bluuebunny/crossref_metadata_2025 huggingface dataset:
Reversible and irreversible magnetization in hybrid magnets
Authors: Emura M. , Cornejo D. R. , Missell F. P.
Published: July 2002

Magnetization reversal was studied in hybrid magnets consisting of ferrite and ribbons of the Magnequench alloy MQP-Q (Nd2Fe14B+α-Fe). Recent studies of demagnetization processes in permanent magnets have used two different methods for determining the reversible magnetization, often arriving at conflicting results. The reversible Mrev and irreversible Mirr magnetizations were determined here by the two different procedures: (i) In the dc demagnetization method, the demagnetization remanence is measured for reverse magnetic fields after saturation in the forward direction. Assuming the change in magnetization along the recoil curve is reversible, the remanent magnetization is identified with the irreversible magnetization Mirr. (ii) Alternatively, the intrinsic reversible susceptibility χrevi was measured by defining it as the mean slope of a small minor loop performed after determining the time dependence of M. The reversible susceptibility was then integrated to obtain Mrev. Although the two procedures for determining Mrev are frequently considered equivalent, we discuss here the differences that arise when one or the other method is used. These differences may be important when magnetic materials are characterized using switching field distributions, Henkel plots, or δM plots.
Unusual static and dynamic magnetic properties of Cd1−yMnyS nanocrystals
Authors: Feltin N. , Levy L. , Ingert D. , Vincent E. , Pileni M. P.
Published: July 2002

Magnetic properties of Cd1−yMnyS nanocrystals have been studied. They vary with the particle size and markedly differ from those observed in the bulk phase, in particular their interactions drastically increase compared to those observed in the latter case. A Mn2+–Mn2+ interaction enhancement with decreasing size involves changes in the magnetic phase diagram. For 4 nm nanocrystals, a spin glass phase can be assumed.
Influence of Sb doping on the luminescent properties of PbWO4 single crystals
Authors: Chen Yonghu , Shi Chaoshu , Hu Guanqin
Published: July 2002

The influence of antimony (Sb) doping on the luminescent properties of lead tungstate single crystals (PWO) has been investigated. Transmission spectra, emission, and excitation spectra of UV-excited luminescence of PWO and PWO:Sb have been reported. Though our measurement based on the surface emission may be different from the light intensity in the crystal body itself, the comparison of spectra between PWO and PWO:Sb showed some significant results. Sb doping improved the luminescence of PWO by enormously increasing its light yield, and the green emission band in luminescent spectra is drastically enhanced, while the red one is restrained. On the ground that the improvement by Sb doping is similar to the air annealing effect on PWO luminescence, the mechanism of Sb doping and the origin of the green emission is briefly discussed. We suggested that Sb3+ ions and Sb5+ ions coexist in PbWO4:Sb crystals and the green emission originate from centers (WO42−+Oi).
Charge and heat collection in a 70 g heat/ionization cryogenic detector for dark matter search
Authors: L’Hôte D. , Navick X. F. , Tourbot R. , Mangin J. , Pesty F.
Published: July 2002

We present detailed studies of the physical mechanisms underlying the collection of the charge carriers and of the thermal energy in a heat/ionization germanium particle detector operated at 20–28 mK, together with a presentation of its main performances. This detector is devoted to the search of dark matter particles, taking advantage of the double signal to reject background induced events. In what concerns the ionization channel, the current–voltage characteristics, energy resolution, time stability, and pulse height bias dependence are presented. A mechanism is proposed to explain the fact that the p-i-n diode maintains some rectifying properties in spite of the very low temperature. The energy resolution is 1 keV full width at half maximum (FWHM) at 86.5 keV, and the stability time is several days. A calculation of the carrier trapping contribution to the energy resolution and bias dependence is performed and its results compared to the data. The bias dependence is interpreted within a “hot” carrier model in which the shape of the trapping cross section as a function of the electric field is investigated. The time stability behavior is interpreted in terms of a space charge evolution due to traps ionization. The reasons why the densities of ionized levels are minimized are discussed. It is shown that one of them is related to the low currents obtained at rather large biases because of the choice of a p-i-n scheme. In what concerns the heat channel, an analysis of the heat flow in the thermal circuit is performed to explain the shape and amplitude of the heat pulses. The risetimes are well accounted for, allowing a determination of the NTD electronic specific heat, (1.80±0.45)×10−6 T J K−1 cm−3. The analysis of the two decay times leads to an interpretation in terms of partial thermalization of the ballistic phonons in the metallic parts of the detector. This mechanism allows to estimate the experimental responsivities and energy resolutions [6 and 20 nV/keV; 1.25 and 2.8 keV (FWHM) for sensor temperatures of respectively 20.5 and 28 mK]. Both the ionization and heat channel results are used to draw guidelines for detector performance improvements and mass increase.
Hypersonic and dielectric properties of (PbZn1/3Nb2/3O3)0.915–(PbTiO3)0.085 single crystal
Authors: Tu Chi-Shun , Tsai C.-L.
Published: July 2002

Both the longitudinal Brillouin backscattering spectra and dielectric permittivities along the [001] direction have been measured as a function of temperature for a relaxor ferroelectric single crystal (PbZn1/3Nb2/3O3)0.915–(PbTiO3)0.085 (PZN–8.5%PT). A sharp ferroelectric phase transition (which is associated with a Landau–Khalatnikov-type phonon damping maximum) was observed near 450 K. As temperature deceases, a diffuse phase transition (which is associated with a broad acoustic phonon damping maximum) was detected near 355 K. This broad damping evolution is attributed to dynamic order-parameter fluctuations. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 355 and 450 K) are diffuse first order and first order, respectively. The frequency-dependent dielectric data εc″(f,T) prove the existence of an electric dipolar relaxation process below 360 K. The activation energy, the Vogel–Fulcher temperature, and attempt frequency corresponding to this relaxation process are also calculated.
Negative ion production by surface ionization at aluminum-nitride surfaces
Authors: Jans S. , Wurz P. , Schletti R. , Fröhlich T. , Hertzberg E. , Fuselier S.
Published: July 2002

In this article, we report on the observation of the formation of negatively charged ions upon reflection from an aluminum-nitride surface. Positive H2 and O2 ions are scattered at small angles of incidence off a single-crystal surface. Charge exchange at the surface yields neutral particles and negative ions in the reflected particle flux. The negative ion fractions are about 15% and 1% for oxygen and hydrogen, respectively. The particle reflection probability is only 2%, which is attributed to the roughness of the surface.
In situ ellipsometry growth characterization of dual ion beam deposited boron nitride thin films
Authors: Franke E. , Schubert M. , Woollam J. A. , Hecht J.-D. , Wagner G. , Neumann H. , Bigl F.
Published: July 2002

Pure hexagonal h, as well as mixed-phase cubic/hexagonal c/h boron nitride (BN) thin films were deposited onto [001] silicon substrates using the dual ion beam deposition technique. The BN thin films were grown under UHV conditions at different substrate temperatures and ion beam bombarding parameters. Thin-film growth was monitored using in situ spectroscopic ellipsometry at 44 wavelengths between 420 and 761 nm. The in situ ellipsometric Ψ and Δ data were compared with two-layer growth model calculations for the mixed-phase c/h BN, and with one-layer growth model calculations for pure h-BN growth. In situ data provide information on the optical properties of deposited h-BN and c/h-BN material, film thickness, and BN growth rates. A virtual interface approach is employed for the optical properties of the silicon substrate. The growth and nucleation of c-BN observed here confirms the cylindrical thermal spike model. The results for composition and thickness of the BN films were compared to those obtained from ex situ infrared transmission measurements and high-resolution transmission electron microscopy investigations.
Recrystallization and electrical properties of MeV P implanted 6H–SiC
Authors: Harada Shinsuke , Motooka Teruaki
Published: July 2002

We have investigated structural changes and electrical activation processes in 8 MeV P+ ion implanted 6H–SiC at room temperature (RT) and 850 °C by means of transmission electron microscopy, secondary ion mass spectrometry, and current–voltage measurements. A buried amorphous layer was produced in the RT implanted sample, while no amorphization occurred and extended defects were formed in the 850 °C implanted sample. The amorphous layer was recrystallized after 1000 °C annealing, but electrical activation of implanted P was very weak. It was found that activation occurred at an annealing temperature of 1300 °C, which is lower than the typical annealing temperature, 1500 °C for ∼100 KeV P+ implantation samples. The resistivity of the RT implanted sample was approximately a half of that in the 850 °C implanted sample probably due to the extended defects, which were stable even after 1300 °C annealing.
Influence of lattice mismatch on magnetization-induced optical second harmonic generation from a magnetic film on a nonmagnetic substrate
Authors: Lyubchanskii I. L. , Jeong Jae-Woo , Shin Sung-Chul , Dadoenkova N. N. , Lyubchanskii M. I. , Rasing Th.
Published: July 2002

The optical second harmonic generation from a thin magnetic film on a nonmagnetic substrate is theoretically investigated for both s- and p-polarized incident light. The contributions of lattice misfit strain as well as of misfit dislocation strain to the second-order nonlinear optical susceptibility are described using a nonlinear photoelastic tensor, together with the contribution of magnetization described by a nonlinear magneto-optical tensor.
Magneto-impedance effect in soft magnetic tubes
Authors: Yu R. H. , Landry G. , Li Y. F. , Basu S. , Xiao John. Q.
Published: July 2002

Soft magnetic crystalline alloys have been fabricated in a tube form by electrodepositing magnetic FeNi and FeNi–Al2O3 onto W fibers with a diameter of 25 μm. Fine Al2O3 particles have also been incorporated into the magnetic matrix to improve mechanical properties. As-prepared materials are not magnetically soft. With heat treatment, the magnetic properties of these composites are similar to commercial bulk soft FeNi alloys. A giant magnetoimpedance value as large as 190% has been found in as-prepared FeNi-W with a magnetic layer thickness of 20 μm. This value is comparable to GMI observed in amorphous magnetic wires. Experiments also show that GMI values decrease when the Al2O3 content increases in a range from 0 to 7.0 at. %. This behavior is due to the increase in electrical resistivity and magnetic permeability of the samples that modifies the skin effect.
Magnetoluminescence in quantum dots and quantum wires of II–VI diluted magnetic semiconductors
Authors: Takahashi Nobuhiro , Takabayashi Kazumasa , Souma Izuru , Shen Jinxi , Oka Yasuo
Published: July 2002

We report optical properties of quantum dots and quantum wires of diluted magnetic semiconductors. The quantum dots of Cd1−xMnxSe (x=0.03) show the exciton luminescence at around 2.4 eV, which indicates a strong confinement effect of the exciton energy corresponding to the dot size of 4–6 nm. The Zeeman shift of the exciton luminescence was observed with an effective g value of 91, showing a significant exchange interaction of the excitons with the Mn ions in the dots. The exciton luminescence from the quantum wires of Cd1−xMnxSe (x=0.08) shifts by 5.2 meV to the higher energy side with decreasing the wire width from 126 to 26 nm. The high energy shift in the narrow wires indicates the influence of the one-dimensional quantum confinement effect for the exciton states. The effective g value of the exciton in these quantum wires is 100–150. The exciton luminescence from the wires is linearly polarized (up to 80%) parallel to the wire direction at zero field, which indicates one-dimensional properties of the quantum wire excitons.
Schottky diode formation and characterization of titanium tungsten to n- and p-type 4H silicon carbide
Authors: Lee S.-K. , Zetterling C.-M. , Östling M.
Published: July 2002

Titanium tungsten (Ti0.58W0.42) Schottky contacts to both n- and p-type 4H silicon carbide were fabricated using sputtering. The n- as well as p-type Schottky contacts had excellent rectifying characteristics after vacuum annealing at 500 °C with a thermally stable ideality factor of 1.06±0.03 for n-type and 1.08±0.01 for p-type. The measured Schottky barrier height (SBH) was 1.22±0.03 eV for n-type and 1.93±0.01 eV for p-type in the range of 24–300 °C. Our results of Ti0.58W0.42 Schottky contacts to both n- and p-type can be explained perfectly by thermionic emission theory and also satisfy the Schottky–Mott model in contrast to earlier works. Capacitance–voltage measurements were also performed and the results were in good agreement with those of current–voltage measurements. In addition, the inhomogeneous behavior with higher ideality factor and lower SBH of p-type Ti0.58W0.42 contacts for as-deposited contacts is explained by using a model with contribution of recombination current originated by lattice defects to thermionic emission current.
Effects of electron emission on steady-state plasma sheaths
Authors: Meng Xian , Wang You-Nian , Ma Teng-Cai
Published: July 2002

Making use of hydrodynamics theory, we have studied effects of electron emission from a solid surface on steady-state argon plasma sheaths in which the emitted electron-neutral atom collisions and the ion-neutral atom collisions are taken into account. Numerical results indicate that the effects of electron emission on the distributions of the sheath parameters such as the sheath electric field and the ion densities are obvious as the neutral gas density increases or the beam current density of the emitted electrons increases.
Critical thickness of a heteroepitaxial film on a twist-bonded compliant substrate
Authors: Obayashi Y. , Shintani K.
Published: July 2002

The critical thickness for misfit dislocation formation in a heteroepitaxial film on a twist-bonded compliant substrate is calculated. The boundary between the twist-bonded compliant substrate and the supporting bulk substrate with a low twist angle is assumed to be represented by a cross grid of straight dislocations. The critical thickness is determined by the zero of that formation energy of a misfit dislocation which consists of the self-energy of the dislocation, the interaction energy between the dislocation and the dislocation arrays in the twist boundary, and the interaction energy between the dislocation and the mismatch strain. Numerical results for the InxGa1−xP film on a twist-bonded GaAs compliant substrate show that the critical thickness of the film on a twist-bonded compliant substrate is considerably larger than that of the film on a free-standing compliant substrate. This difference in critical thicknesses for the twist-bonded compliant substrate and the free-standing substrate is due to the interaction of the misfit dislocation with the dislocation arrays in the twist boundary. From the comparison with the experimental observations in the literature, it is shown that the approximation in the present analysis is valid for the compliant substrates with low twist angles.
CoSi
2
formation in the Ti/Co/SiO2/Si system
Authors: Detavernier C. , Van Meirhaeghe R. L. , Cardon F. , Maex K. , Bender H. , Zhu Shiyang
Published: July 2002

Cobalt silicide formation has been studied in the presence of a thin, chemically grown interfacial SiO2 and a Ti capping layer. It is found that for ex situ annealing without a capping layer (Co/SiO2/Si system), no silicide is formed. In the presence of a Ti capping layer (Ti/Co/SiO2/Si system), CoSi is formed, followed by CoSi2 at higher temperature. The CoSi formation temperature is dependent on the capping layer thickness. The reaction mechanism has been studied in detail. It is found that the function of the Ti capping layer is twofold: first of all the capping layer protects the silicidation reaction from oxygen contamination. Second, Ti from the cap is able to diffuse through the unreacted Co and to transform the interfacial SiO2 diffusion barrier into a CoxTiyOz diffusion membrane. The CoSi2 layer has a preferential epitaxial orientation with the (100) silicon substrate. The epitaxial quality is dependent on the annealing temperature and the thickness of the Co and Ti layers. It is shown that CoSi2 layers formed from a Ti/Co/SiO2/Si system have a better thermal stability and more ideal electrical characteristics than the polycrystalline CoSi2 formed in the standard Co/Si reaction.
Mechanism of ellipsometry monolayer oscillations during metalorganic vapor-phase epitaxy
Authors: Lee Jeong-Sik , Sugou Shigeo , Masumoto Yasuaki
Published: July 2002

Ellipsometry signal oscillations were clearly seen during metalorganic vapor-phase epitaxy (MOVPE) of III–V materials under high sample rotation at 1400 rpm. The ellipsometric signal oscillated at a period corresponding to 1 ML of MOVPE growth. Oscillations correlated with the formation of islands due to nucleation on terraces during the growth of 1 ML. Different surface reconstructions between (100) terraces (As stable) and (111)A step edges (Ga stable) result in different dielectric functions. The effective medium dielectric response of rough and smooth surfaces appears to be responsible for different ellipsometry signals. Layer thickness and InGaAs composition were precisely determined in situ. Critical layer thickness and In composition for InGaAs quantum-dot formation were also evaluated directly from in situ ellipsometry data.
Refractive index of colored films of molybdenum trioxide
Authors: Reyes-Betanzo C. , Herrera-Pérez J. L. , Cocoletzi Gregorio H. , Zelaya-Angel O.
Published: July 2002

We apply a film thickness-independent technique to measure the index of refraction n=n+ik for amorphous thin films of molybdenum trioxide. The films were grown by thermal evaporation and then colored by light irradiation for different exposure times. Data for the reflectivity and differential reflectivity yield n and show no dependence on the density of color centers. We find that small variations of the absorption coefficient induce changes in the differential reflectance line shape at angles different from the Brewster angle.
Electrode effect on photohole generation in smectic phenylnaphthalene liquid crystalline photoconductor
Authors: Zhang Hong , Hanna Jun-ichi
Published: July 2002

The photohole generation process in the smectic phases of a liquid crystalline photoconductor, 2-(4′-octylpheny)-6-dodecyoxylnaphthalene (8-PNP-O12) with different illuminated contact electrodes was investigated through steady-state and transient photocurrent measurements. It was revealed that the photogeneration of holes was governed by two different processes according to the electrode materials: the Onsager type of photocarrier generation in the bulk and the electrode-enhanced hole photoinjection with a delay of μs when Al, and either Pt or In2O3–SnO2 electrodes are applied, respectively. In the latter process, the photogeneration yield was one order of magnitude larger than that in the bulk generation process, but decreased as the molecular ordering degraded from the smectic B phase to the smectic A phase, then to the isotropic phase. According to the spectral response and the results of additional experiments on the effect of chemical doping with electron acceptors, the photohole injection was concluded to be attributed to the exciton decay at the interface of the electrode and liquid crystal, i.e., the charge transfer from photoexcited 8-PNP-O12 molecules to the electrode. The experimental results were analyzed according to a one-dimensional Onsager model and the diffusion length of excitons in the SmB phase was determined to be 30±10 nm.
Effect of porosity on the effective magnetostriction of polycrystals
Authors: Nan Ce-Wen , Huang Yong , Weng G. J.
Published: July 2002

Explicit relations for determining the effective magnetostriction of polycrystals with cubic magnetostrictive crystallites and pores are derived based on the Green’s function technique. For comparison, two extreme approximations-the Reuss and Voigt-type approximations-are also given. To illustrate the technique, the influence of porosity on the effective saturation magnetostriction of common magnetostrictive metals (Fe and Ni), giant magnetostrictive alloy terfenol-D, and Fe–Co spinel are illustrated. The theoretical estimates show an interesting nonmonotonic concentration dependence of magnetostrictive behavior of the polycrystals with such magnetostrictive microcrystallites as Ni and Fe–Co spinel, whose magnetic easy axes are 〈100〉 axes rather than 〈111〉 axes.
Calculation of theoretical capacitance–voltage characteristics of 6H–SiC metal–oxide–semiconductor structures
Authors: Raynaud C.
Published: July 2002

The effect of nonuniform interface trap distributions on capacitance–voltage (C–V) characteristics of 6H–silicon carbide metal–oxide–semiconductor (MOS) capacitors has been investigated. Theoretical C–V curves have been calculated in order to study the influence of: (i) the nature (donor or acceptor) of the traps, (ii) the interface state density peak in the band gap and the peak magnitude. The incomplete ionization of dopants and the depletion in the polysilicon gate have also been taken into account to fit experimental data. A good agreement is observed between the interface state spectrum obtained in our calculation and the one obtained by the Terman’s method. Thus, exact parameters of the MOS structures can be obtained. A peak of donor states is detected at Ev+0.65 eV, and an effective oxide charge is measured to 4.9×1012 cm−2, which denotes a poor SiO2–SiC interface quality.
Temperature–entropy diagram for an irreversible absorption refrigeration cycle
Authors: Chua H. T. , Toh H. K. , Ng K. C.
Published: July 2002

This article develops the theoretical foundation for the temperature–entropy (T–s) diagram for irreversible absorption chillers that employ either a volatile or nonvolatile working pair. The representation of a real absorption refrigeration cycle on a T–s diagram can directly depict the energetic superiority of one design over another. For practical usage, this diagrammatic approach only requires as inputs the inlet and outlet state points that can be computed based on the corresponding temperatures, pressures, and component concentrations of each of the heat-and-mass exchanger modules within a chiller system and can therefore also be employed as a useful tool for system analysis and diagnosis. The same method is also applicable to any continuously operating thermodynamic system that is wholly or partially driven by thermal power.
Energy flow and excimer yields in continuous wave rare gas–halogen systems
Authors: Salvermoser M. , Murnick D. E. , Wieser J. , Ulrich A.
Published: July 2002

A stable, continuous wave (cw), electron beam at 14 keV has been used to study energy flow, reaction rates, and radiative decay in rare gas plus halogen systems at high pressure. Steady state solutions to rate equations were used to isolate parameters which affect both the transient and steady state yield of 193 nm radiation from ArF* and 157 nm radiation from F2*. The scaling of pumping power density to the inverse 4.25 power and cube of the pressure allowed a wide range of reaction times, from nanoseconds to seconds, to be considered. The spectra and yields as a function of partial pressures were used to obtain energy transfer efficiencies near 10% for 193 nm in a Ne:Ar:F2,1:0.008:0.0004 mixture and near 5% for 157 nm in a Ne:F2,1:0.002 mixture at 2–3 bar pressure. Scaling to high brightness lamps and near cw lasers is possible.
Growth, structure, and performance of depth-graded W/Si multilayers for hard x-ray optics
Authors: Windt D. L. , Christensen F. E. , Craig W. W. , Hailey C. , Harrison F. A. , Jimenez-Garate M. , Kalyanaraman R. , Mao P. H.
Published: July 2002

We describe the development of depth-graded W/Si multilayer films prepared by magnetron sputtering for use as broad-band reflective coatings for hard x-ray optics. We have used specular and nonspecular x-ray reflectance analysis to characterize the interface imperfections in both periodic and depth-graded W/Si multilayer structures, and high-resolution transmission electron microscopy (TEM) and selected area electron diffraction (SAED) to characterize the interface structure and layer morphology as a function of depth in an optimized depth-graded multilayer. From x-ray analysis we find interface widths in the range σ=0.275–0.35 nm for films deposited at low argon pressure (with a slight increase in interface width for multilayers having periods greater than ∼20 nm, possibly due to the transition from amorphous to polycrystalline metal layers identified by TEM and SAED), and somewhat larger interface widths (i.e., σ=0.35–0.4 nm) for structures grown at higher Ar pressures, higher background pressures, or with larger target-to-substrate distances. We find no variation in interface widths with magnetron power. Nonspecular x-ray reflectance analysis and TEM suggest that the dominant interface imperfection in these films is interfacial diffuseness; interfacial roughness is minimal (σr∼0.175 nm) in structures prepared under optimal conditions, but can increase under conditions in which the beneficial effects of energetic bombardment during growth are compromised. X-ray reflectance analysis was also used to measure the variation in the W and Si deposition rates with bilayer thickness: we find that the W and Si layer thicknesses are nonlinear with the deposition times, and we discuss possible mechanisms responsible for this nonlinearity. Finally, hard x-ray reflectance measurements made with synchrotron radiation were used to quantify the performance of optimized depth-graded W/Si structures over the photon energy range from 18 to 212 keV. We find good agreement between the synchrotron measurements and calculations made using either 0.3 nm interface widths, or with a depth-graded distribution of interface widths in the range σ=0.275–0.35 nm (as suggested by 8 keV x-ray and TEM analyses) for a structure containing 150 bilayers, and designed for high reflectance over the range 20 keV<E<70 keV. We also find for this structure good agreement between reflectance measurements and calculations made for energies up to 170 keV, as well as for another graded W/Si structure containing 800 bilayers, designed for use above 100 keV, where the peak reflectance was measured at E=212 keV to be R=76.5±4% at a graze angle of θ=0.08°.
Persistent layer-by-layer sputtering of Au(111)
Authors: Ramana Murty M. V. , Couture A. J. , Cooper B. H. , Woll A. R. , Brock J. D. , Headrick R. L.
Published: July 2002

Persistent layer-by-layer removal of Au(111) during Ar+ ion irradiation was observed in a real-time x-ray scattering study. Over 100 specular beam intensity oscillations were measured. For a given ion energy, a smoother surface morphology is obtained when the ion flux is reduced. For a fixed erosion rate, ion energy in the range of 70–500 eV does not have a strong influence on the evolution of surface morphology. Diffuse scattering measurements show the development of features with a characteristic lateral length scale on the surface during ion irradiation.
Vibrational energy transfer between CO and CH4, CD4, and CF4 in liquid Ar
Authors: Abdel-Halim Hamzeh , Ewing George E.
Published: July 2002

Vibrational relaxation of CO (υ=1) by CH4, CD4, and CF4 in liquid Ar at 87 K has been studied. The IR fluorescence decay from CO (υ=1) excited by a blackbody source was used to monitor the rate of vibrational energy transfer. Order of magnitude calculations for the variety of V–V and V-T processes possible in these systems were used to select reasonable relaxation channels. These feasibility calculations together with decay measurements on several concentrations of CO and its collision partners enabled liquid state relaxation rate constants to be assigned. For the CO–CD4 and CO–CF4 systems, near-resonance energy transfer and conditions of concentration enabled us to measure the V-T rate constants for the first time for the relaxation of CD4 and CF4 by liquid Ar. These V–V and V-T rate constants were compared with the gas phase values obtained at the same temperature. We find that the liquid phase and gas phase rate constants are essentially the same. These results are found to be in reasonable accord with the predictions of simple models of vibrational relaxation in the liquid state.
Electronic states of CuO
Authors: Madhavan P. V. , Newton M. D.
Published: July 2002

A b initio SCF and CI calculations are reported for the ground state and low-lying excited electronic states of CuO (i.e., within ∼20×103 cm−1 of the X̃ 2Π ground state), employing an ab initio effective potential for the Cu atomic core (1s–3p), and including comparisons of several atomic orbital basis sets. The calculated bond lengths, energy separation, and population analyses for the lowest two electronic states, X̃ 2Π and Y 2Σ+ [corresponding roughly to Cu+ (3d10) and O− (2p5), where the 2p oxygen hole is, repectively, 2pπ and 2pσ], do not vary appreciably among the basis sets tested (minimal and split valence bases on Cu, and a split valence basis on oxygen, with and without d-polarization functions and diffuse p functions on oxygen) and are in reasonable agreement with previous experimental and theoretical estimates. Most of the remaining excited states of CuO below ∼20×103 cm−1 may be rationalized in terms of atomic-like excitations originating from the

X̃ 2Π ground state: 3d(Cu) → 4s(Cu) and 3d(Cu) → 2 pπ (O) (the latter formally corresponds to Cu2+O2−, though covalent 2p‘gs–4s bonding reduces the charge to ∼Cu+O−). Close agreement (rms deviation of ∼103 cm−1) is obtained between calculated (CI) and observed adiabatic transition energies for ten Σ, Π, and Δ doublet excited states (relative to the X̃ 2Π state), after the calculated excitation energies are reduced by ∼5×103 cm−1, a term which corrects primarily for limitations in the Cu atom basis set, and whose magnitude can be obtained both in terms of calculated and observed atomic quantities and by direct least-squares fitting of calculated and observed CuO transition energies. The calculations yield the first comprehensive assignment of Kronig symmetry (±) for the observed 2Σ states (δ 2Σ−, A 2Σ−, A′ 2Σ+, and G 2Σ+) and one which is consistent with recent analyses of experimental data. The extent of specific molecular correlation effects is analyzed, and found to be important in the assignment of several of the observed transitions. The CI mixing of different principal configurations may be significant for states involving the ‘‘Cu2+O2−’’ configuration (as also suggested in previous studies), and the assignments based on the present CI results for these states are only tentative.
Evidence of quantum ergodicity in stimulated emission pumping spectra of acetylene
Authors: Sundberg Robert L. , Abramson Evan , Kinsey James L. , Field Robert W.
Published: July 2002

We investigate the stimulated emission pumping (SEP) spectra of the acetylene X̃ 1∑+g state near 27 900 cm−1 of vibrational excitation for evidence of quantum ergodicity. The spectra consists of clumps of narrowly spaced lines which arise from transitions into levels with the same lower state J″ value. Thus, the structure within each clump is purely vibrational. We apply criteria for spectral ergodicity and examine nearest-neighbor spacing distributions to test the spectra for signs of ergodic molecular dynamics. The criteria applied suggest that the molecular dynamics is nearly ergodic.
Energy and angular momentum diffusion theory of dissociation rate constants
Authors: Borkovec Michal , Berne Bruce J.
Published: July 2002

We evaluate the dissociation rate constant of a molecule in the low pressure and the weak collision limit. We take the full coupling between energy and angular momentum into account by asymptotic techniques. We applied this result to a diatomic molecule using the Kramers frictional Fokker–Planck model. The predicted temperature dependence of the recombination rate constant is in agreement with experimental data on iodine recombination at high temperatures.
The adsorption, interconversion, and dissociation of CO on Fe(111)
Authors: Bartosch C. E. , Whitman L. J. , Ho W.
Published: July 2002

Adsorption of CO and the coadsorption of CO with low coverages of K on Fe(111) at temperatures as low as 83 K have been studied with low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and high resolution electron energy loss spectroscopy (HREELS). The results indicate that CO fills three different adsorption sites and is mobile enough to access all binding sites after striking the surface at 83 K. A previously unobserved, very low CO stretch vibration is reported at 1325 cm−1 on the unpromoted surface. In addition, a missing metal–CO vibration is reported for an unpromoted surface. The screening mechanism for the missing Fe–CO stretch vibration is discussed in relation to promoted metal surfaces and to other adsorbates which display similar behavior.
Interaction of Xe+ and Cl− ions and their formed molecules with a Xe solid matrix
Authors: Last Isidore , George Thomas F.
Published: July 2002

The aim of the present work is the calculation of the energy of ionic xenon–chlorine systems which can be formed in solid Xe by irradiation. The energy levels of these ionic systems differ from those in the gas phase due to polarization and dispersion interactions with solid Xe atoms. It is shown that the Xe+2Cl− molecule is responsible for experimentally observed emission. The activation energy of the Xe+2Cl− formation is found to form a broad band.
Floquet–Liouville supermatrix approach. II. Intensity-dependent generalized nonlinear optical susceptibilities
Authors: Wang Kwanghsi , Chu Shih-I
Published: July 2002

We present a practical nonperturbative method for exact treatment of intensity-dependent generalized nonlinear optical susceptibilities χ(ω) in intense polychromatic fields, valid for arbitrary laser intensities, detunings, and relaxation. By means of the many-mode Floquet theory, the time-dependent Liouville equation can be transformed into an equivalent time-independent infinite-dimensional Floquet–Liouville supermatrix (FLSM) eigenvalue problem. It is then shown that the nonlinear optical susceptibilities χ(ω) can be completely determined simply from the supereigenvalues and eigenfunctions of the Floquet–Liouvillian  L̂F. In addition to this exact FLSM approach, we have also presented higher-order perturbative results, based on the extension of the Salwen’s nearly degenerate perturbation theory, appropriate for somewhat weaker fields and near-resonant multiphoton processes, but beyond the conventional perturbative or rotating wave approximation (RWA). In the case of two-level systems, for example, the implementation of Salwen’s method in the time-independent L̂F allows the reduction of the infinite-dimensional FLSM into a 4×4 dimensional effective Hamiltonian, from which essential analytical formulas for intensity-dependent χ(ω) can be obtained. These methods are applied to a detailed study of intensity-dependent spectral line shapes (such as hole burning and extra resonance peaks at the line center, and the effects of saturation, detuning, and radiative and collisional damping, etc.) and subharmonic structures in nonlinear multiple wave mixings χ[(m+1)ω1−mω2] for two-level systems in intense linearly polarized bichromatic fields.
The influence of adsorbate–adsorbate interactions on surface structure: The coadsorption of CO and H2 on Rh(100)
Authors: Richter Lee J. , Gurney Bruce A. , Ho W.
Published: July 2002

The coadsorption of CO and H2 on Rh(100) at 100 K has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), and temperature programmed EELS (TP-EELS). The preferred binding sites, long ranged order, and degree of segregation are dependent on the order of adsorption. When H2 is exposed to a CO preexposed surface, segregation of the surface species (atomic H and CO) is observed. The postdosed H2 causes isolated CO molecules to change from the top site to the bridge site, and compresses the c(2×2) CO islands that develop during the CO preexposure. When CO is exposed to a H2 saturated surface (one hydrogen per surface Rh atom) an intimately mixed c(2×2) CO and H structure is formed with all the CO molecules occupying the top site. Strong repulsive CO–H interactions in this mixed adlayer result in two new low temperature H2 TPD states. During the desorption of the lowest temperature H2 TPD peak, the c(2×2) LEED pattern streaks and the CO molecules shift from the top site to the bridge site. It is proposed that the preferred binding site for hydrogen in the c(2×2) bridge bonding CO structure is different from the fourfold hollow site preferred in the c(2×2) top bound CO structure. After the second H2 TPD peak, the remaining adsorbed H and CO segregate, and the CO regions are compressed. The compression is relaxed as the hydrogen desorbs. The development of the surface structures and their influence on the H2 TPD can be qualitatively understood in terms of precursor adsorption of both CO on H covered surfaces and H2 on CO covered surfaces, strong CO–H repulsions, and local poisoning of H2 dissociation by CO.
A laser-induced fluorescence study of bands of the red system of gaseous CoO: Evidence for a 4Δi ground state
Authors: Adam A. G. , Azuma Y. , Barry Judith A. , Huang Gejian , Lyne Michael P. J. , Merer A. J. , Schröder J. O.
Published: July 2002

Three bands of the ‘‘red’’ system of gaseous CoO have been rotationally analyzed using laser-induced fluorescence techniques. The available evidence indicates that the lower levels are the Ω=7/2 and 5/2 spin–orbit components of a 4Δi electronic state, which is assigned as the ground state of the molecule. The cobalt nuclear hyperfine splittings are small in the ground state, which suggests that no unpaired electron in a σ molecular orbital derived from the Co 4s atomic orbital is present, so that the electron configuration is presumably σ2δ3π2; the ground state bond length (r0) is 1.631 Å. The upper electronic levels are heavily perturbed, both rotationally and vibrationally, and their hyperfine structures, though following case (a) behavior, are large and irregular. This work on CoO completes the determination of the ground state symmetries and bond lengths for the whole series of the 3d transition metal monoxides; some comparisons for the members of the series are given.
Rotational spectroscopy of molecular ions using diode lasers
Authors: Liu Di-Jia , Ho Wing-Cheung , Oka Takeshi
Published: July 2002

We report here the first application of tunable diode lasers to study the pure rotational spectroscopy of molecular ions. High J rotational transitions of molecular cations ArH+, NeH+, HeH+, OH+, H2O+, and anion OH− have been observed.
Effect of electron correlation on the topological properties of molecular charge distributions
Authors: Gatti Carlo , MacDougall Preston J. , Bader Richard F. W.
Published: July 2002

This paper reports on the effect of electron correlation on the topological properties of the charge density and its associated gradient vector and Laplacian fields. The properties of these fields define the atoms, their reactivity, and the structure of a molecular system within the theory of atoms in molecules. The singlet and triplet states of CH2, CF2, and SiH2 are investigated using a configuration interaction method which includes all single and double excitations with respect to suitable zero-order reference wave functions, together with a number of hydrocarbon molecules including unsaturated and geometrically strained systems for which the correlation is introduced via the generalized valence bond approach. It is found that the correlated charge distributions possess the same number and kind of critical points in both the ρ(r) and ∇2ρ(r) fields as are found for SCF charge distributions. Thus the topology of a charge distribution and the structure it defines are unaffected by the addition of Coulomb correlation. The quantitative changes in the properties of the charge density at the critical points in both ρ(r) and ∇2ρ(r) induced by correlation are found to be small in magnitude and to be more pronounced for shared or covalent atomic interactions than for systems with pronounced charge transfer between the atoms. The properties of the atoms in these molecules also exhibit correspondingly small changes in value.
Chemical laser grating selection measurements of initial vibrational energy distribution by reactions of F atoms with H2, CH4, C2H6
Authors: Lu Shaoping , Liu Chengshan , Yang Xuefeng , Li Kaiba , Gu Yukun , Tao Yusheng
Published: July 2002

The complete product vibrational energy distribution from F+H2, CH4, C2H6 reactions was determined by the chemical laser grating selection (CLG) technique. Three important factors—vibrational relaxation and secondary reaction, rotational nonequilibrium, and superradiance were carefully investigated by time-resolved laser spectroscopy and analyzed by computer modeling. It was found that by using lower reactant pressure, these disadvantageous effects, which may have influenced the results of previous chemical laser studies, could be minimized or eliminated experimentally. It was also found that for these reactions, the accurate values of N2/N1 and N3/N2 could be obtained from the experiment or modeling calculation; in contrast, the value of N1/N0 was very high and was sensitive to vibrational relaxation and secondary reaction processes. In general, the results of the CLG method, at least for these reactions, are of the same degree of accuracy as those of the infrared chemiluminescence method. The rotational equilibrium assumption is still valid and can be used without apparent effect on the experimental results.
Factorization of the direct correlation function. III. Integral equations and thermodynamics
Authors: Wertheim M. S.
Published: July 2002

Integral equations for pair correlations are one of the standard methods for calculating the thermodynamics of fluid systems. One of the routes to the thermodynamic quantities involves the evaluation of a certain functional of the direct correlation function(s), namely the sum of ring graphs. We show that this functional can be evaluated directly in terms of the factorizing functions ξij and their first two derivatives at their lower cutoff. In numerical computations, this is much simpler and more accurate than the previous method of evaluation using Fourier transform techniques. For certain equations of Percus–Yevick type, where analytic solution is feasible, it provides a much faster route to the compressibility pressure. We give a very general derivation for molecular fluids, using the closed form of the factorizing functions ξij. The range of validity includes recently derived integral equations for fluids of molecules with singly bondable bonding sites. The resulting subdivision of molecular species into subspecies representing different bonding states is absorbed into highly condensed notation. The concept of a support plane of the hard core of a molecule was identified in previous work as fundamental in defining the lower cutoff of the factorizing functions. The same idea is found to be the key to arriving at the evaluation of the ring functional obtained here.
Representation of several complex coordinate methods by similarity transformation operators
Authors: Moiseyev N. , Hirschfelder J. O.
Published: July 2002

The complex coordinate method (CCM) can be presented by carrying out a similarity transformation of the Hamiltonian Ŝ+1ĤŜ−1 in order to correct the asymptotic behavior of the resonance eigenfunction, such that Ŝψres→0, whereas ψres→∞, as r→∞. Therefore, after the similarity transformation, the number of the particles is conserved in the coordinate space for any given time. Here, several different possibilities for Ŝ are presented, emphasizing the advantage of the representation of CCM by similarity transformation operators. A new extension of the complex coordinate method is that Ŝ∼exp[−θf 1/2(r)(∂/∂r) f 1/2(r)], where f(r) can be any function for which f(r)/r→1 as r→∞. Whereas, in the conventional CCM, f(r)=r. This new method enables one to select the ‘‘optimal’’ path in the complex coordinate plane, which gets past the intrinsic nonanalyticities of the potential and provides the most stable resonance solution which can be obtained by a given number of basis functions.
A velocity reset method of simulating thermal motion and damping in gas–solid collisions
Authors: Riley Merle E. , Coltrin Michael E. , Diestler Dennis J.
Published: July 2002

We present a velocity reset procedure for the approximate description of the molecular dynamics of a tractable subset of the atoms composing a macroscopic solid which is subjected to collisions. The coupling of the subset to the remainder (the reservoir) is taken into account in a stochastic manner by periodically resetting the velocities of subset particles which interact with the reservoir. The Cartesian velocity components are reset to vnew =(1−θ)1/2vold +θ1/2vT, where vold is the previous velocity, vT is a random velocity chosen from a Maxwellian distribution at temperature T, and θ is a parameter which controls the strength of the reset. In the limit θ=1 and all subset particles are reset, the method is similar to Andersen’s thermostat procedure [J. Chem. Phys. 72, 2384 (1980)]. In the double limit that θ→0 and the interval between resets Δtrs →0 such that β=θ/2Δtrs is fixed, the equations of motion for the subset reduce to Langevin form, where β is the frictional damping rate. This partial velocity reset method is a computational procedure allowing for (1) relaxation dynamics which are equivalent to the frictional damping theories, (2) inclusion of nonzero temperature effects on damping, (3) rapid generation of initial states selected from a canonical ensemble in preparation for individual transient scattering events, and, (4) simulations akin to molecular dynamics. We show that the velocity reset method reproduces previous calculations of the energy accommodation for the collision of an atom with a simple cubic lattice. Two new simulations of the Ag fcc 111 crystal face are done using a pairwise Lennard-Jones interaction. These involve thermostating to a fixed temperature and computation of spectral densities and autocorrelations.