bluuebunny/crossref_metadata_2025 huggingface dataset:
Reversible variation of donor concentrations in high-purity InP by thermal treatment
Authors: Kubota Eishi , Ando Koshi , Yamada Syoji
Published: July 2002

The donor concentration in high-purity InP bulk crystals was found to be reversibly changed by thermal treatment. At a level of 0.5–2.0×1015 cm−3, the concentration decreased below 340 °C and increased above 380 °C. Far-infrared photoconductivity measurements revealed that shallow donors with a binding energy of ∼7.5 meV were made to disappear and appear by low and high temperature treatment, respectively. Two possible mechanisms responsible for these phenomena are discussed in connection with the extrinsic and intrinsic donor origin. One probable mechanism is that shallow extrinsic donors, assigned to Si, are electrically passivated by some kind of defect, such as atomic hydrogen, and reactivated by low and high temperature treatment, respectively.
Analytic model of direct tunnel current through ultrathin gate oxides
Authors: Khairurrijal  , Mizubayashi W. , Miyazaki S. , Hirose M.
Published: July 2002

A theoretical model for tunnel leakage current through 1.65–3.90-nm-thick gate oxides in metal-oxide-semiconductor structures has been developed. The electron effective mass in the oxide layer and the Fermi energy in the n+ poly-Si gate are the only two fitting parameters. It is shown that the calculated tunnel current is well fitted to the measured one over the entire oxide thickness range when the nonparabolic E-k dispersion relationship for the oxide band gap is employed. The electron effective mass in the oxide layer tends to increase as the oxide thickness decreases to less than 2.80 nm presumably due to the existence of compressive stress in the oxide layer near the SiO2/Si(100) interface.
Comparison of nonlinear and nonstationary response of conventional and resonant cavity enhanced p-i-n photodiode
Authors: Matavulj Petar S. , Golubović Dušan S. , Radunović Jovan B.
Published: July 2002

High-power illumination of a p-i-n photodiode results in nonlinear operating conditions and, of course, in nonlinear electric response. This article presents an investigation and comparison of the nonlinear response of conventional and resonant cavity enhanced (RCE) p-i-n photodiodes constructed using a two-valley semiconductor (GaAs). The presented results are obtained through numerical simulation of the complete phenomenological model for a two-valley semiconductor in the submicron region for several bias voltages and several values of Dirac pulse optical powers. The nonlinear behavior is analyzed through carrier transit and response time. Nonstationary effects exist in two-valley semiconductors in the presence of adequate perturbation of the electric field in the absorption layer. The present investigation shows that the RCE p-i-n photodiode reaches the nonlinear operating regime for smaller incident optical irradiances.
Low threshold field emission from nanoclustered carbon grown by cathodic arc
Authors: Satyanarayana B. S. , Robertson J. , Milne W. I.
Published: July 2002

Nanocluster carbon films grown using a cathodic arc process at room temperature in the presence of background gases such as He are found to be good electron emitters. The variation in the surface morphology and the corresponding emission characteristics of the films with change in helium partial pressure (5×10−4–50 Torr) during the film growth are reported. The effect of helium partial pressure on clustering was studied for films grown at nitrogen partial pressures of 10−4 and 10−3 Torr. The surface morphology of the films grown, varied from smooth through clusters (with sizes 50–200 nm), to fibrous films. The threshold field varied from 1 to 10 V/μm for an emission current density of 1 μA/cm2. These films exhibit an emission site density of ∼104–105/cm2 at an applied field of 5 V/μm.
Characterization of deep impurities in semiconductors by terahertz tunneling ionization
Authors: Ziemann E. , Ganichev S. D. , Prettl W. , Yassievich I. N. , Perel V. I.
Published: July 2002

Tunneling ionization in high frequency fields as well as in static fields is suggested as a method for the characterization of deep impurities in semiconductors. It is shown that an analysis of the field and temperature dependences of the ionization probability allows to obtain defect parameters like the charge of the impurity, tunneling times, the Huang–Rhys parameter, the difference between optical and thermal binding energy, and the basic structure of the defect adiabatic potentials. Compared to static fields, high frequency electric fields in the terahertz-range offer various advantages, as they can be applied contactlessly and homogeneously even to bulk samples using the intense radiation of a high power pulsed far-infrared laser. Furthermore, impurity ionization with terahertz radiation can be detected as photoconductive signal with a very high sensitivity in a wide range of electric field strengths.
Charge carrier generation in a dye sensitized liquid crystal
Authors: Bilke R. , Schreiber A. , Bleyl I. , Haarer D. , Adam Dieter
Published: July 2002

The photocarrier generation and recombination mechanism in double layer cells of dye sensitized liquid crystalline hexahexylthiotriphenylene (HHTT) have been studied by steady-state photoconduction measurements. From the dependence of the photocurrent on light intensity we could determine the charge carrier generation mechanism in an azo pigment 4,4’-[(2,5-diphenyl1,3,4-oxadiazol)-bis(azo)]-bis[3-hydroxy-(2H-benzimidazo[2,1-a]benz[d,e]isoquinolin-7-one)] as surface enhanced whereas in titanylphthalocyanine charge generation takes place in the bulk of the dye. We investigated the actionspectra of a single HHTT cell and an azo sensitized cell in the different phases of HHTT to show the influence of azo on the spectral response of the photocurrent. The quantum efficiency in a single HHTT cell as well as in a double layer cell with azo as charge generation layer was determined. By solving the diffusion equation for excitons we carried out an estimation of the exciton diffusion length in azo.
Preparation and properties of lead-zirconate-titanate ferroelectric thin films using radio frequency planar magnetron sputtering
Authors: Chang C. C. , Tang C. S.
Published: July 2002

Lead-zirconate-titanate [Pb(Zr52Ti48)O3,PZT] thin films, 0.5 μm thick, were deposited onto Pt/Si substrate at room temperature using the radio frequency planar magnetron sputtering technique. A perovskite structure in the PZT thin film was obtained after the annealing processes. The annealing temperature varied from 650 to 750 °C in this experiment to find the optimized annealing temperature. Using x-ray diffraction analysis, the lowest full width of the half maximum (110) plane was 0.23° for the sample at 650 °C annealing temperature. The values of the remanent polarization Pr and coercive field Ec of the PZT thin film were 100 nC cm−2 and 0.6 kV cm−1, respectively, at 60 Hz. The measured pyroelectric coefficient in 0.5 μm thin films was 3.12×10−4 C/m2 K at 50 °C. Their dielectric constant and loss tangent were 494 and 0.072, respectively, at 1 kHz. The surface structure of the PZT thin film was examined using scanning electron microscopy and the grain size was in the range of 0.08–0.14 μm.
Thermal wave reflections of a pulsed stripe heat source from a plane boundary
Authors: Ouyang Zhong , Favro L. D. , Thomas R. L.
Published: July 2002

An experimental and theoretical study is carried out of thermal wave reflections from a plane boundary, such as the back wall of a solid slab, following pulsed heating of the front surface of the slab. The heating pattern is a long, uniform stripe source of finite width. Imaging is carried out using a high spatial resolution, high frame rate focal plane array infrared camera to monitor the surface temperature, through its emission of IR radiation in the 3–5 μm spectral band. We consider the spatial temperature distribution as a function of time, not only on the back surface of the slab, but also on the front surface. The theory is based on the assumption of thermal wave reflections from the two insulating (solid/air) boundaries of the slab, and its predictions are in excellent agreement with experimental data for both surfaces of a pure isotropic slab (Cu) and a highly anisotropic slab (uniaxial carbon-fiber-reinforced polymer composite). The unheated region of the front surface shows two peaks as a function of time — one resulting from the direct propagation of the thermal pulse along the surface from the edge of the source to the observation point, the other resulting from the highly damped reflections from the back surface of the slab. Because of the fact that the first peak depends only on the in-plane diffusivity, and the second peak is strongly dependent on the through-slab diffusivity, the technique can be used to make single-sided measurements of all three principal components of the thermal diffusivity.
Direct laser fabrication of dense microlens arrays in semiconductor-doped glass
Authors: Smuk Andrei Y. , Lawandy Nabil M.
Published: July 2002

A direct laser writing technique in semiconductor-doped glasses is used to fabricate microlens arrays with fill factors exceeding the close-packing limit for circular microlenses. This is achieved by both structural and thermal interactions between adjacent lenses during fabrication, which result in the formation of microlenses shaped in accordance with the translational symmetry of the array. The effect of nearest-neighbor interaction on various time scales is studied and shown to be critical to the distortions observed.
Electronic and magnetic properties of La0.4Ba0.1Ca0.5MnO3−y thin films
Authors: Hong Nguyen Hoa , Sakai Joe , Imai Syozo
Published: July 2002

We investigated the transport and magnetic properties of La0.4Ba0.1Ca0.5MnO3−y thin films to see the effect of substitution of the small atom Ca by Ba, which is much bigger. La0.4Ba0.1Ca0.5MnO3−y thin films have been fabricated by a pulsed laser deposition technique. The electrical resistance has been measured in the range of 20–200 K under the field from zero up to 9 T. Under zero field, at T∼37 K, an insulator-to-metal (IM) transition was observed, as expected. However, we found that the magnetic field dependence of the resistivity seems to be anomalous since TIM shifts to a lower temperature when the magnetic field is increased and the magnetoresistance at low temperature is positive. Magnetization measurements imply that in the metallic region, canted antiferromagnetic and ferromagnetic states coexist. Phase separation is suggested.
Design of a large-scale vertical open-structure cylindrical shield employing magnetic shaking
Authors: Sasada Ichiro , Paperno Eugene , Koide Hiroyuki
Published: July 2002

The shield developed consists of four concentric magnetic shells positioned on the outer surfaces of paper pipes of ∼2.7 m length, ∼1 cm thickness, and with outer diameters of 67, 72, 82.2, and 97.4 cm, respectively. The first (innermost) shell is a Permalloy shell of 2.1 mm thickness and 1.8 m length. The second, third, and fourth shells are made of ∼50 mm wide, ∼22 μm thick Metglas 2705M amorphous ribbons. The second shell, which is a 2.2 m long helical structure, consists of 48 layers of Metglas ribbon divided into four equal sections by ∼1 cm thick flexible Styrofoam sheets. The third shell, 2.43 m in length, and fourth shell, 2.7 m in length, consist of 26 and 30 layers, respectively. A thin polyethylene film is tightly wound on each section of the second shell as well as on the third and fourth shells. It increases the friction between the Metglas ribbons and prevents them from sliding down; there is no foreign material in between the layers of the ribbon. All shells are enclosed by toroidal coils which are used to demagnetize the Permalloy shell and to apply magnetic shaking to the amorphous magnetic shells. The gross weight of the shield is ∼400 kg including ∼65 kg of Permalloy and ∼110 kg of Metglas. An ∼105 transverse shielding factor and a relatively large ∼380 axial shielding factor, despite the effect of the openings, are achieved for a 10 μT external field in the extremely low frequency region. The measured shaking leakage and magnetic noise field strengths at the shield’s center are less than 1 nT. As these low field strengths, it is possible to operate highly sensitive SQUID magnetometers for biomagnetic measurements.
Stress-induced leakage current generation kinetics based on anode hole injection and hole dispersive transport
Authors: Riess P. , Ghibaudo G. , Pananakakis G.
Published: July 2002

An approach giving a physical understanding of the stress-induced leakage current (SILC) generation kinetics based on anode hole injection and hole dispersive transport in the oxide is presented. It is shown that the SILC is not directly correlated to the defects responsible for oxide breakdown. However, it is suggested that trapped holes can impact on the defect creation rate and, in turn, on the breakdown generation.
Structural and magnetic studies of exchange bias films of Ir(20)Mn(80)
Authors: Cheng S.-F. , Lubitz P.
Published: July 2002

Thin films of IrMn were made by magnetron sputtering, using nominally 20 at. % Ir, and their structural and magnetic properties were evaluated for use as an exchange bias material. The magnetic properties of different combinations of Py and CoFe underlayers were determined by magnetometry and by using ferromagnetic resonance (FMR) in the temperature range 4–350 K; x-ray analyses and atomic force microscopy were used to assess the structures. The IrMn films have the AuCu3 structure with strong 〈111〉 texture. The internal interfaces of the multilayer structures are very smooth. They induce relatively strong exchange bias in Py and CoFe overlayers but can have negligible coercivity. The bias field is strongly dependent on the direction of the moment during cooling, even well below 300 K, and coercivity and FMR linewidth increase with cooling. Isotropic shifts of FMR are small at 300 K but increase rapidly below 150 K. Our data suggest that spin-flop models for exchange bias may not apply to IrMn, and that small spin structures dominate low temperature behavior.
Controlling spontaneous generation of optical beam spots in a liquid crystal device
Authors: Iino Yoshiki , Davis Peter
Published: July 2002

An analysis is made of spot patterns generated in a liquid crystal spatial light modulator with optical feedback from the point of view of controlling generation of spots. The conditions for forming solitary spots were analyzed using a theoretical model which fits the experimental device well. Phenomena observed in experiments such as spontaneous birth, motion, and merging of spots is reproduced in numerical simulation with the model. It is shown how parameters characterizing nonlinearity, diffraction, and diffusion can be designed for stable spots and stable spot motion.
Calculation of gas heating in direct current argon glow discharges
Authors: Bogaerts Annemie , Gijbels Renaat , Serikov Vladimir V.
Published: July 2002

A model is developed for self-consistently calculating the gas temperature in a direct current argon glow discharge, used for analytical spectroscopy. The power input into the argon gas due to elastic (i.e., kinetic energy transfer) collisions of Ar+ ions, and fast Ar atoms, sputtered Cu atoms and electrons with the argon gas atoms is calculated with Monte Carlo models. This power input is used in a heat transfer model to calculate the gas temperature. The amount of power input, the contributions of the various input sources, and the resulting gas temperature are calculated for a wide range of voltages, pressures, and currents, typically applied in analytical spectroscopy. It is found that the temperature can increase significantly at high voltages, pressures, and currents (up to a factor of 3 compared to absolute room temperature).
Compositional, structural, and electrical characterization of plasma oxidized thin aluminum layers for spin-tunnel junctions
Authors: Gillies M. F. , Kuiper A. E. T. , Coehoorn R. , Donkers J. J. T. M.
Published: July 2002

In this paper we present results on how the plasma oxidation of a thin (1.5 nm) Al layer proceeds. Transmission electron microscopy of a Co/Al–oxide multilayer was used to determine the thickness of the oxides and Rutherford backscattering spectrometry and elastic recoil detection were utilized in order to determine the oxygen content. The oxide was also characterized via ac impedance measurements. These measurements indicated that the oxidation of Al on Co occurs in three discrete steps.
Optical gain on the I2 (B3Πo+→X3Σg+) system produced by a visible wavelength diode laser
Authors: Davis S. J. , Holtzclaw K. W.
Published: July 2002

We report the results of a measurement of subdoppler gain profiles in molecular iodine produced by a low power tunable diode laser. Using a sensitive, two beam detection method, we have measured absorptions and amplifications that are only a few parts in 104. Amplification was observed for pump laser powers of only 200 μW, and the optical gain observed varied linearly with pump power. Comparisons with a simple, steady-state model are also discussed.
Local structure of In0.5Ga0.5As from joint high-resolution and differential pair distribution function analysis
Authors: Petkov V. , Jeong I.-K. , Mohiuddin-Jacobs F. , Proffen Th. , Billinge S. J. L. , Dmowski W.
Published: July 2002

High resolution total and indium differential atomic pair distribution functions (PDFs) for In0.5Ga0.5As alloys have been obtained by high energy and anomalous x-ray diffraction experiments, respectively. The first peak in the total PDF is resolved as a doublet due to the presence of two distinct bond lengths, In–As and Ga–As. The In differential PDF, which involves only atomic pairs containing In, yields chemical specific information and helps ease the structure data interpretation. Both PDFs have been fit with structure models and the way in that the underlying cubic zinc-blende lattice of In0.5Ga0.5As semiconductor alloy distorts locally to accommodate the distinct In–As and Ga–As bond lengths present has been quantified.
The electronic structure of La0.66Ca0.33MnO3 and La1.2Sr1.8Mn2O7 studied by angle resolved photoemission
Authors: Liu Rong , Tonjes W. C. , Olson C. G. , Joyce J. J. , Arko A. J. , Neumeier J. J. , Mitchell J. F. , Zheng H.
Published: July 2002

We report angle resolved photoemission studies of La0.66Ca0.33MnO3 and La1.2Sr1.8Mn2O7 using single crystal samples. The Mn 3p–3d resonance photoemission data of La0.66Ca0.33MnO3 show that the states at 2.5 eV binding energy have predominantly Mn 3d character, qualitatively consistent with the predictions of local spin density approximation calculations except for a 1 eV shift toward higher binding energy. Band dispersions are observed in the normal emission data. The spectra of La1.2Sr1.8Mn2O7 show well defined features and strong matrix element effects, indicating excellent surface quality.
Microscopic analysis of generation-recombination noise in semiconductors under dc and time-varying electric fields
Authors: Pérez S. , González T. , Delage S. L. , Obregon J.
Published: July 2002

We present a microscopic analysis of current fluctuations in a semiconductor sample in the presence of trapping–detrapping processes and conventional scattering mechanisms. An ensemble Monte Carlo simulation is used for calculations. To ensure the linearity of the system, we use a model where the characteristic times of the different microscopic mechanisms are considered as energy independent. We analyze the behavior of thermal and generation-recombination noise spectra under static (dc field) and time-varying (ac field) conditions. Under dc bias we confirm the validity of the microscopic model by comparing the results of the simulation with analytical predictions. When an ac field is applied, amplitude modulation of the semiconductor response takes place due to generation-recombination processes. This modulation leads to the upconversion of the low-frequency generation-recombination spectrum, which is evidenced (even in the absence of dc current) and analyzed under different physical conditions.
Calculations of hole transport characteristics in bulk GaSb with comparisons to GaAs
Authors: Damayanthi P. , Joshi R. P. , McAdoo J. A.
Published: July 2002

Field dependent drift velocity results are presented for hole transport in bulk gallium antimonide material based on a Monte Carlo model which includes energy band warping. Transient drift velocities are demonstrated to be higher than for gallium arsenide. The steady-state characteristics are also shown to be superior. The material appears to have potential for high-speed photodetection.
The effect of magnetic interactions on low temperature saturation remanence in fine magnetic particle systems
Authors: Pike Christopher R. , Roberts Andrew P. , Verosub Kenneth L.
Published: July 2002

In studies of fine magnetic particle systems, saturation remanence is often measured during warming from liquid helium temperature in order to determine the distribution of blocking temperatures. These data have usually been treated as if they are unaffected by magnetic interactions. However, this treatment is often inconsistent with the experimental data. Furthermore, the thermal decay of saturation remanence often gives values for the mean blocking temperature that are inconsistent with other measurements, such as low temperature ac susceptibility and zero-field-cooled magnetization curves. As an alternative interpretation of these remanence data, we suggest that interactions destabilize the saturation remanence state and accelerate its decay with increasing temperature. As a result, the blocking temperatures associated with the thermal decay of remanence are effectively reduced. We have modeled the effects of interactions on low temperature saturation remanence data using a simple mean interaction field model. This model produces remanence curves that have a steep slope at low temperatures, consistent with experimental curves frequently reported in the literature.
Theoretical analysis of unstable two-phase region and microscopic structure in wurtzite and zinc-blende InGaN using modified valence force field model
Authors: Takayama Toru , Yuri Masaaki , Itoh Kunio , Baba Takaaki , Harris James S.
Published: July 2002

A model to predict material characteristics of the InGaN ternary system, which is useful for blue and green light emitting and laser diodes, with respect to an unstable two-phase region in the phase field and the first neighbor anion–cation bond length is developed. The unstable region is analyzed using a strictly regular solution model. The interaction parameter used in the analysis is obtained from a strain energy calculation using the valence force field (VFF) model, modified for both wurtzite and zinc-blende structures to avoid overestimation of the strain energy. The structural deviation from an ideal wurtzite structure in GaN and InN is also taken into account in our model. The critical temperatures found in our analysis for wurtzite InGaN and zinc-blende InGaN are 1967 and 1668 K, respectively. This suggests that, at typical growth temperatures around 800 °C, a wide unstable two-phase region exists in both wurtzite and zinc-blende structures. The modified VFF model can also predict the microscopic crystal structure, such as first neighbor anion–cation bond lengths. In order to validate our calculation results for zinc-blende structures, we compare the calculated and the experimental results in terms of the interaction parameter and the first neighbor anion–cation bond lengths in the InGaAs system. For the wurtzite structure, we compare the calculated and the experimental results for the first neighbor anion–cation bond lengths in the InGaN system. The calculated results agree well with the experimental results.
Simulation of trench homogeneity in plasma immersion ion implantation
Authors: Keller G. , Rüde U. , Stals L. , Mändl S. , Rauschenbach B.
Published: July 2002

The time-dependent evolution of the potential, the electrical field, and the particle movement surrounding two-dimensional trenches during a high voltage pulse in the context of plasma immersion ion implantation is studied by a particle-in-cell simulation. The numerical procedure is based on the solution of Poisson‘s equation on a grid and the determination of the movement of the particles on the grid. This simulation is combined with simulation codes for the calculation of depth profiles and sputtering yields. The retained ion dose and the depth resolved concentration distribution were determined in dependence on the rise time of the pulse between 0.1 and 2 μs, pulse durations between 1 and 10 μs and the ion mass (m=20–131, i.e., Ne,…,Xe) for trenches with two different aspect ratios (η=3:1 and 3:2). The results are discussed on the basis of the temporal evolution of the energy of the ions and the impact angle of the ions during the pulse.
The finite size effect in a planar inductively coupled plasma
Authors: Chung Chin Wook , Kim S. S. , Seo S. H. , Chang H. Y. , Yoon N. S.
Published: July 2002

Plasma parameters from the measured electron energy distribution are obtained with changing the chamber height at low pressure 2 mTorr. It is observed that electron density has a local peak at a certain chamber height while electron temperature decreases monotonously with increasing chamber height. The chamber height with the maximum electron density is shifted according to the bounce resonance condition when the driving frequency is changed. The electron kinetic model well agrees with the experiment. This shows that the electron density peak against the plasma size is due to the electron bounce resonance that has been theoretically discussed.
High resolution density of states spectroscopy in semiconductors by exact post-transit current analysis
Authors: Main C. , Reynolds S. , Badran R. I. , Marshall J. M.
Published: July 2002

We show that the analysis of post-transit photocurrent i(t) to determine the energy distribution g(E) of trapping states in a semiconductor is capable of much finer energy resolution than has hitherto been realized. Existing methods use a Laplace inversion of i(t) data to find g(E) but employ a delta function approximation for trap release times. In this article we retain the exponential distribution function for the release time and solve the rate equations directly. The analysis is performed on computer generated post-transit data for distributed and discrete traps, and compared with the earlier method and other related transform methods for determining the density of states, g(E).
Radiative recombination mechanisms in aluminum tris(8-hydroxyquinoline): Evidence for triplet exciton recombination
Authors: Curry R. J. , Gillin W. P.
Published: July 2002

The photoluminescence of aluminum tris(8-hydroxyquinoline) (AlQ) has been studied as a function of temperature and excitation wavelength. It was found that as the temperature and excitation energy is reduced the peak of the photoluminescence moves to longer wavelengths and broadens significantly. The photoluminescence spectra obtained at all temperatures and excitation energies can be deconvolved into three distinct peaks originating from three levels within the molecule. A rate-equation approach has been used to model the observed behavior and to obtain the relative lifetimes of the three processes responsible for the photoluminescence. From this we infer that at low temperatures and excitation energies the radiative recombination of triplet excitons is responsible for a significant amount of the photoluminescence of AlQ. It is this process which is responsible for the low energy tail seen in the photoluminescence of AlQ but which is not present in the electroluminescence.
Density-determined orthonormal orbital approach to atomic energy functionalsa)
Authors: Ghosh Swapan K. , Parr Robert G.
Published: July 2002

Orthonormal orbitals systematically constructed from the electron density are employed to obtain various closed expressions for approximate atomic energy functionals. A three-dimensional generalization of a construction originally due to Harriman is proposed. Numerical assessments are made of several new density functionals by evaluating them using accurate Hartree–Fock densities and by solving the corresponing Euler equations for electron density. The molecular virial theorem is stated and proved in a form particularly suitable for density functional theory.
Single photon double ionization studies of CS2 with synchrotron radiation
Authors: Lablanquie P. , Nenner I. , Millie P. , Morin P. , Eland J. H. D. , Hubin-Franskin M. J. , Delwiche J.
Published: July 2002

Single photon double ionization of CS2 has been investigated, using mass spectrometric coincidence techniques: both metastable and dissociative CS++2 states have been observed; three different dissociation pathways of CS++2 have been demonstrated, including one bond (S++CS+) and two bonds (S++C+S+ and S++C++S) breakings; simulation of the observed dissociation provided some insight into the dissociation mechanisms. Monochromatized synchrotron radiation enabled us to measure the excitation spectra of these relatively intense processes, in the 25–75 eV photon energy range. Our results provide an approach to the spectroscopy of the doubly charged CS++2 ion; comparison with a SCF-LCAO-MO calculation leads to a tentative assignment of the observed states.
Quantum chemistry by random walk: A faster algorithm
Authors: Anderson James B.
Published: July 2002

Two extensions of the fixed-node random walk method of solving the Schrödinger equation are described. A simple iterative procedure is found to reduce time-step error and the use of two trial wave functions in place of one for importance sampling is found to reduce computation effort. In combination the two modifications reduce computation effort for a fixed accuracy by a factor of about 10.
Configuration interaction studies of low-lying valence and Rydberg states of NS
Authors: Lie George C. , Peyerimhoff Sigrid D. , Buenker Robert J.
Published: July 2002

Using a better than double-zeta-plus-polarization basis set, the multireference-double-excitation-configuration-interaction (MRD-CI) method has been employed to calculate the potential curves for the X 2Π, a 4Π (unobserved), b 4Σ− (unobserved), B 2Π, 2Φ (unobserved?), A 2Δ, C 2Σ+ (Rydberg), H 2Π, I 2Σ+, and J 2Σ+ (Rydberg) states of the NS radical. Except for the B′ 2Σ+ state, which is not found in our calculations, all the calculated Te and re values for experimentally observed states are found to be in good agreement with experiment, whereas the calculated ωe’s are only satisfactory. Arguments have been advanced to identify the experimental observed B′ 2Σ+ state as the calculated 2Φ state. The radiative lifetime for the v′=0 level of the C 2Σ+ state has been calculated to be about 30 ns, indicating the level predissociates since the experimental radiative lifetime is only 6.5 ns.
The A(2Πi)–X(2Σ+) transition of the SiN radical by infrared diode laser spectroscopy
Authors: Yamada Chikashi , Hirota Eizi
Published: July 2002

The A(2Πi)–X(2Σ+) 1,0 band of SiN has been observed in the 5 μm region by diode laser spectroscopy in a silane/nitrogen electrical discharge plasma. A detailed analysis of about 170 observed lines has led to a v=1 term value for the A state of 1972.443 73(24) cm−1 with 2.5 standard deviations in parentheses, which, when combined with optical data, was converted to the equilibrium term value of 993.9(2) cm−1. The analysis has clearly confirmed that the spin-orbit coupling constant in the A state is negative. The Λ-type doubling constants p and q in the A state have been determined for the first time, and other parameters including the rotational and spin-orbit coupling constants have been improved in accuracy by a factor of 100.
Laser-induced-fluorescence study of the reaction of N2O with hot hydrogen atoms from 248 nm excimer laser photolysis of HIa)
Authors: Hollingsworth W. E. , Subbiah J. , Flynn G. W. , Weston Ralph E.
Published: July 2002

The reaction of hot hydrogen atoms with N2O to form OH and N2 has been studied. The hot hydrogen atoms were generated by the photolysis of HI at 248 nm which produces a bimodal distribution of hydrogen atoms having 43.6 and 22.0 kcal/mol of translational energy. The OH produced was monitored by laser-induced fluorescence in the Ã–X̃ transition [(0,0) band]. The nascent rotational distribution of this hot reaction was found to be characterized by a Boltzman temperature of 4700±300 K.
NO(B 2Πr and A 2Σ+) chemiluminescence from the reaction C+NO2 → CO+NO at 300 K
Authors: Dorthe G. , Caille J. , Burdenski S. , Caubet P. , Costes M. , Nouchi G.
Published: July 2002

The C+NO2→CO+NO reaction at 300 K has been shown to exhibit a strong ultraviolet chemiluminescence from NO (B 2Πr) and a weak one from NO(A 2Σ+). The NO (B 2Πr)v′=0 production rate was found to be approximately ten times greater than that of NO (A  2Σ+)v′=0. No chemiluminescence either from NO(C 2Πr) or NO (D 2Σ+), however energetically accessible, could be observed. These features are in close agreement with the Cs symmetry correction diagram for this reaction. The nascent vibrational distribution of NO(B 2Πr) was found to be much cooler than the prior statistical one.
Internal coordinate formulation for the vibration-rotation energies of polyatomic molecules. III. Tetrahedral and octahedral spherical top moleculesa)
Authors: Clodius William B. , Quade C. Richard
Published: July 2002

The theory of vibration-rotation interactions in tetrahedral and octahedral spherical top molecules has been developed using curvilinear internal coordinates for the vibrational degrees of freedom. Integral to the development has been a derivation of the vibrational kinetic energy for configurations away from equilibrium when redundant coordinates are present in the symmetry coordinate basis. General expressions for the inertial coefficients, Coriolis coupling coefficients, and vibrational kinetic energy anharmonicity coefficients have been derived for both the tetrahedral and octahedral molecules. A Van Vleck transformation is used to obtain both the diagonal and off-diagonal theoretical vibration-rotation and vibration anharmonic spectroscopic coefficients to second order. Comparisons with the earlier work on methane by Gray and Robiette are good. Application is made in the partial determination of the anharmonic potential energy coefficients for SF6 and CF4. As has been found in our previous work, the curvilinear internal coordinate approach has definite advantages over the Cartesian displacement approach for the angle bending modes in the molecules.
Photodissociation of the SO2⋅SO+2 dimer in the visible region of the spectrum: Product relative kinetic energy distributions and product angular distributions
Authors: Jarrold Martin F. , Illies Andreas J. , Bowers Michael T.
Published: July 2002

The results of an investigation of the photodissociation of the SO2⋅SO+2 cluster ion in the visible wavelength range (458–590 nm) are presented. The studies were performed using a crossed fast ion beam/laser beam experiment. Product relative kinetic energy distributions and product center of mass angular distributions are reported. Methods used to extract the angular distributions from the experimental measurements are described. The only ionic product observed from the photodissociation of SO2⋅SO+2 was SO+2. The product relative kinetic energy distributions are characteristic of statistical energy disposal. However, statistical phase space theory substantially overestimates the fraction of available energy partitioned into relative kinetic energy. The product angular distributions are discussed in terms of the asymmetry parameter β. β increases with the products relative velocity. A simple phase space theory model for β is developed for statistical dissociation with complete rotational averaging. The model accounts for the variation of β with the products relative velocity. β also varies with wavelength. The variation of β with wavelength is ascribed to changes in the lifetime of the cluster. At 458 nm the values of β indicate that complete rotational averaging has not occurred. At 590 nm the data are consistent with complete rotational averaging, implying a lifetime>5 average rotational periods. The results are interpreted in terms of a transition to a low lying bound excited state of the SO2⋅SO+2 cluster which arises from one of the low lying excited states of SO+2 and the ground state of SO2.
High resolution electron energy loss spectroscopy of NH3 in the 5.5–11 eV energy range
Authors: Furlan M. , Hubin-Franskin M-J. , Delwiche J. , Roy D. , Collin J. E.
Published: July 2002

New data have been obtained about the singlet excited states of ammonia in the 5.5–11 eV energy range by electron energy loss spectroscopy. The 50 eV excitation spectrum has been analyzed with high energy resolution (15 meV) at several scattering angles ranging from 4° to 40°. Some known vibrational progressions have been completed by new additional terms with high quantum numbers v′. The differential cross section for the five lowest energy singlet states have been measured at 50 eV impact energy as a function of the scattering angle. These data lead to discuss the classification of the Rydberg series.
Raman and far infrared spectra of the two solid phases of methyl bromide: CH3Br and CD3Br
Authors: Binbrek O. S. , Anderson A. , Torrie B. H.
Published: July 2002

The phase transition in crystalline methyl bromide: CH3Br and CD3Br, has been studied by Raman spectroscopy. Spectra of the higher temperature α phase are reported here for the first time. The transition in CD3Br is found to occur near 158 K, about 16 K lower than in CH3Br. Raman and far infrared spectra of the low temperature β phase at 20 K are also reported. Additional peaks in the lattice region and new splittings in the υ5 and υ6 regions have been observed. The spectra are assigned by means of a group theoretical analysis based on the known crystal structure of this phase and good agreement is obtained.
Ab initio MO study on relationships between the electronic state and out-of-plane displacement of the iron atom in four-coordinate Fe-porphine
Authors: Saito Minoru , Kashiwagi Hiroshi
Published: July 2002

The potential energy curves as a function of the out-of-plane displacement of the iron atom in Fe-porphine were calculated for various spin states and oxidation states of the iron atom by the ab initio SCF MO method. It was found that porphine attracts the iron atom to the center of the porphine plane for all the electronic states. The force constant for the out-of-plane motion of the iron atom is decreased by the occupation of the dx2−y2 orbital and increased by oxidation of the iron atom. It was also found that the force constant is strongly correlated with the overlap population between the dx2−y2 orbital and the pyrrole–nitrogen orbitals.
Photoelectron angular distributions from resonant multiphoton ionization of atomic carbon
Authors: Pratt S. T. , Dehmer J. L. , Dehmer P. M.
Published: July 2002

Atomic carbon in both the 3P ground state and the 1D first excited state is prepared by UV multiphoton dissociation of carbon tetrachloride. Photoelectron angular distributions were obtained following two photon resonant, three photon ionization via the 3p 1S0 ← 1D2 transition using linear polarized light and via the 3p 3D2 ← 3P0 transition using both linear and circular polarized light. The results illustrate that resonant multiphoton ionization of atomic carbon must be discussed in terms of the complex atom, and not within a single electron framework.
The excited states of Zn2 and Zn3. Inclusion of the correlation effects
Authors: Tatewaki Hiroshi , Tomonari Mutsumi , Nakamura Takashi
Published: July 2002

This paper deals with the electronic structure of the Zn2 and Zn3 clusters. Independent ab initio SCF calculations are performed for the ground and excited states of Zn2, and the SCF MOs are determined for respective states. By using these MOs, CI calculations taking account of the correlation effects among 4s- and 4p-like electrons are carried out. This is an improvement over previous CI calculations, which used the occupied and virtual (or improved virtual) MOs computed for the ground state, and thus disregarded the reorganization effect. The calculated excitation energies for Zn2 are 0.8–1.3 eV smaller than experiment. The inclusion of the correlation between the inner (3d-like) and valence (4s- and 4p-like) shells ameliorates the result only slightly (by 0.03–0.05 eV). The remaining discrepancy is attributed to relatively poor description of the ground state as compared with that of the excited states. Similar calculations are performed for the electronic structure of Zn3. Calculated excitation energies are compared with the absorption spectra of Zn aggregates (size unknown) in an argon matrix.
An infrared spectroscopic study of the rotation of methane in solid nitrogen
Authors: Nelander Bengt
Published: July 2002

Infrared spectra of CH4, CH3D, and CD4 in solid nitrogen have been recorded with 0.05 cm−1 resolution at temperatures between 5.5 and 20 K. The data show that methane rotates in nitrogen. The proton spin states of CH4 interconvert very slowly at 5.5 K but rapidly at 20 K.
2-Naphthol: A simple example of proton transfer effected by water structure
Authors: Lee J. , Griffin R. D. , Robinson G. W.
Published: July 2002

Using picosecond spectroscopic techniques, the proton transfer process of 2-naphthol in its first excited singlet state is investigated in water/methanol mixtures at different temperatures. The proton transfer rate is found to increase as the temperature increases, and to decrease as the methanol concentration increases. A Markov random walk theory previously used to electron transfer kinetics is employed to analyze the data. By this method, a water cluster containing 4±1 members is shown to be the proton acceptor. Suggestively, a ‘‘four-cluster’’ structure (H9O4)+ has been hypothesized as the most probable hydrating unit for the proton in acid–base equilibria, charge transfer, and other chemical systems. The observed activation energy of 3.45 kcal/mol in pure water is attributed to the energy required for rearrangement of the hydrogen bonding in the normal water structure to form the proton accepting cluster. Proton transfer kinetics thus exactly parallels electron transfer kinetics in aqueous media. The critical size of 4±1 molecules and a specific structure of the water cluster apparently determine the controlling rates for both electron and proton charge transfer processes in aqueous media.
Intramolecular vibrational energy flow in model four-atom systems
Authors: Sumpter Bobby G. , Thompson Donald L.
Published: July 2002

Intramolecular energy transfer from local bond modes is studied by using quasiclassical trajectories. The energy in a local mode as a function of time is computed to determine the nature and time scale for the decay of local mode excitation. Variations of four basic models are examined: HCCC, HOOH, HNNH, and HC ≡ CH. Potential-energy surfaces for the models are sums of Morse functions for bond stretches and harmonic bending functions. The potential for HOOH torsional motion is represented by a truncated Fourier cosine series. The emphasis of the study is on H2O2. The decay of OH overtone excitation is characterized by slow, irreversible flow of energy out of the OH as a function of time. The rate of energy transfer is strongly dependent upon the OOH bending and the torsional motions.
Theoretical calculation of photodissociation cross sections for the Ar–H2 van der Waals complex
Authors: Kidd I. F. , Balint-Kurti G. G.
Published: July 2002

Accurate benchmark calculations of the photodissociaton cross sections for the Ar-H2 van der Waals molecule are presented. Both the bound ground state of the van der Waals molecule and the predissociative (or continuum) upper state are treated by solving sets of coupled differential equations. Calculation of the photodissociation cross section involves computation of the matrix element of the dipole moment function between the wave functions corresponding to these two states. These matrix elements are efficiently evaluated using the ‘‘artificial channel method.’’ Absolute values of photodissociation cross sections are presented for a large number of predissociative spectroscopic transitions. Cross sections for both vibrational and rotational predissociation processes are studied. The line shapes of the spectral absorption lines are obtained naturally by mapping out the total photodissociation cross sections over the width of the spectral transition. In some cases corresponding to high rotational states of the H2 photofragment, non-Lorentzian line shapes are observed. The quantum state distribution of the product photofragments are also reported.
Predominance of CF-stretch vibrational excitation in state resolved inelastic scattering of protons from 11 small fluorohydrocarbon molecules
Authors: Gierz U. , Noll M. , Toennies J. Peter
Published: July 2002

Time-of-flight spectra of H+ scattered from all ten different fluorinated hydrocarbon molecules of type CHmFn and C2HmFn (m+n=4) as well as C2F6 have been measured for small scattering angles (θlab≤15°) at 9.8 eV collision energy. For the pure hydrocarbons the spectra show a complicated structure suggesting unresolved excitation of several modes and a significant effect of charge transfer collisions. In contrast all the spectra for fluorine containing molecules exhibit more or less sharp maxima indicating high overtone excitation of predominantly one vibrational mode with ωv≂1200 cm−1 corresponding to a CF-stretch vibration. At some scattering angles the overtone transition probabilities deviate somewhat from a Poisson distribution suggesting small unresolved contributions from additional vibrational modes. Over- all, however, there is good agreement for the low symmetry molecules, some of which have sizable dipole moments, with the predictions of the simple forced oscillator model previously tested only for the spherically symmetric molecules CF4 and SF6. The relative magnitudes of the observed energy transfers are consistent with absolute infrared intensities where these are available from the literature.
Multiphoton ionization of ammonia clusters and the dissociation dynamics of protonated cluster ions
Authors: Echt O. , Dao P. D. , Morgan S. , Castleman A. W.
Published: July 2002

Ammonia clusters, produced via supersonic expansion, are subjected to multiphoton ionization using a tunable pulsed laser. The products of ionization and subsequent dissociation processes are investigated using a time-of-flight mass spectrometer equipped with a reflectron (reflecting electric field). Cluster ions containing more than 65 ammonia molecules were formed via internal ion molecule reactions taking place within the initially ionized aggregate. The proton transfer reaction which ensues leads to substantial excess internal energy and subsequent cluster decomposition. Dissociation due to both collisional and unimolecular processes was identified in the field-free region. Studies of the dynamics of dissociation were made for species up to the protonated 16-mer; the loss of up to at least five monomer units following cluster ion formation was observed. Results of an investigation of the influence of laser fluence and the energy of the ionizing photons are also presented.
Geometrical structure and vibrational frequencies of several electronic states of Si2C
Authors: Grev Roger S. , Schaefer Henry F.
Published: July 2002

The ground state of the Si2C molecule is found to be a closed-shell C2v symmetry structure with unusually short Si–C bonds. In these theoretical studies a double zeta plus polarization (DZ+P) basis set was used initially. At the self-consistent-field (SCF) level of theory the DZ+P C2v symmetry structure collapses to the linear 1Σ+g state. Extension of the basis set to include a second set of d functions (DZ+2P) on each atom, however, shows the SCF linear state to be a transition state and the C2v structure to be the minimum. Optimization of the linear and C2v symmetry states using configuration interaction (CI) wave functions in conjunction with the DZ+2P basis gives a final prediction that the C2v structure lies 2.1 kcal/mol below linear Si–C–Si.