bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical modeling of charge collection in semiconductor gamma-ray spectrometers
Authors: Nemirovsky Y.
Published: July 2002

Charge collection efficiency and the variance of the collected charge of semiconductor gamma-ray spectrometers are analytically modeled for the case of a uniform electric field. The model is based on a rigorous statistical approach. In contrast to previously reported models for the variance of the collected charge, the present statistical model simultaneously takes into account a random point of photon absorption (i.e., a nonuniform absorption) and a random drift time for each generated carrier. Analytical expressions are obtained for the variance of the collected charge as a function of photon energy, applied bias voltage, electron and hole mobility-lifetime products, and the direction of irradiation. Since at present the performance of high-Z room-temperature spectrometers is mainly limited by the charge transport properties of the semiconductors, it is interesting to calculate the limit for the expected resolution. The statistical model presented here yields physical insight and determines quantitatively the expected spectrometer resolution as a function of the semiconductor transport parameters and device parameters, as well as the radiation parameters. Several limiting cases are calculated and discussed. The methodology presented here, which is based on conditional probabilities, can be extended to spectrometers with nonuniform fields.
Laser-stimulated luminescence of yttria-stabilized cubic zirconia crystals
Authors: Petrik N. G. , Taylor D. P. , Orlando T. M.
Published: July 2002

The kinetics of laser-stimulated luminescence (LSL) of yttria-stabilized cubic zirconia single crystals is investigated. Excitation of ZrO2⋅9.5%Y2O3(100) and (110) using ns pulses of 213 nm (5.82 eV), 266 nm (4.66 eV), and 355 nm (3.49 eV) photons produce LSL bands with Gaussian profiles and peak maxima at 460 nm (2.69 eV), 550 nm (2.25 eV), and 600 nm (2.07 eV), respectively. LSL involves a single-photon process for energy densities below ∼1.0 MW/cm2. Decay times vary from 0.1 to 100 μs depending on the excitation energy and temperature. Decay kinetics are hyperbolic indicating that all LSL bands result from recombination. The LSL quenches with increasing temperature and activation energies obtained using the Mott approximation are 0.10±0.01, 0.20±0.02, and 0.45±0.04 eV for the 2.69, 2.25, and 2.07 eV LSL bands, respectively. The various activation energies, decay kinetics, and excitation/emission energies correspond to the presence of several emission centers which can be associated with anion vacancies. We tentatively assign these to intrinsic F centers and extrinsic F-type centers. The latter are associated with one and two Y3+ ions in the nearest neighborhood positions. Since the normalized temperature dependencies of the decay coefficients are similar for all the LSL bands, we suggest that recombination primarily involves electrons, trapped at intrinsic and extrinsic defect sites, and mobilized holes.
Nonmechanical magnetothermal wind blower by a superconducting magnet
Authors: Uetake Hiromichi , Nakagawa Jun , Horota Noriyuki , Kitazawa Koichi
Published: July 2002

A magnetothermal wind blower without mechanical driving parts is demonstrated up to a velocity of about 0.5 m/s (4 l/min). The wind was created in a ceramic tube, 13 mm i.d., with an external heating coil, and placed in a superconducting magnet with a maximum field of 8 T. The heat zone was at an off-maximum position of the magnetic field distribution. The magnetic forces exerted on the slightly paramagnetic air by the field gradient at the two sides of the magnet center were out of balance due to the asymmetrical heating, thereby creating a net air flow which was detected by the slanting of a candle flame and by measurements of the temperature distribution in the air. The wind direction was such that the warmer air was expelled from the magnet while the cold air was sucked into it. The wind velocity could be quantitatively interpreted by considering the viscous flow of the air driven by the magnetic force imbalance.
A comprehensive thermodynamic analysis of native point defect and dopant solubilities in gallium arsenide
Authors: Hurle D. T. J.
Published: July 2002

A detailed analysis of the role of charged native point defects in controlling the solubility of electrically active dopants in gallium arsenide is presented. The key roles of (a) positively charged arsenic vacancies (VAs+) in determining the doping range over which the solubility curve is linear and (b) multiply negative charged gallium vacancies (VGam−) determining annealing and diffusion behavior in n+ material are demonstrated. An equilibrium thermodynamic model based on these concepts is shown to accurately describe the doping behavior of Te, Zn, Sn, Ge, Si, and C and the formation and annealing of the deep level denoted EL2 (assumed to be the arsenic antisite defect AsGa) in melt- and solution-grown crystals. The model provides a much more comprehensive and accurate description of dopant solubility than the widely cited Schottky barrier model of bulk nonequilibrium dopant incorporation. It is unambiguously shown that partial autocompensation of donor dopants by the donor–gallium vacancy acceptor complex occurs for both group IV and group VI donor dopants. The deduced concentrations of arsenic vacancies grown into the crystal during melt growth are shown to be in excellent agreement with values determined by titration and by density/lattice parameter measurements. The obtained data are used to plot the Ga–As solidus. Due to the presence of charged native point defect species (notably, VAs+), the free-carrier concentration at high temperatures is greater than the intrinsic concentration. The calculated concentration is shown to be in excellent agreement with published experimental data. The utility of an equilibrium thermodynamic model in seeking an understanding of doping behavior under conditions of high supersaturation, such as occur with organometallic vapor phase epitaxy and molecular beam epitaxy, is demonstrated. Finally, some suggestions are made as to the likely native point defect equilibria in indium phosphide.
Wavelength dependent of laser shock-wave generation in the water-confinement regime
Authors: Berthe L. , Fabbro R. , Peyre P. , Bartnicki E.
Published: July 2002

The generation of high-intensity shock waves by laser plasma in the water-confinement regime has been investigated at 1.064, 0.532, and 0.355 μm laser wavelengths. Experimental characterizations of pressures induced by laser plasma have been performed with a velocimetry interferometer for any reflector. For each incident laser wavelength, above a laser power density threshold, maximum pressure levels saturate and the pressure durations are reduced due to parasitic plasma occurring in water. However, it is shown that this threshold is lower at the 0.532 and 0.355 μm wavelengths than at the 1.064 μm wavelength. The generation of the parasitic plasma in water is easier with a short wavelength because it would be dominated by multiphotoionization mechanisms. Below the saturation pressure threshold, the pressure levels are significantly higher at the 0.532 and 0.355 μm wavelengths than at the 1.064 μm wavelength. Unlike the detrimental effect of short laser wavelengths on water breakdown plasma, the confined laser interaction is shown to be more efficient in ultraviolet than in infrared laser irradiation.
Self-consistent microwave field and plasma discharge simulations for a moderate pressure hydrogen discharge reactor
Authors: Hassouni K. , Grotjohn T. A. , Gicquel A.
Published: July 2002

A self-consistent two-dimensional model of the electromagnetic field and the plasma in a hydrogen discharge system has been developed and tested in comparison to experimental measurements. The reactor studied is a 25 cm diameter resonant cavity structure operating at 2.45 GHz with a silica belljar of 10 cm diameter and 17 cm height contained within the microwave cavity. The inside of the belljar where the discharge occurs contains a substrate holder of 5 cm diameter that is used to hold substrates for diamond deposition. The electromagnetic field model solves for the microwave fields using a finite difference time-domain solution of Maxwell’s equations. The plasma model is a three energy mode (gas, molecular vibration, and electron) and nine species (H2, H, H(n=2), H(n=3), H+, H2+, H3+, H−, electron) model which accounts for non-Boltzmann electron distribution function and has 35 reactions. Simulated characteristics of the reactor in two dimensions include gas temperature, electron temperature, electron density, atomic hydrogen molar fraction, microwave power absorption, and microwave fields. Comparisons of the model are made with close agreement to several experimental measurements including coherent anti-Stokes Raman Spectroscopy measurement of H2 temperature versus position above the substrate, Doppler broadening optical emission spectroscopy (OES) measurements of H temperature versus pressure, actinometry measurements of the relative H atom concentration, Hα OES intensity measurements versus position, and microwave electric field measurements. The parameter range studied includes pressures of 2500–11 000 Pa, microwave powers of 300–2000 W, and three vertical positions of the substrate holder.
Grazing incidence small angle x-ray scattering from free-standing nanostructures
Authors: Rauscher Markus , Paniago Rogerio , Metzger Hartmut , Kovats Zoltan , Domke Jan , Peisl Johann , Pfannes Hans-Dieter , Schulze Jörg , Eisele Ignaz
Published: July 2002

We develop the theory for grazing incidence small-angle x-ray scattering (GISAXS) from nanometer-sized naked islands on a flat substrate in the framework of the distorted-wave Born approximation (DWBA). The scattered wave amplitude is composed of four terms, including all combinations of scattering from the islands and reflection from the substrate. We apply this theory to x-ray measurements on Ge islands grown on Si(111), and show that we can determine the full triangular symmetry of these islands. The results also show that the DWBA must be used for smooth substrates near the angle of total external reflection. We finally discuss the advantages of GISAXS as compared to transmission small angle x-ray scattering for determining the symmetry of nanostructures.
Photoionization of deep centers in Al2O3
Authors: Whitley Von H. , McKeever S. W. S.
Published: July 2002

The optical threshold energies for photoionization of several different electron traps in Al2O3 have been determined using photoconductivity and optically stimulated luminescence (OSL). Stimulation of the photoconductivity and OSL is observed over a wide range of optical energies. The localized trapping states giving rise to the signals are found to be distributed in energy over a wide energy range, with major distributions between 1.7 and 2.5, 3.0 and 3.8, and 3.8 and 4.9 eV. From annealing measurements we show that the lowest energy distribution (1.7–2.5 eV) correlates with the thermoluminescence and thermally stimulated conductivity signals appearing in the temperature range of 400–500 K. The annealing measurements show that these traps contribute >90% of the OSL signal.
Magnetic properties and microstructure of FePt–Si3N4 nanocomposite thin films
Authors: Kuo Chih-Ming , Kuo P. C.
Published: July 2002

( Fe 50 Pt 50 ) 100−x –( Si 3 N 4 ) x  (x=0–50 vol. %) nanocomposite thin films are prepared by dc and rf magnetron cosputtering of FePt and Si3N4 targets on silicon wafer substrates, then annealed in vacuum at various temperatures. The effects of Si3N4 volume fraction, film thickness, and annealing temperatures on the magnetic properties are investigated. Transmission electron microscopy analysis indicated that structurally the film is an amorphous Si3N4 matrix with spherical FePt particles dispersed in it. The particle size of FePt increases with the annealing temperature but decreases with increasing Si3N4 content. Magnetization measurements indicated that maximum in-plane squareness and coercivity occurs at 30 vol. % of Si3N4 after annealing the film at 750 °C for 30 min. The average particle size of FePt in this film is about 40 nm. Saturation magnetization of the FePt–Si3N4 film is independent of film thickness but inversely proportional to the Si3N4 volume fraction. Variation of the films’ coercivity with film thickness is small. In contrast, the magnetic hardening mechanism and coercivity of the FePt–Si3N4 composite film are dependent on the Si3N4 volume fraction.
Low temperature dependencies of the elastic properties of Ti4AlN3, Ti3Al1.1C1.8, and Ti3SiC2
Authors: Finkel P. , Barsoum M. W. , El-Raghy T.
Published: July 2002

In this article we report on the temperature dependencies of the longitudinal and shear sound velocities in Ti4AlN3, Ti3Al1.1C1.8, and Ti3SiC2. The velocities are measured using a phase sensitive pulse-echo ultrasonic technique in the 90–300 K temperature range. At room temperature, Young’s, ERT, and shear, μRT, moduli and Poisson’s ratio of Ti4AlN3 are 310±2, 127±2 GPa, and 0.22, respectively. The corresponding values for Ti3AlC2 are 297.5±2, 124±2 GPa, and 0.2. Both moduli increase slowly with decreasing temperature and plateau out at temperatures below ≈125 K. A least squares fit of the temperature dependencies of the shear and Young’s moduli of Ti4AlN3 yield: μ/μRT=1−1.5×10−4(T−298), and E/ERT=1−0.74×10−4(T−298), for T>125 K. The corresponding relationships for Ti3Al1.1C1.8 are: μ/μRT=1−1.2×10−4(T−298), and E/ERT=1−0.84×10−4(T−298) for T>125 K. The acoustic Debye temperatures calculated for Ti4AlN3 and Ti3AlC2, as well as Ti3SiC2, are all above 700 K, in agreement with values calculated from low temperature heat capacity measurements.
Ellipsometric study of polycrystalline silicon films prepared by low-pressure chemical vapor deposition
Authors: Petrik P. , Lohner T. , Fried M. , Biró L. P. , Khánh N. Q. , Gyulai J. , Lehnert W. , Schneider C. , Ryssel H.
Published: July 2002

Polysilicon layers with thicknesses between 8 and 600 nm deposited by low-pressure chemical vapor deposition at temperatures ranging from 560 to 640 °C were characterized by spectroscopic ellipsometry (SE) to determine the layer thicknesses and compositions using multilayer optical models and the Bruggeman effective-medium approximation. The dependence of the structural parameters on the layer thickness and deposition temperature have been investigated. A better characterization of the polysilicon layer is achieved by using the reference data of fine-grained polysilicon in the optical model. The amount of voids in the polysilicon layer was independently measured by Rutherford backscattering spectrometry (RBS). The SE and RBS results show a good correlation. The comparison of the surface roughness measured by SE and atomic force microscopy (AFM) shows that independently of the AFM window sizes, a good correlation of the roughness determined by SE and AFM was obtained.
Low-frequency noise in AlGaN/GaN heterojunction field effect transistors on SiC and sapphire substrates
Authors: Rumyantsev S. , Levinshtein M. E. , Gaska R. , Shur M. S. , Yang J. W. , Khan M. A.
Published: July 2002

Low frequency noise has been investigated in gallium nitride/gallium–aluminum nitride GaN/GaAlN field effect transistors grown on sapphire and silicon carbide (SiC) substrates under identical conditions. GaN/AlGaN heterostructures grown on SiC substrate have a lower level of 1/f noise and a higher electron mobility compared to samples grown on sapphire. The noise of the gate leakage current Ig can give the main contribution to the output noise of the drain current Id even at Ig/Id ratios as small as 10−4–10−5. For the structures grown on SiC, a very weak temperature dependence of the low frequency noise was found in the temperature range 300<T<550 K. For the structures grown on sapphire, the contribution of generation–recombination noise of the local level with energy activation ΔE=0.42 eV became important at T>320 K. The effect of band-to-band illumination on the low-frequency noise is similar to that for silicon and gallium arsenide (GaAs) based transistors. The Hooge parameter α for the wafers grown on SiC can be as small as α=10−4. This value of α is comparable with the value of α for commercial GaAs field effect transistors.
Morphotropic domain structures and phase transitions in relaxor-based piezo-/ferroelectric (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 single crystals
Authors: Ye Z.-G. , Dong M.
Published: July 2002

The domain structures and phase transitions of relaxor-based piezo-/ferroelectric (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 [PMNT] single crystals, grown from high temperature solutions, have been analyzed by polarized light microscopy using an optical crystallographic approach. The substitution of Ti4+ ions for the B-site complex (Mg1/3Nb2/3)4+ ions in the relaxor PMN results in a long-range symmetry breaking, as revealed by the formation of birefringent macrodomains. The domain structures of the PMNT80/20 crystals indicate a primitive trigonal symmetry at room temperature with a weak birefringence. PMNT crystals with 50% Ti show tetragonal domain structures, reflecting the symmetry of PbTiO3. Single crystals of PMNT65/35 exhibit complex morphotropic domain structures, which are composed of both the trigonal and the tetragonal phase intimately mixed together. The domain structures, the sequence and temperature of phase transitions have been interpreted in relation to the morphotropic phase boundary behavior of the PMN-PT system. It is shown that in situ analysis of domain structures and phase transitions can be used as a nondestructive analytical tool for evaluating the local morphotropic composition and the properties, which sensitively depend upon the individual domains and the Ti4+ concentration. The analysis of morphotropic domain structures and phase transitions provides a better understanding of the microscopic mechanisms of the enhanced piezoelectric properties recently disclosed in PMNT and other relaxor-based piezocrystals.
Enhancement of critical temperature in fine La0.7Ca0.3MnO3 particles prepared by mechanochemical processing
Authors: Muroi M. , Street R. , McCormick P. G.
Published: July 2002

Fine La0.7Ca0.3MnO3 powders have been synthesized by mechanochemical processing, involving high-energy ball milling of LaCl3, CaCl2, MnCl2, and Na2CO3 and subsequent heat treatment. Single-phase La0.7Ca0.3MnO3 particles, dispersed in a NaCl matrix, were obtained for heat-treatment temperatures (Tht) of 750 °C or above, with the crystallite size increasing with Tht, from about 20 nm at 750 °C to about 1 μm at 1200 °C. Magnetic measurements showed that as the crystallite size decreases, the saturation magnetization at low temperature decreases from the theoretical value for full polarization of Mn spins, 3.7 μB per Mn ion, while Tc increases from 250 up to 278 K for the crystallite size of ∼20 nm; and that there exists a narrow temperature range near Tc in which the magnetization for a larger crystallite size is smaller in low fields but becomes larger in high fields than for a smaller crystallite size. These observations are discussed in terms of the ferromagnetic transition unique to perovskite manganites, involving the formation and growth of magnetic polarons above Tc.
Magnetic and structural properties of (CoxFe100−x)50Pt50 alloy thin films
Authors: Kanazawa H. , Lauhoff G. , Suzuki T.
Published: July 2002

A study of magnetic and structural properties of (CoxFe100−x)50 (0≦x≦100) alloy thin films as functions of composition and substrate deposition-temperature Ts is carried out. The intrinsic perpendicular magnetic anisotropy Ku starts to increase with deposition temperature Ts for all the films (0≦x≦100) at about TS=250–300 °C, and then becomes nearly constant. The maximum values of Ku at room temperature thus obtained are 6×107, 4×107, and 2×107 erg/cc for Fe50Pt50, (Co43Fe57)50Pt50 and Co50Pt50, respectively. Also, the Ku of Fe50Pt50 films increases with the order parameter (S), being closely related with the tetragonality c/a. These results indicate that the evolution of Ku is due to the occurrence of the ordered fct phase.
Phenomenology of giant magnetic-field-induced strain in ferromagnetic shape-memory materials (invited)
Authors: O’Handley R. C. , Murray S. J. , Marioni M. , Nembach H. , Allen S. M.
Published: July 2002

Ferromagnetic shape-memory alloys have recently emerged as a new class of active materials showing very large magnetic-field-induced extensional strains. Recently, a single crystal of a tetragonally distorted Heusler alloy in the NiMnGa system has shown a 5% shear strain at room temperature in a field of 4 kOe. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this effect are described. Ferromagnetic shape-memory alloys strain by virtue of the motion of the boundaries separating adjacent twin variants. The twin-boundary motion is driven by the Zeeman energy difference between the adjacent twins due to their nearly orthogonal magnetic easy axes and large magnetocrystalline anisotropy. The twin boundary constitutes a nearly 90° domain wall. Essentially, twin-boundary motion shorts out the more difficult magnetization rotation process. The field and stress dependence of the strain are reasonably well accounted for by minimization of a simple free energy expression including Zeeman energy, magnetic anisotropy energy, internal elastic energy, and external stress. Models indicate the limits to the magnitude of the field-induced strain and point to the material parameters that make the effect possible. The field-induced strain in ferromagnetic shape-memory alloys is contrasted with the more familiar phenomenon of magnetostriction.
Polarization and Fowler–Nordheim tunneling in anodized Al–Al2O3–Au diodes
Authors: Hickmott T. W.
Published: July 2002

Polarization in anodic Al2O3 films is measured by using quasi-dc current–voltage (I–V) curves of Al–Al2O3–Au diodes. A reproducible polarization state is established by applying a negative voltage to the Au electrode of a rectifying Al–Al2O3–Au diode. The difference between subsequent I–V curves with Au positive is a measure of polarization in the sample. The magnitude of polarization charge in Al2O3 depends on the anodizing electrolyte. Al2O3 films formed in H2O-based electrolytes have approximately ten times the polarization charge of Al2O3 films formed in ethylene glycol-based electrolyte. Anodizing conditions that produce greater polarizing charge in anodic Al2O3 result in voltage–time curves during anodization under galvanostatic conditions that are nonlinear. Anodic films with greater polarizing charge also have a greater apparent interface capacitance which is independent of Al2O3 thickness. I–V curves of Al–Al2O3–Au diodes for increasing voltage are dominated by polarization. I–V curves for decreasing voltage are reproducible and parallel but depend on the maximum current and voltage reached during the measurement. There is no single current corresponding to a given voltage. I–V curves for decreasing voltage are analyzed assuming that the conduction mechanism is Fowler–Nordheim (FN) tunneling. There is a qualitative difference between the FN tunneling parameters for Al2O3 films formed in H2O-based electrolytes and those formed in ethylene glycol-based electrolyte. For the former the value of the exponential term in the FN analysis increases as the value of maximum voltage and current in an I–V characteristic increases, while the value of the pre-exponential term is nearly constant. For the latter, the exponential term is nearly constant as maximum voltage and current increase, but the pre-exponential term decreases by about 5 decades. Thus polarization charge incorporated during formation of anodized Al2O3 strongly affects the formation of the insulating film, the stability of the films under bias, and their conduction characteristics.
Rapid thermal oxidation of silicon in ozone
Authors: Cui Zhenjiang , Madsen Jonathan M. , Takoudis Christos G.
Published: July 2002

Rapid thermal oxidation (RTO) of Si in ozone gas is studied at temperatures between 200 and 550 °C, and the properties of the resulting ultrathin oxides are characterized using in situ mirror-enhanced reflection Fourier transform infrared (IR) spectroscopy. Thus, the frequency and intensity of the longitudinal optical vibrational mode of the Si–O–Si asymmetric stretching from ultrathin oxide films (<30 Å) are probed in different processing environments and related to the oxidation kinetics and interfacial layer properties. The oxidation rate in ozone is found to be comparable to the one in pure oxygen at approximately 200 °C higher temperature. Analyses of the oxidation in ozone show a fast oxidation regime followed by a slow one with activation energies of 0.13±0.01 and 0.19±0.04 eV, respectively. Two regions are also observed for the oxidation in pure O2 with activation energies of 0.20±0.03 eV for the fast oxidation regime and 0.36±0.04 eV for the slow one. X-ray photoelectron spectroscopy results and IR spectral feature frequency shifts suggest that the RTO of silicon in ozone ambient results in a thinner, less-stressed interfacial layer than the one obtained in pure O2. Preliminary electrical characterization using surface charge analyses indicates that the oxides formed in ozone are of superior quality.
Thermal properties of Ti4AlN3
Authors: Barsoum M. W. , Rawn C. J. , El-Raghy T. , Procopio A. T. , Porter W. D. , Wang H. , Hubbard C. R.
Published: July 2002

In this article we report on the atomic displacement parameters, lattice expansions, heat capacity, and thermal conductivity of samples of Ti4AlN3 in the 298–1370 K temperature range. Rietveld refinement of high temperature neutron diffraction data shows that the nitrogen is substoichiometric and the formula is Ti4AlN2.9. In this structure, the atomic displacement parameters of the Al atoms are higher than those of either the Ti or N atoms. The Ti–N bonds adjacent to the Al planes are about 2.5% shorter than the Ti–N bonds in the inner layers. The thermal expansion coefficients along the a and c axes are, respectively, (9.6±0.1)×10−6 and (8.8±0.1)×10−6 K−1. The unit cell expansivity, (9.4±0.1)×10−6 K−1, is in agreement with the dilatometric bulk thermal expansivity (9.7±0.2)×10−6 K−1. The heat capacity, cp, is 150 J/mol K at ambient temperatures and extrapolates to ≈220 J/mol K at 1300 K. At all temperatures cp equals four times the molar heat capacity of TiN. The room temperature thermal conductivity is 12 W/m K and increases linearly to ≈20 W/m K at 1300 K.
Effect of the excitation wavelength on Raman scattering of microcrystalline diamond prepared in a low pressure inductively coupled plasma
Authors: Okada Katsuyuki , Kanda Hisao , Komatsu Shojiro , Matsumoto Seiichiro
Published: July 2002

Microcrystalline diamond films have been prepared in a 13.56 MHz low pressure inductively coupled plasma. The bonding in the films obtained were studied by Raman spectroscopy with 514, 325, and 244 nm excitation wavelengths. 514 nm excited Raman spectra exhibit peaks at ∼1355 and ∼1580 cm−1 corresponding to sp2 bonding, and a peak at ∼1150 cm−1 assigned to sp3 bonding. The 1332 cm−1 diamond peak due to zone center optical phonon mode with F2g symmetry clearly appears and the peak at ∼1580 cm−1 is remarkably enhanced with 325 nm excitation. In 244 nm excited Raman scattering, the peak at 1332 cm−1 is only enhanced whereas the peak at ∼1580 cm−1 is correspondingly diminished. These features of the Raman spectra are discussed in terms of resonant Raman scattering due to the electronic π–π* transition in sp2-bonded carbon with 514 nm (2.4 eV) excitation, and the possible enhancement of the peak due to the sp3-bonded carbon with 325 (3.8 eV) and 244 nm (5.1 eV) excitations.
The interaction between platinum films and silicon substrates: Effects of substrate bias during sputtering deposition
Authors: Shi J. , Kojima D. , Hashimoto M.
Published: July 2002

The reactions between platinum and silicon, both during platinum deposition at elevated temperature and during a thermal annealing process, have been investigated using x-ray diffraction, cross-sectional transmission electron microscopy, and x-ray photoelectron spectroscopy. It has been found that sputtering deposition of platinum on a silicon substrate at 200 °C results in the formation of PtSi at the Pt–Si interface. But the reaction cannot fully proceed at this temperature for a platinum film with a thickness of 35 nm. Further annealing at 450 °C causes the platinum film to transform to PtSi completely. A substrate bias of −90 V during sputtering deposition leads to the formation of platinum films with larger columnar grains, instead of finer grains as being formed without substrate bias. In such a case, oxygen diffusion toward the interface was enhanced through the boundaries of these columnar grains, and this results in an accumulation of oxygen and oxide formation at the interface. As a result, the reaction between platinum and silicon was inhibited during the further annealing process for the Pt/Si films deposited with substrate bias.
Order–disorder transitions in Langmuir–Blodgett monolayers. I. Studies of two-dimensional melting by infrared spectroscopy
Authors: Naselli C. , Rabolt J. F. , Swalen J. D.
Published: July 2002

A two step melting process in Langmuir–Blodgett (LB) monolayer assemblies of cadmium arachidate (CdA) has been studied by infrared spectroscopy in conjunction with thermal measurements. These techniques have allowed, for the first time, the investigation of detailed structural changes which occur during heating, melting, and annealing cycles. Prior to the melting point, a pretransitional disordering of the hydrocarbon tails was observed. This was followed by a catastrophic breakup of the cadmium lattice at a temperature corresponding to the melting point as measured by differential scanning calorimetry (DSC).
The malonaldehyde equilibrium geometry: A major structural shift due to the effects of electron correlation
Authors: Frisch Michael J. , Scheiner Andrew C. , Schaefer Henry F. , Binkley J. Stephen
Published: July 2002

Complete theoretical optimizations of the equilibrium geometry of malonaldehyde have been carried out within the framework of the self-consistent-field (SCF) approximation. Both Huzinaga–Dunning double zeta plus polarization (DZ+P) and Pople 6–31G** basis sets have been used, resulting in very similar results. The predicted O ⋅ ⋅ ⋅ H hydrogen bond distance is 1.88 Å, in poor agreement with the value 1.68 Å deduced from experiment. It appears that the Hartree–Fock approximation is incapable of describing the equilibrium geometry of malonaldehyde in a qualitatively correct manner. However, second-order perturbation theory yields a structure (O ⋅ ⋅ ⋅ H distance 1.69 Å) in good agreement with experiment. The structures of the keto tautomer and the transition state for symmetric intramolecular hydrogen transfer have also been determined, as have harmonic vibrational frequencies for all stationary points.
An electron diffraction study of alkali chloride vapors
Authors: Mawhorter R. J. , Fink M. , Hartley J. G.
Published: July 2002

Monomers and dimers of the four alkali chlorides NaCl, KCl, RbCl, and CsCl have been studied in the vapor phase using the counting method of high energy electron diffraction. Nozzle temperatures from 840–960 K were required to achieve the necessary vapor pressures of approximately 0.01 Torr. Large mean amplitudes of vibration (as big as 0.26 Å) and the similarity of the monomer and dimer bonded distances complicate the analysis, although the refined structures yield good fits to the data, especially for the stronger scatterers RbCl and CsCl. Using harmonic calculations for the monomer and dimer l values, a consistent set of structures for all four molecules is obtained. The corrected monomer re distances reproduce the microwave values very well. The planar rhombic structure of the dimers shows a uniform decrease of the Cl–M–Cl angle as the alkali M increases in size. For the better data sets some of the l values were refined to gain perspective on the validity of the calculated ones. The experiment also yields information on the amount of dimer present in the vapor, and these results are compared with thermodynamic values.
Observation of high frequency quantum oscillations in elastic differential cross sections: A critical test of the Ne–Ar interaction potential
Authors: Beneventi Laura , Casavecchia Piergiorgio , Volpi Gian Gualberto
Published: July 2002

The high frequency quantum oscillations superimposed on the falloff of the main rainbow structure have been clearly resolved in the elastic differential cross section for Ne–Ar at a collision energy of 65.5 meV. These new, high resolution data have allowed us to discriminate between two recently proposed multiproperty potentials for the Ne–Ar system, which only differ by a few percent in their range parameters. The data are found to be inconsistent with the predictions derived from the potential of Aziz and van Dalen [J. Chem. Phys. 81, 779 (1984)] (ε=5.96 meV, rm =3.414 Å, σ=3.068 Å) and confirm a previous potential by Candori et al. [Chem. Phys. Lett. 90, 202 (1982)] (ε=5.74 meV, rm =3.52 Å, σ=3.133 Å) in the whole attractive part and in the low repulsive region.
Spectrum and decay of the doubly charged water ion
Authors: Richardson P. J. , Eland J. H. D. , Fournier P. G. , Cooper D. L.
Published: July 2002

Ion–ion coincidence measurements on H2O, HDO, and D2O ionized by Heii light show that the main ion-pair dissociations from low-lying states of H2O2+ lead to OH++H+ and H++H++O. Distinct states of H2O2+ are deduced to be at 36.5±1 and at 42±1 eV above the H2O ground state. Double charge transfer measurements show two sets of singlet states of H2O2+ at 41.4 and 45 eV in agreement with the Auger spectrum. New ab initio calculations predict a vertical appearance energy of 39.2 eV for the H2O2+ ion in its 3B1 ground state, but show a very strong geometry dependence, and give an adiabatic value near 36.3 eV, in agreement with experiment.
Calculation of eigenvalues through recurrence relations
Authors: Fernández F. M. , Ogilvie J. F. , Tipping R. H.
Published: July 2002

A simple, fast, and accurate method is developed to calculate eigenvalues when the secular equation can be written as a (2m+1)-term recurrence relation. Several physically interesting examples are discussed to show that the present algorithm compares favorably with the most efficient ones.
Microwave kinetic spectroscopy of reaction intermediates: O+ethylene reaction at low pressure
Authors: Endo Yasuki , Tsuchiya Soji , Yamada Chikashi , Hirota Eizi , Koda Seiichiro
Published: July 2002

A microwave spectroscopic method has been developed to study elementary reactions in real time through in situ observation of rotational spectra of reaction intermediates such as free radicals with lifetime as short as 1 ms. This method was applied to the O(3P)+ethylene reaction in order to assess the roles of (a) vinoxy+H and (b) CH3+CHO channels in the initial process. The reaction was initiated by irradiating an N2O/C2H4 mixture containing a trace amount of mercury with the 253.7 nm mercury resonance line, and the time evolution of vinoxy, HCO, and H2CO was followed by measuring their microwave absorption intensities as functions of time. The branching ratio was thus determined to be 0.4±0.1 and 0.5±0.1 for (a) and (b), respectively, at the sample pressure of 30 mTorr. The present result agrees with those obtained by Hunziker et al. [J. Photochem. 17, 377 (1981)] using much higher pressures of samples, but is not compatible with the observation of Buss et al. [J. Photochem. 17, 389 (1981)] that (a) is dominant in collision-free conditions.
Purely rotational coherence effect and time-resolved sub-Doppler spectroscopy of large molecules. I. Theoretical
Authors: Felker Peter M. , Zewail Ahmed H.
Published: July 2002

In this and the accompanying paper we present a theoretical treatment and experimental study, respectively, of the phenomenon termed purely rotational coherence. This phenomenon has been demonstrated to be useful as a time domain means by which to obtain high resolution spectroscopic information on excited state rotational levels of large molecules [Felker et al., J. Phys. Chem. 90, 724 (1986); Baskin et al., J. Chem. Phys. 84, 4708 (1986)]. Here, the manifestations in temporally resolved, polarization-analyzed fluorescence of coherently prepared rotational levels in samples of isolated symmetric and asymmetric top molecules are considered. These manifestations, for reasonably large molecules at rotational temperatures characteristic of jet-cooled samples, take the form of polarization-dependent transients and recurrences with temporal widths of the order of tens of picoseconds or less. The transients, which arise from the thermal averaging of many single molecule coherences, are examined with respect to their dependences on molecular parameters (rotational constants, transition dipole directions) and experimental parameters (polarization directions and temperature). A physical picture of rotational coherence as a reflection of the time-dependent orientation of molecules in the sample is developed. And, the influence of rotational coherence in experiments designed to probe intramolecular energy flow is discussed. In the accompanying paper, we present experimental results for jet-cooled t-stilbene and anthracene. For t-stilbene we determine rotational constants for vibrational levels in the S1 electronic state (from the recurrences) and we monitor the trends in rotational coherence vs vibrational coherence as the total energy in the molecule increases.
High-temperature photochemistry and BAC-MP4 studies of the reaction between ground-state H atoms and N2O
Authors: Marshall Paul , Fontijn Arthur , Melius Carl F.
Published: July 2002

The H+N2O reaction has been investigated using the high-temperature photochemistry (HTP) technique. H(1 2S) atoms were generated by flash photolysis of NH3 and monitored by time-resolved atomic resonance fluorescence with pulse counting. The bimolecular rate coefficient for H-atom consumption, leading essentially to N2+OH, from 390 to 1310 K is found to be given by k1(T)=5.5×10−14 exp(−2380 K/T)+7.3×10−10 exp(−9690 K/T) cm3 molecule−1 s−1; the accuracy is assessed as approximately 25% at the 2σ confidence level. Above 750 K, k1 closely follows the Arrhenius behavior of the second term alone. Distinct curvature is evident below 750 K. k1 is compared to theoretical BAC-MP4 predictions and good agreement is found for a model involving rearrangement of an HNNO intermediate coupled with tunneling through an Eckart potential barrier, which dominates at the lower temperatures. The branching ratio for the channel leading to NH+NO is discussed in the context of recent thermochemical information and a maximum rate coefficient of <1×10−9 exp(−15800 K/T) cm3 molecule−1 s−1 is set for temperatures up to 2000 K.
Microscopic observation of ordered colloids in sedimentation equilibrium and important role of Debye-screening length. II. Monodispersed polystyrene spheres (specific gravity=1.05) in aqueous and methanol aqueous suspension
Authors: Okubo Tsuneo
Published: July 2002

A metallurgical microscope is used to observe directly the ordered structures of monodispersed polystyrene spheres (specific gravity=1.05) in water and methanol–water mixtures. Sedimentation occurs for all the spheres examined for which the diameters are between 369 and 1020 nm. The interparticle distances (2Dexp) increase with time after the latex sample is set into the cell, and reach equilibrium values after seven to ten days, which shows that complete deionization is essential to get reliable data for 2Dexp. 2Dexp continues to decrease as the initial concentration of the spheres increases and from these data Young’s modulus for the ordered lattices is estimated to be 1 dyn cm−2 for spheres of 0.5 μm. The influences on 2Dexp by the concentration and charge density of the spheres, and ionic strength show that the electrostatic interparticle repulsion and the elongated Debye-screening length around the spheres are essential for the ordering phenomenon.
Electron localization in water clusters. II. Surface and internal states
Authors: Barnett R. N. , Landman Uzi , Cleveland C. L. , Jortner Joshua
Published: July 2002

Electron attachment and localization in small water clusters (H2O)n (n=8–128) is studied using path-integral molecular dynamics simulations. The electron-water molecule interaction is described via a pseudopotential which includes Coulomb, polarization, exclusion and exchange contributions. Different electron localization modes are found depending on cluster size. For small and intermediate size clusters (n=8–32), the energetically favored localization mode involves a surface state and the calculated excess electron binding energies are in agreement with experimentally measured values. In larger clusters, n=64, 128, internal localization (solvation) is energetically favored. In both cases the localization of the excess electron is accompanied by large cluster molecular reorganization. The cluster size dependence of the localization mode, the energetics, structure, and excess electron distributions in the negative molecular anions (H2O)−n, and the dependence on temperature are explored.
Correlation-consistent singlet–triplet gaps in substituted carbenes
Authors: Carter Emily A. , Goddard William A.
Published: July 2002

A b initio GVB-CI (generalized valence bond with configuration interaction) and MCSCF (multiconfiguration self-consistent-field) wave functions are used to calculate electronic state splittings for the lowest singlet and triplet states of substituted carbenes. The calculations emphasize correlation consistency between the two electronic states, resulting in short CI expansions. The singlet–triplet gaps (ΔEST) for CH2, CH(SiH3), CF2, CCl2, CHF, and CHCl are reported. They are in good agreement with available experimental data.
Surface crossings and predissociation dynamics of methyl iodide Rydberg states
Authors: Donaldson D. J. , Child Mark S. , Vaida Veronica
Published: July 2002

We report the results of a calculation of the adiabatic surface crossings between the B and C Rydberg states of CH3I and the dissociative A state. The bound potentials are derived empirically; the dissociative states are based on recent empirical and ab initio results. The surface crossings predissociate the B state at an energy below that of v=1 in the C–I stretching mode, and near the classical turning point for v=2 of the methyl umbrella mode. Our model predicts that at least two dissociative potentials are of importance in the B state predissociation. These results are in agreement with experimental predictions. Classical trajectory results suggest that the predissociation might exhibit mode specificity. The formation of methyl iodide dimers shifts the surface crossings to higher energy, which allows the absorption to v=1 of the C–I stretch and v=2 of the umbrella mode to be observed. This result is also in agreement with experiment.
On the luminescence of Cs2NaGdCl6
Authors: de Vries A. J. , Blasse G.
Published: July 2002

The luminescence properties of Cs2 NaGdCl6 are reported. In the emission, as well as the excitation spectra of the Gd3+ luminescence, vibronic lines are present due to coupling with the ν3 , ν4, and ν6 vibrational modes of the GdCl3−6 octahedron. The magnetic–dipole character of the zero-phonon transitions is confirmed by the long decay time of the 6 P7/2 emission. Energy migration among the Gd3+ ions is absent, due to the large distance (7.6 Å) between nearest-neighbor Gd3+ ions. The emission from two extrinsic Gd3+ ions shows a coupling with the O–H stretching and bending modes.
Frequency dependent specific heat
Authors: Zwanzig Robert
Published: July 2002

Birge and Nagel reported measurements of the frequency dependent specific heat of supercooled liquids. It is shown here that the quantity they measured is directly related to a frequency dependent longitudinal viscosity.
Hydrogen atom abstraction by O(3P) from diborane and ethane
Authors: Borchardt D. B. , Choi J.-G. , Suzuki K. , Bauer S. H.
Published: July 2002

We measured, via LIF, time-dependent concentrations of OH resulting from the reaction of O(3 P) with B2H6 and C2H6. The oxygen atoms were generated by titrating microwave discharged N2 /He with NO to the chemiluminescent end point. The operating pressures in the flow reactor ranged from 5 to 15 Torr; the mixtures consisted of He/O(3 P)/fuel in the approximate ratios 100/1/0.1 to 100/1/1. Flow conditions were such that in the low pressure experiments the controlled residence time prior to detection were 0.8–17 ms; under the higher pressure conditions, the time interval covered was 2–35 ms. We estimated that the temperature of the reaction region was ≈350 K, based on rotational emission temperatures measured for BO*, generated under closely similar conditions. First a complete mechanism was derived for ethane for a specified set of experimental parameters. For ethane, a single set of experimental conditions was selected for ratioing the recorded intensity to the computed OH density; this was cross checked with other runs for ethane. Finally, a mechanism was developed for B2H6, which quantitatively checked our experimentally determined profiles both in shape and magnitude, for three sets of conditions, and within a factor of 2 for the high concentration runs (100% B2H6 feeding into the reactor at 14 Torr).
Structure and adsorption at gas–solid interfaces: Layering transitions from a continuum theory
Authors: Ball P. C. , Evans R.
Published: July 2002

Packing effects in liquid-like films, adsorbed from bulk gas at the interface with an attractive solid substrate, may cause the fluid to adopt layer-like structure near the substrate. Such structure is inherent in the lattice gas model of adsorption where it is known to give rise to a sequence of layering transitions at low temperatures, provided the substrate (wall) is sufficiently attractive. We have used a nonlocal density functional theory to describe a continuum fluid near strongly attractive structureless walls. The theory yields oscillatory density profiles arising from packing effects. At low temperatures T (typically 0.5≲T/Tc ≲0.6, where Tc is the bulk critical temperature) the adsorption may increase with pressure in a series of discontinuous jumps, each corresponding to the appearance of a new dense layer in the adsorbed film. The transitions are first order, with very small metastable regions. As in the lattice gas model the transitions exhibit critical points; above the critical temperature of a given layer the adsorption increases smoothly with increasing pressure. However, we anticipate certain qualitative differences between the transitions we observe in the continuum theory and those of the lattice gas model, since the latter overemphasizes the role of hard-core repulsion. Layering transitions also occur for fluids confined in narrow pores. We find that in cylindrical pores the transitions are shifted to lower pressures. Moreover, capillary condensation (the first order transition to a ‘‘liquid’’ that fills the pore) competes with layering resulting in a rapid truncation of the sequence of layering transitions. The implications of our results for the interpretation of adsorption measurements performed near a bulk triple point are discussed and possible surface phase diagrams are suggested.