these molecules—and are characterized by a variety of local minima and saddle points. Stretching each molecule results in planar zig-zag and linear geometries with overlapping conduction bands. The energy differences between helical and planar minima are small (∼0.2 eV).
Viscosity of dilute polyelectrolyte solutions
Authors: Cohen J. , Priel Z. , Rabin Y.
Published: July 2002

We have developed an apparatus which enables us to perform accurate measurements of the shear viscosity of low ionic strength, dilute polyelectrolyte solutions, down to polymer concentrations below one part per million. We have shown that a theoretical expression for the viscosity of such solutions can be derived using the mode–mode coupling approximation to the hydrodynamics of charged Brownian spheres. Very good agreement between the predicted and observed polymer and salt concentration and molecular weight dependence of the viscosity is observed in the low-added salt, dilute solution range. Furthermore, it appears that the theory gives a qualitatively correct description of the viscosity of semidilute solutions, indicating that independent of polyion concentration, the hydrodynamics of low ionic strength polyelectrolyte solutions is dominated by electrostatic repulsion between polyions.
Femtosecond real-time probing of reactions. II. The dissociation reaction of ICN
Authors: Dantus Marcos , Rosker Mark J. , Zewail Ahmed H.
Published: July 2002

Experimental results obtained for the dissociation reaction ICN*→[I⋅⋅⋅CN]‡*→I+CN using femtosecond transition-state spectroscopy (FTS) are presented. The process of the I–CN bond breaking is clocked, and the transition states of the reaction are observed in real time. From the clocking experiments, a ‘‘dissociation’’ time of 205±30 fs was measured and was related to the length scale of the potential. The transition states live for only ∼50 fs or less, and from the observed transients we deduce some characteristics of the relevant potential energy surfaces (PES). These FTS experiments are discussed in relation to both classical and quantum mechanical models of the dynamical motion, including features of the femtosecond coherence and alignment of fragments during recoil. The observations are related to the radial and angular properties of the PES.
Ordering of the O–O stretching vibrational frequencies in ozone
Authors: Scuseria Gustavo E. , Lee Timothy J. , Scheiner Andrew C. , Schaefer Henry F.
Published: July 2002

The ordering of ν1 and ν3 for O3 is incorrectly predicted by most theoretical methods, including some very high level methods. The first systematic electron correlation method based on one-reference configuration to solve this problem is the coupled cluster single and double excitation (CCSD) method. However, a relatively large basis set, triple zeta plus double polarization (TZ+2P), is required. Comparison with other theoretical methods is made.
Direct measurements of energy transfer between identical chromophores in solution
Authors: Kim Y. R. , Share P. , Pereira M. , Sarisky M. , Hochstrasser R. M.
Published: July 2002

The rate of intramolecular electronic excitation energy transfer in 9,9′ bifluorene has been investigated in a variety of solvents, using both time-correlated single photon counting and femtosecond fluorescence upconversion technique. The kinetics of energy transfer were determined in both cases by time dependent fluorescence anisotropy measurements. The energy transfer dynamics between fluorene moieties has been found to occur on a time scale of approximately 600 fs in different solvents and has been correlated with the T2 value calculated from the absorption linewidth and the β value obtained from jet measurements. The dihedral angle between the fluorene moieties was also calculated from the anisotropy measurement and compared with the values obtained from a solution phase NMR determination.
Three point extension for hypernetted chain and other integral equation theories: Numerical results
Authors: Lozada-Cassou Marcelo , Díaz-Herrera Enrique
Published: July 2002

In a previous paper [M. Lozada-Cassou, J. Chem. Phys. 80, 3344 (1984)], we have proposed a three point extension for integral equation theories. Here we apply our formalism to the interaction of two charged plates of infinite extension, immersed in an electrolyte, and solve the three point extension to the hypernetted chain/mean spherical theory. We calculate the ionic profile around the plates and the pressure between the plates, as a function of distance between them, for a 1:1 and 2:2 electrolyte for different concentrations and potentials on the plates. We compare our results with the Verwey–Overbeek (VO) theory. We find excellent agreement with the VO theory for low potentials and concentrations. However, there is qualitative disagreement for higher potentials and/or concentrations. The interaction force between the plates becomes attractive at a sufficiently high potential and/or concentration. Since the VO force is always repulsive, in our theory the attraction is a consequence of the ionic size. As one would expect from an electrostatically correct theory, the charge on the plates depends on the distance between the plates.
A recursive density functional formalism for nonuniform fluids
Authors: Zhang M. Q. , Percus J. K.
Published: July 2002

The free energy of a classical nonuniform fluid is developed in a density functional expansion defined recursively from mean field theory as a starting point. Capillary wave contributions to the structure of a two-fluid interface appear explicitly in the first order. The first order result is worked out as well in the general case of slowly varying density. Comparison with potential ensemble theory is also made.
Vibrations in small Mg clusters
Authors: Lee Timothy J. , Rendell Alistair P. , Taylor Peter R.
Published: July 2002

The equilibrium geometries, binding energies, and harmonic frequencies of Mg3 and Mg4 have been determined using large atomic natural orbital basis sets in conjunction with high levels of electron correlation. The correlation treatments comprise multireference configuration interaction (MRCI), singles and doubles coupled-cluster (CCSD) theory and the CCSD(T) extension that includes a perturbational estimate of connected triple excitations. As with our previous studies of small Be clusters, the CCSD(T) method is found to reproduce the MRCI results with a remarkable degree of accuracy and at a fraction of the computational cost. Using the CCSD(T) method, full quartic force fields for Mg3 and Mg4 have been determined and anharmonic analyses have been performed using second-order perturbation theory. Vibration–rotation interaction constants and centrifugal distortion constants have also been determined. The fundamental vibrational frequencies are predicted to occur at ν1 (a′1)=96 and ν2 (e′)=104 cm−1 for Mg3, and ν1 (a1)=184, ν2 (e)=143, and ν3 (t2)=167 cm−1 for Mg4. Infrared (IR) intensities, evaluated for the IR active modes via the double harmonic approximation, are 0.2 and 2.4 km/mol for the e′ (Mg3) and t2 (Mg4) modes, respectively.
Optical–optical double resonance of NO2 in the region of 612–614 nm: The role of 2A1 vibronic levels as dark and perturbing state
Authors: Nagai Hidekazu , Shibuya Kazuhiko , Obi Kinichi
Published: July 2002

An optical–optical double resonance (OODR) technique has been applied to the rotational analysis and vibronic assignment of NO2 absorption band in the region of 612–614 nm. The two step excitation through 2 2B2←2B2←X̃ 2A1 has allowed us to determine rotational quantum numbers (NKa,Kc) for 73 eigenstates with B2 vibronic symmetry, lying at 16 306–16 465 cm−1 above the ground state. Although they are severely perturbed and irregular in the rotational structure and spin doubling, we can classify the rovibronic levels as four stacks; two Ka=0 stacks with subband origins of 16 306.2 and 16 321.0 cm−1, and two Ka=1 stacks with origins of 16 312.5 and 16 326.0 cm−1. A near-prolate asymmetric top approximation is used to obtain the term values and rotational constants. Extraordinary large DN measured for 2B2 vibronic levels can be understood by well-known, strong vibronic coupling between Ã 2B2 and highly excited vibrational levels of X̃ 2A1. Among a number of perturbations observed, the spin–orbit (and/or orbital–rotation) coupling between ‘‘light’’ 2B2 and ‘‘dark’’ 2A1 vibronic levels is clearly shown for the first time by analyzing intensity patterns of the ν2 -scanned (second step) OODR spectra.
New method for calculating wave packet dynamics: Strongly coupled surfaces and the adiabatic basis
Authors: Manthe U. , Köppel H.
Published: July 2002

The nuclear dynamics on potential energy surfaces with a conical intersection is investigated on the basis of exact (numerical) integration of the time-dependent Schrödinger equation. The ethylene cation is chosen as a typical realistic model system. Complementing earlier work we study the dynamics also in the adiabatic basis, which will be seen to allow for a more profound understanding of the decay and dephasing processes occurring in the system. The computational effort exceeds considerably that of propagation in the diabatic basis, to which previous related studies have been confined. To solve the resulting computational problems we develop and present a special multidimensional adaptation of the finite basis set method utilizing the product structure of the basis. It allows us to calculate propagation in a general potential including three vibrational modes. For the time integration a fourth order differencing scheme is introduced which is faster than the second order differencing-scheme and predictor–corrector approaches.
Binding energies and bond distances of Ni(CO)x, x=1–4: An application of coupled-cluster theory
Authors: Blomberg Margareta R. A. , Siegbahn Per E. M. , Lee Timothy J. , Rendell Alistair P. , Rice Julia E.
Published: July 2002

The accuracy of the single and double excitation coupled-cluster (CCSD) method that includes a perturbational estimate of connected triple excitations, denoted CCSD(T), has been tested for some representative transition metal complexes. For both the binding energy and metal to ligand bond distance of NiCO and Ni(CO)2, the CCSD(T) method yields results in very good agreement with multireference averaged coupled-pair functional (ACPF) calculations. The results are much better than those obtained using either the coupled-pair functional (CPF) or modified CPF (MCPF) methods. The contribution of connected triples to the binding energy is significant for all Ni(CO)x, x=1–4 ranging from 15 kcal/mol for NiCO to 30 kcal/mol for Ni(CO)4. In contrast, for the geometries the connected triples are only of minor importance. In this case, the correct treatment of disconnected quadruple excitations appears to be more important. For Ni(CO)4, the calculated binding energy is 125 kcal/mol (expt. 140 kcal/mol) and the bond distance is 3.46 a0 (exp. 3.45 a0). Virtually all of the remaining discrepancy, relative to experiment, should be due to limitations in the one-particle basis set. In addition, some test calculations were performed for Ni(C2H4), where near degeneracy effects are even more severe than for the nickel carbonyls and the CCSD(T) method gives very accurate results in this case also.
Dissociative chemisorption of H2 on Ni surface: Time-dependent quantum dynamics calculation and comparison with experiment
Authors: Sheng Jia , Zhang John Z. H.
Published: July 2002

A time-dependent quantum wave packet method has been applied to studying the process of activated dissociative chemisorption of H2 on Ni(100) surface. The Ni surface is treated as static and the effect of weak surface correlation is neglected in our dynamics calculation. The three-dimensional calculation is fully quantum mechanical without any reduced dimension approximation. An empirical London–Erying–Polanyi–Sato (LEPS) potential surface has been used and modified in our dynamics calculation to produce a reasonable barrier height compatible with experiment. Sticking probabilities have been computed as a function of initial normal incident kinetic energy and are compared to experimental results as well as another 3D quantum dynamics calculation. Good agreement has been found between our theoretical calculation and molecular beam experiment in the energy dependence of the dissociation probability. It is also found that vibrational excitation of H2 enhances dissociation probability, in agreement with the prediction of an earlier theoretical calculation. Snapshots of the wave function are plotted that provide intimate details of the dissociation dynamics in time and space.
Measurement of the ν8 intermolecular vibration of (D2O)2 by tunable far infrared laser spectroscopy
Authors: Pugliano N. , Saykally R. J.
Published: July 2002

The first accurate measurement of an intermolecular vibration of the water dimer is reported. Five vibration–rotation-tunneling (VRT) bands of the perdeuterated isotope, located near 84 cm−1, have been assigned to the A1/E/B1 tunneling components of the Ka=0←0 and Ka=1←0 subbands. The vibration involves large amplitude motion of the hydrogen bond acceptor and is assigned as the ν8 acceptor wag. The spectra indicate strong coupling of both the donor–acceptor interconversion and donor tunneling motions to the excited vibrational coordinate. This measurement provides a benchmark for future efforts toward the determination of an accurate potential energy surface for the water dimer.
A path integral Einstein model for characterizing the equilibrium states of low temperature solids
Authors: Li Daohui , Voth Gregory A.
Published: July 2002

A variational Einstein model for describing low temperature solids is developed from a Feynman path integral perspective. The theory can be used to predict fully quantum mechanical values for the thermodynamics (e.g., free energy, entropy, internal energy, etc.) and the equilibrium structure (e.g., pair and angular correlation functions) of a solid. The theory has also been generalized to treat low temperature solids which contain impurity species. The independent harmonic oscillator assumption implicit in the Einstein model allows the results to be cast in a straightforward analytic form. Additionally, the path integral formulation of the model yields solutions which explicitly depend on the path integral discretization parameter P. One can thus systematically examine the equilibrium behavior of a solid ranging from the classical to the quantum limits. The Einstein model is applied to examine the behavior of solid hydrogen and solid hydrogen containing lithium impurities.
The rotational spectrum of trans-HOCO and DOCO
Authors: Radford H. E. , Wei Wang , Sears Trevor J.
Published: July 2002

Part of the rotational spectrum of the hydroxyformyl radical, HOCO, and its deuterated analog has been detected at frequencies between 230 and 300 GHz. The radical was formed in a flow system by the reaction between chlorine atoms and formic acid. Analysis of the spectra yields reliable estimates of the rotational, centrifugal distortion and spin–rotational parameters describing the ground state of this species. The rotational constants derived for HOCO and DOCO are consistent with a planar equilibrium structure and are used to confirm that the carrier of the spectrum is the trans-geometrical isomer.
Spin polarization in restricted electronic structure theory: Multiconfiguration self-consistent-field calculations of hyperfine coupling constants
Authors: Fernandez Berta , Jo/rgensen Poul , Byberg Jo/rgen , Olsen Jeppe , Helgaker Trygve , Jensen Hans Jo/rgen Aa.
Published: July 2002

It is shown that the isotropic hyperfine coupling constant Aiso associated with the Fermi contact interaction can be accurately calculated from conventional multiconfiguration self-consistent-field wave functions if, in addition to the expectation value of the contact operator, one also includes the terms originating from the response of the wave function. These response terms are nonvanishing only for perturbation operators that are nonsymmetric in spin space. Calculations of Aiso for N and BH2 illustrate the importance of the response terms and also indicate that a good estimate of the spin polarization of the core orbitals may be obtained from the response terms without correlating the core.
Transverse relaxation in supported and nonsupported phospholipid model membranes and the influence of ultraslow motions: A 31P-NMR study
Authors: Dolainsky Christoph , Möps Alfred , Bayerl Thomas M.
Published: July 2002

The effect of motions which are slow on the NMR time scale (in particular, lateral diffusion) on the 31P-NMR transverse relaxation behavior of POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) phospholipid membranes was studied by experiment and computer simulation. Two new membrane model systems (spherical supported vesicles) which exhibit some unique geometrical features and physical properties as well as the commonly used multilamellar vesicles, were applied for the measurement of the 31P-NMR anisotropic transverse relaxation times. The known geometry of the new membrane model systems under study enables a direct comparison of the experimental transverse relaxation spectra with those obtained by a spectral simulation technique which considers random diffusion on the surface of a sphere as the dominant relaxation mechanism, where the sphere size corresponds to that used in the experiments. The transverse relaxation times obtained experimentally for the three model systems studied differ drastically, where the multilamellar vesicles exhibit the longest T2 and single spherical supported bilayers show the shortest relaxation time. Both experiment and simulation indicate that lateral diffusion causes an anisotropic transverse relaxation which scales with the angle θ between molecular director axis and magnetic field as 1/T2∼sin2θ cos2θ. An analysis of the experimental data obtained for supported systems suggests that diffusion of the individual molecules along the surface of the sphere is superimposed by diffusion over an ‘‘adiabatic rough’’ surface (a ‘‘quasistationary ’’ roughness on the time scale of the echo experiment). This provides a significant additional contribution to the transverse relaxation time T2. A comparison of single bilayers and multibilayers on a solid support, respectively, indicates that this roughness is mainly a property of the single supported bilayers, most likely due to an area mismatch between the bilayer and the surface of its solid support.