bluuebunny/crossref_metadata_2025 huggingface dataset:
Tetragonal to rhombohedral transformation in the lead zirconium titanate lead magnesium niobate-lead titanate crystalline solution
Authors: Gupta Surya M. , Viehland Dwight
Published: July 2002

Evidence of a transformation from tetragonal to rhombohedral phases in a 80/20 mixture of lead ziconate titanate (PZT) (53/47) and lead magnesium niobate-lead titanate (PMN-PT) (65/35) was observed by x-ray diffraction, microstructural analysis, dielectric spectroscopy, and electrically-induced strain measurements. An increase in the electrically-induced strain and remanent polarization were found near 175 °C, which was significantly below the transition temperature (370 °C). Near this temperature, dielectric loss measurements revealed small changes, x-ray diffraction studies demonstrated a reduction in tetragonality, and hot-stage electron diffraction patterns revealed a change in spot splitting along the 〈110〉. At room temperature, bright-field imaging revealed that normal size domains coexisted with polar nanodomains. These results are then compared to corresponding investigations of PZT and PMN-PT morphotropic phase boundary compositions.
Magnetic ordering of Ce in the heavy-fermion compound Ce3Al
Authors: Li W.-H. , Peng J. C. , Lin Y.-C. , Lee K. C. , Lynn J. W. , Chen Y. Y.
Published: July 2002

Neutron diffraction and specific heat measurements have been performed to study the magnetic ordering of the Ce ions in the heavy-fermion compound Ce3Al. Detailed crystal structure analysis, determined using high resolution neutron diffraction patterns and Rietveld method, shows that the Ce ions may be grouped into two types: one in the Ce–Al chain, the other in the Ce–Ce chain. The specific-heat data reveal an anomaly at T≈2.2 K, but the calculated magnetic entropy is much smaller than the expected R ln 2 if all Ce spins ordered. Low temperature neutron diffraction measurements confirm that the transition at 2.2 K is magnetic and is associated with the ordering of the Ce spins. The magnetic unit cell is double the nuclear one along the a and c axes, and contains 48 Ce ions. Only the Ce ions in the Ce–Al chains participate in the ordering at 2.2 K, and they are coupled antiferromagnetically.
Ruby R-line shifts for shock compression along (11̄02)
Authors: Hyun Jin-Kee , Sharma Surinder M. , Gupta Y. M.
Published: July 2002

Experimental measurements of the shifts of ruby R lines, for shock loading along the low symmetry r direction, were obtained to evaluate our earlier theoretical formulation regarding the response of R lines in ruby crystals subjected to arbitrary deformations. The experimental results show good agreement with the calculated values based on symmetry adapted irreducible strains and, therefore, make a strong case for the predictive capability of the theoretical model. Additionally, the parameters in the theory were augmented to incorporate the previously observed nonlinear shifts of the R lines for shock compression along the c and a axis. With the general validation of our theoretical model, oriented ruby chips can now be utilized in diamond anvil cell experiments to obtain quantitative estimates of stress deviators at high pressures. These, in turn, can be used to evaluate material strength, and the role of nonhydrostatic stresses on sample response, at large compressions.
Signals induced in semiconductor gamma-ray imaging detectors
Authors: Eskin J. D. , Barrett H. H. , Barber H. B.
Published: July 2002

The signal induced in a readout circuit connected to a pixel electrode in a semiconductor gamma-ray imaging array is calculated by solving the Laplace equation. Two approaches are presented that use Green functions in solving the boundary value problem: decomposition into basis functions, and construction of an infinite series of image charges. Another approach is developed based on the Ramo–Shockley theorem, which makes use of weighting potentials. These potentials may be readily calculated in three dimensions using a Fourier-transform propagation technique. An analytic solution is found for the special two-dimensional case of a strip detector. Experiments on CdZnTe square-pixel test structures using alpha radiation confirm the expected trends in pulse shape as a function of pixel size. Signals observed simultaneously on adjacent pixels also follow the predicted division of currents. Trends with pixel size are also confirmed in the shape of pulse-height spectra taken using a Tc99m source.
Ferromagnetic resonance in ferrite nanoparticles with uniaxial surface anisotropy
Authors: Shilov V. P. , Bacri J.-C. , Gazeau F. , Gendron F. , Perzynski R. , Raikher Yu. L.
Published: July 2002

Magnetization oscillations in a single-domain spherical ferromagnetic particle with uniaxial surface and bulk anisotropies are studied. In a linear approximation we consider a weakly nonuniform precession mode and derive the dispersion relationships for the cases of the applied field either parallel or normal to the particle easy axis. It is shown that in both situations the surface can produce a considerable shift of the precession frequency as observed with the conventional ferromagnetic resonance (FMR) technique. The data obtained for fine particle assemblies (frozen magnetic fluids based on γ-Fe2O3 nanosize grains) support the conclusion. Moreover, in the systems under study the surface anisotropy contribution to the FMR resonance field appears to entirely dominate the bulk one.
Structural and optical properties of type II GaSb/GaAs self-assembled quantum dots grown by molecular beam epitaxy
Authors: Suzuki K. , Hogg R. A. , Arakawa Y.
Published: July 2002

We report structural and optical properties of GaSb/GaAs self-assembled quantum dots (QDs) grown by molecular beam epitaxy. The QDs, with nanometer-scale dimensions, were characterized by atomic force microscopy. Furthermore, in photoluminescence (PL) measurements the feature from the QDs was observed at ∼1.1 eV, clearly separated from that of the wetting layer at ∼1.3 eV. With increasing excitation power, the peak from the QDs displayed a large shift towards higher energy. In addition, the temperature dependence of PL yielded a large thermal activation energy, 130 meV, confirming the strong localization of excitons in the QDs.
Dopant activation of heavily doped silicon-on-insulator by high density currents
Authors: Chen Chih , Huang J. S. , Liao C. N. , Tu K. N.
Published: July 2002

We have activated heavily boron-doped and arsenic-doped silicon-on-insulator (SOI) strips by applying electrical current. The SOI strips were implanted with 40 keV BF2+ or As+ at a dosage of 5×1015 ions/cm2. Without postimplantation annealing, these implanted SOI strips can be activated by applying a current up to 1×106 A/cm2, the resistance decreased from 8.80 to 0.61 kΩ for a 10 μm wide, 50 μm long, and 0.2 μm thick n+ silicon strip, for instance. This reduction of resistance is close to that obtained by the conventional postimplantation annealing at 900 °C for 30 min. To separate the effect of Joule heating from that of current activation, the temperature of the SOI strips during the current activation has been measured by Pt sensors. The result indicates that the temperature rise due to Joule heating is low and cannot explain the observed activation. We conclude that implanted dopants in Si can be activated by current stressing. To confirm it, carrier concentration obtained by Hall measurement is presented.
Valence state and local atomic structure of copper in Cu-implanted silica glass
Authors: d’Acapito F. , Mobilio S. , Battaglin G. , Cattaruzza E. , Gonella F. , Caccavale F. , Mazzoldi P. , Regnard J. R.
Published: July 2002

Copper implanted fused silica samples were investigated for the first time by depth-selective x-ray absorption techniques. X-ray absorption near-edge structure and extended x-ray absorption fine structure measurements were performed using both x-ray fluorescence yield and conversion electron yield detection configurations, with the aim to discriminate the contribution of the copper ions at different depths. The copper species were found in both oxidized and metallic states. A dependence of the oxidized copper amount on the depth was detected. Compositional analyses were made by means of secondary ion mass and Rutherford backscattering spectrometries. A matrix-damage related oxidation effect on copper atoms was confirmed.
Asymmetry in the dark current low frequency noise characteristics of B–B and B–C quantum well infrared photodetectors from 10 to 80 K
Authors: Kolev P. , Deen M. J. , Liu H. C. , Li J. M. , Buchanan M. , Wasilewski Z. R.
Published: July 2002

We present experimental results demonstrating the significant effect that the type of intersubband transition has on the dark current noise of quantum well infrared photodetectors containing 32 Si-doped GaAs quantum wells separated by ∼350 Å thick AlxGa1−xAs barriers. The experimentally observed asymmetry in the noise characteristics is similar but larger than the asymmetry between dark currents under forward and reverse bias voltages. We are able to control the ratio between the components of the dark current by biasing with constant current and varying the temperature over an extended range well below the intended operational point. Our results indicate a clear relationship between the excessive low-frequency generation-recombination noise and the thermally assisted tunneling component of the dark current. We have also found indications that the excessive 1/f like noise component can be related to the tunneling process in direct well-to-well tunneling and to thermally assisted tunneling dark current mechanisms.
Drude conductivity of highly doped GaAs at terahertz frequencies
Authors: Huggard P. G. , Cluff J. A. , Moore G. P. , Shaw C. J. , Andrews S. R. , Keiding S. R. , Linfield E. H. , Ritchie D. A.
Published: July 2002

Time domain spectroscopy has been used to measure the room temperature transmission of highly doped GaAs in the frequency range from 0.2 to above 3 THz. We studied n- and p-type layers, with carrier densities between 1016 and 2×1018 cm−3, which had been grown on undoped GaAs substrates. Transmission spectra could be fitted within experimental error by using a Drude model for the conductivity. Fitted carrier densities for both carrier types and mobilities for p-type GaAs were in good agreement with the results of Hall measurements. In the case of n-GaAs, the optically determined mobility appeared to underestimate slightly the Hall mobility.
Aging behavior and recovery of polarization in Sr0.8Bi2.4Ta2O9 thin films
Authors: Chen San-Yuan , Lee Ving-Ching
Published: July 2002

Ferroelectric thin films of bismuth-containing layered perovskite Sr0.8Bi2.4Ta2O9 (SBT) have been prepared by both metalorganic decomposition (designated as MOD–SBT film) and magnetron sputtering (designated as sputtering-SBT film) processes. SBT thin films are well known to exhibit free-fatigue behavior with electrical field cycling. However, it was found that after the SBT films were applied with electrical field display, the polarization reduction with time was observed. The aging rate was related to microstructure, which in turn was dependent on the film processing. The sputtering-SBT films show a slower degradation rate than MOD–SBT films. The suppressed polarization can be near-completely recovered by applying either thermal treatment above 150 °C or electric-field cycling. There exists a minimum required thermal energy to restore the polarization. The easy recovery exhibited by SBT films in suppressed polarization reveals that a relatively weak domain pinning may exist between domain wall and electronic defects in SBT. The SBT shows little fatigue behavior during electric field cycling but exhibits an aging phenomenon after applied electric field.
Magnetic depth profiling Co/Cu multilayers to investigate magnetoresistance (invited)
Authors: Unguris J. , Tulchinsky D. , Kelley M. H. , Borchers J. A. , Dura J. A. , Majkrzak C. F. , Hsu S. Y. , Loloee R. , Pratt W. P. , Bass J.
Published: July 2002

The magnetic microstructure responsible for the metastable high resistance state of weakly coupled, as-prepared [Co(6 nm)/Cu(6 nm)]20 multilayers was analyzed using polarized neutron reflectivity and scanning electron microscopy with polarization analysis (SEMPA). This article focuses and expands on the SEMPA measurements. In multilayer structures such as these, SEMPA can be combined with ion milling to directly image the layer-by-layer magnetization and quantitatively depth profile the interlayer magnetic domain correlations. We found that in the as-prepared Co/Cu multilayer, the domains are about 1 μm in size and the magnetizations in adjacent layers are almost completely oppositely aligned. The relative magnetoresistance derived from this measured degree of anticorrelation is in agreement with the measured magnetoresistance.
Structure and magnetic properties of L1 CoPt(Ag/MgO,MgO) thin films
Authors: Jeong Sangki , Hsu Yu-Nu , McHenry Michael E. , Laughlin David E.
Published: July 2002

Disordered fcc (face-centered-cubic) [100] fiber-textured CoPt thin films with thicknesses of 20–40 nm were deposited on a nonmagnetic sputtered MgO seed layer with and without a Ag intermediate layer. These were subsequently annealed at 600–750 °C for 5–10 min using rapid thermal annealing (RTA). The structural variants were determined to coexist in the in-plane and normal to the plane directions after the RTA process. The MgO underlayer without the intermediate layer revealed strong in-plane anisotropy by magnetic hysteresis measurement. Selected area diffraction (SAD) by TEM and XRD measurements for these samples confirmed the preferential growth of in-plane variants of c axis compared to the perpendicular direction. High remanence and coercive squareness (S*) were obtained due to strong in-plane texture and especially due to exchange coupling among the grains. Evidence for this was provided by ΔM curves. Initial magnetization curves for samples annealed above 600 °C, showed the possibility of assembled single domain or domain pinning mechanism but with negligible free wall motion inside of the grains.
Effect of sputter heating in ionized metal physical vapor deposition reactors
Authors: Lu Junqing , Kushner Mark J.
Published: July 2002

Ionized metal physical vapor deposition (IMPVD) is a process in which sputtered metal atoms from a magnetron target are ionized by a secondary plasma, accelerated into the substrate, and deposited with moderately anisotropic fluxes. The momentum and energy transfer from the sputtered metal atoms and ion-produced reflected neutrals to the background gas, sputter heating, produces rarefaction which influences the operating characteristics of the discharge. To address these processes, a model was developed to simulate the sputtering of metal atoms and their transport in IMPVD reactors. The model accounts for the ion-energy-dependent yield and kinetic energy of the sputtered and reflected atoms, and for sputter heating. The model was validated by comparing its results to experimentally measured metal atom densities and the ionization fraction of the deposition flux. Sputter heating as a function of auxiliary ionization and magnetron power in an inductively coupled plasma IMPVD reactor for Al deposition was then investigated. Sputter heating produces rarefaction of the buffer gas which results in a redistribution of Al species in the reactor compared to the absence of sputter heating. Consequently, the ionization fraction of the depositing metal flux decreases, while the magnitude of the flux increases. The minimum Ar density due to sputter heating is regulated by heat transfer to the target. The electron density increases significantly with the addition of a small amount of metal atoms to the plasma.
Magnetic coupling induced increase in the blocking temperature of γ-Fe2O3 nanoparticles
Authors: Dai Jianbiao , Wang Jian-Qing , Sangregorio Claudio , Fang Jiye , Carpenter Everett , Tang Jinke
Published: July 2002

In this article, we report the magnetic properties of surfactant coated γ-Fe2O3 nanoparticles which are pressed under different pressures. With increasing pressure, the sample volume decreases, density increases, and a 55% density change has been achieved. The blocking temperature is increased from 50 to 80 K. Analyzing the data of blocking temperature versus densities, which exhibits linear relationship, and comparing the magnetic properties, the increase in blocking temperature is understood in terms of increased magnetic interactions between neighboring nanoparticles, which is due to the reduced average interparticle distance by the applied pressure.
Structure of nitrogenated carbon films prepared from acetylene and nitrogen mixture in electron cyclotron resonance plasma
Authors: Bhattacharyya Somnath , Vallée C. , Cardinaud C. , Turban G.
Published: July 2002

Amorphous nitrogenated carbon (a-CNx) films have been prepared from a mixture of acetylene and nitrogen gas in an electron cyclotron resonance plasma and characterized by electron energy-loss spectroscopy (EELS), spectroscopic ellipsometry (SE), Fourier transformed infrared (FTIR) spectroscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy. From EELS and SE, a significant change in the π plasmon peak position and a reduction of its area is observed in the carbon films after nitrogen introduction, which suggest that there is no further development of graphitic structure. The features of D and G peaks observed from Raman as well as FTIR spectra support a decrease in the amount of sp2 bonded carbon in the a-CNx films. Valence band spectra using He I and He II excitations show that the p-π band becomes less intense upon nitrogen addition. A comparative study between the characteristics of these films and the films deposited from a methane-nitrogen mixture using an identical procedure is also presented. It is found that the structural changes in these films upon nitrogen incorporation are different, indicating a definite role of the precursors on the film structure.
Magnetic properties and domains in Co–Ag granular thin films
Authors: Chen Y. J. , Suzuki T. , Kanazawa H.
Published: July 2002

The intrinsic perpendicular magnetic anisotropy and the domain patterns in granular CoxAg1−x films with 17%⩽x⩽62% are studied. Magnetic stripe domains are found in samples with x⩾45% and to change to in-plane domain patterns with increasing annealing temperature. This is consistent with the fact that perpendicular magnetic anisotropy becomes negative with annealing.
Solution to the Boltzmann equation for layered systems for current perpendicular to the planes
Authors: Butler W. H. , Zhang X.-G. , MacLaren J. M.
Published: July 2002

Present theories of giant magnetoresistance (GMR) for current perpendicular to the planes (CPP) are based on an extremely restricted solution to the Boltzmann equation that assumes a single free electron band structure for all layers and all spin channels. Within this model only the scattering rate changes from one layer to the next. This model leads to the remarkable result that the resistance of a layered material is simply the sum of the resistances of each layer. We present a solution to the Boltzmann equation for CPP for the case in which the electronic structure can be different for different layers. The problem of matching boundary conditions between layers is much more complicated than in the current in the planes (CIP) geometry because it is necessary to include the scattering-in term of the Boltzmann equation even for the case of isotropic scattering. This term couples different values of the momentum parallel to the planes. When the electronic structure is different in different layers there is an interface resistance even in the absence of intermixing of the layers. The size of this interface resistance is affected by the electronic structure, scattering rates, and thicknesses of nearby layers. For Co–Cu, the calculated interface resistance and its spin asymmetry is comparable to that measured at low temperature in sputtered samples.
The effect of microstructural changes on magnetic Barkhausen noise and magnetomechanical acoustic emission in Mn–Mo–Ni pressure vessel steel
Authors: Song Y. Y. , Park D. G. , Hong J. H. , Ahn Y. S. , Kim G. M.
Published: July 2002

The effect of microstructural changes on magnetic Barkhausen noise (BN) and magnetomechanical acoustic emission (MAE) in Mn–Mo–Ni pressure-vessel steel with various microstructures was investigated. The BN and MAE signals were strongly influenced by the microstructural features which were characterized by transmission electron micrograph observation. BN energy and the measured area under the rectified signal envelope varied inversely with hardness, and changed with the microstructure. It indicates that they are closely related to the dislocation density and residual stress which act as a barrier to irreversible domain wall motion.
Topology of amorphous gallium arsenide on intermediate length scales: A molecular dynamics study
Authors: Ebbsjö Ingvar , Kalia Rajiv K. , Nakano Aiichiro , Rino Jose P. , Vashishta Priya
Published: July 2002

Structural correlations in amorphous gallium arsenide are investigated with molecular-dynamics simulations using a new interatomic potential function. The calculated static structure factor, in particular the height and width of the first peak which is a signature of the intermediate-range correlations, is in excellent agreement with x-ray diffraction experiments. Atomistic topology on intermediate length scales is elucidated through the analyses of shortest-path rings, partial static structure factors, and bond-angle distributions. The calculated energy difference between crystalline and amorphous systems is also in good agreement with electronic-structure calculations.
Sol-gel derived Ba(Fe, Ti)O3 ferroelectric materials for infrared sensors
Authors: Ray (Mal) Jayati , Hing Peter
Published: July 2002

In this article we report the development of Fe modified BaTiO3 using sol-gel technique and present results of structural and dielectric studies to establish its suitability for pyroelectric sensor applications. Fine powders of Fe modified BaTiO3 were prepared by sol-gel process using barium acetate, iron isopropoxide, and titanium isopropoxide. X-ray structural analyses of the BaTiO3 with different proportion of Fe ion suggest the formation of single-phase compounds having tetragonal structure at a relatively low temperature around 750 °C for 4 h. The particle size of the powder was found to be in nm range. The sintering temperature of 1250 °C for the above calcined powder was considerably lower than the powders made by the conventional methods. The ferroelectric to paraelectric phase transition of Ba(Fe, Ti)O3 ceramics was studied using the dielectric measurement. The value of dielectric constant for Fe modified BaTiO3 was found to be lower than pure BaTiO3 and the transition temperature shifted towards the lower temperature side with an increase in Fe ion concentration. A combination of low dielectric constant and a low transition temperature makes Fe modified BaTiO3 materials attractive for room temperature pyroelectric sensor applications.
Quantum and classical vibrational chaos in floppy molecules
Authors: Farantos Stavros C. , Tennyson Jonathan
Published: July 2002

Classical and quantum mechanical calculations on the vibrational motions of LiCN using a realistic potential are presented. These, together with recent results for KCN, are analyzed using indicators that have been proposed for identifying quantum chaos. These are nodal structure, dominant coefficient, overlapping avoided crossings, second differences, and spectral distribution. We find an early onset of chaos in LiCN and KCN with good agreement between the indicators. Localized quasiperiodic trajectories and regular states are found well into the chaotic region for both isomers of LiCN. LiCN is a weakly coupled system in contrast to strongly coupled KCN. The utility of the indicators is discussed in the light of these results. The link between floppy molecules and chaos suggests that floppy systems are suitable for the experimental investigation of vibrational chaos.
Kinetics of the reactions of NCO radicals with H2 and NO using laser photolysis–laser induced fluorescence
Authors: Perry R. A.
Published: July 2002

Absolute rate constants for the reaction of NCO radicals with hydrogen (k6; T=592–913K) and nitric oxide (k7;T=294–538K) were measured using excimer laser photolysis-laser induced fluorescence at 416.8 nm. The following Arrhenius expressions summarize the data: k6=1.43×10−11e(−9000±1500)/RT k7=1.69×10−11e(390±320)/RT cm3 molecule−1 s−1. The experimental technique and results are discussed, and the importance of these reactions to combustion chemistry is addressed.
Molecular beam photoionization study of S2
Authors: Liao C. L. , Ng C. Y.
Published: July 2002

Higher resolution photoionization efficiency data for the formation of S+2 and S+ from S2 in the wavelength region of 600–1350 Å have been obtained using the supersonic oven beam method. The ionization energy (IE) of S2 is determined to be 9.356±0.002 eV (1325.2±0.3 Å). The measured appearance energy of 14.732±0.005 eV (841.6±0.3 Å) for the dissociative ionization process S2+hν→S++S+e−, together with the IE of S2, yields a value of 5.376±0.005 eV for the bond dissociation energy of S+2. Two Rydberg series converging to the b̃ 4Σ−g state (13.20 eV) of S+2 are observed. Window resonances resolved in the wavelength region of 670–850 Å are assigned as members of the Rydberg series converging to the c̃ 4Σ−u (17.70 eV) and 2Πu (or 2Σ−u) (18.66 eV) states of S+2.
Interfacial surface statistics arising in diffusion and flow problems in porous media
Authors: Torquato S.
Published: July 2002

A study is made of the surface–void, surface–surface, and surface–particle correlation functions that arise in expressions for transport properties associated with diffusion-controlled reactions and flow in porous media consisting of impenetrable spherical particles distributed randomly throughout void. The relationship between these three different correlation functions is noted for this model. We present an exact low-density expansion of the surface–particle correlation for such a distribution. All of the two-point correlation functions are computed, for the first time, from integral relations that we derived elsewhere, as a function of the distance between the two points for various sphere volume fractions up to 94% of the random close-packing value.
The C̃→Ã emission in water: Theory and experiment
Authors: Engel V. , Meijer G. , Bath A. , Andresen P. , Schinke R.
Published: July 2002

The C̃→Ã emission spectra for H2O and D2O are measured and calculated. The theoretical model is based on an exact treatment of the dissociation dynamics of the Ã state using a calculated potential energy surface. Agreement with the measurements is excellent. The spectra extend from λ∼380 nm up to λ∼600 nm with maxima around 425 (H2O) and 440 nm (D2O).
Vibrational level structure of highly excited SO2 in the electronic ground state as studied by stimulated emission pumping spectroscopy
Authors: Yamanouchi Kaoru , Yamada Hideyuki , Tsuchiya Soji
Published: July 2002

The stimulated emission pumping (SEP) spectroscopy is applied to SO2 cooled rotationally in a supersonic free jet to investigate the vibrational and rotational level structure in the 17 300–17 900, 21 400–21 500, and 22 200–22 500 cm−1 regions of the electronic ground state. It is concluded that respective vibrational levels are found to couple with each other by the network of Fermi and Coriolis interactions on the basis of the following grounds: (1) there are a number of Fermi pairs of the vibrational levels, and (2) the number of vibrational levels identified as the final states of the SEP transitions increases for rotational levels with larger rotational quantum numbers J and Ka. The distribution of the nearest neighbor vibrational level spacing shows that the vibrational quantum dynamics in the observed regions is almost quasiperiodic and the onset of the quantum chaos is estimated to be above 17 900 cm−1.
Predissociation-induced rovibrational autoionization and zero-energy photoelectrons
Authors: Chupka William A. , Miller Paul J. , Eyler Edward E.
Published: July 2002

A recent model of molecular autoionization involves an intermediate neutral dissociative state to explain extensive vibrational structure in the photoelectron spectrum. As an extension of this previous work, we propose a modification of the model by invoking a continuity condition in order to explain the ‘‘zero-energy’’ electron phenomenon. The condition requires that any photoelectron spectrum extending smoothly to zero energy with finite probability must involve the excitation not only of the continuum, but also of high Rydberg states. Rotational autoionization of these states will produce a very narrow photoelectron peak at zero energy superimposed on a broader peak due to vibrational autoionization.
Site specific fragmentation in molecules: Auger-electron ion coincidence studies on N2O
Authors: Murphy R. , Eberhardt W.
Published: July 2002

We have studied the electronic decay of an oxygen 1s core hole in N2O and the subsequent fragmentation into ions. Coincidence studies between the energy resolved Auger electrons and the ions reveal the individual decay channels for the various doubly charged ionic configurations populated in the Auger decay of the core hole. The site specificity in these events is not absolutely confined to breaking only the O–NN bond, even though the excitation of a core hole at the oxygen site in the initial absorption event is localized. This localization is to some extent preserved in the Auger decay, which favors the participation of valence electrons having wave functions with a large overlap at the site of the initial oxygen core hole. However some of these electrons are also crucial in forming the ON–N bond or the bonding structure of the molecule as a whole, such that removal of these electrons in the Auger decay results in the total destruction of the molecular bond.
High resolution laser spectroscopy of the c 3Σ+←X 1Σ+ and b 3Π←X 1Σ+ forbidden transitions in NaK
Authors: Kowalczyk P.
Published: July 2002

The ‘‘spin-forbidden’’ c 3Σ+←X 1Σ+ and b 3Π←X 1Σ+ band systems have been studied in the excitation spectrum of the 23Na39K molecule in the range of 560–600 nm. Single mode continuous wave (cw) dye laser excitation in a supersonic molecular beam yielded Doppler-free spectra and allowed resolution of the hyperfine structure of lines. Mutual perturbations of the c 3Σ+, b 3Π, and B 1Π states were studied. The observed vibrational variation of perturbation matrix elements for c 3Σ+–B 1Π interaction was used to assign the absolute vibrational numbering in the c state. The deperturbed molecular constants of this state have been found and its fine structure examined. The hyperfine structure in the c 3Σ+ and b 3Π states was studied for several vibrational levels and through a wide range of J values. For the c 3Σ+ state, fits of known theoretical expressions for the hyperfine energies to the experimental data provided accurate values of the hyperfine coupling constants. The results are discussed in terms of the electronic structure of the c 3Σ+ state.
Fourier transform spectroscopy of the 1 3Σ+g–a 3Σ+u transition of the 6Li2 molecule
Authors: Linton C. , Murphy T. L. , Martin F. , Bacis R. , Verges J.
Published: July 2002

The 1 3Σ+g–a 3Σ+u transition of 6Li2 has been observed via collisionally induced fluorescence, excited by visible lines of an argon-ion laser and detected at high resolution with a Fourier–transform spectrometer in the 8200–10 100 cm−1 region. By combining the results with previously obtained data on 7Li2 [F. Martin, R. Bacis, J. Vergès, C. Linton, G. Bujin, C. H. Cheng, and E. Stad, Spectrochim. Acta Part A 44, 1369 (1988)], an accerate, isotopically consistent description of both states has been obtained for 1≤v′≤7 and 0≤v″≤7. Equilibrium constants, Rydberg-Klein-Rees potential curves, and dissociation energies have been determined and found to be in good agreement with ab initio calculations. From the analysis, the following positions and dissociation energies of the two states were found. For 1 3Σ+g, Te (cm−1) is 16 328.8(1.7) and De (cm−1) is 7091.6(1.2). For a 3Σ+u, Te (cm−1) is 8183.8(1.5) and De (cm−1) is 333(1).
Adiabatic dynamics of the solvated electron in liquid ammonia
Authors: Sprik Michiel , Klein Michael L.
Published: July 2002

The adiabatic dynamics of an electron in a polar solvent has been studied by means of a molecular dynamics technique for the coupled electron–solvent system. The method hinges on a separation of time scales for the solvent motion and certain classical dynamical degrees of freedom which represent the ground-state wave function of the electron. For the solvated electron these variables are taken to be the positions and widths of Gaussians that comprise a distributed basis set. The method is applied to an electron in liquid ammonia, and the conditions for the validity of the classical separation of time scales are investigated. Time correlation functions describing the electron diffusion coefficient, and the fluctuation and dissipation of electron energy, are compared with analogous quantities for a classical solute (a chloride ion) in the same solvent. The electron diffusion coefficient in ammonia is calculated to be about twice that of a chloride ion. Experimental mobility data on the electron–ammonia system suggest that the real diffusion coefficient is a factor of 3 larger than calculated for the model used here. Possible reasons for this discrepancy are noted.
Vibrational relaxation of water molecules in H2O+Ar collisions between 200 and 1000 K: 001→020, 020→010, and 010→000 transitions
Authors: Ree J. , Shin H. K.
Published: July 2002

The vibrational relaxation of the stretching levels of H2O in the H2O+Ar collision is studied using a model of vibration-to-rotation (VR) energy transfer in the temperature range of 200–1000 K. The dominant relaxation pathway from the rapidly equilibrating ν1 and ν3 levels is deexcitation to the 2ν2 level followed by the 020→010 and 010→000 relaxation processes. At 300 K, the probabilities of 001→020, 020→010, and 010→000 are 2.2×10−3, 2.6×10−3, and 1.4×10−3, respectively. In the model the energy release ΔE of each deexcitation process is preferentially removed by rotation. Transition probabilities calculated by a semiclassical procedure indicate that when ΔE is large, the VR mechanism is of major importance at lower temperatures. The VR model correctly predicts both the temperature dependence and magnitude of the 020→010 probability over the temperature range of 200–1000 K. A model which assumes the removal of the energy release by translation seriously underestimates the probability of this process, especially at lower temperatures, and predicts a temperature dependence which is too steep. The contribution of translational motion to the relaxation process becomes important at higher collision energies, especially for 001→020, where ΔE is small. Calculations are extended to the D2O+Ar collision in order to study isotope effects.
On the different representations of the hole-correlation functions in the Hartree–Fock and the Hartree–Fock–Slater methods and their influence on bond energy calculations
Authors: Tschinke Vincenzo , Ziegler Tom
Published: July 2002

We have investigated the change in shape of the Hartree–Fock Fermi-hole correlation function upon bond formation. Our analysis indicates that the molecular Fermi-hole correlation function is on the whole considerably more diffuse than its atomic counterpart. It is shown that this imbalance gives rise to HF bond energies which in many instances are much smaller than the experimental values. The imbalance is related to the so-called near degeneracy error, a well known feature of the HF method. It can be removed by introducing a limited configuration interaction calculation which ensures a proper dissociation limit as the bond is broken. The Fermi-hole correlation function adopted by the Hartree–Fock–Slater method does not introduce the same imbalance between molecular and atomic hole functions. The calculated bond energies are, as a consequence, much larger and in better agreement with experiment. It is suggested that the Hartree–Fock–Slater method in part introduces correlation by avoiding the near degeneracy error through a balanced description of the molecular and atomic hole functions.
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o study of the intermolecular potential of Ar–H2O
Authors: Chal/asiński G. , Szczȩśniak M. M. , Scheiner S.
Published: July 2002

The combination of supermolecular Mo/ller–Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential-energy surface of Ar–H2O. The surface is very isotropic with the lowest barrier for rotation of ∼35 cm−1 above the absolute minimum. The lower bound for De is found to be 108 cm−1 and the complex reveals a very floppy structure, with Ar moving freely from the H-bridged structure to the coplanar and almost perpendicular arrangement of the C2 –water axis and the Ar–O axis, ‘‘T-shaped’’ structure. This motion is almost isoenergetic (energy change of less than 2 cm−1 ). The H-bridged structure is favored by the attractive induction and dispersion anisotropies; the T-shaped structure is favored by repulsive exchange anisotropy. The nonadditive effect in the Ar2–H2O cluster was also calculated. Implications of our results on the present models of hydrophobic interactions are also discussed.
Electrokinetic flow in ultrafine slits: Ionic size effects
Authors: González-Tovar Enrique , Lozada-Cassou Marcelo , Olivares Wilmer
Published: July 2002

The equilibrium density profiles for the restricted primitive model (RPM) for electrolytes, which takes into account the ionic size, are used to calculate several electrokinetic quantities for a 1:1 ionic solution in a charged slit. The three-point-extension hypernetted-chain–mean-spherical equation is used to calculate the RPM density profiles. The results are compared to those obtained through the Poisson–Boltzmann equation for a point-ion model. Important quantitative and qualitative differences are found for ultrafine slits. These differences, as well as the general behavior of the electrokinetic quantities, are explained in terms of the structure of the electrical double layer inside the slit.
Dynamics of the chemisorption of O2 on Pt(111): Dissociation via direct population of a molecularly chemisorbed precursor at high incidence kinetic energy
Authors: Rettner C. T. , Mullins C. B.
Published: July 2002

We have used the thermal desorption spectroscopy of the O/O2+CO→CO2 system to probe the chemical nature of oxygen that remains on a Pt(111) surface following exposure to a supersonic O2 beam under various conditions. We find that for a surface temperature of 90 K, the resulting CO2 formation thermal desorption spectrum is the same for all beam kinetic energies employed up to 1.1 eV at normal incidence, in all cases resembling that assigned to the O2+CO co-adsorbate system. This spectrum is clearly distinct from the O+CO case, where atomically chemisorbed oxygen is obtained either by thermal dissociation of O2 on the surface or by exposing the 90 K surface to a beam containing O atoms. These results imply that the dissociative chemisorption of O2 on Pt(111) proceeds by way of a molecular precursor even at relatively high incidence kinetic energies, at least as high as 1.1 eV. This interpretation readily accounts for the strong surface temperature dependence associated with dissociation under these conditions but contrasts with previous assignment of a direct (or quasidirect) dissociation process at high energies. We have also reexamined a number of previous observations in terms of this new picture, including the initial decline in dissociation probability with increasing kinetic energy. This falloff is attributed to a decrease in the trapping probability into a physisorption state, as recently suggested by Luntz et al. Considering the present results in the light of other recent studies, it now seems clear that the physisorption state is then a precursor to the molecular chemisorption state which can also be accessed directly at high kinetic energy. In this picture the molecular chemisorption state is then a precursor to dissociation even at high kinetic energy, and the dissociation probability depends on the (temperature-dependent) branching ratio between the dissociation and ultimate desorption of these species.
Relaxation of spin quantum number S=3/2 under multiple-pulse quadrupolar echoes
Authors: van der Maarel J. R. C.
Published: July 2002

The time evolution of the spin S=3/2 density operator due to relaxation in the presence of a sequence of (π/2) pulses has been calculated using perturbation theory in an interaction representation in which no external time-dependent rf fields occur. It is shown that the relaxation under the effect of the pulse train is similar to T1ρ relaxation. If the magnetization is sampled between the pulses, outside extreme narrowing conditions the observed signal is characterized by a biexponential decay. The amplitude ratio of the fast and slowly relaxing component equals 0.8:0.2. The rate of the fast relaxing component is sensitive to the pulse cycle time in the presence of slowly fluctuating electric-field gradients. A full expression of the relevant spectral density function valid for (π/2) pulses with arbitrary pulse width has been derived. In this derivation, the processes involved in determining the loss of correlation of the quadrupolar interaction are assumed to be independent and characterized by exponential correlation functions. Due to relaxation under the effect of the (π/2) pulse train triple-quantum coherences may be excited. The latter coherences can be monitored by applying an additional coherence transfer pulse after the pulse train. The relaxation of sodium in an ion exchange resin in the presence of the (π/2) pulse sequence has experimentally been studied and agrees with the theoretical analysis.
Adiabatic population inversion in I2 vapor with picosecond laser pulses
Authors: Melinger J. S. , Hariharan A. , Gandhi Suketu R. , Warren W. S.
Published: July 2002

Frequency modulated picosecond laser pulses are applied to the X→B electronic transition in I2 vapor. In the adiabatic limit, these pulses generate more excitation in I2 vapor than do ‘‘transform-limited’’ laser pulses. Multilevel effects make it important to match the pulse bandwidth to the material absorption spectrum, as demonstrated experimentally.
Isothermal coverage dependent measurements of NH3 and ND3 desorption from Cu(001)
Authors: Wu K. J. , Kevan S. D.
Published: July 2002

We present time-resolved electron energy loss spectroscopy measurement of isothermal desorption of NH3 and ND3 from Cu(001) as a function of coverage. The desorption process is characterized by pseudo-first-order kinetics. The pseudo-first-order rate constant increases approximately exponentially as a function of adsorbed ammonia coverage. The results of an analysis based on a quasiequilibrium approximation which expresses the desorption rate in terms of a virial expansion in adsorbate coverage are reported. This demonstrates that the intermolecular interactions are dominated by short range repulsive lateral interactions, in qualitative accord with expectations of dipolar interactions. The results also suggest the existence of longer-range attractive interactions.
Long time fluctuation of liquid water: 1/f spectrum of energy fluctuation in hydrogen bond network rearrangement dynamics
Authors: Sasai Masaki , Ohmine Iwao , Ramaswamy Ramakrishna
Published: July 2002

The power spectrum of the potential energy fluctuation of liquid water is examined and found to yield so-called 1/f frequency dependence (f is frequency). This is in sharp contrast to spectra of simple liquids (e.g., liquid argon), which exhibit a near white spectrum. This indicates that there exists an extended multiplicity of hydrogen bond network relaxations in liquid water. A simple model of cellular dynamics is proposed to explain this frequency dependence. On the other hand, the cluster dynamics of argon also involves energy fluctuations of a 1/f type, resulting from various relaxation processes at core and surface.
Theoretical study of the BeLi, BeNa, MgLi, MgNa, and AlBe molecules and their negative ions
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry
Published: July 2002

The alkaline earth–alkali diatomics are found to have weak bonds, because the diffuse alkali valence s orbitals cannot form a bond of sufficient strength to pay the promotion energy of the alkaline-earth atoms. This leads to van der Waals bonding in the neutrals as well as the negative ions. In fact, the negative ions have larger binding energies than the neutrals as a result of the much larger polarizability of the negative ion. The binding energy of AlBe is significantly larger than the Be–alkali molecules, due to a covalent contribution to the bonding. The binding energy in AlBe− is considerably larger than AlBe; the binding energy of the X 3Σ− state of AlBe− is computed to be 1.36 eV, as compared with 0.57 eV for the X 2Π state of AlBe.
Comparison of the Brueckner and coupled-cluster approaches to electron correlation
Authors: Lee Timothy J. , Kobayashi Rika , Handy Nicholas C. , Amos Roger D.
Published: July 2002

The equilibrium structures, dipole moments, harmonic vibrational frequencies, and infrared intensities of NH3, FON, Be3, BeC2, and BeO2 have been determined using the coupled-cluster and Brueckner electron correlation methods. The singles and doubles coupled-cluster (CCSD) and the Brueckner doubles (BD) methods have been employed and the corresponding methods that include a perturbational estimate of connected triple excitations [i.e., CCSD(T) and BD(T)] have also been investigated. The T1 diagnostic [Int. J. Quantum Chem. Symp. 23, 199 (1989)] is found to provide a good indication of the magnitude of the difference between the results obtained with the coupled-cluster and Brueckner methods. For NH3, the T1 diagnostic is small and so the differences between results obtained from coupled-cluster and Brueckner theories are quite small. For the other four molecules the T1 diagnostic is larger, and so the differences between the coupled-cluster and Brueckner methods become larger. However, it is found for all of the molecules considered in this study that inclusion of the contribution from connected triple excitations is more important than the differences between the Brueckner and coupled-cluster correlation methods.
Nonlinear programming approach to locally constrained variational calculations: He and H− in the Hartree–Fock approximation
Authors: King Frederick W. , Dykema Kenneth J. , Dalke Brian D.
Published: July 2002

Nonlinear programming procedures are employed to carry out constrained variational calculations on the helium atom and the hydride ion in the Hartree–Fock approximation. The inequality constraints imposed to improve the local behavior of the wave function are based on the reduced local energy, evaluated at selected points in configuration space. Improved moments 〈rn〉 are obtained for the helium atom and the hydride ion using this procedure. The calculations provide empirical support for the fact that the reduced local energy does reflect errors in the wave function.
X-ray photoemission investigations of the interface formation of Ca and poly(p-phenylene vinylene)
Authors: Gao Y. , Park Ken T. , Hsieh Bing R.
Published: July 2002

We have investigated the interface formation of Ca with poly(p-phenylene vinylene) (PPV) using x-ray photoemission spectroscopy (XPS). The most astonishing result of the investigations is that the Schottky barrier formation in Ca/PPV is a slow process possibly caused by the oxygen and sulfur impurities segregated on the PPV surface.