bluuebunny/crossref_metadata_2025 huggingface dataset:
A thermionic-field-diffusion model for Npn bipolar heterojunction phototransistors
Authors: Frimel S. M. , Roenker K. P.
Published: July 2002

This article describes the development of a thermionic-field-diffusion model for use in the analysis and design of abrupt, single heterojunction bipolar phototransistors (HPTs) for use in HBT-based optical receivers. In particular, included in this approach are the effects of a dc base bias on the optical gain and device performance. Taking into account the optical generation, the excess electron concentration at the emitter end of the quasi-neutral base is initially determined using a matching of the thermionic field emission across the emitter-base heterojunction with the diffusion current at the emitter end of the base. At the collector end of the base region, a finite electron concentration is used based on the collector current density. The results are used to determine the electron profile in the quasi-neutral base region and the diffusion components to the emitter and collector currents. To determine the device’s optical gain, the photocurrent and the small signal current gain are calculated. The dominant component in the photocurrent is found to be due to optical absorption in the base-collector space charge region. For backside illumination, the collector component becomes larger. The base width and electron diffusion length control the current gain and thereby the optical gain. The model is used to quantify the effects of the device’s structure on the optical gain.
A self-consistent technique for the analysis of the temperature dependence of current–voltage and capacitance–voltage characteristics of a tunnel metal-insulator-semiconductor structure
Authors: Cova P. , Singh A. , Masut R. A.
Published: July 2002

A new formalism is reported for the analysis of the current–voltage (I–V) characteristics of a tunnel metal-insulator-semiconductor (MIS) device, which considers a bias dependent distribution of interface states and barrier lowering due to the image force. Our theoretical expression for the I–V characteristics is general in the sense that it is applicable even under conditions when both the thermionic emission and the diffusion mechanisms of current transport compete with each other. The method is ideal for new epitaxial materials and devices where the carrier density is not known precisely beforehand. A self-consistent method of analysis is reported to determine the characteristic parameters of MIS diodes, using simultaneously the I–V and capacitance–voltage data as a function of temperature. This computational analysis has been used to examine the current transport mechanism in an Au/p-InP epitaxial MIS diode. The experimental verification of the theory and computational analysis is done by comparing the values of the interface state density distribution in thermal equilibrium with the semiconductor Nss, obtained from the forward I–V characteristics, with those directly measured by the multifrequency admittance method. Excellent agreement from these comparisons strongly supports the validity of the theory. Over the temperature range of 200–393 K, our results indicate that the interfacial layer-thermionic emission was clearly the dominant mechanism of the forward current transport in an MIS fabricated on a lightly doped InP:Zn epitaxial layer. The transmission coefficient through the insulator layer obtained from the reverse I–V characteristics was θp=1.43×10−3±7% from which we estimate an oxide thickness of 2.2 nm. The analysis of the barrier height φb0 versus temperature, obtained from 1 MHz C–V data provided a value φ0=1.06 V±10% for the zero bias and zero temperature barrier height.
Theory of second harmonic generation in composites of nonlinear dielectrics
Authors: Hui P. M. , Stroud D.
Published: July 2002

We consider the effective nonlinear susceptibility tensor d(−2ω;ω,ω) for second harmonic generation in a nonlinear composite medium. We derive a simple expression for this susceptibility in terms of the position-dependent tensor d, and three factors that describe the local field enhancement in a corresponding linear medium. The resulting expression can be used to calculate the local-field enhancement of d in many geometries. In the dilute limit, the general expression reduces to a result previously derived.
Determination of lateral stresses in shocked solids: Simplified analysis of piezoresistance gauge data
Authors: Feng R. , Gupta Y. M.
Published: July 2002

Determining lateral stresses in shocked solids is an important need. Toward this end, a study was carried out to ascertain the use of simplified analyses to relate lateral piezoresistance gauge data to sample lateral stresses, with reasonable accuracy, for a class of solids. Dynamic, two-dimensional calculations were used to compute the resistance change histories of lateral manganin foil gauges in a shocked ceramic. This rigorous solution served as a bench mark for the simplified approaches. Results from several simplified methods, including those in the literature, were examined and error bars evaluated by comparing them with the ideal solution. Difficulties with simplified analyses are discussed. A combination of a simple forward analysis and a conditional lateral gauge calibration appears to provide an approach that is both efficient and consistent. Since simplified analyses are useful only in particular situations, applicability of these methods is discussed.
Tunneling currents through ultrathin oxide/nitride dual layer gate dielectrics for advanced microelectronic devices
Authors: Yang H. Y. , Niimi H. , Lucovsky G.
Published: July 2002

Direct and Fowler–Nordheim tunneling currents through oxide and dual layer silicon oxide–silicon nitride dielectrics are investigated for substrate and gate injection. The calculations include depletion effects in the heavily doped (n+) polysilicon gate electrodes as well as quantization effects in the less heavily doped n-type substrates. The Wentzel–Kramers–Brillouin (WKB) effective mass approximation has been compared with exact calculations for the tunneling probability, and based on these comparisons it has been found that the WKB approximation is adequate for single layer dielectrics, but is not for the dual layer dielectrics that are the focus of this article. Using exact tunneling transmission calculations, current-voltage (I–V) characteristics for ultrathin single layer oxides with different thicknesses (1.4, 2.0, and 2.3 nm) have been shown to agree well with recently reported experiments. Extensions of this approach demonstrate that direct tunneling currents in oxide/nitride structures with oxide equivalent thickness of 1.5 and 2.0 nm can be significantly lower than through single layer oxides of the same respective thickness.
Electrical properties of TiO2 thin films formed on self-assembled organic monolayers on silicon
Authors: Shin Hyunjung , De Guire Mark R. , Heuer Arthur H.
Published: July 2002

TiO 2 thin films were synthesized onto sulfonated self-assembled organic monolayers from Ti4+ aqueous solutions at low temperature (80 °C). The deposited TiO2 thin films were characterized using Rutherford backscattering spectroscopy and transmission electron microscopy. The electrical properties of the TiO2 films were measured using capacitance–voltage (C–V) and current–voltage techniques. From C–V measurements, the dielectric constants were calculated to be in the range of 24–57 for the as-deposited films and 67–97 for films annealed at 400 °C. For the as-deposited samples, the breakdown voltage was 2.7 MV/cm, the leakage current was 4.5×10−6 A/cm2, the resistivity was 1–1.5×109 Ω cm, and the interface trap density ∼1×1012 cm−2 eV−1. The leakage current behavior was consistent with the Poole-Frenkel mechanism of conduction.
Hydrostatic pressure studies of GaAs tunnel diodes
Authors: Beji L. , el Jani B. , Gibart P. , Portal J. C. , Basmaji P.
Published: July 2002

GaAs (doped with Zn and Sn in the p and n parts, respectively) tunnel diode I–V characteristics were studied under hydrostatic pressure. The decrease in peak and valley current was explained using the band-gap pressure dependence and the fact that the Fermi energy is pinned by the DX level at normal pressure for heavy Sn doping. These findings are verified by calculating theoretical normalized peak and valley currents, which substantially agree with the experimental results.
Raman scattering from fully strained Ge1−xSnx (x⩽0.22) alloys grown on Ge(001)2×1 by low-temperature molecular beam epitaxy
Authors: Rojas-López M. , Navarro-Contreras H. , Desjardins P. , Gurdal O. , Taylor N. , Carlsson J. R. A. , Greene J. E.
Published: July 2002

Fully strained single-crystal Ge1−xSnx alloys (x⩽0.22) deposited on Ge(001)2×1 by low-temperature molecular beam epitaxy have been studied by Raman scattering. The results are characterized by a Ge–Ge longitudinal optical (LO) phonon line, which shifts to lower frequencies with increasing x. Samples capped with a 200-Å-thick Ge layer exhibit a second Ge–Ge LO phonon line whose position remains close to that expected from bulk Ge. For all samples, capped and uncapped, the frequency shift ΔωGeSn of the Ge–Ge LO phonon line from the Ge1−xSnx layer, with respect to the position for bulk Ge, is linear with the Sn fraction x (ΔωGeSn=−76.8x cm−1) over the entire composition range. Using the elastic constants, the Grüneisen parameter, and the shear phonon deformation parameter for Ge, we calculate the contribution of compressive strain to the total frequency shift to be Δωstrain=63.8x cm−1. Thus, the LO phonon shift in Ge1−xSnx due to substitutional-Sn-induced bond stretching in fully relaxed alloys is estimated to be Δωbond =−140.6x cm−1.
Large electrical conductivity enhancement of WO3 thin films produced by ion implantation
Authors: Miyakawa Masashi , Kawamura Ken-ichi , Hosono Hideo , Kawazoe Hiroshi
Published: July 2002

Ion implantation induced dc electrical conductivity changes in polycrystalline WO3 thin films was examined. Helium, argon, and tungsten ions were implanted under the following conditions: helium at 90 keV, argon at 360 keV, and tungsten at 300 keV. The fluence range studied was from 1×1017 to 3×1017 cm−2 for helium and argon, and from 5×1014 to 5×1015 cm−2 for tungsten. The 300 K conductivities after implantation at a 1×1017 cm−2 fluence increased from ∼10−5 to ∼10−1 S cm−1 for helium implantation and to ∼102 S cm−1 for argon implantation. Also, implantation induced a broad optical absorption feature at ∼1000 nm and a negative thermopower coefficient indicating n-type conduction. The implanted samples exhibited low conductivity activation energies, which were consistent with degenerate conduction. Tungsten implantation induced conductivities were higher than those resulting from either helium or argon implantation as a conductivity of ∼102 S cm−1 was obtained at a 5×1015 cm−2 fluence. These conductivity changes are correlated to the displacements per atom resulting from the various ion implantations. Several mechanisms are responsible for the observed donor species increase: (1) in the proton case the implanted species becomes a donor, (2) in the helium and argon cases a charged oxygen vacancy may be formed by the nuclear collisions that occur during implantation, and (3) in the tungsten case the subsequent oxidation of the implanted tungsten results in the formation of a donor species.
Vacuum ultraviolet application of Li2B4O7 crystals: Generation of 100 fs pulses down to 170 nm
Authors: Petrov V. , Rotermund F. , Noack F. , Komatsu R. , Sugawara T. , Uda S.
Published: July 2002

We investigate and characterize the newly grown crystal Li2B4O7 which is transparent down to 160 nm for nonlinear optical conversion into the vacuum ultraviolet using sum frequency mixing with femtosecond pulses. This material exhibits excellent properties below 180 nm and makes possible the generation of wavelengths down to 170 nm with an all solid state laser system. The most important advantage of Li2B4O7 in this spectral range turns out to be the possibility of utilizing noncritical phase matching with maximized effective nonlinearity. We demonstrate generation of nearly transform limited 100 fs pulses between 170 and 185 nm with conversion efficiency of 4%. Their peak powers range from 200 kW at 170 nm to >2 MW at 185 nm.
New modified Sr-ferrite particles for high density magnetic recording
Authors: Fang Qingqing , Zhong Wei , Jin Zhiqiang , Du Youwei
Published: July 2002

Platelike particles of SrFe12O19 with epitaxially grown Fe3O4 overlayer were prepared by a wet method. The experimental results have shown that modified sample has an exchange coupling between epitaxial layer and Sr-ferrite core. As Fe2+ quantity added, coercive force decreased and saturation magnetization increased, and these values can be controlled by changing ferrous ions ratio with respect to core material, simultaneously, reflection intensity of magnetoplumbite phase has been weaker and that of spinel phase stronger. Moreover, the variations of particle morphology changing from the hexagonal platelet of Sr-ferrite to the square platelet of Fe3O4 were observed by transmission electron microscope. The origin of coercive force decrease quickly may be understood by the nucleation and growth of reverse magnetic domains when Fe2+ content x<0.4 wt %.
Optical and photorefractive properties of Nb-doped BaTiO3
Authors: Chang J. Y. , Chu C. F. , Huang C. Y. , Yueh R. R.
Published: July 2002

The photorefractive properties of BaTiO3 are gradually modified by the addition of Nb which change the charge carrier from hole to electron. Before the compensation point, the dark conductivity decreases and the response time increases with increasing Nb concentration. Two beam coupling gain can be explained by using the deep and shallow trap model with hole as the major carrier. Near the compensation point, the deep and shallow trap model is needed, in addition to the electron–hole competition to explain this. After the compensation point, an absorption around 600 nm evolves, the dark conductivity increases very fast, and the electron becomes the major carrier.
Analysis of the complex permeability of a ferromagnetic wire
Authors: Britel M. R. , Ménard D. , Ciureanu P. , Yelon A. , Rouabhi M. , Cochrane R. W. , Akyel C. , Gauthier J.
Published: July 2002

We have measured the magnetoimpedance of ferromagnetic wires placed as internal conductors in shorted coaxial lines. We have also modeled the impedance by simultaneously solving Maxwell’s equations and the Landau–Lifshitz equation of motion for the magnetization in a cylindrical magnetic conductor under the appropriate boundary conditions. The permeability of Ni45Co25Fe6Si9B13Mn2 amorphous wires, deduced from the impedance, is compared with the model on plots of the imaginary part of the permeability versus the real part, with the dc magnetic field as implicit parameter. This approach emphasizes the region of ferromagnetic resonance and facilitates the investigation of the permeability locus. A number of intrinsic parameters of the magnetic wire, such as the exchange constant, the damping constant, and the gyromagnetic ratio, have been evaluated.
Fluence effects on the magnetic properties of Fe81B13.5Si3.5C2 metallic glass produced by pulsed laser deposition
Authors: Sorescu M. , Tsakalakos L. , Sands T.
Published: July 2002

A metallic glass target of Fe81B13.5Si3.5C2 ribbon was ablated under pulsed focused laser irradiation at a high fluence, Φ=3, 5, 6, 7.5, and 10 J/cm2. X-ray diffraction was performed on the resulting films, demonstrating the amorphous nature of the samples. Hysteresis loop measurements were recorded at 4.2 K in an applied magnetic field of 1.5 T to show differences in the saturation magnetic moment and coercivity between the as-cast ribbon and laser-ablated films. Conversion electron Mössbauer spectra were used to obtain the fluence dependence of the hyperfine magnetic field and magnetic texture parameter. At low laser fluences, the magnetic moments were predominantly oriented in the plane of the film. For intermediate values of the laser fluence, a random distribution of magnetic-moment directions was achieved. For high laser fluences, however, an out-of-plane orientation of the magnetic moments resulted. The films have prospective applications in magnetic sensors, switches, and concentrators.
Simple interpretation of metal/wurtzite–GaN barrier heights
Authors: Bermudez V. M.
Published: July 2002

Photoemission data for the dependence of the Schottky barrier height on the metal work function, for n-type wurtzite GaN, are discussed in terms of the Cowley–Sze model [J. Appl. Phys. 36, 3212 (1965)] for a uniform density of surface states in the band gap. It is suggested that, in the context of this model, such barrier heights can be expressed largely as a sum of the “bare-surface barrier height” (i.e., the band bending before contact formation) and a Mott–Schottky term.
Electroluminescence in silicon oxynitride films
Authors: Price K. J. , Sharpe L. R. , McNeil L. E. , Irene E. A.
Published: July 2002

We report electroluminescence (EL) from 50 nm silicon oxynitride films on p-type crystalline silicon substrates in a Au/silicon oxynitride/Si structure. The EL intensity has a peak below 2.45 eV, and is consistent with radiative recombination of injected carriers. The EL is present only in annealed samples, and the emission is similar to the photoluminescence from the same samples. The current–voltage behavior is indicative of space charge-limited current. No polarity or field dependence of the EL peak energy is observed. This phenomenon is attributed to the relaxation of carriers down the band tails before recombination.
Grain boundary electrical barriers in positive temperature coefficient thermistors
Authors: Hayashi Katsuro , Yamamoto Takahisa , Ikuhara Yuichi , Sakuma Taketo
Published: July 2002

Resistance–temperature, current–voltage, and capacitance–voltage characteristics have been measured in niobium-doped barium titanate bicrystals. The analysis by device simulation reveals that potential barrier with a semiconductor–insulator–semiconductor structure can consistently interpret the observed characteristics rather than the double Schottky barrier. Absence of sub-ohmic behavior in the current–voltage characteristics and excess low-frequency capacitance at zero bias are caused by the spatially distributed traps at the insulating layer.
Sputter deposition of cathodes in organic light emitting diodes
Authors: Hung L. S. , Liao L. S. , Lee C. S. , Lee S. T.
Published: July 2002

Sputter deposition was employed for cathode preparation in organic light emitting diodes (OLEDs). A thin film of copper phthalocyanine (CuPc) was found to be an effective buffer layer in preventing sputter damage to the OLED layer structure. However, the CuPc layer forms an electron-injection barrier with the underlying Alq layer, resulting in increased electron-hole recombination in the nonemissive CuPc layer, and thus a substantial reduction in electroluminescence efficiency. Incorporation of Li at the CuPc/Alq interface from a sputter-deposited Al (Li) cathode was found to reduce the injection barrier at the interface and make the overall device efficiency comparable to a device having an evaporated MgAg cathode. The devices exhibited good operational stability with a half lifetime greater than 3800 h at 20 mA/cm2.
Light emitting porous silicon diode based on a silicon/porous silicon heterojunction
Authors: Pavesi L. , Chierchia R. , Bellutti P. , Lui A. , Fuso F. , Labardi M. , Pardi L. , Sbrana F. , Allegrini M. , Trusso S. , Vasi C. , Ventura P. J. , Costa L. C. , Carmo M. C. , Bisi O.
Published: July 2002

A new structure is proposed to improve the external quantum efficiency of porous silicon (PS) light emitting diodes (LED). It is based on a heterojunction between n-type doped silicon and PS. The heterojunction is formed due to the doping selectivity of the etching process used to form PS. The improvement of the proposed LED structure with respect to usual metal/PS LED is demonstrated. This is thought to be due to a different injection mechanism for which carriers are injected directly into conduction band states. Anodic oxidation experiments show further improvements in the LED efficiency.
Scanning Hall probe microscopy with shear force distance control
Authors: Schweinböck Thomas , Weiss Dieter , Lipinski Martin , Eberl Karl
Published: July 2002

We describe a new type of scanning Hall probe microscope operating at room temperature for quantitative and noninvasive measurements of magnetic stray fields. The probe-sample distance is controlled by piezoelectrical detection of the shear forces acting on an oscillating cantilever. The Hall probes are manufactured from prepatterned GaAs wafers overgrown with a GaAs/AlGaAs heterostructure containing a high-mobility two-dimensional electron gas a few ten nm below the surface. The active Hall area is defined by optical and electron-beam lithography with a junction width of 0.6 μm yielding in a resolution of approximately 0.4 μm. The Hall coefficient of the sensor at room temperature is 0.23 Ω/G with a noise level of ∼0.1 G/Hz1/2. We show measurements of the stray field pattern of bits written on a magnetic hard disk.
A Monte Carlo method for small signal analysis of the Boltzmann equation
Authors: Kosina H. , Nedjalkov M. , Selberherr S.
Published: July 2002

An approach for analysis of the small signal response of carriers in semiconductors is presented. The response to an electric field impulse is explained in terms of a relaxation process governed by a Boltzmann equation. New Monte Carlo algorithms for the direct simulation of the impulse response are presented and existing algorithms are discussed in a unified way.
Magnetothermopower of cobalt/copper multilayers with gradient perpendicular to planes
Authors: Baily S. A. , Salamon M. B. , Oepts W.
Published: July 2002

The magnetothermopower and magnetoresistance of two copper/cobalt multilayers grown on grooved indium phosphide were measured between 100 K and 295 K. One sample was deposited at an angle onto a grooved substrate such that transport properties are measured between layers rather than parallel to them (CPP); the other was deposited perpendicular to the substrate, so that the transport properties are measured at an angle to the planes (CAP). The data show the magnetothermopower to vary inversely with the resistivity, as predicted by the Mott formula if the magnetoresistance is determined by the ratio α of majority to minority densities of states at the Fermi energy EF in the magnetic metal. Using this result, we can collapse the thermopower data at various temperatures to find that α′/α is 0.6 eV−1 for the CPP sample and 0.7 eV−1 for the CAP sample, where α′≡(∂α/∂E)EF.
On the exchange biasing through a nonmagnetic spacer layer
Authors: Thomas Luc , Kellock Andrew J. , Parkin Stuart S. P.
Published: July 2002

We present results on the magnetic coupling between a ferromagnetic (F) thin film and an antiferromagnetic (AF) thin film through a nonmagnetic metallic spacer (S) layer. Multilayered structures have been grown on silicon substrates using dc magnetron sputtering. We have studied the structure AF/S/F for different materials. AF is Ir22Mn78, F is Fe16Co16, and the spacer S is Al, Ag, Au, Si, Pd, Ru, and Ti. In most cases, both the exchange-bias and the coercive fields decrease exponentially with the spacer thickness with a decay length of a few angstroms, depending slightly on the material. Similar decay lengths are observed for the reversed structure F/S/AF. In some specific cases, we observe a nonmonotonic variation of the exchange-bias field with the spacer thickness. For example, the exchange-bias field increases when a thin Ag layer is inserted between the F film and an AF film grown at high sputter pressure.
On the relevance of Kondo disorder in the non-Fermi-liquid behavior of UCu4Ni
Authors: López de la Torre M. A. , González J. A. , Izquierdo A. , Vieira S. , Ellerby M. , McEwen K. A.
Published: July 2002

The electrical, magnetic and thermal properties of UCu4Ni show temperature dependencies suggestive of non-Fermi-liquid (NFL) behavior. In this work, we present an interpretation of these properties in terms of a Kondo disorder model for NFL behavior. The overall magnitude and temperature dependence of the magnetic susceptibility and the specific heat are found to be consistent with this model, assuming reasonable values of the relevant parameters, the average Kondo temperature (TK), the Kondo coupling constant (λ) and the distribution width w. These results are discussed in comparison with those obtained for other NFL systems, as UCu4Pd and UCu3.5Pd1.5.
Getting the information in and out: The channel (invited)
Authors: Howell Thomas D. , McEwen Peter A. , Patapoutian Ara
Published: July 2002

Magnetic recording channels perform several signal processing tasks in the process of storing and retrieving data. Among them are precompensation, equalization, timing recovery, detection, and coding. A range of algorithms is available for each of these tasks with different levels of complexity and performance. We describe how each of these tasks is done today and indicate how they are likely to change as requirements and implementation technologies evolve.
Fluorescence polarization in pyrazine, pyrimidine, and s-triazine in the vapor phase
Authors: Ohta Nobuhiro , Sekiguchi Osamu , Baba Hiroaki
Published: July 2002

The fluorescence polarization in the vapor phase has been observed for excitation along the rotational contour of the 0–0 bands of pyrazine and pyrimidine and the 610 and 620 bands of s-triazine in the S0→S1 transition. In all the cases, the resonance fluorescence exhibits a remarkable variation in the degree of polarization along the rotational contour, with larger values for the Q branch excitation than for the R or P branch excitation. On the other hand, the broad emission of s-triazine vapor originating from levels to which a nonradiative transition occurs from the initially prepared vibronic level does not show polarization.
Collective vibrational modes in molecular assembly of DNA and its application to biological systems. Low frequency Raman spectroscopy
Authors: Urabe Hisako , Hayashi Hideyuki , Tominaga Yasunori , Nishimura Yoshifumi , Kubota Kenji , Tsuboi Masamichi
Published: July 2002

The origin of low frequency modes in a molecular assembly of DNA has been investigated by changing the assembly conditions of DNA molecules. The ∼85 cm−1 broadband in B form DNA was found to be insensitive of the assembly condition, and was ascribed mainly to motions of base pairs bound with hydrogen bonds. The lowest frequency mode was found, on the contrary, to be sensitive to the assembly condition and therefore considered to include motions of backbone chains. Through the observation of the lowest frequency mode in B-form DNA in biological systems such as lambda phage, chromatin and nucleosome core, we have concluded that from the dynamical point of view the DNA molecules in biological systems is more likely in a solid state than in an aqueous solution state.
Raman spectra of solid CH4 under high pressure. III. New high pressure phases in solid CH4 and CD4
Authors: Thiéry M.-M. , Fabre D. , Kobashi K.
Published: July 2002

We have observed the Raman spectra of the intramolecular vibrations in solid CH4 and CD4 at T=4.2 K under high pressure. For solid CH4, the spectra changed markedly at P≂9 kbar, indicative of a new phase VI above this pressure. For solid CD4, an analogous change was observed at P≂5.4 kbar, indicating a phase transition from phase III to a new phase IV. The spectra of these new phases, CH4 VI and CD4 IV, are very similar so that it is concluded that the crystal structures of the phases are isomorphous. The observed spectra seem to indicate that the molecular orientations of methane molecules in the new high pressure phases are fully ordered.
High order crystal field invariants and the determination of lanthanide crystal field parameters
Authors: Yeung Y. Y. , Newman D. J.
Published: July 2002

The relationship between high order J-multiplet splitting moments and crystal field invariants is investigated. Expressions for corrections due to crystal field intermultiplet coupling are obtained. The formalism is applied to determine the crystal field parameters of lanthanide ions in cubic sites from ground multiplet energies alone, without fitting energy levels.
Acetaldehyde photochemistry: The radical and molecular dissociations
Authors: Yadav Janardan S. , Goddard John D.
Published: July 2002

The dissociation of acetaldehyde to radical products, CH3+HCO and H+CH3CO, and to the molecular limit of CH4+CO has been investigated by ab initio SCF calculations. Effects of zero point energy corrections and of electron correlation on energy differences have also been considered. The computed T1 origin of acetaldehyde is in good agreement with recent experimental determinations. On the T1 surface, CH3CHO faces activation barriers of 12–15 kcal mol−1 for dissociation into radicals. The existence of and the quantitative determination of the barrier heights have been accurately computed for the first time. Vibrational excitation of T1 acetaldehyde formed with energy equal to that of its spectroscopic origin is anticipated to play an important role in acetaldehyde photochemistry as has been inferred experimentally. The barrier to the unimolecular dissociation to molecular products on the S0 surface is slightly lower than the T1 radical barriers. If hot ground state acetaldehyde can be formed with energies of ∼85 kcal mol−1 by some process then the molecular dissociation channel should also be accessible.
Non-Markoffian theory of quantum beats in molecular fluorescence
Authors: Fujimura Y. , Akiyama T. , Nakajima T. , Kono H. , Lin S. H.
Published: July 2002

We develop a theory of quantum beats in molecular fluorescence taking into account collision effects (such as pure dephasing due to elastic collision) without invoking the Markoff approximation. Model calculations of the time dependent fluorescence from molecules with two excited levels are performed in order to see how the non-Markoffian character in the collision process exerts influence upon the quantum beat pattern. It is suggested that if the pure dephasing between the excited states is a Markoffian ‘‘the degree of modulation of the beats’’ decreases exponentially with time; if it is in the static limit the degree of modulation of the beats decreases as a Gaussian function (the non-Markoffian behavior in the quantum beats). A given case can be classified as either Markoffian or non-Markoffian, depending on the relative magnitudes of the collision duration time τc, the beat frequency ω, and the coupling strength v which is characteristic of the pure dephasing process. The Markoff approximation holds only if the condition ω<v<1/τc is satisfied; the non-Markoffian behavior appears in the quantum beat pattern when ω>v, even if v2τ2c<1 .
Rotational and vibrational state dependence on intramolecular coupling and dynamics in the S1 state of pyrazine
Authors: Amirav Aviv , Jortner Joshua
Published: July 2002

In this paper we report on the absolute fluorescence quantum yields, Y, from photoselected rotational and vibrational states within the S1 manifold of pyrazine cooled in planar, pulsed, supersonic expansions of Ar. The Y data from the S1 origin exhibit a marked J′ dependence, which can be fit by the empirical relation Y=0.124/(2J′+1) for J′=5–22. The vibrational state dependence of Y on the excess vibrational energy, EV, exhibits an approximate exponential decrease with increasing EV in the range EV=0–1300 cm−1 with a possible saturation at EV=1300–1800 cm−1. The low absolute Y values can be attributed to intramolecular nonradiative decay of S1–T molecular eigenstates. The marked rotational state dependence of Y from the S1 origin can be interpreted in terms of the intermediate level structure (ILS) provided that the dilution factor is Neff=16(2J′+1) for J′=5–22. This Neff exhibits a qualitatively different J′ dependence than the number of effectively coupled states originating from a single J′K′ doorway state N(spectroscopic)=12–15 for J′=0–3, which was inferred by Kommandeur et al. [Chem. Phys. Lett. 92, 565 (1980)] from high resolution spectroscopic studies. In order to reconcile our Y data, together with the available psec time-resolved data and the spectroscopic information in terms of the ILS model, one has to assert that the range J′=3–5 marks a crossover from J′ independent to J′ dependent Neff.
Testing the Lorentz–Lorenz relation in the near-critical binary fluid mixture isobutyric acid and water
Authors: Andrew W. V. , Khoo T. B. K. , Jacobs D. T.
Published: July 2002

The refractive index in each phase of the binary fluid mixture isobutyric acid and water was measured at temperatures below the system’s upper consolute point. This data was combined with existing density data to test the Lorentz–Lorenz relation in a near-critical binary fluid mixture. The Lorentz–Lorenz relation is verified within experimental error (0.5%) when the volume change on mixing the components is taken into account. The density coexistence curve data is reanalyzed to determine the critical exponent  β and amplitude B. By allowing the order parameter to be a definition of the volume fraction that includes volume loss on mixing, a very symmetric coexistence curve is obtained which can be described by simple scaling with  β=0.326±0.003 and B=1.565±0.021. This exponent agrees with theoretical predictions while the amplitude, when combined with existing turbidity data, confirms two-scale-factor universality. The amplitude obtained by analyzing the coexistence curve when the refractive index is the order parameter also combines with turbidity data to confirm two-scale-factor universality, but does not require knowledge of the volume loss on mixing or the composition dependence of the refractive index.
An unsuspected Fermi perturbation in the acetylene Ã 1A
u 3ν3 level
Authors: Scherer George J. , Chen Yongqin , Redington Richard L. , Kinsey James L. , Field Robert W.
Published: July 2002

A previously undetected perturbation in the HCCH Ã 1Au 3ν3 (trans-bend) level has been found and analyzed. The perturbing level is identified as the Ã 1Au ν2+2ν4 level. Locally deperturbed molecular constants for the 3ν3 and perturbing level are obtained, along with the perturbation matrix element and the J-dependent mixing coefficients. The Ã-state torsional fundamental, ν4≈850 cm−1, is estimated from the energy of perturbing level. The presence of the perturbation explains large differences between the rotational line intensity patterns observed in the absorption and fluorescence excitation spectra. The perturbed 3ν3 and extra lines are especially prominent in a fluorescence excitation spectrum of the Ã 1Au–X̃ 1Σ+g 33 0 K10 subband of acetylene rotationally cooled in a supersonic jet.
Quantitative analysis of covalent bonding between substituted silanes and inorganic surfaces
Authors: Miller James D. , Ishida Hatsuo
Published: July 2002

Diffuse reflectance Fourier transform infrared spectroscopy was used to study the interfacial chemical reaction between gamma-methacryloxypropyltrimethoxysilane and a low surface area particulate lead oxide. The surface reaction results in a covalent interfacial bond formed between the solid surface and the hydrolyzed alkoxysilane. Quantitative information about the interfacial bond is available from the spectra. Using digital subtraction techniques spectra of the adsorbate species and only the adsorbate/adsorbent interaction are produced. From these spectra it is possible to analyze quantitatively the covalent bonding between adsorbate and adsorbent at surface coverages near and much less than one equivalent molecular layer. Covalent bonding occurs with the substrate up to an adsorbate surface coverage of 1.50 mg/m2. The intensity of the interfacial covalent bond is linear with adsorbate concentration up to 1.50 mg/m2 and behaves similarly to theoretical prediction based on a simple model of bonded and non-bonded species. An example of the time-dependent formation of the adsorbate/adsorbent covalent bond is illustrated and suggests the ability to monitor kinetically the surface reaction. The loss of free hydroxyl groups on the substrate surface is also quantitatively monitored.
The O2–O2 dimer: Magnetic coupling and spectrum
Authors: van der Avoird A. , Brocks G.
Published: July 2002

A theoretical analysis has been made of the van der Waals vibration–rotation–electron spin states of the O2–O2 dimer in its 3Σ−g –3Σ−g electronic ground state. This analysis is based on a Hamiltonian that includes a spin-dependent O2–O2 interaction potential and it involves also the permutation–inversion symmetry of the system. We have constructed some hindered internal rotor models for the vibrational states of the O2–O2 dimer which correspond with different equilibrium geometries, and for each of these models we have numerically calculated the spin-rotation fine structure. This fine structure appears to be determined not only by the Heisenberg exchange interaction between the O2 monomer triplet states, but also by intramolecular spin–orbit and spin–spin coupling and, to a smaller extent, by the intermolecular spin–spin (magnetic dipole) interaction and by the Coriolis terms in the kinetic energy. The resulting fine-structure spectrum is very complex, and very sensitive to the geometry of the O2–O2 dimer, to the nature of its internal motions and to the various magnetic couplings. This implies that detailed measurements of this spectrum, which can be interpreted with the help of the theory presented here, will yield interesting information on the properties of the O2–O2 dimer and, at the same time, verify our assumptions on the magnetic interactions between O2 molecules which have important consequences for the properties of solid oxygen.
Tunneling motion in ArH+3 and isotopomers from the analysis of their rotational spectra
Authors: Bogey M. , Bolvin H. , Demuynck C. , Destombes J. L. , Van Eijck B. P.
Published: July 2002

The millimeter and submillimeter wave spectra of different H/D isotopomers of ArH+3 were investigated. The ionic clusters were produced inside a negative glow extended by a magnetic field, in Ar/H2/D2 mixtures. Most of the observed rotational lines were split by internal motion and spin statistical weights as well as intensity ratios for the components were determined from symmetry considerations. The splittings were interpreted in terms of internal motion of the H+3 triangle. In a first step, the usual rigid top-rigid frame model was used to reproduce the order of magnitude of the splittings. In a second step, a more refined model, which accounts for the flexibility of the top and the frame, led to a better calculation of the observed splittings in ArH+3 and ArD+3.
Magnetic field effects on yields and decays of fluorescence of pyrimidine vapor: Excited rovibronic level dependence
Authors: Ohta Nobuhiro , Takemura Takeshi , Fujita Masahisa , Baba Hiroaki
Published: July 2002

External magnetic field effects on yields and decays of fluorescence of pyrimidine vapor on excitation into various rotational levels belonging to the vibrationless level or the 6a1 level of S1 have been studied in a supersonic jet or in a bulk gas at room temperature with a field strength of 0–150 G. The fast component of fluorescence is not affected by an external magnetic field, whereas the slow fluorescence is quenched by a field except for excitation at the R(0) line belonging to the 0–0 transition. The fluorescence quenching is more effective at the 6a1 level than that at 00, indicating that the level density of the triplet state coupled to the singlet state plays an important role in the magnetic mixing of the triplet spin sublevels, in terms of which the fluorescence quenching by a magnetic field is interpreted. The excited rotational level dependence of the fluorescence quenching by a magnetic field is attributed to K scrambling in the triplet manifold following intersystem crossing.
Concentration fluctuation of stiff polymers. III. Spinodal decomposition
Authors: Shimada Takayuki , Doi Masao , Okano Koji
Published: July 2002

We discuss the dynamics of the formation of liquid crystalline phase of rod-like polymer solutions, particularly the behavior of spinodal decomposition which should be observed when the system is suddenly brought into a thermodynamically unstable region. The growth rate of the concentration fluctuation is calculated as a function of the wave number. It is shown that these growth rates are governed by the translational motion, not the rotational motion. The dynamical structure factor in the isotropic phase is also calculated.
Gaussian functions in Hylleraas-configuration interaction calculations: Potential curves for the e 3∑+u and the f 3∑+u states of hydrogen
Authors: Preiskorn A. , Lie G. C. , Frye D. , Clementi E.
Published: July 2002

The explicitly correlated wave function method (Hylleraas-CI) with Cartesian Gaussian basis sets has been used to calculate the potential curve for the e3∑+u and f 3∑+u states of the H2 molecule. Potential barriers of 215 and 10 300 cm−1 are predicted to exist in the e3∑+u and f 3∑+u states, respectively, and their nature analyzed. Spectroscopic constants for H2 and D2 derived from the curves are compared with experiments and other theoretical calculations. It is concluded that the present potential curves are the best theoretical ones to-date and their accuracy is probably as good as the scanty experimental data. No o3∑+u state below the f 3∑+u state is found in our calculations.
Beyond the linear approximations of the conventional approaches to the theory of chemical relaxation
Authors: Bianucci Marco , Grigolini Paolo , Palleschi Vincenzo
Published: July 2002

The nonlinear coupling between the reacting system and its molecular bath results in a generalized Langevin equation with a memory kernel which is nonstationary as well as dependent on the reaction coordinate. In a preceding paper by Grigolini [J. Chem. Phys. 89, 4300 (1988)] a theory was developed to determine the reaction rate of a physical system characterized by a nonlinear interaction between system and bath. It is here shown that the local linearization adopted in that paper extends to this nonlinear condition the linear theory of Grote and Hynes, disregards also nonlinear effects, which does not conflict with the conservation of the Smoluchowski structure necessary to apply the standard first passage time approach. Here a clear distinction is made between the second-order local linearization (SOLL) and the infinite-order local linearization (IOLL). When deriving the Kramers equation from a microscopic description, it is possible to go beyond the SOLL approximation without contravening the basic requirement of keeping our description within a standard Fokker–Planck form.

Thus, the influence of nonstationary memory kernel as well as that of the anharmonic contribution of the reaction potential can be conveniently described. The next step, of basic importance for a simple expression of the chemical reaction rate in the space diffusion regime to be found, consists of deriving the Smoluchowski equation. This must be taken in a careful way so that in the linear case the Grote and Hynes theory is recovered. The study of the simple linear case shows indeed that the contraction over the variable velocity of a Kramers equation which is not fully renormalized does not lead to a correctly renormalized Smoluchowski equation, even if the IOLL is applied. A simple rule to take into account the effects of higher-order terms is then found. In the linear case, this simple rule leads to a result coincident with the exactly renormalized structure. In the nonlinear case, at the second order in the interaction between system and bath, the novel expression coincides with the results provided by the current methods to take into account the anharmonic effects produced by colored noises. The final step of our approach consists of deriving the Smoluchowski equation from this fully renormalized Kramers equation by adopting the IOLL aproximation. The final result is more general than those previously derived by Grigolini, thereby also naturally including the Grote and Hynes theory.
Vibrational energy transfer in OH X 2Πi, v=2 and 1
Authors: Raiche George A. , Jeffries Jay B. , Rensberger Karen J. , Crosley David R.
Published: July 2002

Using an infrared pump/ultraviolet probe method in a flow discharge cell, vibrational energy transfer in OH X 2Πi has been studied. OH is prepared in v=2 by overtone excitation, and the time evolution of population in v=2 and 1 monitored by laser-induced fluorescence. Rate constants for vibrational relaxation by the colliders H2O, NH3, CO2, and CH4 were measured. Ratios of rate constants for removal from the two states, k2/k1, range from two to five.
Temperature dependent quenching of the A 2Σ+ and B 2Π states of NO
Authors: Raiche George A. , Crosley David R.
Published: July 2002

Collisional quenching of the v′=0 vibrational levels of the A 2Σ+ and B 2Π states of nitric oxide has been studied over the temperature range 300 to 750 K. The pressure dependence of the time decay of laser-induced fluorescence, in a slowly flowing heated cell, furnished the quenching cross sections σQ. NO and O2 quench the A state rapidly but with no temperature dependence; σQ=37 and 21 Å2, respectively. σQA for H2O drops from 105 Å2 at 300 K to 34 Å2 at 750 K. σQB for O2 is independent of temperature but σQB for NO drops twofold and for H2O decreases by a factor of 3 over the temperature range studied. This variation among these colliders cannot be explained by a uniform, simple picture of the collision dynamics. Evidence is seen for B→A transfer proceeding through an intermediate state, perhaps a 4Π.
Theoretical study on the ground and excited states of MnO−4
Authors: Nakai Hiromi , Ohmori Yutaka , Nakatsuji Hiroshi
Published: July 2002

The symmetry adapted cluster (SAC) and SAC-configuration interaction (SAC-CI) theories are applied to the calculations of the ground and excited states of MnO−4. With the use of three different active spaces, we examine the convergence of the results. Electron correlations work to relax charge polarizations of the Mn–O bonds in the ground state. The experimental spectrum of MnO−4 is well reproduced by the present calculations. All of the observed peaks are assigned to the electronic allowed transitions to the 1T2 excited states. They are characterized as the excitations from the nonbonding orbitals of oxygen to the antibonding orbitals between the metal and oxygens. For reasonable descriptions of the ground and excited states of MnO−4, electron correlations should be considered with the use of large enough active space for expressing the relaxations of valence electrons. In this sense, all of the previous theoretical assignments of the spectra are unreliable, since they are not the results of calculations including sufficient amount of electron correlations.
Electron attachment to the fluoro-, bromo-, and iodomethanes studied by means of electron transmission spectroscopy and Xα calculations
Authors: Modelli Alberto , Scagnolari Francesco , Distefano Giuseppe , Jones Derek , Guerra Maurizio
Published: July 2002

The electron transmission spectra of the fluoro-, bromo-, and iodomethanes (except CH2F2 ) and of some monohaloalkanes are reported. The resonance energies for electron attachment to the halomethanes have also been evaluated by means of bound and continuum multiple scattering Xα calculations, which closely reproduce the low-energy experimental data. A large electron affinity increase is observed on going from the fluorine to the heavier halogen derivatives, while within each series of halides the electron affinity increases with increasing number of halogen atoms. The ground anion state is found to be stable in CHBr3, CBr4, and in all the iodomethanes except CH3I. The fragment anion production, as a function of the electron impact energy, has been measured for some of the compounds under investigation, in order to obtain experimental information on anion states very close to zero energy or slightly stable, which are not accessible in electron transmission.