bluuebunny/crossref_metadata_2025 huggingface dataset:
A magnetic and structural study of Mn, Co, and Ni substituted Fe3Ge2 hexagonal germanides
Authors: Albertini F. , Pareti L. , Deriu A. , Negri D. , Calestani G. , Moze O. , Kennedy S. J. , Sonntag R.
Published: July 2002

Structural (x-ray and neutron diffraction), Mössbauer, and magnetic investigations were performed in order to study Mn, Co, and Ni substituted Fe3.34Ge2 hexagonal germanides. From an x-ray diffraction analysis of Fe3.34Ge2 single crystal data Fe was found to enter exclusively 2a and 2d sites of the defective structure, while vacancies only occupy the 2d site in the congruent composition. It was also found that Ge atoms are slightly displaced from 2c site towards an adjacent vacancy in a 2d site. The preferential entrance of Mn, Co, and Ni was determined by neutron powder diffraction and Mössbauer data in the paramagnetic region. The complex Mössbauer spectra in the ferromagnetic region were interpreted in terms of a distribution of crystal field gradient directions which has its origins from the splitting of the coordinates of Ge atoms due to the presence of vacancies at 2d site. No magnetic contribution was found for Ni, while a small magnetic moment was found to be supplied by Co. In the 2a site (the only one occupied by Mn) a magnetic moment of the same order as that of Fe was found for the host atom. The studied substitutions all reduce the intrinsic magnetic characteristics (Curie temperature, Tc, anisotropy field, Ha, and saturation magnetization, Ms) of the system. The overall anisotropy is planar for all the investigated compounds. The observed Fe planar anisotropy is the result of a competition between an axial anisotropy of Fe in the 2a site and a nine times higher 2d planar contribution. The origin of the observed relevant Fe anisotropy seems to be due to a residual nonquenched orbital moment. Its composition behavior was well described in the frame of a localized model. The temperature behavior of the anisotropy constant for the different compounds substantially follows the Callen and Callen power law with n=3. This suggests the possibility of a single-ion nature for the transition metal anisotropy in these metal-metalloid based compounds.
Charge injection into light-emitting diodes: Theory and experiment
Authors: Arkhipov V. I. , Emelianova E. V. , Tak Y. H. , Bässler H.
Published: July 2002

An analytic theory is presented for dark injection from a metallic electrode into a random hopping system, e.g., a conjugated polymer or a molecularly doped polymer. It encompasses injection of a charge carrier from the Fermi level of the electrode into tail states of the distribution of hopping states of the dielectric followed by either return of the charge carrier to the electrode or diffusive escape from the attractive image potential. The latter process resembles Onsager-type geminate pair dissociation in one dimension. The theory yields the injection current as a function of electric field, temperature and energetic width of the distribution of hopping states. At high electric fields it resembles that the current calculated from Fowler-Nordheim tunneling theory although tunneling transitions are not included in the theory. Good agreement with experimental data obtained for diode structures with conjugated polymers as a dielectric is found.
Microwave losses in incipient ferroelectrics as functions of the temperature and the biasing field
Authors: Vendik Orest G. , Ter-Martirosyan Leon T. , Zubko Svetlana P.
Published: July 2002

The dielectric constant of ferroelectric materials can be controlled by an applied electric field. That is promising for applications using the microwave technique. The widespread use of ferroelectric materials at microwaves is retarded by a considerable high dielectric loss by the materials at high frequencies. The goal of this article is to model the essential mechanisms of losses in incipient ferroelectrics at microwave frequencies. The following materials are studied: strontium titanate (SrTiO3) and potassium tantalate (KTaO3). The following loss mechanisms are considered: fundamental loss determined by multiphonon scattering of the soft ferroelectric mode and transformation of microwave electric field oscillations into acoustic oscillations due to residual ferroelectric polarization.
Hysteretic giant magneto impedance
Authors: Sinnecker J. P. , Tiberto P. , Kurlyandskaia G. V. , Sinnecker E. H. C. P. , Vázquez M. , Hernando A.
Published: July 2002

Giant magneto impedance (GMI) has been measured in Fe77.5Si13.5B9 wires and Co67Fe3Cr3Si15B12 ribbons. The samples were annealed in order to induce specific anisotropies. An hysteretic behavior of the GMI is observed in both cases. Results show that this hysteretic behavior is related to irreversible changes in the magnetization processes of the samples.
Magnetostriction of Tb–Dy–Fe crystals
Authors: Mei Wu , Okane Toshimitsu , Umeda Takateru
Published: July 2002

〈111〉-oriented twin free Tb–Dy–Fe single crystals, 〈112〉- and 〈110〉-oriented twinned “single” Tb–Dy–Fe crystals were prepared using floating zone melting technique. Magnetostrictive performances of the crystals were investigated. Better low-field properties were observed in the 〈110〉 twinned crystals than in the 〈112〉 crystals. The highest properties were achieved in the 〈111〉 twin free single crystals. Even though there are still oxidized particles in the present 〈111〉 single crystals, a large magnetostrictive jump of 1700 ppm and a very low saturation magnetic field of 500 Oe were obtained. To understand magnetization and magnetostriction of different Tb–Dy–Fe crystals, theoretical modeling was carried out based on a simplified domain rotation model. Magnetization moment rotation paths of different domains were simulated and hence the resultant magnetostriction was obtained, which can well account for the experimental results of different crystals. The limitation of the domain rotation model was also discussed.
Ion-beam induced amorphization and dynamic epitaxial recrystallization in α-quartz
Authors: Dhar Sankar , Bolse Wolfgang , Lieb Klaus-Peter
Published: July 2002

We report on the evaluation of ion-beam induced damage in α-quartz and its dynamic annealing behavior in the temperature range between 80 and 1050 K using Rutherford backscattering spectrometry in channeling geometry. The results illustrate that the critical temperature for inhibiting amorphization during irradiation is about Tc≈940 K. The critical fluence φc for amorphization is independent of the temperature up to 550 K, but strongly increases at higher temperatures. The activation energy for the diffusion of defects in the collision cascade or at the amorphous/crystalline interface is found to be 0.28±0.02 eV. The dynamic annealing mechanism is explained by the vacancy out-diffusion model of Morehead and Crowder.
Thermal wave reflection and refraction: Theoretical and experimental evidence
Authors: Bertolotti M. , Liakhou G. L. , Li Voti R. , Paoloni S. , Sibilia C.
Published: July 2002

This article describes and proves the basic phenomena which take place when thermal waves approach an interface between two media: the reflection and the refraction. In synthesis the Snell law for plane thermal waves is proved, both theoretically and experimentally, by means of the mirage technique.
Nanoheteroepitaxy: The Application of nanostructuring and substrate compliance to the heteroepitaxy of mismatched semiconductor materials
Authors: Zubia D. , Hersee S. D.
Published: July 2002

This article describes an approach to the heteroepitaxy of lattice mismatched semiconductors, that we call nanoheteroepitaxy. The theory developed here shows that the 3D stress relief mechanisms that are active when an epilayer is nucleated as an array of nanoscale islands on a compliant patterned substrate, will significantly reduce the strain energy in the epilayer and extend the critical thickness dramatically. Calculations show that with the scale of patterning that is achievable with advanced lithography (10–100 nm) we can eliminate mismatch dislocations from heterojunctions that are mismatched by as much as 4.2%.
“Contact” solid immersion lens near-field optical recording in magneto-optical TbFeCo media
Authors: Chekanov A. , Birukawa M. , Itoh Y. , Suzuki T.
Published: July 2002

The solid immersion lens (SIL) near-field magneto-optical (MO) recording technique offers a significant increase in areal density through near-field resolution enhancement. Since the near-field recording scheme requires close proximity of the optical recording head and the magnetic medium, the SIL MO recording setup is usually changed to a “first surface” recording. This requires further optimization of the optical flying head design and a MO medium. In this work marks as small as 150 nm using a SIL with a 680 nm laser source and a TbFeCo magneto-optical medium were recorded. The minimum size of a recorded mark is strongly dependent on the slider flying height, the flying attitude, and lens centering. It should be emphasized that recording power as small as 4 mW is sufficient enough to achieve carrier-to-noise ratio of more than 30 dB.
Magnetic structure at copper–permalloy interfaces
Authors: Oparin A. B. , Nicholson D. M. C. , Zhang X.-G. , Butler W. H. , Shelton W. A. , Stocks G. M. , Wang Yang
Published: July 2002

First principles local spin density calculations of the magnetic structure of interdiffused Cu/Py multilayers were performed. It was possible to solve the Kohn–Sham [Phys. Rev. 140, A1133 (1965)] equations for both canted and collinear magnetic arrangements to a high level of convergence. We found multiple, metastable, noncollinear magnetic structures that showed a reduced contribution to the saturation magnetization consistent with roughly one “magnetic dead” layer per interface. These canted interface layers were stable with respect to collinear interfaces by about 2 mRy per atom at the interface. The calculated noncollinear total spin magnetic moments as a function of the number of Py layers were in good agreement with the experiment.
Resistance and spin-direction memory loss at Nb/Cu interfaces
Authors: Baxter David V. , Steenwyk S. D. , Bass J. , Pratt W. P.
Published: July 2002

A new method is described for measuring the loss of spin-direction memory (spin relaxation) in nonmagnetic materials and at nonmagnetic interfaces. The method involves inserting the material of interest into an exchange-biased spin valve and monitoring the associated reduction in the spin valve’s magnetoresistance measured with the current perpendicular to the planes. This technique is tested by investigating spin relaxation near Nb/Cu interfaces. A surprisingly large loss per interface is found, between 15% and 25% with larger values being associated with thicker Nb layers.
Morphological and magnetic characteristics of monodispersed Co-cluster assemblies
Authors: Yamamuro S. , Sumiyama K. , Kamiyama T. , Suzuki K.
Published: July 2002

Co-cluster-assembled films have been prepared using a size-controllable cluster beam deposition system, by which monodispersed Co clusters with a mean diameter, d=6–13 nm are available. Their morphology and magnetic properties have been studied by scanning electron microscopy (SEM), small-angle x-ray scattering (SAXS) and magnetization measurements. The SEM images show that the film has a porous structure consisting of fine grains without a columnar texture and its density is about 25% of the bulk Co. The SAXS measurements indicate that monodispersivity of the incident clusters is maintained through their assembling process only for d=13 nm. All the specimens exhibit ferromagnetic behavior at room temperature and the magnetic coercive field Hc rapidly increases with decreasing temperature: Hc=168 kA/m (2.1 kOe) at 5 K. Such an enhancement in Hc is ascribed to the exchange anisotropy which arises from the antiferromagnetic Co–oxide layers covering the Co clusters, and to the assemblies of single-domain ferromagnetic clusters with the structure modification and magnetic interaction among them. The monotonic increase in Hc at 300 K with increasing d is simply understood in terms of the single-domain particle theory.
Photorefractive damage of LiNbO3 quasiphase matched wavelength converters
Authors: Xu C. Q. , Okayama H. , Ogawa Y.
Published: July 2002

Photorefractive damage (PRD) of LiNbO3 quasiphase matched (QPM) wavelength converters is studied by a novel two-beam method, in which the second-harmonic generation (SHG) tuning curve of a broadband probe beam is monitored. The QPM condition and wavelength conversion efficiency, which are characterized by the peak wavelength and peak intensity of the SHG tuning curve respectively, are investigated in detail with respect to the irradiation power and irradiation time. It is found that the QPM condition and wavelength conversion efficiency change markedly even for low irradiation powers (<20 mW), indicating that the PRD effect is non-negligible in LiNbO3 QPM wavelength converters intended for practical applications.
Quantitative interpretation of the magnetoresistive response (amplitude and shape) of spin valves with synthetic antiferromagnetic pinned layers
Authors: Dieny B. , Li M. , Liao S. H. , Horng C. , Ju K.
Published: July 2002

We present an analytical calculation of the shape of the magnetoresistive response of spin-valve structures with synthetic antiferromagnetic pinned layer, i.e., of the form buffer/ferromagnet1/Cu/ferromagnet2/Ru/ferromagnet3/antiferromagnet. The magnetization reversal in the three magnetic layers is assumed to occur via coherent rotation. An analytical expression of the whole hysteresis loop is given as a function of the characteristic parameters of the system (coupling strength through the Ru spacer, ferromagnet3/antiferromagnet pinning energy). We also extended a code based on the Boltzmann equation of transport to calculate the giant magnetoresistance (GMR) amplitude in these structures from the microscopic transport parameters. In order to explain the relatively high GMR amplitude experimentally observed in such spin valves, it is shown that some degree of specular reflection must be introduced at the ferromagnet2/Ru interface. Good agreement with both the shape and amplitude of the experimental magnetoresistance curves can be obtained.
Thermal stability and breakdown strength of carbon-doped SiO2:F films prepared by plasma-enhanced chemical vapor deposition method
Authors: Lubguban J. , Kurata Y. , Inokuma T. , Hasegawa S.
Published: July 2002

Carbon-doped SiO2:F films were deposited by changing the CH4 flow rate, [CH4], using a plasma-enhanced chemical vapor deposition technique from SiH4/O2/CF4/CH4 mixtures. The thermal stability for the films deposited with a fixed temperature of 300 °C was investigated through the changes in the dielectric constant (εS), IR spectra and stress with annealing temperature, TA, from 400 to 800 °C. The breakdown strength for films as-deposited were also measured. The addition of carbon to SiO2:F films resulted in the films with low εS (up to 3.0) and high water resistivity. Addition of high [CH4] in the films resulted in stable εS for TA=500 °C or less. The CH4 addition initially decreased the breakdown strength but then increases again. Under the application of sum rule to the analysis of the vibrational absorption, it was suggested that the decrease in εS with [CH4] results from the decrease in the effective dynamic charge of Si–O dipoles along with the direct contribution of Si–F bonds incorporated to SiO2. The decrease in Si–F and Si–O peak frequencies based on the analysis of the charge transfer model suggested an increase in the Si–F and Si–O bond length, in addition to a decrease in the Si–O–Si bond angle.
Effects of fatigue-induced changes in microstructure and stress on domain structure and magnetic properties of Fe–C alloys
Authors: Lo C. C. H. , Tang F. , Biner S. B. , Jiles D. C.
Published: July 2002

A study of the effects of microstructural changes on domain structure and magnetic properties as a result of fatigue has been made on Fe–C alloys subjected to either cold work, stress-relief annealing, or heat treatment that produced a ferritic/pearlitic structure. The magnetic properties varied with stress cycling depending on the initial condition of the samples. Variations in coercivity in the initial stage of fatigue were closely related to the changes in dislocation structure. In the intermediate stage of fatigue the observed refinement of domain structures was related to the development of dislocation cell structures and formation of slip bands. In the final stage of fatigue the remanence and maximum permeability decreased dramatically, and this rate of decrease was dependent on the crack propagation rate.
Magnetic properties and magnetization reversal of α-Fe nanowires deposited in alumina film
Authors: Peng Yong , Zhang Hao-Li , Pan Shan-Lin , Li Hu-Lin
Published: July 2002

Uniform arrays of Fe nanowires were prepared by electrochemical deposition of iron into nanoporous anodic aluminum oxide films. The microstructure and crystal structures of the nanowires were studied by transmission electron microscopy and electron diffraction. It was found that each nanowire looked like a chain of dots and each dot in the chain was supposed to be a single crystal of α-Fe. Each dot was shown to be a single magnetic domain. The magnetic properties of a uniform array of Fe nanowires and the magnetization reversal in a Fe nanowire were investigated by Mössbauer spectroscopy and vibrating sample magnetometry, which demonstrated that the film of Fe nanowires in alumina had superior perpendicular magnetic characteristics. The magnetic studies also revealed that the moments of each single domain dot were oriented along the chain. Experimental results could be interpreted by the reversal model of “chains of spheres” with the symmetric fanning mechanism.
Disk recording beyond 100 Gb/in.2: Hybrid recording? (invited)
Authors: Ruigrok J. J. M. , Coehoorn R. , Cumpson S. R. , Kesteren H. W.
Published: July 2002

A new method for recording above 100 Gb/in.2 is discussed. We call this method “hybrid recording,” a form of thermally-assisted recording that combines thermo-magnetic writing and magnetic reading. In order to increase the stability of the recorded information, writing is carried out at an elevated temperature on a medium with a very high coercivity at room temperature. In our proposal write and read heads with extremely narrow trackwidths are used, so the trackwidth is not determined by the optical spot size and the written bits have a rectangular shape, in contrast to the schemes proposed by others. Preliminary experiments are shown. The applicability of today’s granular and MO type media for hybrid recording is discussed. It is calculated that hybrid recording on optimized media can give an increase of the areal density of a factor 2.9 in areal density or 7 dB (2.2×) medium SNR improvement in case of Poisson noise and 11 dB (3.4×) in case of transition noise. Practically a factor of about 2 in density is more realistic, pushing limiting densities for longitudinal recording to 100–200 Gb/in.2. Typical limitations at very high densities arise from heat dissipation in the head and thermal instability of the medium. Based on simplified model calculations including realistic limitations on medium, head and leads, and today’s practical limitations on electronics, comparisons are made between read heads containing a tunnel junction magnetoresistive (TMR) element and containing a giant magnetoresistive element with sense current in the plane (CIP-GMR) or perpendicular to the plane (CPP-GMR) of the sensor films. They show that the signal-to-noise ratio of TMR sensors for areal densities above 15 Gb/in.2 is not advantageous over GMR sensors with sense current in the plane as long as the junction’s tunnel resistance is not drastically reduced to below 10 Ω μm2. The CPP-GMR heads are disadvantageous with respect to CIP-GMR heads until the highest densities, 300 Gb/in.2, considered.
Interparticle interactions in magnetic recording media as obtained from high-order measurements by a Preisach model
Authors: Stancu Alexandru , Bissell Philip R. , Chantrell Roy W.
Published: July 2002

Generalized ΔM plots, which allow interactions to be investigated for any starting remanence state, have been used in the analysis of interactions in particulate magnetic recording media for three samples [very high speed (VHS) videotape, Ba-ferrite dispersed particles oriented and nonoriented]. The results have been interpreted using a generalized moving Preisach model. This gives a good fit to the hysteresis loops and ΔM plots. However, agreement with the generalized ΔM plots is less complete. This is interpreted in terms of a requirement for a magnetization dependent standard deviation in the generalized moving Preisach model.
Basic properties of GaAs oxide generated by scanning probe microscope tip-induced nano-oxidation process
Authors: Okada Yoshitaka , Iuchi Yoshimasa , Kawabe Mitsuo , Harris James S.
Published: July 2002

The basic properties of GaAs oxide generated by atomic force microscope (AFM) tip-induced nano-oxidation process have been investigated. The chemical analysis of the AFM tip-generated GaAs oxide was performed by using scanning microprobe x-ray photoelectron spectroscopy, and the main constituents of GaAs anodic oxide were determined to be Ga2O3 and As2O3. The electrical characterization showed that the electron transport across a GaAs oxide nanodot of ∼5.7 nm thickness, from a doped n+-Si tip into the n+-GaAs substrate follows the Fowler–Nordheim tunneling mechanism over a range of applied bias. Further, the tip-generated GaAs oxide nanodots were found to withstand moderate thermal treatments, but some volume reduction was observed.
Rotational analysis of the Ã 2E1/2(0,1,7)←X̃ 2E1/2 (0,0,0) transition in CH3I+
Authors: McLoughlin Russell G. , Morrison James D. , Smith Dianne L. , Wahrhaftig Austin L.
Published: July 2002

A computer simulation of the partially resolved rotational structure observed in the photodissociation spectrum of methyl iodide ion was undertaken in an attempt to determine the extent of rovibronic interaction in this polyatomic ion. From a series of calculations a set of rotational constants have been derived which give a simulated spectrum strongly resembling that obtained experimentally for the Ã 2E1/2(0,1,7)←X̃ 2E1/2(0,0,0) band of CH3I+. The series of prominent peaks observed in the experimental spectrum are reproduced in the simulated spectrum and are due to P branch progressions, within the q subbands, which nearly coincide in energy. The corresponding R branch ‘‘lines’’ are weak by comparison and are lost in the surrounding unresolved rotational lines. The encouraging fit obtained when Hund’s case (c) coupling is assumed for the Ã state is indicative of strong spin-orbit coupling in the Ã state, caused by interaction between the Ã and B̃ states of the ion. The rotational constants obtained from the simulated spectrum are: A′v=5.07±0.05 cm−1, B′v=0.1848±0.0005 cm−1. The nonquantized component of electronic angular momentum along the symmetry axis for the Ã state of CH3I+ is ζ′e=0.5 and the rotational temperature of the ions at the point of photon interaction is 350 K±50 K.
The Na2 a 3Σ+u state. Rotationally resolved OODR 3Πg–a 3Σ+u fluorescence spectroscopy
Authors: Li Li , Rice Steven F. , Field R. W.
Published: July 2002

Thirteen bound vibrational levels (v=0–12) and a portion of the continuum of the Na2 a 3Σ+u state have been observed in rotationally resolved fluorescence spectra from single rotation-vibration levels of two 3Πg states. These 3Πg states were populated by A 1Σ+u∼b 3Πu perturbation-facilitated optical–optical double resonance from the X 1Σ+g state. Analysis, using a LeRoy–Bernstein long-range expression for the energy levels, produced an RKR potential curve from which were derived, in combination with recent results obtained by Barrow et al. on the X 1Σ+g state, principal constants for the a 3Σ+u state of re=5.0911(30) Å, De=174.45(36) cm−1, ωe=24.47(21) cm−1, and Te=5848.21(35) cm−1. In addition, the exchange interaction between two ground state Na atoms was found from the difference between the X 1Σ+g and a 3Σ+u potential curves to be well represented by an exponential, A exp(-BR), A=9.44(71)×105 cm−1, B=1.448(14) Å−1 for R≳5.0 Å.
Rovibronic absorption analysis of the Ã ← X̃ transition of ammonia
Authors: Ziegler L. D.
Published: July 2002

The rovibronic absorption lines shapes of the predissociated Ã ← X̃ transition of NH3 and ND3 are analyzed. All of the observed features in the room temperature and jet-cooled absorption spectra are fit by a single Franck–Condon progression in ν2 with systematically varying linewidths. The N–H equilibrium bond length is determined to be 1.08 Å. Vibronic lifetimes of ND3 and NH3 are compared and consequences for the mechanism of photodissociation are discussed.
Quenching of the 1Σ+g state in liquid oxygen isotopes
Authors: Klingshirn H. , Maier Max
Published: July 2002

A nonexponential decay of the 1Σ+g state in liquid oxygen is observed for high number densities (≫1017 cm−3) of excited O2 molecules. At low excitation intensities the decay of O2(1Σ+g) is exponential with rate constants 1.2×105 and 6.7×103 s−1 for liquid natural O2 and 18O2, respectively (at T=77 K). There is direct experimental evidence that O2(1Σ+g) is quenched to the 1Δg state. Calculations of the ratio of the transition probabilities for collisional deactivation of 16O2(1Σ+g) and 18O2(1Σ+g) are in satisfactory agreement with the experimental results.
Absolute total cross section for the scattering of low energy electrons by methanea)
Authors: Jones Robert K.
Published: July 2002

The absolute total electron scattering cross section for methane (inelastic+elastic, integrated over all angles) was measured with a time-of-flight electron transmission spectrometer for incident energies from 1.3 to 50 eV. The major discrepancies that exist between recent absolute measurements of the total cross section are discussed with careful consideration of probable error limits. Comparisons are also made with recent calculations of the elastic cross section.
The geometric and electronic structure of small copper clusters Cun and Cu+n (n=1–3) by an effective core potential method
Authors: Wang Sheng-Wei
Published: July 2002

Electronic structure calculations have been carried out for the total energy of the ground state of Cun and Cu+n (n=1–3) clusters. The Cu atom is treated as a one-electron system and the effect of the core is approximated by a shape consistent pseudopotential and a semiempirical core-valence, core–core polarization potential. The exchange and correlation energies among the valence electrons are treated by the local-spin-density-functional approximation with and without the self-interaction corrections (SIC). The binding energy and bond distance of Cu2 calculated with SIC are in almost exact agreement with experimental data. The calculated ionization potentials exhibit even–odd oscillations as a function of n. The potential energy surfaces of Cu3 show substantial differences when obtained with and without SIC. The most stable geometry predicted by the latter is an acute triangle with an apex angle of 47° and a long bond length of 5.41 a.u. In terms of a distortion parameter ρ with respect to an equilateral triangle with an equilibrium bond length of 4.78 a.u. we have ρ=0.435 a.u. This is in excellent agreement with the experimental value of ρ=0.472 a.u. The calculated binding energy of Cu3 with respect to fragmentation into Cu atoms is 2.259 eV in comparison with the experimental value of 3.108±0.135 or 3.058±0.151 eV. The 27% error is probably due to the use of local exchange-correlation energies and the somewhat small basis set.
Magic numbers for water–ammonia binary clusters: Enhanced stability of ion clathrate structures
Authors: Shinohara Hisanori , Nagashima Umpei , Tanaka Hideki , Nishi Nobuyuki
Published: July 2002

The formation of water–ammonia binary clusters (H2O)n(NH3)mH+ (q≲40, q=n+m), have been investigated employing a neutral supersonic nozzle expansion of premixed water–ammonia gas with molecular-beam-mass spectrometry. The analysis of the mass spectra reveals that the number of water-rich clusters is greatly increased as the cluster size is increased. Mass spectroscopic evidence for the existence of enhanced structural stabilities (‘‘magic numbers’’) has been found at the protonated clusters (H2O)20(NH3)mH+ (m=1–6) and (H2O)27NH+4 . Considerations for the magic number stabilities are presented within the framework of ion clathrate (ion-centered cage) structures. Monte Carlo simulations are also presented for ionized (protonated) clusters around n=20 and n=27 with m=1. The clusters (H2O)20NH+4 and (H2O)27NH+4 have greater binding energies per molecule than their neighbors, in agreement with the mass spectroscopic observations. The calculated structure for (H2O)20NH+4 also indicates the stability of pentagonal rings of water molecules and deformed dodecahedral structure with an NH+4 ion trapped inside; the cluster is especially stable due not only to the strong Coulombic interaction (ionic hydrogen bonding) between the NH+4 ion and the surrounding 20 water molecules but to hydrogen bonding networks forming the cage structure. The proposed ion-centered cage model can also explain satisfactorily the well-known stability of the water cluster of (H2O)20H3O+.
Diffusion of hydrogen, deuterium, and tritium on the (110) plane of tungsten
Authors: Wang S. C. , Gomer R.
Published: July 2002

Diffusion coefficients for thermally activated and tunneling diffusion as well as mean square fluctuation data are presented for 1H, 2H, and 3H on W(110), as functions of coverage θ and temperature. If D is written as D0 e−E/kT for thermally activated diffusion, there are only small isotope effects, consistent with zero point energy differences for E, which increases slightly with coverage. There are dramatic isotope effects in D0, which can be approximated by ke−cM−1/2 at low θ, k and c being constants. It is suggested that this behavior results from the large vibron spacings relative to phonon energies, and that adsorbate induced changes in the surface phonon spectrum of W(110) are responsible for the changes in D0 with θ which are also very substantial. For tunneling diffusion isotope effects are remarkably small. This is rationalized in terms of additive mass corrections resulting from adsorbate–substrate interactions, which make the effective masses of the three isotopes very nearly the same. The variation in Dtunneling with θ is explained in terms of increasing barrier heights and slight decreases in effective mass as θ increases. Differences in the behavior of 1H and 3H on the one hand and 2H on the other in the tunneling regime at θ>0.6 are tentatively explained in terms of the statistics of the isotopes. The effect of oxygen on tunneling diffusion is also presented and discussed. Mean square fluctuations, proportional to the two-dimensional compressibility of the adlayer show isotope effects, decreasing monotonically with increasing mass. Mixtures were also examined. In the activated regime D0 varies very strongly with composition at total constant θ=0.9, going through minima when the mole fraction of the lighter isotope in each binary mixture is ∼75%. Evidence for phase segregations in the mixtures is also presented: Diffusion and fluctuation results for mixtures are most easily interpreted by assuming complete miscibility of any two isotopes above 133 and below 90 K but segregation into two phases between these temperatures, the two phases coming closest to being pure at 115–118 K.
Hyperfine quantum beats and Zeeman spectroscopy in the polyatomic molecule propynal HC≡CCHO
Authors: Dubs M. , Mühlbach J. , Bitto H. , Schmidt P. , Huber J. Robert
Published: July 2002

Quantum beats in the fluorescence decay of two single rovibronic levels of optically excited propynal (HC≡CCHO) have been measured in a supersonic jet apparatus. The complex beat pattern was explored using laser light of linear and circular polarization under zero-field conditions (nulled earth magnetic field) or in conjunction with an applied magnetic field. Theoretical aspects of molecular quantum beats were derived which include hyperfine coupling, magnetic field effects, and polarized excitation and detection. Singlet–triplet coupling matrix elements, Landé g-factors of the triplet hyperfine levels, and singlet and triplet decay rates have been determined.
A study of the unimolecular decomposition of the (C2H4)+3 complex
Authors: Tzeng W.-B. , Ono Y. , Linn S. H. , Ng C. Y.
Published: July 2002

The energetics and dissociation dynamics of the (C2H4)+3 complexes have been studied by photoionization of neutral van der Waals ethylene trimers. The major product channels identified in the unimolecular decomposition of (C2H4)+3 in the total energy [neutral (C2H4)3 heat of formation plus excitation energy] range of ∼260–336 kcal/mol are C3H+6 +C3H6, C3H+7 +C3H5, C4H+7 +C2H5 (or C2H4+H), C4H+8 [or (C2H4)+2 ]+C2H4, C5H+9 +CH3, and C6H+11+H. The fact that these product channels are similar to those observed in the unimolecular decompositions of (C3H6)+2 and (c-C3H6)+2 is consistent with the interpretation that the (C2H4)+3 , (C3H6)+2, and (c-C3H6)+2 loose complexes rearrange to similar stable C6H+12 ions prior to fragmenting. The ionization energies (IE) of (C2H3)3 and (C2H4)4 are determined to be 9.465±0.036 (1310±5 Å) and 9.287±0.034 eV (1335±5 Å), respectively. Using the known IE’s of (C2H4)n, n=2, 3, and 4, and the estimated binding energies of (C2H4)2 ⋅ C2H4 and (C2H4)3 ⋅ C2H4, the bond dissociation energies for (C2H4)+2  ⋅ C2H4 and (C2H4)+3  ⋅ C2H4 are deduced to be 9.2±1 and 4.6±1 kcal/mol, respectively.
Theoretical study of the electronic properties and crystal structure of poly(perinaphthalene): On the origin of high observed conductivities
Authors: Brédas J. L. , Baughman R. H.
Published: July 2002

Combining the quantum chemical valence effective Hamiltonian (VEH) technique and crystal packing methods, we have investigated the electronic properties and crystal structure of poly(perinaphthalene), PPN. The goal is to understand the origin of high conductivities observed for a pyrolysis product which contains PPN chains. VEH band structure calculations for an isolated PPN chain predict a band gap which is small, 0.44 eV. The highest occupied and lowest unoccupied (HOMO and LUMO) bands have a combined width of about 9 eV. Analogous calculations for poly(perianthracene), PPA, yield a band gap (2.26 eV) which is much larger than that calculated for PPN. The symmetries of the HOMO and LUMO bands provide an explanation for the band gap difference between PPN and PPA. Crystal packing analysis predicts the existence of two phases for PPN, a phase in which PPN molecules are arranged as overlapping dimer pairs and a nondimeric phase in which there is little intermolecular overlap. The observed diffraction patterns for PPN suggests the packing of overlapping molecules in a third type of structure, possibly stabilized by a degree of irregular reaction during synthesis. Quantum chemical calculations indicate that interchain overlap decreases the band gap of PPN, from the 0.44 eV isolated chain value, to 0.29 eV.
Electron correlation in the ground and low-lying excited states of alkaline earth atoms
Authors: Krause Jeffrey L. , Berry R. Stephen
Published: July 2002

Variational wave functions are constructed for the ground and several low-lying excited states of the alkaline earth atoms Be, Mg, Ca, Sr, and Ba. Effective core potentials are employed to treat these atoms as pseudo-two-electron systems. Conditional probability distributions ρ(r2,θ12‖r1=ζ), representing the probability of finding one electron at a distance r2 from the nucleus with interelectronic angle θ12, given that the other electron is at a distance r1 from the nucleus, are plotted for several states of the alkaline earths at various values of ζ. We find that the molecular model which so successfully classifies and describes the collective rotational and vibrational states of doubly excited He carries over well to these ground and low-lying states of the lighter alkaline earths. For the heavier atoms, though the angular correlation remains strong, the application of the molecular model becomes less clear. Some states exhibit behavior intermediate between independent particle-like and collective, and ‘‘interloper states’’ appear which have no immediate interpretation in terms of a single vibrator-rotator model or relationship to the states of doubly-excited helium.
Formation and dissociation of doubly ionized methane
Authors: Fournier P. G. , Fournier J. , Salama F. , Richardson P. J. , Eland J. H. D.
Published: July 2002

Translational energy spectra of H− ions arising from double charge transfer to 1.5–7.0 keV protons from methane have been recorded and interpreted as the single and double ionization spectra of CH4. The excited states of CH++4 are not resolved, but a large broad peak of 4 eV half-width is observed, starting at around 35 eV, with a long tail extending to 44 eV. The peak maximum is at 38.6 eV. Four dissociation reactions of CH2+4 have been observed using photoion–photoion coincidence techniques with Heii light. All the principal discrepancies in comparisons between the experimental and theoretical work on CH2+4 can now be reconciled.
Electron attachment to carbon dioxide clusters at very low electron energies
Authors: Stamatovic A. , Leiter K. , Ritter W. , Stephan K. , Märk T. D.
Published: July 2002

Electron attachment to CO2 clusters formed by nozzle expansion was investigated in a crossed molecular-beam–electron-impact–mass spectrometer system. In addition to cluster ions previously observed at 3–4 eV electron energy we observe presently cluster ions produced at around zero electron energy. Some of these ions are likely produced by a less dissociative production mechanism allowing the probing of cluster beams with better reliability than previously.
Dissociative electron attachment to SO2
Authors: Spyrou S. M. , Sauers I. , Christophorou L. G.
Published: July 2002

The total rate constant ka(〈ε〉,T) for dissociative electron attachment to SO2 has been measured as a function of the mean electron energy 〈ε〉 (1.9–4.8 eV) and temperature T (300–700 K) using an electron swarm technique. From the measured ka(〈ε〉,T), the total electron attachment cross sections σda(ε,T) were determined. The peak value (at ∼4.5 eV) of σda increases by more than a factor of 2 when T is increased from 300 to 700 K, but the peak position and onset of σda shift to lower energy only slightly, indicating a dissociative attachment process with a vertical onset. The magnitude of σda(ε,T≂300 K) was also determined in a time-of-flight mass spectrometric study. These measurements resolve the existing disparity in the literature concerning the magnitude of σda(ε) for this molecule.
Isomers and excitation energies of C4
Authors: Magers David H. , Harrison Robert J. , Bartlett Rodney J.
Published: July 2002

Coupled-cluster (CC) and many-body perturbation theory (MBPT) studies of the rhombic and linear structures of C4 are reported. For each isomer, the electronic spectra is obtained, and comparisons are made with experimental matrix-isolated ESR and electronic spectra. The closed-shell 1Ag rhombic ground state is found to be more stable than the 3∑−g state of the linear isomer by 5 kcal/mol at the highest level of calculation performed (CCSDT-1). However, the predicted spectrum for linear C4 is in reasonable agreement with the observed results. An allowed electronic transition for the rhombus is predicted to lie in the same region, suggesting the possibility that both isomers could coexist in the experiment. Finally, vibrational frequencies for the rhombic isomer are calculated using analytical second-order MBPT second derivatives to aid in the experimental identification of this transient species.
The structure of small metal clusters
Authors: Bauschlicher Charles W. , Pettersson Lars G. M.
Published: July 2002

One metal atom surrounded by its 12 nearest neighbors is considered for both D3d (face-centered cubic-like) and D3h (hexagonal close-packed-like) geometries. For Al and Be, the neutral cluster and the positive and negative ions are considered for idealized (all bonds equal) and distorted geometries. The D3d geometry is found to be the lowest for Be13, while the D3h geometry is lower for Al13. This is the reverse of what is expected based upon the bulk metal structures, Be(hcp) and Al(fcc). Al13 is found to have only small distortions, while Be13 shows large distortions for both the D3d and D3h geometries. The ions have geometries which are similar to those found for the neutral systems. Both all-electron and effective core potential calculations were carried out on the X13 clusters; the agreement is very good.
Homogeneous nucleation rates for n-alcohol vapors measured in a two-piston expansion chamber
Authors: Strey R. , Wagner P. E. , Schmeling T.
Published: July 2002

Homogeneous nucleation rates J in supersaturated n-alcohol vapors (methanol through n-hexanol) were measured in a two-piston expansion chamber as functions of supersaturation S and temperature T. The measured nucleation rates were compared with the classical nucleation theory. We found that the slopes of the J–S curves are in good agreement with the theoretical prediction. However, the actual values of experimental and theoretical rates were generally found to differ significantly, Jexptl/Jtheor ranging from about 10−10 (methanol, 273 K) to 107 (n-hexanol, 257 K). In particular, the experimental nucleation rates show a significantly weaker temperature dependence as compared to the classical nucleation theory, the difference being more pronounced for the higher alcohols. Thus, a temperature dependent correction to the classical nucleation theory is needed. For the lower alcohols, in particular methanol, we found that the change of the cluster distribution during the expansion strongly influences the nucleation process. The heat released during the formation of oligomers causes a significant temperature increase in the system and thus alters the nucleation conditions dramatically. For methanol vapor under the considered experimental conditions this effect leads to a reduction of the theoretically predicted nucleation rates by factors up to about 108. Since oligomer formation is prerequisite to nucleation, this effect has to be taken into account generally for nucleation experiments in adiabatically closed systems.
Theoretical dissociation energies for the alkali and alkaline-earth monofluorides and monochlorides
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Partridge Harry
Published: July 2002

A b initio calculations using self-consistent-field and correlated wave functions are used to determine accurate spectroscopic parameters (re, ωe, D0) for the alkali and alkaline-earth monofluorides and monochlorides. Numerical Hartree–Fock (NHF) calculations are performed on selected systems to ensure that the extended Slater basis sets employed are near the Hartree–Fock limit. Because the bonding is predominantly electrostatic in origin, there is a strong correlation between the dissociation energy (to ions) and re. By dissociating to the ionic limits, most of the differential correlation effects can be embedded in the accurate experimental electron affinities and ionization potentials. With this model, correlation effects are relatively small (0.0–0.4 eV), but invariably increase D0. The importance of correlating the electrons on both the anion and the metal is discussed. For the heavier alkali chlorides, RbCl and CsCl, we show that a core-valence treatment, which excludes the double excitations out of the metal (n−1) shell, gives significantly better re and D0 parameters than does the full singles plus doubles configuration-interaction CI(SD) calculation. The theoretical studies combined with the experimental literature allow us to recommend D0 values that are accurate to 0.1 eV for all systems considered.
Theoretical and experimental studies of the triatomic doubly charged ions CO2+2, OCS2+, and CS2+2
Authors: Millie P. , Nenner I. , Archirel P. , Lablanquie P. , Fournier P. , Eland J. H. D.
Published: July 2002

The term schemes of the doubly charged ions CO2+2, OCS2+, and CS2+2 have been calculated by the CIPSI method using a new and economical basis set of polarized atomic orbitals and difference orbitals. The calculated energies are compared with existing and new experimental data obtained from Auger spectra, double charge transfer, and photoionization including the PIPICO technique. A complete assignment of the manifolds of singlet states is achieved with good agreement between theory and experiment. Only the lowest doubly charged ion states are well described by simple two-hole configurations, while three-hole one-particle configurations are important at higher energy. It is confirmed that the triplet ground states of these ions are well populated by photoionization, while the excited singlet states are revealed most clearly by the double charge transfer technique.
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o calculation of near-equilibrium potential and multipole moment surfaces and vibrational frequencies of H+3 and its isotopomers
Authors: Meyer Wilfried , Botschwina Peter , Burton Peter
Published: July 2002

H+3 potential energies and multipole moments are calculated from a full CI with a 10s, 4p, 2d GTO hydrogen basis. 69 calculated energy points with energies of up to 25 000 cm−1 above the minimum are fitted by a power series expansion in terms of a Morse-type coordinate with a mean square error of less than 1 cm−1. Rotationless vibrational states with energies of up to 12 000 cm−1 above equilibrium are calculated variationally for ten isotopomers. The resulting band origins for the seven analyzed fundamental transitions show a mean deviation of less than 2 cm−1. For the other predicted frequencies, the errors are expected to be below 0.1% also. The equilibrium bond length of H+3 is predicted to be 0.8732(2) Å.
Angle-resolved photoelectron spectroscopy of the valence orbitals of SiCl4 as a function of photon energy from 14 to 80 eV
Authors: Carlson Thomas A. , Fahlman Anders , Krause Manfred O. , Whitley Timothy A. , Grimm Frederick A. , Piancastelli M. N. , Taylor James W.
Published: July 2002

Angle-resolved photoelectron spectroscopy coupled with synchrotron radiation have been used to measure partial cross sections and angular distribution parameters, β, from a photon energy of 14 to 80 eV for SiCl4. Parallel to these measurements, calculations have been made using the continuum multiple scattering Xα method. The results have been examined, primarily in terms of the phenomena of the Cooper minimum and shape resonances. Minima in both the cross sections and β values were found for each of the first five orbitals of SiCl4: 2t1, 8t2, 2e, 7t2, and 7a1. These minima were examined for their energy positions and, in the case of the β values, the depth of the minimum. Shape resonances were calculated in the photoionization of each of the orbitals, and a number of experimental features due to shape resonances are identified. The results, both experimental and theoretical, are compared with earlier work on CC14.
Absolute total cross sections for the scattering of low energy electrons by CCl4, CCl3F, CCl2F2, CClF3, and CF4
Authors: Jones Robert K.
Published: July 2002

Absolute total electron scattering cross sections were measured with a time-of-flight electron transmission spectrometer for incident electron energies ranging from 0.6 to 50 eV. The halomethanes studied exhibit low energy structure which can be associated with electron capture processes. Comparisons are made with recent experiments and calculations.