bluuebunny/crossref_metadata_2025 huggingface dataset:
Reverse-field reciprocity for conducting specimens in magnetic fields
Authors: Sample H. H. , Bruno W. J. , Sample S. B. , Sichel E. K.
Published: July 2002

A new static-electromagnetic reciprocity principle is presented, extending ordinary resistive reciprocity to the case of nonzero magnetic fields by requiring the magnetic field to be reversed when the reciprocal measurement is made. The principle is supported by measurements on various types of specimens, including those which exhibit the quantum-Hall effect. A derivation using elementary electromagnetic theory shows that the principle will hold provided only that the specimen is electrically linear (Ohmic), and that the Onsager form for the conductivity tensor applies throughout. The principle has important implications for electrical measurements on semiconductors in applied-magnetic fields.
Shock consolidation of diamond and graphite mixtures to fused polycrystalline diamond
Authors: Potter David K. , Ahrens Thomas J.
Published: July 2002

The production of fused compacts of polycrystalline diamond was achieved by subjecting porous (35%–49% porosity) mixtures of diamond crystals plus graphite (13–16 wt. %) to dynamic shock pressures of 10–18 GPa. The recovered material from an initial mixture of 4–8-μm diamond crystals plus graphite revealed a very homogeneous texture with little evidence of original grain boundaries. The preconsolidation addition of graphite also allowed ultrafine (<5 μm) diamond crystals to be consolidated; this was not previously possible with the use of diamond crystals alone. The results are consistent with calculations which suggest that a thin layer of graphite surrounding a diamond crystal delays thermal equilibrium between the surface and interior of the diamond crystal, thus allowing greater surface heating. Consolidation is also probably enhanced by conversion of graphite to diamond, possibly via the liquid state.
CeSi: A non-Kondo trivalent Ce compound
Authors: Shaheen S. A.
Published: July 2002

The magnetic properties of CeSi, which crystallizes in the FeB-type orthorhombic structure, have been investigated. CeSi orders antiferromagnetically at 5.6 K. Its ground-state magnetic moment attains a value of 1.47 μB in a field of 57 kOe at 3.0 K, which is far above the highest value ever observed for any Ce compound. The magnetic properties of this compound are like conventional stable moment rare-earth compounds and differ from other nearly trivalent Ce compounds. More specifically, unlike nearly trivalent Ce intermetallic compounds, no evidence for Kondo effect is apparent in this compound.
Magnetic properties of Co-R (R=Pr,Nd,Sm,Gd,Dy,Er) amorphous alloys
Authors: Yang Xingbo , Miyazaki T.
Published: July 2002

Magnetization and susceptibility have been measured in the temperature range 4.2–300 K for amorphous Co100−x Rx (R=Pr,Nd,Sm,Gd,Dy,Er; 50≤x≤70) alloys prepared by a liquid-quenching technique. All alloys show a normal ferromagnetic behavior at low temperatures and a Curie–Weiss-type temperature dependence of susceptibility in the paramagnetic regime except for the Co-Sm alloy system. In the case of the Co-Sm alloys, a field-cooling effect at lower temperatures and a temperature dependence of paramagnetic susceptibility similar to that of the Van Vleck ion are observed. From analysis of the average effective moment, it is concluded that the Co no longer sustains a moment above 60 at. % R.
Electron spin resonance in electron-irradiated 3C-SiC
Authors: Itoh Hisayoshi , Hayakawa Naohiro , Nashiyama Isamu , Sakuma Eiichiro
Published: July 2002

Electron-irradiation-induced defects in epitaxially grown 3C-SiC crystals have been studied by electron-spin-resonance (ESR) measurements. The results indicate the presence of an isotropic ESR center that consists of five lines equally spaced at about 1.5 G and has a g value of 2.0029±0.0001. Isochronal and isothermal annealing of electron-irradiated 3C-SiC showed that this center was annealed at three stages (150, 350, and 750 °C) and that the 750 °C stage exhibited first-order reaction with an activation energy of 2.2±0.3 eV.
Generation and propagation of threading dislocations in GaAs grown on Si
Authors: Tsai H. L. , Kao Y. C.
Published: July 2002

The generation and propagation of threading dislocations in GaAs grown on Si was studied by transmission electron microscopy. This study was conducted by analyzing dislocation structures in GaAs at various growth stages. The dislocation structure changes as the GaAs grows from initial islands to a thick film. These changes are explained by the following physical processes: (1) nucleation of dislocations in GaAs islands, (2) generation and propagation of threading dislocations during coalescence of initial GaAs islands and their subsequent growth, and (3) bending of threading dislocations under misfit-strain force and their interactions. The second process results in threading dislocations extending from the interface to the surface in thin films. The third process causes an improvement in near-surface quality in thick films. The iplication of this study in confining threading dislocations to near the interface is discussed.
Removal of the surface contamination layer from CF4 plasma etched GaAs(100) substrate by thermal annealing in hydrogen
Authors: Bernstein R. W. , Grepstad J. K.
Published: July 2002

Examination with x-ray photoelectron spectroscopy (XPS) of CF4 plasma etched GaAs(100) wafers unveils a residual surface reaction layer composed of Ga-fluoride and Ga-oxide. Efficient removal of this contamination layer by a brief immersion in a dilute NH4OH wet etch is demonstrated. The formation of a thin native oxide upon exposure to atmosphere of the clean substrate surface cannot be avoided, however. Prospective replacement of this wet etch processing by in situ thermal annealing in hydrogen was investigated. The recorded XPS spectra show almost complete desorption of fluorine after annealing at 200 °C, whereas a temperature of ∼600 °C is required for entire removal of residual surface Ga-oxide. Heat treatment in H2 also compares favorably with vacuum annealing, for which a noticeable reduction of the surface contamination layer was found only after annealing at 600 °C. The cleaning efficiency of hydrogen processing may be attributed to the reactive nature of this ambient.
N and Ar ion-implantation effects in SiO2 films on Si single-crystal substrates
Authors: Mazzoldi P. , Carnera A. , Caccavale F. , Favaro M. L. , Boscolo-Boscoletto A. , Granozzi G. , Bertoncello R. , Battaglin G.
Published: July 2002

The chemistry of an argon-ion-irradiated interface between an amorphous silicon dioxide film and a silicon single-crystal substrate was studied by determining the kind and depth distribution of compounds formed after nitrogen implantation at a depth more shallow than the SiO2 film thickness. With this study we intended to obtain some insight into the chemical and physical processes involved in the formation of silicon oxynitrides in silica as a consequence of nitrogen ion implantations. Samples were mainly characterized by x-ray photoelectron and Fourier-transform infrared spectroscopies. Scanning electron microscopy, Rutherford backscattering spectrometry, nuclear reaction analysis, and secondary-ion mass spectrometry techniques were also used to complete the set of results. The experimental evidences are consistent with a picture of an argon-induced radiation damage in terms of Si—O and Si—Si bond breaking in the SiO2 and in the silicon substrate regions, respectively. The subsequently implanted nitrogen atoms are drawn toward the interface by a chemical driving force, and there interact with unsaturated silicon bonds to produce SiOxNy or SiNz compounds. The formation of a large amount of these compounds at the interface is responsible for very rough surface morphological features.
Magnetic properties and microstructure of Fe4N and (Fe,Ni)4N
Authors: Chen S. K. , Jin S. , Tiefel T. H. , Hsieh Y. F. , Gyorgy E. M. , Johnson D. W.
Published: July 2002

Single-phase Fe4N and (Fe1−xNix)4N compounds have been synthesized in a continuous form by heat treating iron and iron-nickel alloy sheets at various temperatures under NH3/H2 atmospheres. The Fe4N sheet has a high room-temperature magnetization value of 179 emu/g (2.14 μB/Fe), which is only slightly less than 218 emu/g (2.19 μB/Fe) observed in pure iron. The magnetic moments of the Fe-Ni alloy nitrides decreased monotonically as x was increased, in contrast to those for the starting alloys Fe1−xNix which exhibited a peak value around x=0.05. The decrease in magnetic moment with nickel content in the alloy nitrides was close to the value anticipated by magnetic dilution from nickel. The coercive force is about 5 Oe and is slightly decreased by the Ni substitution. The Fe-nitride offers a significantly improved corrosion resistance over pure iron. Even further improvement is obtained in the (Fe1−xNix)4N system with only slight sacrifice in magnetic moment. The addition of nickel has been found to noticeably improve the mechanical ductility of the normally brittle Fe4N compound. Theses nitrides also exhibit significantly increased electrical resistivity and wear resistance, and may be useful for a variety of technological applications.
The determination of the elastic and electric fields in a piezoelectric inhomogeneity
Authors: Benveniste Y.
Published: July 2002

The transformed inclusion problem in a piezoelectric solid of general anisotropy is studied. An anisotropic piezoelectric medium contains a subregion Ω of a different piezoelectric solid which undergoes constant eigenstresses and spontaneous electric polarizations. An analytical solution is constructed for ellipsoidal shapes Ω in an unbounded surrounding matrix. A related problem is that in which no eigenstresses and polarizations are prescribed anywhere, but instead the inhomogeneity-matrix assembly is subjected to an external electromechanical loading. The electric and elastic fields in this case are shown to follow from the solution of the transformed inclusion problem.
Infrared spectroscopy of yttrium aluminum, yttrium gallium, and yttrium iron garnets
Authors: Hofmeister A. M. , Campbell K. R.
Published: July 2002

All 17 infrared lattice modes for yttrium aluminum, yttrium gallium, and yttrium iron garnets Y3R2(RO4)3 are obtained from single-crystal reflection and absorption spectroscopy, and the powder technique. Assignments for 12 modes are firmly establish based on chemical substitution among these garnets and within related series. Frequencies of the modes associated with translation of yttrium are nearly constant across the series. Frequencies of all other modes decrease quasilinearly as cell volume increases. Values the high and low frequency dielectric constants are in good agreement with independent measurements; also, the Lyddane–Sachs–Teller relationship was found to hold. Calculations of bulk moduli from vibrational spectra using a modification of Brout’s formula linearly correlate with experimental values for KT.
High-energy-product rapidly solidified Fe–Nd–B alloys with additions of Nb and Si
Authors: Manaf A. , Leonowicz M. , Davies H. A. , Buckley R. A.
Published: July 2002

The ternary Fe82.4Nd13.1B4.5 alloy, which exhibits enhanced remanence (0.92 T) in the rapidly solidified state, was further modified by partial substitution of Nb or Si, or both Nb and Si, for Fe. The latter alloys containing Nb and Si, while maintaining remanence and coercivity values comparable with those obtained for ternary and quaternary compositions, exhibit higher (BH)max of ∼150 kJ m−3. This results from improved squareness of the demagnetization curve in the second quadrant of the B-H loop and is due to a more homogeneous distribution of very small (∼20–30 nm) crystallites through the ribbon thickness. This can be attributed to the effect of increased undercooling of the liquid alloys containing Nb and Si on quenching.
A simple accurate expression of the reduced Fermi energy for any reduced carrier density
Authors: Van Cong H. , Debiais G.
Published: July 2002

Based on the two correct asymptotic forms of the reduced Fermi energy η, its simple accurate approximate expression for any reduced carrier density is proposed. It is suggested that (i) its maximal relative error in absolute value is found to be equal to 2.11×10−4 at fairly large η (=3.6), (ii) and, in particular, in the neighborhood of η=0, its relative errors in absolute values are small and regular, while whose obtained from the formulas by Nilsson [Phys. Status Solidi A 19, K75 (1973)], and Chang and Izabelle [J. Appl. Phys. 65, 2162 (1989)] are found to be large and unstable to different degrees.
On the role of Zn in Sm2Fe17Nx permanent magnets
Authors: Wendhausen P. A. P. , Eckert D. , Handstein A. , Müller K.-H. , Leitner G. , Skomski R.
Published: July 2002

Coercivities up to 1.9 T were achieved by bonding micron-size Sm2Fe17Nx particles with zinc. X-ray analysis of Zn-bonded samples indicates the presence of the ZnFe Γ phase and no unreacted Zn, after heat treatment at about 390 °C for 100 min for samples with Zn content varying from 5 to 35 wt. %. The increase of coercivity occurs only for a more intense heat treatment, e.g., at 390 °C for 300 min or 425 °C for 100 min. Differential scanning calorimetry (DSC) shows two exothermic peaks at 388 and 426 °C, but no endothermic peak related to the melting of Zn (at 419.6 °C) was observed. In dilatometric measurements two peaks were observed for the rate of length change near the same temperatures as the DSC peaks. Polarization-versus-field curves point to an additional ferromagnetic phase with (i) a Curie temperature near 180 K and (ii) 2% of the net magnetization at low temperatures. The corresponding dip in low-temperature demagnetization curves, however, is about 10% in depth. Most of the Sm2Fe17Nx grains in the demagnetized Zn-bonded magnets are multidomain. We found γ≊3.4×10−2 J/m2 and Dc≊0.3 μm for the wall energy and for the critical single-domain particle diameter, respectively.
Carrier lifetime in amorphous semiconductors
Authors: Shen D. S. , Conde J. P. , Chu V. , Wagner S.
Published: July 2002

In amorphous semiconductors, because of the band-tail and -gap states, the excess carrier lifetime becomes sensitive to many experimental details. An attempt is made to clarify the relationship between carrier lifetime, density of states, and measurement details. The results show that in a steady-state photoconductivity measurement, the loss of carriers by recombination is determined by the density of deep states and the position of the quasi-Fermi level. In a transient measurement, the limited observation time and the contact also play important roles, besides the density of states. The shallow states in the energy band control the drift mobility and thus affect the lifetime indirectly. In an extreme case, the lifetime can become longer as the defect density increases. These points are illustrated with the electron lifetime derived from photoconductivity, time of flight, and delayed-field charge collection for hydrogenated amorphous silicon and silicon-germanium alloys.
Superconductivity in PtSi ultrathin films
Authors: Oto K. , Takaoka S. , Murase K. , Ishida S.
Published: July 2002

Normal and superconducting properties of PtSi films with thicknesses of d=2–20 nm have been investigated. The superconducting transition has been observed on the thin films down to d=4 nm. The systematic reduction of the transition temperature with decreasing d (increasing the sheet resistance) has been explained by the localization and Coulomb interaction effects on superconductivity. The temperature dependence of the upper critical field indicates that the PtSi films behave as homogeneous superconductors.
Effects of Al substitution in Nd2Fe17 studied by first-principles calculations
Authors: Huang Ming-Zhu , Ching W. Y.
Published: July 2002

We have studied the effect of Al substitution in Nd2Fe17 compound by means of first-principles calculations. We first obtain the site-decomposed potentials for Fe from self-consistent calculation on Y2Fe17 and the atomiclike potentials in the crystalline environment for Al and Nd. Calculations are carried out for a single Al substituting one Fe at four different Fe sites (6c), (9d), (18f ), and (18h), two Al substituting two Fe (18h), and four Al substituting three Fe (18h) and one Fe (18f ). Our results show that the Al moment is oppositely polarized to Fe. The average moment per Fe atom actually increases for Al substituting Fe (18h) and Fe (18f ) is about the same for Al substituting Fe (6c), and is drastically reduced when replacing Fe (9d). Experimentally, Al is shown to be excluded from the (9d) sites because of the small Wigner–Seitz volume. When two Fe atoms are replaced by two Al atoms, the total moment is only slightly less than when only one Fe atom is replaced, and the Ms per Fe site actually increases, in agreement with the Mössbauer data. These results are analyzed in terms of the local atomic geometry and the charge transfer effect from the neighboring Fe to Al.
Effect of heat treatment and Co on the giant magnetoresistance of Fe-60 Cr–XCo alloy thin films
Authors: Chang H. C. , Chang Y. H. , Yao S. Y.
Published: July 2002

This article presents the giant magnetoresistance (GMR) effect and microstructure of Fe-60Cr-XCo ternary heterogeneous alloy films. After ion beam sputtering, the films are heat treated at a temperature within the miscibility gap. The separated phases and lattice images of the α1 (Fe–Co rich phase) and α2 (Cr rich phase) are further determined by high resolution transmission electron microscopy. In the granular alloy, the GMR effect is obtained by the interface scattering between the phases of α1 and α2. By isothermal aging at 550 °C for 80 min, the largest MR ratio of −62% appears for the film with the composition of Fe-60Cr-15Co at 77 K under a field of 14 kOe. For the same treatment, the MR ratios of Fe-60Cr-10Co and Fe-60Cr-20Co are −39% and −43%, respectively. The Co content and heat treatment condition in the alloy system obviously affect the MR ratio.
Electrical properties of Pb1-xSnxTe layers with 0⩽x⩽1 grown by molecular beam epitaxy
Authors: Abramof E. , Ferreira S. O. , Rappl P. H. O. , Closs H. , Bandeira I. N.
Published: July 2002

In this work, the electrical properties of Pb1−xSnxTe epitaxial layers with Sn composition covering the whole range were investigated. The samples were grown on (111)BaF2 substrates in a molecular beam epitaxy system using PbTe, SnTe, and Te solid sources. As the alloy composition varies from PbTe to SnTe, the hole concentration increases exponentially from 1017 to 1020 cm−3 for Te-rich sources and from 1017 to 1019 cm−3 for stoichiometric ones. The resistivity of the samples, which depends mainly on their hole concentrations, shows an exponential dependence on the temperature with a slope which decreases as x goes from 0 to 1. For all Pb1−xSnxTe samples with x in the range of 0.35–0.7, the resistivity curve shows a very well defined minimum at low temperatures. This anomalous behavior is supposed to be related to the band crossing, where the energy gap temperature coefficient changes sign. The temperatures where the minimum in the resistivity occurs only agree with the ones predicted by the band inversion model around x=0.4, exhibiting a large deviation to lower temperatures as x increases.
Scaling limit of digital circuits due to thermal noise
Authors: Natori Kenji , Sano Nobuyuki
Published: July 2002

The error probability at a node of a digital circuit exposed to thermal noise agitation is investigated and the minimal dissipation–reliability relation for practical electronic circuits is derived. The digital circuit is modeled by an inverter chain with ideal transfer characteristics, and the error probability due to spurious data transfer caused by the thermal noise fluctuation is evaluated as a function of the node switching energy. The maximal error probability at each node allowed by the reliability requirement of the total system leads us to the minimal node energy dissipated per logical switching, which amounts to around 12 eV in the future 1010 gate system operated at a 10 GHz clock rate with a 104 FIT level reliability. In view of the device size-scaling trend of large-scale integrated circuits, the minimal node energy is expected to be reached at a feature size of 10–20 nm.
Enhanced magnetic stability in spin valves with synthetic antiferromagnet
Authors: Dimitrov D. V. , van Ek J. , Li Y. F. , Xiao J. Q.
Published: July 2002

The nucleation field in a synthetic antiferromagnet has been studied as a function of the stabilization strength. The results can be used to predict the magnetic stability in a GMR read-back head, that utilizes a synthetic antiferromagnet instead of a single pinned layer. Experimental studies showed substantially improved magnetic stability at higher temperature in synthetic antiferromagnet spin valves.
Energy shift correction for the reflection approximation
Authors: Lee Soo-Y.
Published: July 2002

The neglect of the kinetic energy term in the usual reflection approximation leads to a shift of the reflection intensity towards lower energy compared to the exact intensity. A time-dependent formulation is used to derive the energy shift correction factors for absorption and Raman scattering. Examples are given to illustrate the new reflection formulas.
Quantum mechanical approach to IR intensities via nuclear electric shielding tensors. I. Water
Authors: Lazzeretti P. , Zanasi R.
Published: July 2002

The connection between the nuclear electric shielding and the atomic polar tensors are shown. The electric shielding tensors are related to the polarizability and the magnetizability, and satisfy a constraint condition for the electrostatic equilibrium which is the mixed length-acceleration Thomas–Reiche–Kuhn sum rule. In addition, they can be successfully used to rationalize experimental IR intensity data, which is verified by extended basis set calculations on the water molecule.
Rates of convergence and error estimation formulas for the Rayleigh–Ritz variational method
Authors: Hill Robert Nyden
Published: July 2002

A general theory of rates of convergence for the Rayleigh–Ritz variational method is given for the ground states of atoms and molecules. The theory shows what functions should be added to the basis set to improve the rate of convergence, and gives explicit formulas for estimating corrections to variational energies and wave functions. An application of this general theory to a CI calculation on the ground state of the helium atom yields an explicit large L asymptotic formula for the ‘‘L’’-limit energies EL. The increments are found to obey the formula EL−EL−1 =−3C1(L+ 1/2 )−4−4C2(L+ 1/2 )−5+O(L−6), where the constants C1 and C2 are given by explicit integrals over the exact wave function evaluated at r12=0. Numerical evaluation of these integrals yields 3C1≅0.0741 and 4C2≅0.0309, in excellent agreement with the empirical results 3C1≅0.0740 and 4C2≅0.031 found by Carroll, Silverstone, and Metzger.
Electron attachment by haloalkenes and halobenzenes
Authors: Olthoff James K. , Tossell John A. , Moore John H.
Published: July 2002

Electron transmission spectra and mass spectra of negative ions from dissociative electron attachment have been obtained for four series of halogenated alkenes: vinyl halides, chloroethylenes, halobenzenes, para-dihalobenzenes. The stabilizing effect of halogen substituents upon anion state energies is obvious. Systematic observations permit assignment of ∑ and ∏ anion states. Dissociative attachment usually proceeds through the ∏ anion state. An enormous enhancement of the cross section for dissociative attachment of threshold electrons is observed in some cases.
Calculations on species relevant to the photoionization of the van der Waals molecule (H2S)2
Authors: Fernández Pelayo F. , Ortiz J. V. , Walters Edward A.
Published: July 2002

A b initio geometry optimizations are performed on the neutral and cation of H2S and (H2S)2. The neutral dimer is a weakly bound complex possessing only shallow minima in the potential energy surface, but the dimer cation has a deeper minimum where a proton has been transferred to give a complex of H3S+ and HS. Dimer binding energies, ionization energies, and appearance potentials are calculated. Intermonomer distances can be quite sensitive to basis set and correlation improvements on the HF/6-31G* level of theory. The potential energy surfaces, however, are flat enough so that the energetic consequences are not large. A simple procedure for estimating energy changes involving ionization is suggested; this method separates an adiabatic energy difference into a vertical ionization followed by a geometric relaxation.
Rotational energy level structure in the local mode limit
Authors: Halonen L. , Robiette A. G.
Published: July 2002

The nature of the rotational energy level structure of the stretching vibrational states in XY2, XY3, and XY4 molecules (belonging to the C2v, C3v, and Td point groups, respectively) has been investigated in a local mode limit with no kinetic or potential energy copling between the bond oscillators and with the fundamental stretching frequencies much larger than the bending frequencies. The Coriolis effects between the rotational levels in the stretching vibrational states disappear in this limiting case, but the H22 resonance terms are seen to be important owing to the multiple vibrational degeneracies. Simple relationships are obtained between the coefficients of the vibrationally diagonal and off-diagonal H22 terms and consequently the number of independent molecular parameters is reduced. The model is compared with experimental data for XHn hydrides, and particularly in the molecules with a heavy central mass the major contribution to the H22 coefficients is well explained.
Interatomic potentials of HgXe van der Waals complex formed in supersonic jets as studied by laser induced fluorescence spectroscopy
Authors: Yamanouchi Kaoru , Fukuyama Junichiro , Horiguchi Hiroyuki , Tsuchiya Soji , Fuke Kiyokazu , Saito Takayuki , Kaya Koji
Published: July 2002

The laser induced fluorescence spectra of HgXe formed in a supersonic free jet of a He/Xe/Hg mixture were observed to determine the interatomic potentials between Hg and Xe in the X 10+, A 30+, and B 31 states. The dissociation energies for the X, A, and B states are 254, 1457, and 172 cm−1, respectively, with the equilibrium interatomic distances of 4.25, 3.25, and 4.47 Å for the respective states. It is found that the potential shape for the X and B states can be represented in good approximation by a Morse function, though this function may not reproduce the potential for the A state especially in the vicinity of high vibrational levels (v′>16).
An a
b i
n
i
t
i
o calculation of spectroscopic properties of the azide anion
Authors: Botschwina Peter
Published: July 2002

Using a basis set of 99 contracted Gaussian type orbitals and Meyer’s coupled electron pair approximation (CEPA), several spectroscopic properties such as vibrational frequencies, rotational, and centrifugal distortion, and vibration–rotation coupling constants were calculated for different isotopomers of the azide anion N−3. The ν3 band (asymmetric stretch) of 14N−3 is predicted to be extremely intense.
Laser and phosphor host La1−xMgAl11+xO19 (x=0.050): Crystal structure at 295 K
Authors: Abrahams S. C. , Marsh P. , Brandle C. D.
Published: July 2002

The laser and phosphor host material with nominal composition LaMgAl11O19 is shown to be nonstoichiometric. Czochralski grown crystals are hexagonal, with space group P63/mmc, and a=5.589 82(5), c=21.9738(12) Å at 295 K. The thermal expansivity has been determined over the range 295 to 860 K: at 295 K, α1=4.97(2)×10−6 K−1 and α3=8.14(3)×10−6 K−1and at 775 K, α1=7.01(1)×10−6 K−1 and α3=10.1(2)×10−6 K−1. The integrated intensities of 20 487 reflections within a reciprocal lattice sphere of radius (sin θ)/λ=1.0 Å−1 were measured on a CAD-4 diffractometer with MoKα radiation, resulting in 18 232 reflections above background. Correction and averaging gave 864 independent structure factors with an internal agreement factor of 0.0145. The crystal structure was solved from the Patterson function and Fourier series and refined by the method of least squares. The final agreement factor R=0.013 98. The composition corresponds to La1−xMgAl11+xO19, with x=0.0502(8): the La deficiency is exactly compensated by excess Al. The principal La site is not fully occupied, but contains 4.76 at. % Al and 5.2 at. % Schottky defects. The remaining 4.94 at. % La and 0.26 at. % Al are distributed over two threefold satellite sites 0.583(3) Å distant from the principal La sites. The Mg atoms share a tetrahedral site equally with Al, while the Al atoms in the trigonal bipyramidal site undergo anharmonic motion. The remaining Al atoms occupy distorted octahedral or trigonal antiprismatic sites, with the latter sharing faces to give an Al–Al distance of 2.6541(9) Å. Other polyhedra share only oxygen atom corners or edges. The superior lasing characteristics are due at least partially to the wide separation of 5.5898 Å between principal La sites. The small average atomic radial vibrational amplitude of 0.071 Å corresponds to a Debye characteristic temperature of 631 K.
An electron–water pseudopotential for condensed phase simulation
Authors: Schnitker Jürgen , Rossky Peter J.
Published: July 2002

A simple electron–molecule pseudopotential is obtained that describes the interaction between an excess electron and a rigid water molecule in the electronic ground state. The potential is completely local and involves only spherically symmetric terms with respect to the three molecular nuclei (interaction site model). The potential is thus suitable for large-scale computer simulations, as well as more analytical theories. A description is given of the contributions included in this potential, as well as the ramifications of alternative choices.
Efficient computation of absolute free energies of binding by computer simulations. Application to the methane dimer in water
Authors: Jorgensen William L. , Buckner J. Kathleen , Boudon Stephane , Tirado-Rives Julian
Published: July 2002

An efficient procedure is noted for computing absolute free energies of binding for complexes in solution. Two series of computer simulations are required in which the substrate is annihilated in the solvent by itself and in the solvated complex. For illustration, the free energy of binding for two methane-like particles at their contact separation of 4 Å has been computed in TIP4P water. Though several alternatives are possible, in this case, Monte Carlo simulations were employed with statistical perturbation theory in the NPT ensemble at 25 °C and 1 atm. The results for the free energy of binding as well as for the potential of mean force are consistent with prior findings from the integral equation theory of Pratt and Chandler.
Vacuum ultraviolet resonance Raman studies of the excited electronic states of ethylene
Authors: Sension Roseanne J. , Hudson Bruce S.
Published: July 2002

A resonance Raman study of ethylene has been performed with the use of excitation wavelengths ranging from 200 to 141 nm. Excitation resonant with the V(ππ*) state results in Raman spectra exhibiting intensity in the C=C stretching vibration, the CH2 symmetric scissors vibration, the CH2 torsional vibration and the CH2 out-of-plane wagging vibrations. These spectra confirm that the V state is strongly twisted about the C–C bond. They also indicate that the C=CH2 groups are no longer planar in the V state. Resonance with the (π3s) and (π3p) Rydberg transitions results in spectra exhibiting much less intensity in the torsional vibration, the CH2 symmetric scissors vibration and the CH2 wagging vibrations. The spectrum obtained in resonance with the (π3d) Rydberg transitions once again exhibits activity in the CH2 symmetric scissors mode.
Avoided crossings and resummation of nearly resonant molecular vibrations: Reconstruction of an effective secular equation
Authors: Fried Laurence E. , Ezra Gregory S.
Published: July 2002

Nondegenerate Rayleigh–Schrödinger perturbation theory diverges when applied to nearly resonant molecular vibrations. Traditionally, this divergence has been avoided by using quasidegenerate perturbation methods, such as partitioning, or Van Vleck’s transformation approach. In this paper, we present a method of resummation, called reconstruction, that is appropriate for nearly degenerate systems displaying a strong mixing between a finite number of zeroth order states. In order to implement the approach, a set of strongly interacting states must be identified. The method then produces a pseudoconvergent power series expansion for an effective secular equation from the divergent perturbation expansion of the energies of the strongly interacting states. The effective secular equation has a low polynomial degree, and may readily be solved numerically to yield resummed values of the energies. We compare reconstruction to diagonal Padé approximation, and find that Padé approximants resum energies in the vicinity of avoided crossings poorly, yielding levels that cross where the actual eigenvalues repel. Reconstruction, however, correctly reproduces the avoided crossing behavior.
The lowest triplet state of 1,3,5-hexatrienes: Quantum chemical force field calculations and experimental resonance Raman spectra
Authors: Negri Fabrizia , Orlandi Giorgio , Brouwer Albert M. , Langkilde Frans W. , Wilbrandt Robert
Published: July 2002

Theoretical and Raman spectroscopic studies are presented of E and Z-1,3,5-hexatriene and their 3,4- and 2,5-dideuteriated analogs in ground and excited triplet states. The T1 potential energy surface is calculated from extended SCF-LCAO-MO-CI theory. Energy minima and equilibrium geometries are determined in T1 . Frequencies and normal modes of vibration are calculated for the minima of the T1 and S0 states. Energies of higher triplet levels are computed and oscillator strengths for the transitions from T1 to Tn are determined. The displacements in equilibrium geometries between the T1 and the Tn level corresponding to the strongest T1→Tn transitions are calculated and are used to estimate the intensities of the resonance Raman spectra of the T1 state under the assumption of a predominant Franck–Condon scattering mechanism. The results indicate that the planar E and Z forms of hexatriene and its analogs are the only ones contributing substantially to the T1→Tn absorption and the T1 resonance Raman spectra found in the present experiments. The existence of a twisted form in the T1 state cannot be ruled out, but its contribution to the resonance Raman spectra corresponding to an electronic T1→Tn transition around 315 nm is likely to be much weaker than that of the E or Z forms. Satisfactory agreement is found between the calculated and experimentally determined resonance Raman spectra. An assignment is obtained for the experimentally determined vibrational modes in T1. The theoretical results indicate a substantial rotation of normal modes from S0 to T1.
Investigations of the temperature dependence of resonance Raman cross sections: Applications to heme proteins
Authors: Schomacker K. T. , Champion P. M.
Published: July 2002

We have measured the resonance Raman cross sections of the predominant modes of ferrocytochrome c as a function of temperature. The data are fit quite successfully with a Kramers–Kronig transform technique and a first-order resonance Raman cross-section expression, which explicitly separates effects due to the thermal properties of the absorption line shape from the Bose–Einstein factor of the Raman mode. The results give experimental support to the assumption that corrections due to higher order scattering processes are negligible for systems that contain many weakly coupled thermally populated modes and verify approximate theoretical expressions for the resonance Raman cross sections at finite temperature. The results also demonstrate that higher order scattering processes cannot account for the recently observed anomalous enhancement of the resonance Rayleigh cross section of ferrocytochrome c. Finally, transient resonance Raman experiments that probe the local temperature of the heme group are suggested and the method of analysis is discussed. These experiments are considered in light of recent molecular dynamics calculations that predict photon absorption will lead to a significant increase in the heme temperature that dissipates within 40 ps. We present expressions, appropriate for absorbing media, that suggest how the mode selective temperature of the heme group can be directly measured by use of the resonant Stokes and anti-Stokes scattering cross sections. Calculations, specific to hot transient states of myoglobin, are presented in order to help quantify the analysis of recent subpicosecond resonance Raman measurements. Recent applications of anharmonic exchange models to obtain heme temperatures are shown to be inappropriate when kT≳ℏΩb.
A study of the structure and dynamics of the hydronium ion by high resolution infrared laser spectroscopy. III. The ν3 band of D3O+
Authors: Petek H. , Nesbitt D. J. , Owrutsky J. C. , Gudeman C. S. , Yang X. , Harris D. O. , Moore C. B. , Saykally R. J.
Published: July 2002

294 transitions were measured by velocity modulation spectroscopy using a difference frequency laser system with a D2/O2 discharge and assigned to the ν3 (antisymmetric stretch) band of D3O+. A simultaneous analysis of this data with the ν2 band spectra reported by Sears, et al. [J. Chem. Phys. 83, 2676 (1985)] yielded a refined set of parameters for both the ground and the excited states of both bands. The ν3 bandcenters are 2629.6512(39) and 2624.2376(47) cm−1 for the s–s and a–a inversion components, respectively.
High pressure x-ray diffraction studies on solid N2 up to 43.9 GPa
Authors: Olijnyk H.
Published: July 2002

Solid N2 was studied by energy-dispersive x-ray diffraction between 5.75 and 43.9 GPa at room temperature. The transition from δ-N2 (Pm3n) to ε-N2 (R3̄c) was observed at 16.3±0.5 GPa. The pressure-volume curves for both phases were determined. A comparison with the results of a priori calculations shows good agreement up to ∼12 GPa, however, at higher pressures the theoretical isotherm becomes increasingly stiffer.
Structure and dynamics of water at the Pt(111) interface: Molecular dynamics study
Authors: Raghavan K. , Foster K. , Motakabbir K. , Berkowitz M.
Published: July 2002

We prescribe an analytical form of the interaction potential between rigid water and a rigid platinum metal surface, which takes into account the surface symmetry and corrugation. Using this potential we perform a molecular dynamics computer simulation on water lamina restricted by two Pt(111) surfaces and investigate the structure and dynamics of water at the Pt interface. At 300 K the water layer adjacent to the metal surface displays solid-like properties. Patches of ice-like structure embedded in this layer are observed in the simulation. The next two layers of water display ordering similar to ice-I. Beyond these three layers the structure and dynamics of water are bulk-like.
Diffusion and reaction in heterogeneous media: Pore size distribution, relaxation times, and mean survival time
Authors: Torquato S. , Avellaneda M.
Published: July 2002

Diffusion and reaction in heterogeneous media plays an important role in a variety of processes arising in the physical and biological sciences. The determination of the relaxation times Tn (n=1,2,...) and the mean survival time τ is considered for diffusion and reaction among partially absorbing traps with dimensionless surface rate constant κ̄. The limits κ̄=∞ and κ̄=0 correspond to the diffusion-controlled case (i.e., perfect absorbers) and reaction-controlled case (i.e., perfect reflectors), respectively. Rigorous lower bounds on the principal (or largest) relaxation time T1 and mean survival time τ for arbitrary κ̄ are derived in terms of the pore size distribution P(δ). Here P(δ)dδ is the probability that a randomly chosen point in the pore region lies at a distance δ and δ+dδ from the nearest point on the pore–trap interface. The aforementioned moments and hence the bounds on T1 and τ are evaluated for distributions of interpenetrable spherical traps. The length scales 〈δ〉 and 〈δ2〉1/2, under certain conditions, can yield useful information about the times T1 and τ, underscoring the importance of experimentally measuring or theoretically determining the pore size distribution P(δ). Moreover, rigorous relations between the relaxation times Tn and the mean survival time are proved. One states that τ is a certain weighted sum over the Tn, while another bounds τ from above and below in terms of the principal relaxation time T1. Consequences of these relationships are examined for diffusion interior and exterior to distributions of spheres. Finally, we note the connection between the times T1 and τ and the fluid permeability for flow through porous media, in light of a previously proved theorem, and nuclear magnetic resonance (NMR) relaxation in fluid-saturated porous media.
Theoretical study of the di-imide (N2H2) molecule in ground and n→π* excited states
Authors: Kim Kyungsun , Shavitt Isaiah , Del Bene Janet E.
Published: July 2002

A number of ab initio approaches have been used to determine the equilibrium structures, energies, and vibrational frequencies of N2H2 in the ground state and in the excited singlet and triplet n→π* states. The methods included restricted Hartree–Fock (RHF) and unrestricted Hartree–Fock (UHF), multiconfiguration self-consistent field (MCSCF), single-reference configuration interaction (SRCI) and multireference configuration interaction (MRCI) including all single and double excitations from the reference configurations, and second-order Mo/ller–Plesset perturbation theory (MP2) using RHF and UHF orbitals for ground and excited states, respectively. Unlike the singlet excited state, for which broken-symmetry solutions were found at the RHF level, no symmetry breaking was encountered for the triplet state. The ground-state MCSCF and MP2 structures of N2H2, which have C2h (trans-planar) symmetry, are in good agreement with the experimental structure. The excited states are predicted to have nonplanar C2 structures with dihedral angles ranging from 96° to 106° for the triplet state and from 105° to 121° for the singlet state. Except for the SRCI singlet adiabatic excitation energy, the effect of configuration interaction is to increase the vertical and adiabatic excitation energies of both excited states relative to the RHF values in single-reference calculations, and to decrease these excitation energies relative to MCSCF values in multireference calculations, bringing the single-reference and multireference CI values into better agreement with each other. The MRCI vertical excitation energies are 2.6 eV for the triplet and 3.6 eV for the singlet, while the corresponding 0–0 transition energies are 1.9 and 2.9 eV, respectively.
Spectroscopic characterization of the X 1Σ+ and C 1Π1 states of the ZnKr molecule
Authors: Wallace Ingvar , Ryter Jarral , Breckenridge W. H.
Published: July 2002

The X 1Σ+0 and C 1Π1 electronic states of the ZnKr van der Waals molecule have been characterized by laser-induced fluorescence spectroscopy. Spectroscopic constants, bond lengths, and dissociation energies are reported for both states. The substantial amount of data now available on M⋅RG ‘‘pure-π’’ excited states, where M=Zn, Cd, and Hg and RG=Ne, Ar, Kr, and Xe is also examined critically. It is proposed that the much larger dissociation energies and shorter bond lengths of the pure-π states compared to the ground states is due primarily to the fact that the RG atom, approaching in the nodal plane of the diffuse M(np) orbital, feels an effective positive charge when near the contracted M(ns) ‘‘core.’’ In the cases where re and De have both been determined, effective core charges are calculated to be 0.85–1.00 using a simple model potential with only a ‘‘core-ion/induced-dipole’’ attractive term. Fundamental vibrational frequencies calculated with the same potential also agree well with values determined experimentally for the pure-π states. Detailed comparisons of the properties of the pure-π states of Na⋅RG vs those of Zn⋅RG, Cd⋅RG, and Hg⋅RG molecules indicate that in the latter cases, the interaction also involves ‘‘back polarization’’ of the M(ns) core electron away from the approaching RG atom, which increases the apparent M(ns) charge.
Vibrational–torsional coupling. High-resolution stimulated Raman spectrum of the ν3 band of ethane (12C2H6)
Authors: Bermejo D. , Santos J. , Cancio P. , Fernández-Sánchez J. M. , Montero S.
Published: July 2002

We report a stimulated Raman spectrum of the Q branch of the ν3 band of ethane recorded with a resolution of about 0.0055 cm−1 and an absolute wave-number accuracy of the order of 0.001 cm−1. This spectrum is analyzed in terms of a vibrational–torsional coupling Hamiltonian that allows for the calculation of the observed wave numbers within the experimental uncertainty. The following spectroscopic parameters (in cm−1) have been obtained: ν30=994.1108(10), B3A1s−B0=−0.006 165(3), B3E3d−B0=−0.006 170(2), B3E3s−B0 =−0.006 184(2), B3A3d−B0=−0.006 257(2), A3A1s−A0=−0.003 037(15), A3E3d−A0 =−0.003 040(12), A3E3s−A0=−0.003 054(12), and A3A3d−A0=−0.003 065(12). A strong dependence of the threefold torsional potential barrier with respect to the q3 normal mode, ∂V3/∂q3=−275.9±1.4 cm−1, is deduced from the present analysis.
Grafted polymer layers with chain exchange: A Monte Carlo simulation
Authors: Lai Pik-Yin
Published: July 2002

By introducing a head-group energy of adsorption with the grafting surface, we simulate the grafted polymer layer including chain exchange with the bulk solution using the bond-fluctuation model. The kinetics of adsorption is relatively rapid in short times and becomes much slower in later time as the layer is formed. The self-adjusted surface coverage is measured for different values of chain lengths and head-group energies. We also found that the polymer chains in a grafted layer are replaced by introducing shorter chains of identical head groups, which is also observed in recent experiments.
Ground state potential surface for van der Waals complexes: Ab initio second-order Mo/ller–Plesset study on benzene...N2 van der Waals molecule
Authors: Hobza P. , Bludský O. , Selzle H. L. , Schlag E. W.
Published: July 2002

The potential energy surface of the benzene...N2 was investigated ab initio using second-order Mo/ller–Plesset (MP2) theory. The calculations were performed with 6-31+G*/[5s4p2d] and 6-31+G*/[5s4p3d] basis sets. Among five structures studied the sandwich structure (N2 lies in the plane parallel to the benzene ring) appeared to be most stable. The calculated intermolecular distance (3.46 Å after vibrational averaging) is in excellent agreement with recent high resolution measurements. Calculated stabilization energy (591 cm−1) and intermolecular vibrational frequencies (stretching, 57 cm−1; torsion, 73 cm−1) are compared with available experimental results.
Molecular dynamics simulations of ring inversion in RDX
Authors: Wallis Eric P. , Thompson Donald L.
Published: July 2002

Molecular dynamics simulations, using the finite volume method of Murrell and co-workers [J. Chem. Phys. 94, 3908 (1991)], have been carried out to study conformational changes in hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in isolation and in dense Xe gas. The configurational distributions for RDX in a Xe bath and in the gas-phase are markedly different. The results show that as the solvent concentration increases, the concentration of RDX molecules in the boat conformation increases by a factor of about 4. The rate constant for the chair→boat ring inversion was calculated as a function of the xenon concentration [Xe]. The rate constant obeys Lindemann behavior at low concentrations, i.e., it increases with increasing solvent density. At [Xe]∼6.2 mol dm−3, the rate constant reaches a maximum (Kramer’s turnover) and becomes a decreasing function of the solvent concentration. For [Xe] above 16.2 mol dm−3, the rate constant again increases as a function of the solvent density.