bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic properties of R(Fe1−xCox)11.3Nb0.7 (R=Y,Ho) compounds
Authors: Shen Y. P. , Wang J. L. , Tang N. , Fuquan B. , Yang D. , Yang F. M. , Zhong X. P.
Published: July 2002

Magnetic properties R(Fe1−xCox)11.3Nb0.7 compounds have been investigated. All of the compounds crystallize in the tetragonal ThMn12 structure. The unit cell volume slightly decreases with increasing Co content for both systems. Curie temperature (TC) almost increases linearly with increasing Co content for both systems. The maximum of saturation magnetization (Ms) appears around x=0.2 in Ho(Fe1−xCox)11.3Nb0.7 and x=0.15 in Y(Fe1−xCox)11.3Nb0.7 compounds at 5 K. The easy magnetization directions of all compounds are along the crystallographic c axis at room temperature. Thermomagnetic analysis shows that there exists no spin reorientation in the whole ordering temperature range. The magnetocrystalline anisotropy field Ba of Y(Fe1−xCox)11.3Nb0.7 exhibits a maximum with x=0.10 at 5 K and x=0.05 at room temperature, while Ba of Ho(Fe1−xCox)11.3Nb0.7 decreases monotonically with the increasing Co content at room temperature. First-order magnetization process (FOMP) has been observed in Ho(Fe1−xCox)11.3 compounds at low temperature.
Structural and magnetic properties of Nd2Fe17−δGaδ (δ⩽2)
Authors: Girt E. , Guillot M. , Swainson I. P. , Krishnan Kannan M. , Altounian Z. , Thomas G.
Published: July 2002

Magnetization measurements of Nd2Fe17−δGaδ (δ=0–2) show that the saturation magnetization, Ms, decreases with an increase in Ga content from 40.4 μB/f.u. for Nd2Fe17 to 36.1 μB/f.u. for Nd2Fe15Ga2 at 4.2 K. Neutron diffraction data at 25 K show that the magnetic moment of Fe depends on its crystallographic site and decreases in the order Fe(6c)>Fe(18f)⩾Fe(18h)⩾Fe(9d). The magnetic moments of Fe(9d), Fe(18f), and Fe(18h) are found to be practically independent of the Ga content. However, the magnetic moment of Fe(6c) decreases from 2.81(9) μB in Nd2Fe17 to 2.14(9) μB in Nd2Fe15Ga2. The decrease of the Fe(6c) moment clearly reduces the exchange interaction between Fe(6c)–Fe(6c) dumbbell pairs which explains the decrease in the anomalous thermal expansion with an increase in Ga concentration. The reduced Fe(6c)–Fe(6c) exchange interaction may also play an important role in increasing the Curie temperature, TC, with Ga content; TC increases from 327 K in Nd2Fe17 to 535 K in Nd2Fe15Ga2.
Nd–Fe–B permanent magnets (thick films) produced by a vacuum-plasma-spraying process
Authors: Rieger G. , Wecker J. , Rodewald W. , Sattler W. , Bach Fr.-W. , Duda T. , Unterberg W.
Published: July 2002

Thick, hard-magnetic Nd–Fe–B films (∼1 mm) were deposited on different substrates (Cu, steel) by a low-pressure plasma-spraying process. The properties of the applied Nd–Fe–B powders (e.g., grain size, composition) and the conditions of the spraying process were optimized with respect to the mechanical and magnetic properties of the films. Film thicknesses up to 1.2 mm were achieved with good adhesive properties (bond strength>40 MPa). Cracks at the interface or within the films during the deposition process could be suppressed by adjusting the temperature profile of the substrate and controlling the deposition rate. Depending on the maximum temperature of the substrate and the thickness of the Nd–Fe–B films, either amorphous or microcrystalline structures were obtained. In general, the magnetic properties were improved by a postdeposition annealing treatment. Coercivities HcJ up to 16 kA/cm and isotropic remanences of about 0.6 T were achieved.
Nd
8
Fe
73
Co
5
Hf
2
B
12
strip cast alloy
Authors: Chiriac H. , Marinescu M. , Castaño F. J.
Published: July 2002

Nd 8 Fe 73 Co 5 Hf 2 B 12 alloys were cast as strips (thick continuous ribbons with thickness t=100–160 μm) by a chill disk melt spinning technique (vdisk=3 m/s) from a master alloy prepared by arc melting (A) and from two prealloyed components (B). Samples obtained by the B procedure revealed in the as-cast state, good magnetic properties with an unusually high value of the coercivity, Hc=10.3 kOe, for the substoichiometric Nd2Fe14B system. A high degree of structural refinement and a homogeneous dispersion of the phases are attained in sample B. The strength and type of magnetic interactions between the grains are presented by δM and irreversible susceptibility plots.
Baseline popping noise of giant magnetoresistive head
Authors: Li Zhaohui , Wang Geng , Kao Andrew S.
Published: July 2002

Baseline popping noise (BLPN) is characterized by the spurious popping of baseline between readback pulses from magnetoresistive/giant MR (MR/GMR) heads. This article focuses on experimental investigation of BLPN from hard-magnet-biased GMR head. It is found that BLPN is sensitive to read bias current in both magnitude and polarity. Also, we found that BLPN is very sensitive to head offset across a written track as a result of interaction between localized BLPN active slice and written track edge. Full-track and microtrack profiling of BLPN is used to substantiate these behaviors. A close-fit model is proposed to further explain the BLPN mechanism. With this model, GMR BLPN is explained by longitudinal hard bias insufficiency on a free layer. When hard bias is not strong enough, due to the demagnetization field, magnetization around edge area rotates either up or down. This skewed magnetization is not stable, a small field will make it flip up and down, which results in jumps in transfer curve and eventually instability in track average amplitude.
Biquadratic interlayer exchange coupling in epitaxial Fe/Si/Fe
Authors: Strijkers G. J. , Kohlhepp J. T. , Swagten H. J. M. , de Jonge W. J. M.
Published: July 2002

We have studied the biquadratic exchange coupling in epitaxially grown Fe/Si/Fe. The temperature and thickness dependence of the biquadratic coupling strength were determined unambiguously by fitting the easy- and hard-axis magneto-optical Kerr effect loops. The origin of the biquadratic coupling can be fully understood in terms of Slonczewski’s loose spins mechanism.
Temperature dependent magnetic domain structure in ultrathin Fe films on Cr(100)
Authors: Hopster H.
Published: July 2002

Magnetic microscopy is used to study the temperature dependent magnetization structures in 2 nm Fe films on Cr(100). Above the Cr Néel temperature, the Fe films can be magnetized into a single domain state. When the films are cooled below the Néel temperature the Fe magnetization has a tendency to turn perpendicular (in-plane) resulting in a spatially varying magnetization direction. The resulting magnetization structures are highly reproducible. The tendency of the Fe magnetization to rotate is attributed to frustration due to atomic steps. It is suggested that the local angle of magnetization rotation reflects the average step density.
Large enhancement of elastic light scattering due to random fluctuations of the magnetization
Authors: Murayama Akihiro , Hyomi Kyoko , Falco Charles M.
Published: July 2002

We have observed the field dependence and enhancement of the intensity of polarized elastic light scattering, from ultrathin magnetic films with perpendicular magnetic anisotropy (PMA). High-resolution Brillouin spectroscopy elucidates that this enhancement reaches its maximum at a critical field (Hcrit) where the magnetization starts to incline toward out-of-plane due to the strong PMA. We attribute this new effect to the microscopic instability due to random fluctuations of the magnetization near Hcrit, since this elastic scattering has no frequency shift from the excitation light and thus the correlation of each spin decays monotonically. The enhancement factor depends on the PMA strength as well as the film thickness, and we obtain an enhancement as large as a factor of 20 at room temperature with a low external field of 0.18 kOe.
Temperature dependent anomalous Hall effect in La–Ca–Mn–O films
Authors: Lin Yeongkuo , Miller Dean J. , Jiang J. S. , Pearson John , Bader S. D.
Published: July 2002

The colossal magnetoresistance of La1−xCaxMnO3 has been reported in many experiments. We present our study of the anomalous Hall effect in epitaxial La0.67Ca0.33MnO3 thin films. We have measured the temperature dependence of resistivity, magnetization, and anomalous Hall effect coefficients between 300 and 5 K for our samples grown on different substrates. From these studies, the relation between the resistivity and anomalous Hall effect coefficient as well as the temperature dependence of the anomalous Hall effect coefficient are explored. Our results show that the direction of the anomalous Hall effect is reversed below approximately 100 K. This sign reversal is discussed in terms of band structure and the coexistence of hole-like and electron-like conduction.
Scattering of a diffusive wave by a subsurface object
Authors: Thibaud J.-B. , Carminati R. , Greffet J.-J.
Published: July 2002

We present a theoretical and numerical study of the scattering of a diffusive wave by an object embedded in a semi-infinite substrate. We derive exact integral equations for the scattered wave, using Green’s theorem and appropriate Green’s functions. We show that two methods can be used, leading either to a purely surface-integral formalism or to a formalism involving a volume integral and a surface term. In the first case, we derive an extinction theorem for diffusive waves and present an efficient numerical procedure to solve exactly the scattering problem. In the second formalism, physically more explicit, we apply an improved Born approximation, and study its range of validity by comparison with rigorous numerical results. Our approach also suggests a simple way to determine the depth of the object. In this article, we focus on thermal waves. Yet the formalism can be applied to photon-density waves.
Positron annihilation study of defects in boron implanted silicon
Authors: Huang M. B. , Myler U. , Simpson P. J. , Mitchell I. V.
Published: July 2002

Defects and their annealing behavior in boron implanted silicon have been studied using positron annihilation spectroscopy (PAS), ion channeling, nuclear reaction analysis, and transmission electron microscopy. Silicon wafers were implanted with 80 keV boron ions to fluences from 1012 to 1015 cm−2. Furnace annealing or rapid thermal annealing (RTA) of the implanted Si samples was conducted to temperatures in the range 750–950 °C in a N2 ambient. For as-implanted samples, the defect profiles extracted from PAS spectra were found to extend beyond the implanted boron distribution given by TRIM calculations. The Sdefect/Sbulk values increased monotonically with increasing boron fluences. For boron fluences ⩾1013 cm−2,Sdefect/Sbulk was found to be >1.04 (the characteristic value for divacancy), while Sdefect/Sbulk was found to be <1.04 for a boron fluence of 1012 cm−2. After annealing at 750 °C, all B-implanted samples had similar S-parameter values in the near-surface region, while in the deep region the S values for high B fluences (φ⩾1014 cm−2) were found to be lower than those for low B fluences (φ⩽1013 cm−2). Annealing at 950 °C did not change the S-parameter data for the lowest boron fluence (1012 cm−2), but caused a slight increase of the S parameters in the deep region for other boron fluences. RTA at 750 °C shows that major defects in B-implanted Si are annealed out within the first 3 s. An interesting transient annealing behavior is observed in which the S value decreases in the initial annealing stage, and then increases to a saturating value after prolonged annealing. Possible effects of electric fields resulting from the electrical activation of implanted boron on the behavior of positron annihilation line shapes after annealing are discussed.
Full band Monte Carlo modeling of impact ionization, avalanche multiplication, and noise in submicron GaAs p+-i-n+ diodes
Authors: Ong D. S. , Li K. F. , Plimmer S. A. , Rees G. J. , David J. P. R. , Robson P. N.
Published: July 2002

A full-band Monte Carlo model is used to investigate the probability distribution functions of impact ionization path length and impact ionization energy for electrons and holes in GaAs. The simulations show that the soft ionization threshold energy in GaAs allows impact ionization to occur at energies much higher than the band gap. As a result, secondary carriers have a shorter dead space than newly injected carriers. The ionization path length distributions narrow at higher fields, producing a more deterministic impact ionization process in thin devices. The model is also used to simulate avalanche multiplication and noise in submicron homojunction GaAs p+-i-n+ diodes. The predicted mean multiplication, 〈M〉 and excess noise factor, F are in quantitative agreement with the experimental results, in which F decreases as the length of multiplication region is reduced.
Effects of relaxation on glass transition and crystallization of ZrTiCuNiBe bulk metallic glass
Authors: Zhuang Yan Xin , Wang Wei Hua
Published: July 2002

Differential scanning calorimetry is used as the main method to investigate the effects of relaxation on the glass transition and crystallization of Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass. It is found that the glass transition and crystallization behavior as well as their apparent activation energy are markedly affected by the preannealing induced relaxation. The relation between the glass transition, crystallization kinetics, and the structural relaxation is explored.
Strain variation with sample thickness in GaN grown by hydride vapor phase epitaxy
Authors: Reynolds D. C. , Look D. C. , Jogai B. , Hoelscher J. E. , Sherriff R. E. , Molnar R. J.
Published: July 2002

High quality GaN crystals can be grown on sapphire by hydride vapor phase epitaxy. The thermal expansion mismatch between sapphire and GaN produces strain in the GaN crystal as it is cooled from the growth temperature to room temperature. The strain is evidenced by shifts in the photoluminescence and reflectance line positions. By analyzing the surface strain as the crystal thickness is increased, the thickness required to obtain zero surface strain can be estimated. This structure might provide a lattice matched and thermally matched substrate for further epitaxial growth of GaN.
Correlation between photoreflectance signal intensity and current gain of InP/InGaAs heterojunction bipolar transistor structures
Authors: Sugiyama Hiroki , Watanabe Noriyuki , Watanabe Kazuo , Kobayashi Takashi , Wada Kazumi
Published: July 2002

InP/InGaAs heterojunction bipolar transistor (HBT) wafers grown by metal–organic vapor-phase epitaxy were characterized by photoreflectance (PR) spectroscopy. We found that the intensity of PR signals from the InP emitter and InGaAs collector layers of the HBT wafer decreases with increasing emitter growth temperature and shows a linear positive correlation with the HBT current gain. On the other hand, the intensity of PR signals from the n-InP single layers scarcely changes with increasing InP growth temperature. Similar tendencies in the PR intensity were also observed with changes in postgrowth annealing temperature. The change in the PR intensity of the emitter and collector layers is expected to reflect the crystal quality of the adjacent InGaAs:C base layer, which determines the HBT current gain. The present PR method is eminently suitable for the nondestructive diagnostics of the crystal quality of InP/InGaAs HBT wafers.
Isotopic branching ratio for the reaction A+HD→AD(H)+H(D)
Authors: Tsukiyama K. , Katz B. , Bersohn R.
Published: July 2002

Several reactions of the form A+HD→AD(H)+H(D) have been studied by a two laser experiment. One laser dissociates a molecule to form a reactive atom A and the second laser monitors the hydrogen atom products by laser induced fluorescence. When A is F(2P), O(1D), and S(1D), the H/D ratios are 0.66±0.10, 1.13±0.08, and 1.91±0.10, respectively. Muckerman and co-workers have given qualitative arguments that abstraction reactions should have an H/D ratio less than one whereas insertion reactions should have H/D greater than one. Inasmuch as extensive previous experimental and theoretical work has shown that F(2P)+H2 procedes by abstraction whereas O(1D)+H2 and S(1D)+H2 proceed largely by insertion, the present results are consistent with the general qualitative conjecture.
Diode laser spectroscopy of CF+
Authors: Kawaguchi Kentarou , Hirota Eizi
Published: July 2002

The fundamental band of CF+ in the X 1Σ+ state has been observed by infrared diode laser spectroscopy with magnetic field modulation of the dc discharge plasma. The CF+ ion was generated by a hollow cathode discharge in a mixture of CF4 and H2. From an analysis of the observed spectra, the following molecular constants were obtained: ν0=1766.3589(9), Be =1.720 366(81), αe =0.018 947(96), De =6.179(17)×10−6 in cm−1 unit and re =1.154 272(35) Å, with three standard errors in parentheses.
Methane–methane isotropic interaction potential from total differential cross sections
Authors: Reid Brian P. , O’Loughlin Michael J. , Sparks Randal K.
Published: July 2002

Total differential cross sections (DCS) for methane–methane scattering at three collision energies were determined using the crossed molecular beams technique. These DCS’s were used along with literature viscosity and second virial coefficient data to determine a spherical methane–methane interaction potential energy function. The potential has a zero crossing point σ of 3.62 Å, a well depth ε of 200 K, and an intermolecular separation at the minimum rm of 4.02 Å.
A photoionization study of SeH and H2Se
Authors: Gibson S. T. , Greene J. P. , Berkowitz J.
Published: July 2002

The photoion yield curve of SeH, prepared by the reaction H+H2Se is presented. The adiabatic I.P. is 9.845±0.003 eV, and autoionization structure is observed, from which higher I.P.’s are inferred. The photoion yield curves of H2Se+, SeH+, and Se+ from H2Se are also measured. The fragmentation thresholds, together with I.P. (SeH), enable one to infer the bond energies D0(HSe−H)=78.99±0.18 kcal/mol and D0(SeH)=74.27±0.23 kcal/mol. The adiabatic I.P. for H2Se (X̃ 2B) is 9.886±0.003 eV.
Raman spectrum of ammonia solid II
Authors: Luo R. K. , Nye C. , Medina F. D.
Published: July 2002

Raman spectra of the orientationally disordered phase II of solid ammonia in the intramolecular and lattice regions are reported for the first time. The spectrum of the intramolecular region contains one line for each vibrational mode, broadened by the orientational disorder. The lattice region contains two broad lines near 50 and 280 cm−1, assigned to translational modes, and two central peaks ascribed to rotational relaxation.
The SiH+(A 1Π–X 1Σ+) emission produced from the thermal energy reaction of He+ with SiH4 under single collision conditions
Authors: Yamaguchi Sumio , Tsuji Masaharu , Obase Hiroshi , Sekiya Hiroshi , Nishimura Yukio
Published: July 2002

A flowing afterglow reactor has been coupled to a low-pressure chamber for an optical spectroscopic study of the charge–transfer reaction of He+ with SiH4 at thermal energy. The SiH+(A 1Π–X 1Σ+) emission was observed in the 380–610 nm region. The nascent vibrational and rotational distributions of SiH+(A) have been determined. The vibrational distribution for 0≤v′≤3 was approximately exponential with an effective vibrational temperature of 820±60 K. The rotational temperature decreased from 600 K for v′=0 to 300 K for v′=3. These data indicated that only about 3% of the excess energy is released as internal energy of SiH+(A). From the emission rate constant, SiH+(A) represents about 25% of the total SiH+ ion in the He++SiH4 reaction.
The structure and bonding of F2O2
Authors: Rohlfing Celeste McMichael , Hay P. Jeffrey
Published: July 2002

The geometry of F2O2 is determined by several ab initio methods, including Mo/ller–Plesset perturbation theory (MP) based on an RHF reference and externally contracted configuration interaction (CCI) based on a complete-active-space (CAS) reference. Basis sets with polarization and diffuse functions are used. Compared to the gas-phase experimental structure, SCF and MP bond lengths are very poor, and the MP geometries vary considerably with basis set changes. Three sizes of CAS are examined and each results in a dissociative surface. Only CCI calculations from a large CAS produce a geometry in reasonable agreement with experiment. After addition of Davidson’s correction, the remaining errors in the O–O and O–F bond lengths are 0.016 and 0.025 Å, respectively.
Rheology of n-alkanes by nonequilibrium molecular dynamics
Authors: Edberg Roger , Morriss G. P. , Evans Denis J.
Published: July 2002

We have simulated sheared liquid n-butane and n-decane by isothermal nonequilibrium molecular dynamics. Both alkanes exhibit nonlinear rheological behavior. Shear thinning at small shear rates, second Newtonian viscosities at moderate shear rates, normal stresses, dilatancy, and birefringence are observed for both fluids. Zero shear rate viscosities were calculated for both fluids: η(γ=0)=0.24 cp for butane at T=292 K, η(γ=0)=0.159 cp for decane at 481 K. These viscosities agree with experimental data for both fluids. The zero shear rate butane viscosity agrees with the Green–Kubo result of Marechal and Ryckaert, Chem. Phys. Lett. 101, 548 (1983). Shear induced molecular rotation, deformation, and alignment are quantitatively described to give a complete picture of molecular behavior in a sheared fluid. Equilibrium and shear simulations of butane with modified dihedral potentials were performed to investigate the effect of molecular conformation on the rheological properties of butane.
Kinetic theory of polymer melts. VIII. Rheological properties of polydisperse mixtures
Authors: Schieber J. D.
Published: July 2002

The Curtiss–Bird model of polymer melts has been extended to include polydisperse mixtures. The formula for the stress tensor derived in the last paper is used here to give explicit expressions for material functions in shear and elongational flow. Predictions are shown for viscosity, first normal stress coefficient, elongational stress growth viscosity, and steady elongational viscosity for Flory–Schulz and log–normal molecular weight distributions. It is also shown how generalizations that are observed experimentally for viscosity are predicted by the Curtiss–Bird model. In the next paper, these calculations are compared with experimental data.
Applications of electron propagator theory to the electron affinities of AsH2, SeH, Br, SbH2, TeH, and I
Authors: Ortiz J. V.
Published: July 2002

Electron propagator theory is applied to calculating vertical electron affinities of the title molecules. Third order and outer valence approximation quasiparticle calculations with and without effective core potentials are compared for molecules with fourth period atoms. Discrepancies between the two sets of results are small and stimulate applications to fifth period analogs. Good agreement with experiment is obtained for all the molecules; a prediction for the still unmeasured electron affinity of SbH2 is made.
Damping of total differential cross sections: Observations and empirical anisotropic potentials for HeC2H2 and HeOCS
Authors: Danielson Laurie J. , McLeod Karen M. , Keil Mark
Published: July 2002

Differential cross section (DCS) measurements are reported for scattering of a He atomic beam by crossed beams of Ar, C2H2, CO2, and OCS. Relative to the HeAr diffractive structure, the HeC2H2 DCS is moderately damped at small scattering angles and severely damped at large scattering angles; the HeCO2 and HeOCS DCS’s are severely damped for all scattering angles. This damping directly reflects increasing anisotropy of the scattering partner from Ar (none) to C2H2 (moderate) to CO2 and OCS (strong). Even though the present data do not resolve elastic from inelastic contributions, the HeCO2 results are consistent with partially resolved inelastic DCS measurements [U. Buck, H. Meyer, M. Tolle, and R. Schinke, Chem. Phys. 104, 345 (1986)] and therefore complement them. Consequently the data are used to obtain empirical anisotropic intermolecular potentials for HeC2H2 and HeOCS within the infinite-order-sudden approximation. These reproduce the total (unresolved elastic +inelastic) DCS measurements very well, even though the angular asymmetry of OCS is ignored in the analysis.
The microwave spectrum of the H2Cl+ ion
Authors: Saito Shuji , Yamamoto Satoshi , Kawaguchi Kentarou
Published: July 2002

Four rotational transitions of the H2Cl+ ion were observed in the frequency region of 270 to 500 GHz by using a source-modulated microwave spectrometer combined with a hollow cathode free space cell. The H2Cl+ ion was generated by a dc discharge in a mixture of HCl, H2, and He. The ion was identified on the basis of the hyperfine structure of the chlorine nucleus. This was further confirmed by the observation of the line intensity decrease with external magnetic field, which is a characteristic of ions in a hollow cathode discharge. The rotational constants A, B, and C, and the centrifugal distortion constant ΔJK were determined. The nuclear quadrupole coupling constants of the chlorine nucleus were obtained for the first time: χaa =−53.44(47) MHz, χbb =−15.71(50) MHz, and χcc =69.15(70) MHz with three standard errors in parentheses. The character of the H–Cl bond orbital is estimated from the observed nuclear quadrupole coupling constants and the molecular structure.
Monte Carlo study of free energy of hydration for Li+, Na+, K+, F−, and Cl− with a
b i
n
i
t
i
o potentials
Authors: Migliore M. , Corongiu G. , Clementi E. , Lie G. C.
Published: July 2002

We present extensive calculations of the free energies of hydration for Li+, Na+, K+, F−, and Cl− ions evaluated from Monte Carlo simulations of dilute aqueous solutions. Two-body ab initio potentials, derived from configuration interaction and self-consistent field calculations, respectively, are used for water–water and ion–water interactions. Free energy differences are calculated with the coupling parameter approach. Different numbers of sampling windows, moves per window, as well as system sizes, are investigated. It is found that: (i) if averages are taken over the forward and the backward directions of perturbation, the errors tend to cancel out; (ii) if the same number of windows and of moves per molecule are used, the spread between the two directions is much smaller for larger systems. Regarding the potentials studied, we find that although the deviation from experimental values goes from about −30% for Cl− to +21% for Na+, the overall trend is consistent with experiments and the Born theory.
Multiple-pulse NMR in inhomogeneously broadened rotating solids: Theory of sideband suppression experiments
Authors: Raleigh D. P. , Olejniczak E. T. , Griffin R. G.
Published: July 2002

A detailed analysis of a wide range of magic angle sample spinning (MASS)/multiple-pulse NMR experiments is presented. The analysis depends upon the calculation of the magnetization trajectories of individual crystallites, and thus is valid for inhomogeneously broadened systems. Using this model we examine a variety of experiments used to control and manipulate rotational sidebands in MASS spectra. This model provides a straightforward analysis of both the two and four π pulse experiments used for total suppression of sidebands (TOSS), and allows us to derive new timings for both sequences. We are able to provide the first proof that the TOSS sequence exactly eliminates sidebands, and that the two-pulse sequence eliminates first-order sidebands. We also demonstrate that the TOSS cycle can actually lead to inverted spectra, and we have accurately quantified intensity losses which occur in both the two- and four-pulse versions. The effect of pulse errors on sideband suppression experiments is also briefly discussed, as well as one possible means to reintroduce information about shift anisotropies into TOSS-type experiments.
The anharmonic force fields of HOF and F2O
Authors: Thiel Walter , Scuseria Gustavo , Schaefer Henry F. , Allen Wesley D.
Published: July 2002

A b initio predictions at the SCF, CISD, and CCSD levels are reported for the title compounds using DZP and TZP basis sets. The calculated geometries, rotational constants, dipole moments, fundamental frequencies, isotopic frequency shifts, vibration–rotation interaction constants, centrifugal distortion constants, Coriolis coupling constants, and infrared band intensities are compared with experimental data (if available). The best agreement is usually found for the CCSD results. The experimentally derived cubic force field of F2O is reproduced well by our results so that the predicted cubic and quartic force fields of HOF and the predicted quartic force field of F2O are also expected to be realistic. On the basis of our theoretical anharmonic constants, a new interpretation is suggested for the anomalous isotopic frequency shift of ν3 in HOF and DOF. Finally, an experimentally derived re structure with R(O–F)=1.4356 Å, r(O–H)=0.9664 Å, and α(H–O–F)=97.72° is proposed for HOF on the basis of the TZP CCSD vibration–rotation interaction constants.
Production and appearance size of multiply charged stoichiometric and nonstoichiometric SO2 cluster ions
Authors: Scheier P. , Walder G. , Stamatovic A. , Märk T. D.
Published: July 2002

We report the first well resolved SO2 cluster ion spectrum showing doubly charged parent and fragment ions. The critical appearance size deduced for doubly charged SO2 clusters is n2=21 and for triply charged SO2 clusters n3=49. This result is compared with theoretical predictions.
Improved dynamic hyperpolarizabilities and field-gradient polarizabilities for helium
Authors: Bishop David M. , Pipin Janusz
Published: July 2002

Accurate and explicitly electron-correlated wave functions have been used to determine the dynamic hyperpolarizability tensor (γ) and the dynamic field-gradient polarizabilities (B and C) for helium. The γ-tensor is related to nonlinear optical processes such as the Kerr effect, second- and third-harmonic generation and degenerate-four-wave mixing. In the static limit our results agree completely with a previous highly accurate variational-perturbation treatment. For second-harmonic generation at λ=4880 Å the ratio γzzzz/γxxzz is 2.950, which may be compared with the experimental value of 2.949.
Electron momentum redistribution during the van der Waals interaction between two hydrogen atoms
Authors: Koga Toshikatsu , Yamazaki Hiroyuki
Published: July 2002

Using a momentum-space method, we first determine the leading second-order perturbation wave function ψ(2)6(p1,p2) (proportional to R−6) of the van der Waals interaction between two ground-state hydrogen atoms. The resultant wave function is used to study the electron momentum redistribution γ6(p) which appears in the second order of the perturbation. The momentum density is shown to have the contractive nature as has been conjectured previously. The accuracy of the present ψ(2)6(p1,p2) is numerically verified through the calculation of the second-order kinetic energy T6, which should be equal to five times the dispersion energy E6 according to the virial theorem.
A radially accurate perturbation theory for molecular liquids such as nitrogen or fluorine
Authors: Goldman Saul
Published: July 2002

We derive a molecular perturbation theory that accurately predicts the centers distribution function for Lennard-Jones diatomic models of N2 and F2 at their triple points. The motivation for this work stems from a pre-existing need in molecular perturbation theory for a theory that would generate accurate values of this function. For N2, the predicted values of this function are essentially quantitative and for F2 they are nearly so. The theory is physically based and contains no adjustable parameters. The derivation is based on a first-order functional Taylor expansion of the background pair correlation function y(12). The expansion functional is selected so as to cause the numbers of nearest neighbors in the isotropic reference fluid and the molecular fluid to be the same. We also report other measures of structures and thermodynamics, and we make comparisons with related theories.
Collisionally induced population transfer effect in infrared absorption spectra. II. The wing of the Ar-broadened ν3 band of CO2
Authors: Boissoles J. , Menoux V. , Le Doucen R. , Boulet C. , Robert D.
Published: July 2002

The absorption beyond the ν3-band head of CO2 broadened by argon has been measured at room temperature. The absorption exhibits a strong sub-Lorentzian behavior (several orders of magnitude) resulting from collisionally induced line interferences which transfer intensity from this wing region to the ν3-band center. This wing absorption region implies detuning frequencies from resonances much larger than the reciprocal duration of collision. Consequently, finite duration of collisions in rotational energy transfers and initial correlations must be included in absorption calculation. A line-by-line coupling theory accounting for both these effects has been recently proposed [J. Chem. Phys. 89, 625 (1988)] and is applied here to a detailed study of the CO2–Ar collisional system. A convenient generalized detailed balance correction is introduced in this theory to overcome the limitation of the assumed resonant character of the energy transfer in the short time limit with respect to the thermal time ( βℏ)−1. The calculated absorption is in quantitative agreement with experiment. The origin and the nature of the empirical correcting factor currently used in similar studies are clearly established on a firm physical basis.
General phase modulation method for stored waveform inverse Fourier transform excitation for Fourier transform ion cyclotron resonance mass spectrometry
Authors: Guan Shenheng
Published: July 2002

A general phase modulation algorithm has been developed for the stored waveform inverse Fourier transform (SWIFT) excitation method used in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR or FTMS). The algorithm, based on the time shifting theorem and the uncertainty principle, shows that the quadratic phase modulation is the theoretically optimal method for square magnitude spectral profiles. For more complicated magnitude spectral profiles, the corresponding phase functions can be generated through the algorithm by using a nonlinear grid on the frequency domain. The degree of dynamic range reduction can be estimated from a simple equation.
Theoretical investigations of small multiply charged cations. III. NeN2+
Authors: Koch Wolfram , Liu Bowen , Frenking Gernot
Published: July 2002

First-order configuration-interaction calculations based on 4–6 σ, 1–2 π complete active space self-consistent-field reference wave functions are reported for the potential-energy curves of the 21 lowest-lying electronic states of NeN2+ which dissociate into (2P)Ne++(3P)N+ or (2P)Ne++(1D)N+. Using the same complete active space self-consistent-field SCF reference function, second-order configuration-interaction wave functions have been calculated for the X2Π ground state of NeN2+. At this level the potential minimum lies 4.57 eV above the (3P)N++(2P)Ne+ dissociation limit and there is a barrier to dissociation of 0.94 eV. All other states of NeN2+ are either repulsive, or exhibit a flat potential curve at some closer interatomic distance. In addition, the X 3Σ− ground-state potential-energy curve of single charged NeN+ was computed at the complete active space SCF + second-order configuration-interaction level. The dissociation energy is predicted as De =0.47 eV at re =3.30 a0. Bonding in NeN2+ and NeN+ is discussed in terms of donor–acceptor interactions between Ne and N2+ or N+, respectively. [For paper II of this series see W. Koch and G. Frenking, J. Chem. Phys. 86, 5617 (1987).]
Dynamics of third-order nonlinear optical processes in Langmuir–Blodgett and evaporated films of phthalocyanines
Authors: Casstevens Martin K. , Samoc Marek , Pfleger Jiri , Prasad Paras N.
Published: July 2002

The results of degenerate four-wave mixing studies performed on evaporated films of metal-free phthalocyanine and Langmuir–Blodgett films of a silicon phthalocyanine using subpicosecond 602 nm pulses are reported. The third-order nonlinearity of all the samples is high permitting the observation of the degenerate four-wave mixing signal from even a monolayer. In both cases, the third-order nonlinearity has a resonant character and the observed time-resolved phase conjugate signals provide information on the dynamics of excitons. Simultaneously monitoring first- and second-order diffractions from the transient gratings formed in our samples allowed us to obtain more-detailed information on the factors influencing the temporal behavior of the nonlinear response. It is concluded that the dynamics of excitons determining the nonlinearity is mainly governed by the presence of bimolecular exciton–exciton interaction. Saturation behavior is observed at high light intensities.
Localized chaos and partial assignability of dynamical constants of motion in the transition to molecular chaos
Authors: Jaffe Charles , Kellman Michael E.
Published: July 2002

The implications of approximate dynamical constants of motion for statistical analysis of highly excited vibrational spectra are investigated. The existence of approximate dynamical constants is related to localized chaos and partial assignability of a ‘‘chaotic spectrum.’’ Approximate dynamical constants are discussed in a dynamical symmetry breaking formulation of the transition from periodic to quasiperiodic motion, and from quasiperiodic to chaotic motion. Level repulsion, leading to a Wigner distribution in the case of a strongly chaotic system, is shown to originate in dynamical symmetry breaking via the noncrossing rule that states of the same symmetry do not cross. It is argued that quantum numbers for dynamical constants must be correctly assigned to detect localized chaos in statistical spectroscopy. Two possible kinds of approximate constants, for a ‘‘total polyad number’’ and a bend normal mode, are discussed in relation to two coupling schemes that could govern the transition to chaos in H2O.
Theoretical study of low-lying electronic states of CoH+
Authors: Anglada J. , Bruna P. J. , Grein F.
Published: July 2002

Potential energy curves and spectroscopic parameters of several electronic states of CoH+ have been calculated using multireference configuration interaction methods. The four lowest-lying states X 4Φ, 1 4Σ−, 1 4Π, and 1 4Δ lie between 0 and 0.6 eV. The 2 4Π state, with a minimum slightly below 2.0 eV at R=2.92 a0, exhibits a second minimum (Re=4.46 a0, Te=2.14 eV) due to an avoided crossing between the Co+ occupations d8 and d7s. Such an interaction is also responsible for the shallow minima predicted for the 2 4Δ and 2 4Φ states. Several doublet states of Σ−, Π, Δ, Φ, and Γ character are bound. Except for 1 2Δ, they are expected to exhibit potential barriers along the dissociation path. All bound states have a d77σ2 configuration (7σ is CoH bonding), with the exception of 1 2Δ which is d8 -like and has a significant dσ(Co) contribution into the metal–hydrogen bond. The sextet states 1 6Δ, 1 6Π, 1 6Σ−, and 1 6Φ are repulsive due to single occupation of 7σ. The dissociation energy De(X 4Φ) of 2.18 eV is in good agreement with experimental estimates. The ionization potential (IP) from X 3Φ (CoH) into X 4Φ(CoH+) (6σ→∞) is calculated to be 7.23 eV, supporting an experimental lower limit of 7.30±0.10 eV for this quantity. The next IPs result from ionization of 3π into 1 4Δ of CoH+ and 1δ into 2 4Π, whereas 7σ ionization is expected to break the CoH bond.
Magnetic ordering in synthetic oligonucleotides. A deuterium nuclear magnetic resonance investigation
Authors: Alam Todd M. , Drobny Gary
Published: July 2002

Deuterium NMR of the [methyl mph type='21'>2H ]-2′-deoxythymidine labeled synthetic oligonucleotide, [d(CGCGAAT*T*CGCG)]2 in a liquid crystal phase are reported. This ordered phase was observed for hydrated samples with DNA concentrations ranging from 490 to 722 mg ml−1. Temperature variation of line shape and quadrupolar echo decay times were investigated, allowing the degree of alignment and changes in dynamics to be determined. At greatly reduced temperatures a cylindrical line shape was observed, as is expected for samples with the DNA helix axis aligned perpendicular to the magnetic field, while at ambient temperatures, motion about the helix axis averages this cylindrical line shape. NMR investigations at magnetic field strengths of 11.7, 9.4, and 4.7 T resulted in only minor variation in the degree of alignment.
Hg(7 1S→7 3S1) spin changing transition and Hg(7 3S1) deactivation upon collisions with Hg, H2, D2, He, and Xe
Authors: Bras N. , El Afghani A. , Butaux J. , Jeannet J. C. , Perrin D.
Published: July 2002

Hg(7 1S0→7 3S1) spin changing transfer and Hg(7 3S1) total deactivation, induced by collisions with Hg, H2, D2, He, and Xe, are studied. Hg vapor, pure or mixed with foreign gas, was excited to the 7 1S0 state by absorption of two photons from a pulsed dye laser. Measurement of the relative intensities of the fluorescence from the 7 1S0 and 7 3S1 states allowed the determination of the collisional rate constants. The velocity-average cross sections thus measured for Hg(7 3S0)+M→Hg(7 3S1)+M are 1.32, 0.11, and 0.28 Å2 for Hg, H2, and D2, respectively. The total deactivation cross sections for 7 3S1 are 4.7 and 9.6Å2 for H2 and D2, respectively.
Theory of multicenter partitioning of molecular energies
Authors: Ewig Carl S.
Published: July 2002

The theory of analyzing molecular energies, as found from ab initio computations, in terms of the contributions from specific sets of atoms or centers is discussed. It is shown that the basis-function expansion of molecular wave functions in general leads to energetic interactions involving only one- through four-center terms. For the special case of self-consistent-field energies the formulas for the energy terms are given explicitly and several numerical properties are presented, including basis-set dependence, the differences between terms that arise in chemical reactions, and the first and second derivatives of individual terms with respect to molecular deformations. Expressions are also given for the multicenter resolution of correlation energies employing second-order many-body perturbation theory. Some numerical examples of the latter are also presented.
Quantum exact three-dimensional study of the photodissociation of the ozone molecule
Authors: Le Quéré Frédéric , Leforestier Claude
Published: July 2002

We present an exact three-dimensional quantum study of the dissociation of the ozone molecule on a single potential-energy surface (1B2). The wave function has been discretized on a three-dimensional grid, in conjunction with a mixed pseudospectral scheme in order to represent the action of the Hamiltonian operator on it. The correlation function (Fourier transform of the total cross section) has been computed within the time formulation. This function was obtained by means of the Lanczos algorithm, without any actual propagation of the initial wave packet on the upper surface. A calculation performed for the total angular momentum value J=0, reproduces the characteristic features of the experimental correlation function (recurrence times).
Theoretical study of GeHn, AsHn, and SeHn: Bond dissociation energies
Authors: Binning R. C. , Curtiss Larry A.
Published: July 2002

A b initio molecular orbital theory (Mo/ller–Plesset perturbation theory to fourth order and new basis sets developed for Ge, As, and Se) is used to calculate the atomization energies of GeHn(n=1–4), AsHn(n=1–3), and SeHn(n=1–2). Good agreement is found with the experimental bond dissociation energies of these hydrides derived from recent photoionization studies. The theoretical energies are combined with experimental data on the isolated atoms to determine ideal enthalpies of formation at 0 K for these hydrides, which are expected to have an accuracy of about ±3 kcal/mol (±0.15 eV).
Intermolecular potential of the methane dimer and trimer
Authors: Szczȩśniak M. M. , Chal/asiński G. , Cybulski S. M. , Scheiner S.
Published: July 2002

The Heitler–London (HL) exchange energy is responsible for the anisotropy of the pair potential in methane. The equilibrium dimer structure is that which minimizes steric repulsion between hydrogens belonging to opposite subsystems. Dispersion energy, which represents a dominating attractive contribution, displays an orientation dependence which is the mirror image of that for HL exchange. The three-body correction to the pair potential is a superposition of HL and second-order exchange nonadditivities combined with the Axilrod–Teller dispersion nonadditivity. A great deal of cancellation between these terms results in near additivity of methane interactions in the long and intermediate regions.
Slow time-resolved photodissociation of p-iodotoluene ion
Authors: Dunbar Robert C. , Lifshitz Chava
Published: July 2002

Using time-resolved photodissociation in the ion cyclotron resonance mass spectrometer, the dissociation of p-iodotoluene ion was time resolved at five wavelengths between 532 and 630 nm. This is the slowest dissociation which has been time resolved for ions of well defined internal energies, giving inverse time constants for fragment ion appearance ranging from 2×102 to 1.1×103 s−1. At the longer wavelengths, dissociation is in clear competition with infrared-radiative relaxation of the photoexcited ion, which is calculated by kinetic modeling of the observed curves to give an energy loss rate of 6.2 eV s−1 (50 000 cm−1 s−1). The dissociation kinetics of this ion are in complete accord with a Rice–Ramsberger–Kassel–Marcus (RRKM) model with E0 of 1.9 eV and ΔS1000 K of −4 eu. It is concluded from the high activation energy that the rate limiting step in the dissociation kinetics is a molecular rearrangement preceding dissociation.
Electronic states, ionization potentials, and bond energies of TlHn, InHn, TlH+n, and InH+n (n=1–3)
Authors: Balasubramanian K. , Tao J. X.
Published: July 2002

Potential energy surfaces of 6 electronic states of TlH2 and InH2 and 8 electronic states of TlH+2 and InH+2 are computed. In addition the ground states of TlH3, InH3, TlH+3, InH+3, TlH, and TlH+ are investigated. A complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by second-order configuration interaction (SOCI) and relativistic configuration interaction (RCI) including spin–orbit coupling calculations are carried out. The step-wise bond energies, De(Hn−1M–H) and adiabatic ionization potentials are computed. The ground states of TlH2 and InH2 are found to be bent (2A1; θe∼121.5 °, 120 °) while the ground states of TlH+2 and InH+2 are linear (1Σ+g). The ground states of TlH3 and InH3 are found to be 1A1 (D3h ) states while the ground states of TlH+3 and InH+3 are Jahn–Teller distorted 2B2(C2v ) states. The unique bond length of TlH+3 and InH+3 is shorter than the two equal bond lengths. The bond angles (H–M–H) for TlH+3 and InH+3 deviate considerably from the neutral θe=120 ° to near 69 °. The TlH+ ion is found to be only 0.04 eV stable. Periodic trends in the geometries, bond energies and IPs are studied. Spin–orbit effects were found to be significant for TlHn species. The IPs of InHn and TlHn exhibit odd–even alternation. The bond energies also show an interesting trend as a function of n.
The quasiperiodic route to chaos in a model of the peroxidase–oxidase reaction
Authors: Steinmetz Curtis G. , Larter Raima
Published: July 2002

We have investigated in detail the transition from stable steady state to chaos in the DOP model of the peroxidase–oxidase reaction. Two consecutive Hopf bifurcations give rise to toroidal oscillations. Chaotic oscillations occur after the circle map associated with the torus becomes noninvertible. The supercritical region of parameter space is characterized by a periodic-chaotic sequence in which the periodic states form severely, but systematically, pruned Farey trees. In terms of both the transition to chaos and the periodic-chaotic sequence, the DOP model appears to share important qualitative characteristics with a variety of oscillating chemical reaction systems and their models.