bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of tribo-charge/emission at the head–disk interface
Authors: Feng Zhu , Shih Chung , Gubbi Vidya , Poon Frank
Published: July 2002

The tribo-current/voltage buildup at head-disk interface during take-off and flying at the landing zone was measured with an electrometer in conjunction with the measurement of acoustic emission. Effects of various parameters including airbearing designs on head, media texture, lubricant and carbon overcoat on the tribo-emission/charge were investigated. It was found that the intensity of tribo-emission/charge increases with the severity of head–disk interactions (generation of charge), and decreases with the conductivity of the carbon overcoat on disks (charge dissipation). There was a marked effect of relative humidity on the measurements for a given head-disk combination. The reasons for the results are discussed.
Spontaneous emission factor for semiconductor superluminescent diodes
Authors: Zhao Yongsheng , Han Weihua , Song Junfeng , Li Xuemei , Liu Yang , Gao Dingsan , Du Guotong , Cao Hui , Chang R. P. H.
Published: July 2002

The spontaneous emission factor β is an important parameter for the characterization of semiconductor light emitting devices. In the analysis of superluminescent diodes, especially in the calculation of the optical intensity using rate equations, most authors have used the estimated value of β taken from laser diodes, despite the conceptual difference involved in each device. In this article, the spontaneous emission factor β for superluminescent diodes is discussed in detail, and a new method in calculating the average value of β is introduced. Based on this method, the values of β for gain-guided and index-guided structures are obtained.
A probe for moisture permeation using metal/organic composite films
Authors: Higginson Keith A. , Papadimitrakopoulos Fotios
Published: July 2002

Composite films made by simultaneous sublimation of metal and organic materials were used as sensors for moisture in overlying polymeric films, and polymer transport properties were obtained from a spatially resolved signal. In the probe film, the molecularly dispersed metal quenched the fluorescence of the organic dye. Upon exposure to water vapor, the metal was oxidized and the fluorescence of the dye recovered. The intensity profile development over time was modeled, and a diffusion coefficient for a ultraviolet-curable adhesive was obtained which compared well to other resins of its type. This technique offers several advantages in characterizing transport properties in confined geometries, such as those used for the passivation of light-emitting diodes or other electronic devices.
Effect of stray-field coupling on active spin-valve elements analyzed by Lorentz transmission electron microscopy
Authors: Portier X. , Petford-Long A. K. , Anthony T. C. , Brug J. A.
Published: July 2002

In situ experiments using Lorentz transmission electron microscopy have been performed on 10 μm×30 μm active spin-valve elements through which a current is applied during observation of the magnetization reversal of the sense layer. It is shown that the reversal mechanism and the change in resistance are very different depending on the relative orientations of the easy axis versus the applied field. The results show clearly that the contributions of the magnetostatic fields, due to either ferromagnetic coupling or stray-field coupling, are higher than that of the field induced by the sensing current. A model is proposed to predict the offset of the giant magnetoresistance curves with respect to zero field. This model has been found to be in good agreement with the experimental results.
Effective energy-loss functions for oxygen-adsorbed amorphous silicon surfaces
Authors: Nagatomi T. , Shimizu R. , Ritchie R. H.
Published: July 2002

Effective energy-loss functions were derived for oxygen-adsorbed amorphous silicon surfaces from a reflection electron energy-loss spectroscopy analysis based on the extended Landau theory. This study has revealed that the intensity of the surface-plasmon-loss peak for a clean surface decreases and its peak position shifts towards the lower-energy losses as oxygen exposure proceeds (⩽1000 L). To understand the above behavior of the surface-plasmon-loss peak, the distribution of the energy losses was calculated using the hydrodynamic model. The decrease and shift of the surface-plasmon-loss peak has been described with considerable success by assuming that the quasifree static electron density in the vicinity of the silicon surface decreases as oxygen adsorption proceeds owing to oxygen’s high electron affinity.
Theoretical model of twisted high Tc superconducting tapes for numerical alternating-current loss calculations
Authors: Banno N. , Amemiya N.
Published: July 2002

A theoretical model of twisted multifilamentary superconducting tapes was developed for numerical estimations of alternating-current (ac) losses. The model consists of anisotropic Ohm’s law representing the twisted structure [Banno et al., Proc. MT-15, 1140 (1998)], where the superconducting property is represented as equivalent conductivity. The electric field–current density characteristic of superconductor is given by a power law, and a dynamic resistivity is also taken into account in the equivalent conductivity. The twisted structure is represented by a tensor equivalent conductivity. The model enables us to estimate ac losses under the condition that the superconducting tape carries ac in ac magnetic field, which is the most possible and general circumstance for the superconducting tapes in practical power apparatuses. This model was applied to finite element analysis to calculate the ac loss in a high Tc multifilamentary superconducting tape.
Photoconductivity measurement of polymers by x-ray absorption fine structure
Authors: Hu T. D. , Xie Y. N. , Liu T. , Ju X. , Ibrahim K. , Jin Y. L. , Chen L. , Yang C. Z.
Published: July 2002

Photoconductivity spectra measurement of polymers at x-ray energies around the Zn and Co K edges shows obvious x-ray absorption fine structure oscillations. The photoconductivity spectra obtained for gapped and sandwiched electrode geometric samples indicate that the shape of photoconductivity spectrum depends on the electrode configuration of the samples. The thickness of the conduction layer can be estimated from the photoconductivity spectrum.
Quantitative characterization of ion-induced SiO2/Si interface traps by means of MeV He single-ion irradiation
Authors: Koh M. , Ohdomari I. , Igarashi K. , Matsukawa T. , Sawara S.
Published: July 2002

The dependencies of MeV He single-ion-induced interface traps on oxide thickness and oxide electric fields Eox during irradiation was investigated quantitatively with a single-ion microprobe. Under a negative electric field, the number of interface traps induced by a single ion becomes constant at about 0.1 regardless of both the oxide thickness and oxide electric field. Under a positive electric field, the generation rate of interface traps per ion for thicker oxide metal–oxide–semiconductor field-effect transistors (MOSFETs) is greater than that for thinner oxide MOSFETs. We also found that the generation rates increase with dependence on the oxide electric field of Eox0.2, regardless of the oxide thicknesses. The field dependence of interface traps under a positive electric field was found to be very similar to that of oxide trapped holes. The causes of this dependence are discussed comprehensibly.
Vortex dynamics of Nb variable-thickness bridges measured by direct flux detection method
Authors: Hirano S. , Hirata Y. , Kuriki S. , Matsuda M. , Morooka T. , Nakayama S.
Published: July 2002

In order to study the dynamical behavior of vortices moving through superconducting thin film devices, we constructed a probe having a concentric thin-film planar gradiometer coupled to a direct current SQUID. The probe was designed to directly detect the flux generated by a single vortex, based on the monopole approximation. Measurements of static current–voltage (I–V) and flux noise characteristics were made for variable-thickness bridges (VTB) made on epitaxial Nb films. The critical current was unambiguously determined from the onset of vortex motion. Multiple step noise of the flux with a minimum step height of 0.28Φ0 was observed above the critical current in the time traces. This step height is in agreement with the numerical calculation for single vortex motion, confirming the validity of the monopole approximation. It is suggested that the multiple step noise is generated by the successive entry of single vortices into the bridge. VTBs made on polycrystalline Nb films were also measured. The I–V characteristic showed asymmetric voltage increases and a multiple peak structure of the flux noise was observed above the critical current, suggesting the successive depinning of trapped vortices as flux bundles from widely distributed pinning potentials.
Contribution of free-electron recombination to the luminescence spectra of thick GaN films grown by hydride vapor phase epitaxy
Authors: Arnaudov B. , Paskova T. , Goldys E. M. , Yakimova R. , Evtimova S. , Ivanov I. G. , Henry A. , Monemar B.
Published: July 2002

Emission spectra of thick unintentionally doped GaN layers grown on sapphire by hydride vapor phase epitaxy have been studied. In addition to sharp exciton emission lines, a broad asymmetric band extending to energies above the band gap is observed in the near band-gap regions of the photoluminescence and cathodoluminescence spectra. We explain this feature as a free-electron recombination band. Using spatially resolved cathodoluminescence we were able to separately record the spectra of lightly doped regions which contain only sharp bound exciton, the spectra of highly doped regions which show the broadband only, as well as the spectra of the areas containing surface defects which show both spectral features simultaneously. Using a model in which the electrons are degenerate and the holes are localized together with the results of Hall-effect measurements we can simulate the spectral shape of the broadband. Raman scattering spectroscopy confirms the coexistence of lightly doped (∼1×1017 cm−3) and heavily doped (∼1019 cm−3) regions in our films.
Dislocation defect based model analysis for the pre-breakdown reverse characteristics of 4H–SiC p+n diodes
Authors: Zheng L. , Joshi R. P. , Fazi C.
Published: July 2002

A model analysis of the reverse characteristics of 4H–silicon carbide diodes in the pre-breakdown regime is carried out to probe the physics and qualitative trends arising from the presence of dislocations and traps in the material. Results compare favorably with recent data. The response is shown to be controlled primarily by the impact ionization process. With dislocation defects, significant increases in the device current, a softer threshold and lower hold-off capabilities are predicted in keeping with measurements.
Comment on “Influence of the doping element on the electron mobility in n silicon” [J. Appl. Phys. 83, 3096 (1998)]
Authors: Fischetti M. V. , Laux S. E.
Published: July 2002

Kaiblinger-Grujin, Kosina, and Selberherr [J. Appl. Phys. 83, 3096 (1998)] have proposed an explanation for the dependence of the electron mobility in n-type silicon on the doping element. We point out that their model presents some questionable aspects: The macroscopic dielectric constant is used even inside the impurity core, screening by valence electrons is double counted, and the distribution of the valence electrons around the impurity is assumed to be isotropic. We modify the model and find that the dependence on the doping element becomes too weak to explain the experimental results.
Characteristics of an extrapolation chamber for β-ray protection level measurements
Authors: Dias Simone K. , Caldas Linda V. E.
Published: July 2002

An absolute determination of the absorbed dose rates to tissue from a Sr90Sr+90Y β-ray secondary standard source was attempted using an extrapolation chamber developed at the Calibration Laboratory of São Paulo. The performance of this chamber and its main characteristics are presented in this work. A comparison of the results obtained with those of the calibration certificate indicates an absorbed dose rate agreement within 1.0%.
Monitoring of hydrogen consumption along a palladium surface by using a scanning light pulse technique
Authors: Löfdahl M. , Lundström I.
Published: July 2002

The hydrogen concentration over a large palladium surface is studied by using a hydrogen sensitive palladium–oxide–semiconductor device and a scanning light pulse technique. This technique makes it possible to laterally resolve the hydrogen response of the device. Due to the water forming reaction on the catalytic palladium surface, the hydrogen concentration shows a significant decrease in the direction of a gas flow containing a mixture of hydrogen and technical air. A one-dimensional model for the hydrogen consumption in laminar flow is presented and found to compare well to measurements. The results show also that diffusion from regions beside the palladium covered part of the surface affects the response of the device. Furthermore, with the scanning light pulse technique both the local gas concentration and the local sensitivity of the device can be studied and the measurements show that local regions with a deviating sensitivity exist initially.
Effect of pulse duration and strain rate on incipient spall fracture in copper
Authors: Johnson J. N. , Gray G. T. , Bourne N. K.
Published: July 2002

Data are presented on real time (VISAR) measurements of the spall fracture of copper for various pulse durations and tensile strain rates at the spall plane. The impactors consist of Teflon, Y-cut quartz, and a tungsten heavy alloy. VISAR data are compared with finite-difference calculations employing a rate-dependent void-growth model. The data and comparisons show little dependence of the onset of void growth on either pulse duration or tensile strain rate. Also, it is shown that hydrodynamics (wave propagation properties) involving the transmission of the spall signal from the spall plane to the free surface (plane of the VISAR measurement) can mask slight differences in the void-growth or fracture response. In addition, new results are presented for the elastic description of planar wave propagation in Y-cut quartz; expressions are given for the six independent stress components to second order in infinitesimal Lagrangian strains. A discussion with regard to additional use of Y-cut quartz in impact experiments is presented.
Operating conditions of continuous wave simultaneous dual wavelength laser in neodymium host crystals
Authors: Shen H. Y. , Su H.
Published: July 2002

In consideration of the hole-burning effects, based on the single longitudinal mode rate equation, the operating condition of continuous wave simultaneous dual wavelength laser (SDWL) in neodymium host crystals is studied. The relationship between the operating threshold of SDWL and thresholds of the single wavelength laser for both wavelengths is ascertained. The effect of multilongitudinal modes on the operation result has been discussed qualitatively. Taking Nd:YAlO3 crystal as an example, the experimental results agree well with our theoretical analysis.
A perturbation model to predict thermal sensitivity of transverse waves on thin quartz plates
Authors: Briot J. B. , Ballandras S. , Martin G.
Published: July 2002

The development of new acoustic wave devices has received increasing interest for various applications, particularly for sensors, but also in the telecommunication domain. The need to increase operating frequencies together with the efficiency of such devices pushes new principles to produce high-quality acoustic wave resonators and filters. Previous work has been done to experimentally measure the thermal sensitivity of transverse waves trapped by metal strip gratings deposited on thin quartz plates. Temperature compensation was found for AT-cut devices in an extended range of frequency, with the turnover temperature varying from 30 to 0 °C. Moreover, thermal compensation was found for higher-order modes in the Z cut, although the first mode exhibited a very high sensitivity to temperature. A theoretical model has been developed to explain these results. It is based on the Sinha–Tiersten perturbation approach combined with a model of unperturbed propagation, taking piezoelectricity into account. Comparison between the theoretical results provided by this model and experimental data is presented. A very good agreement is found for Z and AT-cut devices, even if second-order effects appear to be predominant in the latter case.
Implant dose dependence in doping Si atoms from a-Si:H films into S+-implanted GaAs
Authors: Yokota Katsuhiro , Nakamura Kazuhiro , Ishizu Tomoyuki , Sakaguchi Masanori , Takano Hiromichi , Kumagai Masao
Published: July 2002

Sulfur (S+) ions were implanted into gallium arsenide (GaAs) at 50 keV at various doses. Hydrogenated amorphous silicon (a-Si:H) films were deposited on the surface of the S+-implanted GaAs using a radio-frequency glow discharge system. The samples were annealed in an Ar atmosphere at 1000 °C for 10 min. During annealing, Si atoms were diffused from the a-Si:H films into the S+-implanted GaAs. The concentration [Si] of the Si atoms increased with the dose [S] of the implanted S+ ions as [Si]=1.6×10−6 [S]1.45.
Mechanism maps for electromigration-induced failure of metal and alloy interconnects
Authors: Andleigh Vaibhav K. , Srikar V. T. , Park Young-Joon , Thompson Carl V.
Published: July 2002

Numerical simulation of electromigration-induced stress evolution provides a versatile technique for analyzing the reliability of interconnects under a wide range of conditions. We study the evolution of stress in confined, layered, stud-terminated, pure metal, and alloy interconnects. Failure times are estimated using different failure criteria associated with different failure modes for broad ranges of line lengths and current densities. The simulation results can be conveniently catalogued through construction of failure mechanism maps that display domains of dominance of different failure modes. Failure mechanism maps are constructed for several different failure criteria, illustrating regimes of line immortality, void-nucleation-limited failure, void-growth-limited failure, and compressive failure as a function of line length and current density. The effects of changes in failure criteria, geometry, and composition are studied for representative interconnect stacks at accelerated and service temperatures. Failure maps may be used to: (i) provide an overview of predicted reliability behavior, (ii) assess how data from accelerated tests can be accurately scaled to service conditions, and (iii) predict the effects of changes in interconnect and shunt-layer materials and dimensions on interconnect reliability.
Structural and magnetic properties of the nanocrystalline alloy Fe86Zr7Cu1B6
Authors: dos Santos D. R. , Torriani I. L. , Silva F. C. S. , Knobel M.
Published: July 2002

A detailed study on the nanocrystallization and its correlation with the attractive soft magnetic properties of the alloy Fe86Zr7Cu1B6 (at %) is presented. Melt-spun amorphous ribbons were submitted to Joule heating for a short time interval (ta=50 s) with different direct current values. The electrical resistance measured during heating showed strong variations associated with the nucleation and growth of α-Fe nanocrystals. From x-ray diffraction line profile analysis we obtained the average grain size, crystalline volume fraction, grain size distribution function, and lattice distortions for different annealing currents. The results display an excellent agreement with the magnetic characterization (ac initial permeability and dc hysteresis loops), showing that the optimum soft magnetic behavior is associated with the formation of about 50% in volume of the α-Fe nanophase. The extended x-ray absorption fine structure (EXAFS) technique was applied to study the evolution of the short-range ordering around Fe atoms. EXAFS spectra also showed the evolution from a disordered Fe environment to the bcc crystalline arrangement. The onset of crystallization determined from both techniques is slightly different, indicating that the nanocrystallization begins at the surface of the studied samples.
On ferro- and antiferromagnetic ordering in ultrafine particles of Fe-rich Fe–Ni and Fe–Mn alloys
Authors: Petrov Yu. I. , Shafranovsky E. A. , Baldokhin Yu. V. , Kochetov G. A.
Published: July 2002

Ultrafine Fe–Ni (28%–32%) and Fe–Mn (30%) particles with an average size of 10–15 nm are studied by combined x-ray diffraction and Mössbauer spectroscopy techniques with the latter being applied at a temperature range from 298 to 4 K. They are produced by evaporation of bulk alloys at 3 Torr Ar pressure. From the x-ray data the ultrafine Fe–Ni (28%–32%) particles are a mixture of bcc and fcc phases, and the ultrafine Fe–Mn (30%) particles contain bcc, fcc, and hcp phases. It is shown that in the former the paramagnetic fcc phase transforms to the antiferromagnetic state with decreasing temperature from 77 down 4 K. As for the latter, the fcc phase is observed to be antiferromagnetic even at room temperature whereas the hcp phase keeps a paramagnetic state right down to 4 K. The results corroborate the Weiss hypothesis that the high temperature face-centered-cubic lattice of Fe-rich alloys can exist in two (ferro- and antiferromagnetic) spin states. The oxide contribution in the spectra is also separated.
Stable microcrystalline silicon thin-film transistors produced by the layer-by-layer technique
Authors: Roca i Cabarrocas P. , Brenot R. , Bulkin P. , Vanderhaghen R. , Drévillon B. , French I.
Published: July 2002

Microcrystalline silicon thin films prepared by the layer-by-layer technique in a standard radio-frequency glow discharge reactor were used as the active layer of top-gate thin-film transistors (TFTs). Crystalline fractions above 90% were achieved for silicon films as thin as 40 nm and resulted in TFTs with smaller threshold voltages than amorphous silicon TFTs, but similar field effect mobilities of around 0.6 cm2/V s. The most striking property of these microcrystalline silicon transistors was their high electrical stability when submitted to bias-stress tests. We suggest that the excellent stability of these TFTs, prepared in a conventional plasma reactor, is due to the stability of the μc-Si:H films. These TFTs can be used in applications that require high stability for which a-Si:H TFTs cannot be used, such as multiplexed row and column drivers in flat-panel display applications, and active matrix addressing of polymer light-emitting diodes.
On the factors governing the energy resolution of Ta-based superconducting tunnel junctions
Authors: Kraft S. , Verhoeve P. , Peacock A. , Rando N. , Goldie D. J. , Hart R. , Glowacka D. , Scholze F. , Ulm G.
Published: July 2002

The spectral response of Ta-based superconducting tunnels junctions to monochromatized synchrotron radiation in the photon energy range between 15 eV and 1.7 keV has been investigated with a view to establishing the physical processes contributing to the energy resolution. The resolution over the whole spectral range is only a factor of two or three above the expected intrinsic tunnel limited resolution. Between 15 and 60 eV peak widths of ∼1 eV have been achieved. Spanning over two orders of magnitude in photon energy the high resolution has allowed us to isolate the various intrinsic components contribution to the spectral response and in essence confirm the validity of models describing the photoabsorption and tunnel processes in a superconducting tunnel junction.
Onset of current self-quenching in a metal/CdS/LaS cold cathode in the presence of inelastic scattering in the CdS layer
Authors: Modukuru Yamini , Cahay Marc , Kolinsky Heidrun , Mumford Philip
Published: July 2002

We use an ensemble Monte-Carlo approach to analyze the importance of space-charge effects in the cathode to anode gap region of a recently proposed metal/CdS/LaS cold cathode and its influence on the energy distribution of the electrons collected at the anode. Our analysis is based on a mean-free path approach to include the effects of inelastic scattering in the CdS layer following Fowler–Nordheim injection across the metal/CdS interface. We investigate in detail the regime of current self-quenching and show the importance of the effects of space charge in narrowing the energy of the electron beam collected at the anode. We compare our results to an analytical treatment of the current self-quenching regime based on a hydrodynamic description of electron transport through the gap.
Production of iron nitrides by mechanical alloying
Authors: Saito Tetsuji
Published: July 2002

The iron nitrides were prepared by mechanical alloying followed by solid state reaction. Mechanical alloying of α-Fe and ε-Fe3N powders resulted in the formation of α-Fe solid solution with the composition of Fe16N2. The mechanically alloyed powders, when annealed at low temperatures (100–200 °C), consisted of α″-Fe16N2, α-Fe, and ε-Fe3N phases and consisted of α″-Fe16N2, α-Fe, and γ′-Fe4N phases when annealed at intermediate temperatures (300–500 °C). On the other hand, the mechanically alloyed powders consisted mostly of α′-martensite phase and showed a higher saturation magnetization than iron powders when annealed at high temperatures (700–800 °C).
Observation of spin polarized transport across a ferromagnet–two-dimensional electron gas interface (invited)
Authors: Hammar P. R. , Bennett B. R. , Yang M. J. , Johnson Mark
Published: July 2002

Spin injection at a ferromagnet–semiconductor interface has been demonstrated by convolving the spin-split density of states of carriers in a high mobility InAs quantum well with the spin polarization of conduction electrons in a thin ferromagnetic film. Transport measurements on a diode structure detected a spin-dependent interfacial resistance, with ΔRi/R̄i≈1%. In a different configuration, open circuit voltage experiments measure the spin-dependent shifts in the chemical potential of the carriers in the quantum well. Results of the two techniques are compared over the temperature range 50<T<296 K, demonstrating both spin injection and detection at the ferromagnet–semiconductor interface.
Role of the density of states in the giant magnetoresistance of magnetic multilayers
Authors: Stanley F. E. , Marrows C. H. , Hickey B. J.
Published: July 2002

The role of the density of states in the giant magnetoresistance (GMR) of ferromagnet 1 (FM1)/nonmagnetic (NM=Cu)/ferromagnet 1 (FM1) sandwiches has been investigated by inserting a thin layer of the second ferromagnetic material (FM2=Co) at the FM1/Cu interfaces. As the thickness of the inserted layer increases, the GMR increases to a value seen for purely Co/Cu/Co sandwiches. By using differing FM1 materials [e.g., Py (permalloy (Ni80Fe20), Ni, Fe] we observe the exponential increase in GMR as more Co is inserted to be dependent on the FM1 material behind it. This is explained by realizing that although the GMR is dominated by the scattering caused by defects like those at the FM2 material/Cu interface, the nature of the scattering depends on the density of states of the FM1 material. This result is discussed in terms of advanced theories on the origin of the GMR in magnetic multilayers.
Corrugated quantum well infrared photodetectors for material characterization
Authors: Choi K. K. , Chen C. J. , Tsui D. C.
Published: July 2002

In this article, we discuss the utilities of corrugated quantum well infrared photodetectors (C-QWIPs) in detector material characterization. By measuring the detector responsivity as a function of corrugation period, several important detector parameters, such as the absorption coefficient α of parallel propagating light and the energy resolved photoconductive gain g, can be directly deduced. For the QWIP material presented, α at the peak was found to be 0.21 μm−1 under the usual operating condition. This value of α corresponds to an absorption length of 4.8 μm. Instead of being a constant, the value of g also varies significantly across the excitation spectrum, and the peak value is larger than the noise gain at large bias. Our results show that the present characterization technique is capable of providing accurate and detailed information on the intrinsic properties of QWIP materials under actual operating conditions. It is extremely useful in detector optimization. In addition, we also show the characteristics of C-QWIPs with an additional vertical trench at the center of each corrugation to gain more insights into the distribution of light intensity in a C-QWIP structure.
Multipolar long-range electrostatic, dispersion, and induction energy terms for the interactions between two identical alkali atoms Li, Na, K, Rb, and Cs in various electronic states
Authors: Bussery B. , Aubert-Frécon M.
Published: July 2002

For the alkali dimers Li2, Na2, K2, Rb2, and Cs2 long-range potential energy curves of the molecular states corresponding to the asymptotes ns+ns and ns+np have been investigated from standard perturbation method up to second order in the multipolar expansion approximation of the interaction energy. Two different cases have been considered: (i) neglecting spin-orbit effects and then values of the long-range expansion coefficients Cn, n=(3), 6, 8, (10) have been determined (ii) including spin-orbit effects and then potential energy curves have been determined numerically and are graphically displayed. Very good agreement is obtained between present curves and very recent long-range experimental ones for the X 1Σ+g state of Li2 and for the X 1Σg+ and 1Πg states of Na2. The position and the energy of purely long-range extrema of the states 1u(2S1/2+ 2P1/2), 1u(2S1/2+2P3/2) and 0g−(2S1/2+2P3/2) have been determined for each alkali dimer investigated.
The coupled pair functional (CPF). A size consistent modification of the CI(SD) based on an energy functional
Authors: Ahlrichs Reinhart , Scharf Peter , Ehrhardt Claus
Published: July 2002

A modification of the CI(SD) energy functional is proposed which leads to size consistency through the use of partial normalization denominators. The method is derived from simple principles: correct description of separated two-electron systems and certain invariance requirements. This approach is connected to CEPA-1. The theoretical framework allows for a simple rationalization of connections between CI(SD), CEPA-1, and the linear version of CP–MET. As demonstrative applications we report comparisons with full CI calculations for BH, NH3, H2O, HF, and Re, De for F2, N2, O2, Cl2, NO, and CO obtained for very large basis sets.
Accurate ground state potential of Cs2 up to the dissociation limit
Authors: Weickenmeier W. , Diemer U. , Wahl M. , Raab M. , Demtröder W. , Müller W.
Published: July 2002

Using the method of Doppler-free double resonance polarization spectroscopy transitions from selectively pumped levels (v′, J′) of the D1Σ+u state down to levels (v″, J″) of the X 1Σ+g state with v″= 0–140 have been measured with high accuracy. A long range analysis, based on 21 outer turning points of the IPA potential with v″=110-130 and on calculated exchange energies and second order van der Waals coefficients Cn with n=6, 8, 10, and 12 leads to a dissociation energy De =3649.5±0.8 cm−1 (0.452 49 eV). The influence of the third order C11 contribution and the hyperfine structure on the correct value of De are briefly discussed.
On the mechanism of dissociative and double photoionization in CO
Authors: Masuoka Toshio
Published: July 2002

Observed branching ratios for dissociative ionization (C+ and O+) and double ionization (CO2+ and C2+) have been differentiated with respect to the incident photon energy in the region between their thresholds and 130 eV. The method provides a new photoion spectroscopy of which results indicate the dissociative ionization and the double ionization channels for the respective ions. The mechanism producing these ions is discussed in the light of the reported energy states of CO+ and CO2+ such as the multielectron bands [C 2Σ+, D 2Π, F, G 2Σ+, and 2Σ+(3σ)−1] and the double ionization states [(5σ−1, 1π−1), (4σ−1, 5σ−1), (5σ−2), (1π−2), (4σ−1, 1π−1), (4σ−2), (3σ−1, 5σ−1), (3σ−1, 1π−1), (3σ−1, 4σ−1), and (3σ−2)] observed in the Auger spectra. It becomes evident that these states play a significant role in producing the fragmented ions. The thermochemical thresholds for various dissociation channels are listed from the known energy levels of the separated atoms and ions in order to deduce the possible energy states of the dissociated fragments.
On the bilinear vibronic coupling mechanism
Authors: Zimmermann Th. , Köppel H. , Cederbaum L. S.
Published: July 2002

The bilinear vibronic coupling mechanism, i.e., vibronic coupling through terms involving the product of two nontotally symmetric vibrational modes, is studied theoretically. For a two-state two-mode model we discuss adiabatic potential energy surfaces and absorption-type spectra which are calculated by perturbation theory and by numerical methods. The inclusion of totally symmetric modes is shown to lead to a biconical intersection of the adiabatic potential energy surfaces and to enhance the nonadiabatic effects. Using ab initio data for the coupling constants the model is applied to study qualitatively the influence of vibronic interactions between the Ã 2Πu and B̃ 2Σ+u electronic states of CO+2 on the absorption and emission spectra. It is shown that bilinear vibronic coupling can explain the experimentally observed anomalies in the B̃→X̃ emission spectrum and the CO+2 Ã/B̃ branching ratio paradox.
Theoretical calculations of carbon-13 NMR chemical shifts via the Xα-scattered wave method
Authors: Freier David G. , Fenske Richard F. , Xiao-Zeng You
Published: July 2002

The Xα-SW molecular-orbital method is applied to the calculation of carbon-13 NMR nuclear shielding constants in a series of small organic molecules. The application of perturbation theory within the Xα framework is discussed in order to place the equations used in the calculation of the shielding constants on firm theoretical ground. Use is made of a highly accurate mixed analytic-numerical method of integration in calculating the necessary matrix elements of 1/r, L/r3, and L. The results for both overlapping and nonoverlapping spheres calculations are shown to reproduce trends in experimental shielding constants remarkably well, but not the correct magnitudes. Calculated shielding constants are shown to be strongly dependent upon the carbon atomic-sphere radius, and the origins of this dependence are discussed in terms of discontinuities in the muffin–tin potential. An established method of choosing sphere radii based upon an initial approximation to the molecular charge density yields results which correlate well with experiment.
Semiclassical calculation of quantum-mechanical wave functions for a two-dimensional scattering system
Authors: Knudson S. K. , Delos J. B. , Bloom B.
Published: July 2002

The semiclassical theory developed by Maslov and Fedoriuk is used to calculate the wave function for two-dimensional scattering from a Morse potential. The characteristic function S and the density Jacobian J are computed in order to obtain the primitive wave function. The incident part shows distorted plane-wave behavior and the scattered part shows radially outgoing behavior. A uniform approximation gives a wave function that is well-behaved near the caustic.
A spectroscopic study of the E(0+g ) state of Cl2 by optical–optical double resonance
Authors: Shinzawa Tsutomu , Tokunaga Atsuto , Ishiwata Takashi , Tanaka Ikuzo
Published: July 2002

An optical–optical double resonance (OODR) technique has been applied to study the E(0+g ) ion-pair state of Cl2 correlating with Cl−(1S)+Cl+(3P2). Observations were made on the v′=0 through v′=23 levels of the 35Cl2 isotope species. A set of Dunham molecular constants was derived by a global weighted least squares fit of 474 transitions (all in cm−1 and 3σ in parentheses): Y00=57 819.366(38), Y10=251.954(15), Y20=−1.0318(19), Y30=2.235(64)×10−3, Y01=0.116 556(62), Y11=−6.403(56)×10−4, and Y02=−4.8(37)×10−8. The emission spectra of the E(0+g )–B 3Π(0+u ) system observed in the OODR experiments were interpreted on the basis of Franck–Condon factor calculations, which established the absolute v′ numbering of the E(0+g ) ion-pair state.
Molecular (Feshbach) treatment of charge exchange Li3++He collisions. II. Cross sections
Authors: Errea L. F. , Martín F. , Méndez L. , Riera A. , Yáñez M.
Published: July 2002

Using the wave functions calculated in the preceding article, and a common translation factor, the charge exchange cross section for the Li3++He(1s2) reaction is calculated, and the mechanism of the process discussed. We show how small deviations from the Landau–Zener model, which are unrelated to Nikitin’s conditions for its validity, lead to a minimum of the cross section at an impact energy E≂1 keV, and to larger values of σ at intermediate nuclear velocities.
Theory of the Liapunov exponents of Hamiltonian systems and a numerical study on the transition from regular to irregular classical motion
Authors: Meyer Hans-Dieter
Published: July 2002

The theory of Liapunov exponents of Hamiltonian systems is reviewed and the connection between vanishing Liapunov exponents and constants of motion is discussed. There are at least 2 N vanishing Liapunov exponents if there are N independent constants of motion. The Liapunov exponents are then used to determine the relative weight of the irregular motion on the energy shell as well as the Kolmogorov–Sinai entropy for two model systems. The trajectory and the stability matrix from which the Liapunov exponent is deduced are integrated with the aid of a Taylor-expansion integration scheme.
The benzene ground state potential surface. I. Fundamental frequencies for the planar vibrations
Authors: Thakur Surya N. , Goodman Lionel , Ozkabak Ali G.
Published: July 2002

The accuracy of vapor phase vibrational data has been improved for all 12 deuterium-labeled benzenes and for 13C12C5H6 and 13C6H6. Many vapor phase fundamental frequencies are observed for the first time. Precise isotopic frequency/splitting patterns for ν1, ν18, and ν19 have been obtained. Isotope induced harmonic mode mixing matrices are given for all 14 labeled benzenes and used to provide detailed description of the fundamental bands observed in the spectra. These descriptions provide numerous reassignments for the fundamental bands, particularily in low symmetry deuterium benzenes. The matrices show that some skeletal modes, such as ν1, gain CH stretching character as a result of deuterium labeling, providing a rationalization for the increased anharmonicity observed in recent jet experiments for C6D6. In addition, a reassessment of Fermi resonance gives 3072.3 cm−1 for the unperturbed frequency (correction +24 cm−1) for the e1u mode ν20 in C6H6 refining the CH local mode anharmonic constant, 2xii, to 117.5 cm−1.
Kinetics of cluster formation in the laser vaporization source: Carbon clusters
Authors: Bernholc J. , Phillips J. C.
Published: July 2002

A general model of cluster formation in the laser vaporization source starting from the atomic vapor is developed and applied to carbon clusters. Two limiting cases of cluster growth exist: the diffusion-limited regime, in which the cluster distribution is essentially featureless, and the reaction-limited regime, in which the most stable clusters show as magic numbers in the cluster distribution. An approximate theory of the aggregation kernel allows for calculation of the cluster distribution in the reaction-limited regime from the formation energies of the reacting clusters. Heat released in cluster fusion allows small and medium size clusters to attain their lowest or almost lowest energy configurations in both the diffusion- and reaction-limited regimes. For larger clusters, crystalline structures are expected for transition metals, while network-forming materials are likely to exibit a multitude of structures. An application to carbon clusters in the n=1–25 range reproduce the experimentally observed cluster distributions and the magic numbers in the n=10–25 regime. The equilibrium structures of the small carbon clusters formed in the nozzle are found to be chains and monocyclic rings. At the reaction temperature, the transition between the chain and ring structures occurs around n=10 for the neutral and the positively charged clusters.
Kinetic studies on the CO oxidation on Pd(111) with low energy electron diffraction (LEED) and angle-resolved thermal desorption
Authors: Matsushima Tatsuo , Asada Hiromu
Published: July 2002

Three different coadsorption structures of CO and oxygen over Pd(111) were analyzed through measurements of the intensity of LEED spots; separate domains of p(2×2)–O and ((3)1/2×(3)1/2)R30°–CO, closely contacted domains of ((3)1/2×(3)1/2)R30°–O with the same structure of CO, and also a mixed phase with (2×1) periodicity. The desorption of CO2 produced in the above adlayers was analyzed in an angle-resolved form. The angular distribution becomes extremely sharp, when the reaction sites are closely surrounded by coadsorbates. The collimation of the desorption along the surface normal is suggested to be due to the collision of desorbing CO2 with the surrounding adsorbates, as well as the repulsive interaction between nascent CO2 and the surface.
Quantitative photoabsorption and fluorescence study of HCl in vacuum ultraviolet
Authors: Nee J. B. , Suto Masako , Lee L. C.
Published: July 2002

The photoabsorption and fluorescence cross sections of HCl were measured in the 106–185 nm region. Sharp absortion bands appear at wavelengths shorter than 135 nm, and fluorescence occurs at several excited states. The fluorescence cross sections are generally quite small, indicating that the excited states are strongly predissociative. The molecular processes for producing the VUV and UV fluorescences are investigated, and the Rydberg characteristics of the strong absorption bands are discussed.
Bound electronic states of HCl−
Authors: ONeil S. V. , Rosmus P. , Norcross D. W. , Werner H.-J.
Published: July 2002

This paper reports ab initio Born–Oppenheimer calculations on the X 1Σ state of HCl and on the 1 2Σ, 2 2Σ, and 2Π states of HCl− for internuclear distances at which these states are electronically bound. The calculations employed square-integrable configuration interaction wave functions which included all single and double replacements from multiconfiguration reference functions. The MC-SCF reference configurations and the Gaussian atomic basis encompass the dominant effects which govern the electronic states studied. In the process of these calculations, we have calculated the static polarizability of Cl− and reproduced the known polarizabilities of H, Cl, and H−. The calculations on the X 1Σ state of HCl yield a De within 0.10 eV of the measured value, and ωe within 3 cm−1 of the measured value. For the molecular ion, we find the potential of the 1 2Σ state of HCl− to be attractive and to undergo substantial charge transfer, and the potentials of the 2Π and 2 2Σ states to be repulsive even at large internuclear separations.
Isotope effects in hydrogen adsorption on Ni(111): Direct observation of a molecular precursor state
Authors: Russell J. N. , Gates S. M. , Yates J. T.
Published: July 2002

The nature of the interaction of hydrogen with Ni(111) is probed by comparing the adsorption and desorption kinetic behavior of H2 and D2. Pure H2 and pure D2 adsorbed on Ni(111) at 140 K exhibit identical desorption behavior. The adsorption rate law depends upon (1-θ) rather than (1-θ)2, suggesting that the rate determining step involves the interaction of an H2 molecule with a single Ni site. The temperature programmed desorption spectra show two desorption features, β1 and β2, which have maxima in desorption rate at 290 and 370 K. When either pure H2 and pure D2 or a mixture of H2 and D2 interact with the Ni(111) surface at 87 K, evidence for kinetic retardation of D2 adsorption is observed. When the H2:D2 mixture is exposed to the Ni(111) surface, held at 87 K, an additional desorption feature, α, is uniquely observed at 100 K for D2. α-D2 desorbs with first order kinetics exhibiting Ed =11.1 kJ/mol and a preexponential factor of 1.2×105 s−1. It may be due to desorption from an intrinsic molecular precursor state. The observed differences between hydrogen and deuterium adsorption indicate the presence of an activation energy barrier for dissociative chemisorption, and a sensitivity of the adsorption process to differences in zero point vibrational energies. A molecular precursor state is proposed to describe the H2+Ni(111) interaction. Dissociative adsorption of hydrogen on intentionally produced surface defects has been shown to destabilize α-D2.
A study of the low-lying electronic states of Fe2 and Co2 by negative ion photoelectron spectroscopy
Authors: Leopold Doreen G. , Lineberger W. C.
Published: July 2002

The anions Fe−2 and Co−2 were prepared and cooled to 300 K in a flowing afterglow ion source, and the low-lying electronic states of the neutral dimers were probed by negative ion photoelectron spectroscopy. Previous ab initio studies of Fe2 and Co2 have predicted single 4s–4s bonds, and extremely high densities of low-lying states due to the small energy cost in transferring electrons among nonbonding 3d orbitals. In contrast to the complex photoelectron spectra implied by these calculations, the observed spectra are remarkably simple. It is argued that this spectral simplicity implies a greater role for the 3d electrons in the iron and cobalt dimer bonds. These data also provide values for the electron affinities of the neutral dimers (0.902±0.008 eV Fe2, 1.110±0.008 eV Co2), the bond elongation on electron attachment (0.08±0.02 Å Fe2, Co2), and the vibrational frequencies of the anions (250±20 cm−1 Fe−2, 240±15 cm−1 Co−2). Related studies of the atomic anions yield improved values for the electron affinities of Fe (0.151±0.003 eV) and Co (0.662±0.003 eV).
Interaction potential for one-phonon inelastic He–Cu(111) and He–Ag(111) scattering
Authors: Eichenauer D. , Harten U. , Toennies J. P. , Celli V.
Published: July 2002

The interaction potential for the elastic and inelastic scattering of low-energy He atoms from the Cu(111) and the Ag (111) crystal surface is derived from the sum of semi ab initio pair potentials modified by a damping function which takes into account the smoothing effect of the conduction electrons at large parallel momentum transfer. By adjusting only two free parameters of the potential model a very good fit to published experimental diffraction intensities and bound state levels is obtained. Without any further adjustment of the potential new experimental data of one-phonon inelastic reflection coefficients can be reproduced within the experimental error by calculations based on the distorted wave Born approximation. This new potential model has made it possible to study the dependence of the inelastic intensities on potential parameters and different experimental conditions.
Cross sections for transfer of rotational angular momentum in CO2 from 13C spin relaxation studies in the gas phase
Authors: Jameson Cynthia J. , Jameson A. Keith , Smith Nancy C. , Jackowski Karol
Published: July 2002

Nuclear spin-lattice relaxation times (T1) have been measured for 13C in 13C16O2 in pure CO2 gas and in CH4, N2, Ar, HCl, Kr, Xe, and SF6 gases as a function of temperature. The relaxation is completely dominated by the spin-rotation mechanism so that empirical values of the cross sections for rotational angular momentum transfer σJ are obtained as a function of temperature. At 300 K the values of σJ/Å2 are 59.9±0.8 (CO2–CO2), 30±1 (CO2–CH4), 26.6±0.8 (CO2–N2), 33.9±0.2 (CO2–Ar), 53.5±0.9 (CO2–HCl), 49±1 (CO2–Kr), 62±2 (CO2–Xe), and 91±3 (CO2–SF6). The temperature dependence of these cross sections in CO2–CO2, CO2–HCl, CO2–Xe, and CO2–SF6 is T−1 as expected, and deviates from T−1 in CO2–CH4, CO2–N2, CO2–Ar, and CO2–Kr.