bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of substrate temperature and heat treatment on the microstructure of diamondlike carbon films
Authors: Bhargava Sanjay , Bist H. D. , Narlikar A. V. , Samanta S. B. , Narayan J. , Tripathi H. B.
Published: July 2002

Nonhydrogenated diamondlike carbon films prepared at a substrate temperature (ST) of 100, 300, and 500 °C by the laser ablation of graphite on a single-crystal silicon substrate have been characterized by scanning tunneling microscopy for the surface structure and Raman spectroscopy for the microstructure. Distorted pentagonal and hexagonal rings are observed on the surface of the film grown at 100 °C while only hexagonal rings are observable for the one grown at 500 °C. The rise in ST is found to increase the surface roughness. To assign the various coexisting carbonaceous species formed at different growth temperatures and to check their thermal stability, heat treatment was performed at up to 1300 °C in vacuum and 600 °C in air. The changes occurring on heat treatment in vacuum in these films around 600 °C have been correlated with the release of defects from the threefold network. Likewise, 950 °C temperature has been associated with the conversion of disordered tetrahedral bonding to a distorted trigonal one. The heat treatment in air shows that the microstructure induced due to lower ST is thermally more stable.
Reactive molecular beam epitaxy of wurtzite GaN: Materials characteristics and growth kinetics
Authors: Kim Wook , Aktas Ö. , Botchkarev A. E. , Salvador A. , Mohammad S. N. , Morkoç H.
Published: July 2002

High quality GaN layer growth by reactive molecular beam epitaxy employing ammonia gas as a nitrogen source and with high growth rates (∼2 μm/h) is described. The high crystalline quality of the layer is evidenced by our recently reported modulation-doped field-effect transistors, GaN/AlGaN separate confinement heterostructures, GaN/AlGaN quantum wells, high quality Schottky contacts, long excitonic lifetime, and GaN epitaxial layers that exhibit only intrinsic transitions even with the second excited states of excitonic transitions visible in the emission spectra. The dependence of background carrier concentration and resistivity on substrate temperature is studied. The hexagonal nature of wurtzite GaN manifests itself as hexagonal features on the film, becoming as large as ∼5 μm with facets at high growth temperatures (e.g., 800 °C). For low V/III ratios, large hexagonal hillocks, with highly strained regions on them, are formed due to the localized preferential growth. The photoluminescence characteristics of the films grown with various V/III ratios are also studied. The PL spectra were analyzed to uncover the effect of substrate temperature on the crystalline quality, as well as on electrical and optical properties of films. The ground and excited state excitons were seen from the layer grown at 800 °C with a growth rate of 1.2 μm/h. The influence of two competing factors, viz., thermal activation of ammonia and thermal desorption of Ga from the surface, on the growth kinetics was investigated.
Temperature and injection dependence of the Shockley–Read–Hall lifetime in electron irradiated n-type silicon
Authors: Bleichner H. , Jonsson P. , Keskitalo N. , Nordlander E.
Published: July 2002

Using three measurement techniques the injection and temperature dependence of the Shockley–Read–Hall (SRH) carrier lifetime was studied. Flying-spot scanning was used for the measurement of lifetimes under low-level injection. Open-circuit carrier decay for the measurement of high-injection lifetime, and deep-level transient spectroscopy for the characterization of active recombination centers. The samples were silicon p-i-n type diodes (p+-n-n+) irradiated with 15 MeV electrons for lifetime control. The measurements consistently show that two defect levels located at Ec−0.164 eV (E1) correlated to the vacancy-oxygen complex and Ec−0.421 eV (E4) correlated to the single-negatively charged state of the divacancy are significant for the SRH lifetime in different injection domains in the low-doped n-type region. The E1 recombination center is dominating the high-injection lifetime and the E4 center is dominating the low-injection lifetime. Similar to other authors, additional defect levels have also been observed, but the E1 and E4 levels seem sufficient to explain the behavior of electron-irradiated semiconductor power devices.
Diffusion parameters of indium for silicon process modeling
Authors: Kizilyalli I. C. , Rich T. L. , Stevie F. A. , Rafferty C. S.
Published: July 2002

The diffusion parameters of indium in silicon are investigated. Systematic diffusion experiments in dry oxidizing ambients at temperatures ranging from 800 to 1050 °C are conducted using silicon wafers implanted with indium. Secondary-ion-mass spectrometry (SIMS) is used to analyze the dopant distribution before and after heat treatment. The oxidation-enhanced diffusion parameter [R. B. Fair, in Semiconductor Materials and Process Technology Handbook, edited by G. E. McGuire (Noyes, Park Ridge, NJ, 1988); A. M. R. Lin, D. A. Antoniadis, and R. W. Dutton, J. Electrochem. Soc. Solid-State Sci. Technol. 128, 1131 (1981); D. A. Antoniadis and I. Moskowitz, J. Appl. Phys. 53, 9214 (1982)] and the segregation coefficient at the Si/SiO2 interface [R. B. Fair and J. C. C. Tsai, J. Electrochem. Soc. Solid-State Sci. Technol. 125, 2050 (1978)] (ratio of indium concentration in silicon to that in silicon dioxide) are extracted as a function of temperature using SIMS depth profiles and the silicon process simulator PROPHET [M. Pinto, D. M. Boulin, C. S. Rafferty, R. K. Smith, W. M. Coughran, I. C. Kizilyalli, and M. J. Thoma, in IEDM Technical Digest, 1992, p. 923]. It is observed that the segregation coefficient of indium at the Si/SiO2 interface is mIn≪1, similar to boron; however, unlike boron, the segregation coefficient of indium at the Si/SiO2 interface decreases with increasing temperature. Extraction results are summarized in analytical forms suitable for incorporation into other silicon process simulators. Finally, the validity of the extracted parameters is verified by comparing the simulated and measured SIMS profiles for an indium implanted buried-channel p-channel metal–oxide–semiconductor field-effect-transistor [I. C. Kizilyalli, F. A. Stevie, and J. D. Bude, IEEE Electron Device Lett. (1996)] process that involves a gate oxidation and various other thermal processes.
Preparation and soft magnetic properties of epitaxial Fe–Si(111) monolayer films and Fe–Si(111)/Cr(111) multilayer films
Authors: Yaegashi Seiji , Kurihara Toshiya , Satoh Kazuyuki
Published: July 2002

Epitaxial Fe–Si(111) monolayer films and [Fe–Si(111)/Cr(111)]4 multilayer films were grown on Si(111) substrates by dc facing-targets sputtering. The thickness dependence of the Fe–Si layer and the Cr layer on the crystallinity and the magnetic properties was studied. In the case of Fe-7.2 wt% Si(111) monolayer films, excellent soft magnetic properties were observed in films thinner than 200 nm. The sudden deterioration of soft magnetic properties was observed at the thickness of 200 nm. Torque measurements of the films revealed that the increase of effective anisotropy energy caused the deterioration of soft magnetic properties. In the [Fe-7.2 wt% Si(111)/Cr(111)]4 multilayer films, permeability increased with increase of the Cr layer thickness, and excellent soft magnetic properties of multilayer films were obtained with a Cr layer thickness of 20 nm. Soft magnetic characteristics were improved in the multilayered structure compared with the monolayer film with identical total thickness of magnetic layers.
Magnetic thin films having a lateral nanostructural periodicity
Authors: Nguyen-Van-Dau F. , Sussiau M. , Schuhl A. , Galtier P.
Published: July 2002

Magnetic and magnetoresistive properties of thin films deposited on periodically structured substrates are studied. We used a thermodynamical heat treatment to activate the step bunching phenomena on vicinal Si(111) substrates misoriented towards [112̄]. Then, Co or FeNi magnetic layers have been grown, with thicknesses ranging between 20 and 100 Å. Transmission electron microscopy experiments reveal a lateral pseudoperiodic variation of the epitaxial relationship of the metallic layers. Magnetometry measurements reveal an in-plane uniaxial magnetic anisotropy with the easy axis parallel to the step direction. We discuss the relative contributions of magnetocrystalline and shape effects to the anisotropy. Surprisingly, the angular dependance of the anisotropic magnetoresistance reveal a single-domain behavior and a magnetization reversal by rotation, with an active area as large as 700 μm×100 μm. We discuss the influence of the magnetic layer thickness on the magnetization reversal process.
Temperature dependent leakage currents in polycrystalline silicon thin film transistors
Authors: Kim Chul Ha , Sohn Ki-Soo , Jang Jin
Published: July 2002

We have studied the origins of the leakage current in polycrystalline silicon (poly-Si) thin film transistors. Temperature dependent transfer characteristics were measured as a function of drain voltage. Three kinds of leakage current were introduced to explain the experimental results. The leakage current may arise from the generation current at very low drain voltage, and may result in the same activation energy between leakage current and conductivity of undoped poly-Si. The leakage current may be due to the thermionic field emission via grain boundary defects in the intermediate drain voltage region. At high drain voltage and high negative gate voltage, the leakage current may result from the field enhanced tunneling of electrons in the valence band to the conduction band via grain boundary traps.
Surface roughening during low temperature Si(100) epitaxy
Authors: Karpenko O. P. , Yalisove S. M. , Eaglesham D. J.
Published: July 2002

Reflection high energy electron diffraction (RHEED) was used to investigate surface roughening during low temperature Si(100) homoepitaxy. The use of RHEED allowed in situ real-time collection of structural information from the growth surface. RHEED patterns were analyzed using a simple kinematic diffraction model which related average surface roughness and average in-plane coherence lengths to the lengths and widths of individual RHEED diffraction features, respectively. These RHEED analyses were quantified by calibrating against cross-section transmission electron microscopy (TEM) analyses of surface roughening. Both the RHEED and TEM analyses revealed similar scaling of surface roughness with deposited thickness, with RHEED analyses resulting in roughness values a factor of ∼2 times lower than those obtained from TEM analyses. RHEED was then used to analyze surface roughening during Si(100) homoepitaxial growth in a range of temperatures, 200–275 °C. Initially, surface roughness increased linearly with deposited thickness at a roughening rate that decreased with increasing growth temperature. At each growth temperature, near the crystalline/amorphous Si phase transition, the rate of surface roughening decreased. This decrease coincided with the formation of facets and twins along Si{111} planes. Surface roughness eventually saturated at a value which followed an Arrhenius relation with temperature Eact∼0.31±0.1 eV. This activation energy agrees well with the activation energy for the crystalline/amorphous Si phase transition, Eact∼0.35 eV, and suggests that limited thickness epitaxy is characterized by this saturation roughness. Once the saturation roughness was reached, no significant changes in surface roughness were detected. In addition, the decay of average in-plane coherence lengths was also temperature dependent. Values of average coherence lengths, at the crystalline/amorphous Si phase transition, also increased with growth temperature. All of these data are consistent with a model that links surface roughening to the formation of critically sized Si{100} facets and the eventual breakdown in crystalline growth.
Influence of gas pressure and cathode composition on ion energy distributions in filtered cathodic vacuum arcs
Authors: Bilek M. M. M. , Martin P. J. , McKenzie D. R.
Published: July 2002

We report measurements of ion energy distributions of ionized species in titanium and aluminium filtered cathodic vacuum arcs operating in oxygen and nitrogen gas atmospheres. The ion energy distributions were recorded using a Hiden mass selected ion energy analyzer. The results show that a significant reduction in ion energies and a change in the shape of ion energy distributions occurs as the gas pressure is increased. The degree of the energy reduction depends on both the type of gas and the metal ions making up the arc plasma. This has important implications for the deposition of thin films, such as titanium nitride, commonly produced using vacuum arcs in reactive gas atmospheres. The ion energy distributions of the cathode ion species in the absence of background gas and at low gas pressures are well fitted by shifted Maxwellian distributions. As the gas pressure rises the distributions consist of a progressively increasing thermalized Maxwellian component and a decreasing shifted Maxwellian. An investigation of energy distributions of species in arcs triggered on alloyed cathodes showed that the relative abundance of ions of different charge states and their energy distributions varied as alloy metals were introduced. This indicates that the electron temperature in the cathode spot and hence the cathode spot dynamics is affected by the presence of the alloy metal.
Precipitation of β-SiC in Si1−yCy alloys
Authors: Guedj C. , Dashiell M. W. , Kulik L. , Kolodzey J. , Hairie A.
Published: July 2002

The infrared modes of annealed Si1−yCy alloys were studied experimentally and theoretically. The alloys were grown on Si(100) substrates by solid-source molecular beam epitaxy and were characterized by Fourier transform infrared spectroscopy. At annealing temperatures above 850 °C, the localized vibrational mode of substitutional C around 605 cm−1 diminished in intensity while another mode due to incoherent silicon carbide precipitates appeared at 810 cm−1. For lower processing temperatures, a peak around 725 cm−1 has been tentatively attributed to a C-rich phase, which is a precursor to SiC precipitation. Theoretical calculations based on the anharmonic Keating model predict that small (1 nm) 3C–SiC coherent precipitates may actually produce a mode at 725 cm−1. This mode occurs if the bonds gradually vary in length between the C-rich region and the host lattice. On the other hand, if the bonds are abruptly distorted at the edges of the precipitate, it becomes elastically isolated from the host lattice, and the 810 cm−1 mode appears. This study yields a picture of the thermal stability of dilute SiC alloys, which is important for the high-temperature processing steps necessary for device applications. Moreover, the coherent precipitation may provide a controllable way to form self-assembled 3C–SiC quantum dots into silicon germanium carbon alloys.
Heteronuclear and homonuclear surface abstraction reactions of Cl, Br, and F
Authors: Kota Gowri P. , Coburn J. W. , Graves David B.
Published: July 2002

Surface reactions of atomic halogen atoms play important roles in various plasma etching processes, commonly used in microlectronics manufacturing. However, relatively little is known about the surface chemistry of these key reactive intermediates. Previous measurements of the recombination coefficients of Cl, Br, and F on various surfaces in a molecular beam apparatus indicated that the recombination reaction is pseudofirst order [G. P. Kota, J. W. Coburn, and D. B. Graves, J. Vac. Sci. Technol. A 16, 270 (1998); 16, 2215 (1998)]. One mechanism that would result in pseudofirst order kinetics is a two-step process in which the first halogen atom adsorbs into a relatively strongly bound chemisorbed state, and the second atom reacts with it either through a direct reaction, or after being physisorbed onto the halogenated surface. In this article, we report experiments in which surfaces are first exposed to a molecular beam of one type of halogen atom, then the surface is exposed to a second type of halogen. During the second exposure, the heteronuclear reaction product is monitored with a mass spectrometer. Finally, the surface is sputtered and the mass spectrometer is used to detect any remaining presence of the original halogen atom. Analogous experiments were also performed with isotopically enriched mixtures of chlorine. These experiments unambiguously demonstrate that halogen atom surface recombination involves a two step adsorption-abstraction mechanism. Under all conditions studied, the surface recombination reactions proceeded at rates on the order of surface collision frequencies. The relative magnitudes of the heteronuclear rates (as a function of surface composition and halogen atom type) scaled in the same way as the homonuclear recombination probabilities measured previously. In every case examined, after the second halogen exposure, the surface retained a significant coverage of the halogen that had been originally exposed to the surface. This leads to the conclusion that only a fraction of the strongly bound surface sites are available for abstraction by free radical attack. Absolute calibration of the incident and evolved species fluxes allowed an estimate to be made of the reactive site densities for several surfaces. These ranged from 1012 to 1015 cm−2 depending on the surface.
Effects of the site symmetry and host polarizability on the hypersensitive transition P3→3F2 of Pr3+ in fluoride glasses
Authors: Tikhomirov V. K. , Naftaly M. , Jha A.
Published: July 2002

Observations and analysis of hypersensitive P03→3F2 transition of Pr3+ ion are reported in a series of fluoride (fluoroindate, fluoroaluminate, and fluorozirconate) glasses. The data in fluoride families are also compared with the data on the selected tellurite and sulfide glasses. The structure of dopant sites in fluoride glasses and the mechanism of hypersensitivity are discussed in relation to the symmetry of dopant sites and host polarizability. The possibilities of quenching, or alternatively, of magnification of the hypersensitive transitions of rare-earth ions in the glassy hosts and their applications are discussed.
Evolution process of cross-hatch patterns and reduction of surface roughness in (InAs)m(GaAs)n strained short-period superlattices and InGaAs alloy layers grown on GaAs
Authors: Samonji K. , Yonezu H. , Takagi Y. , Ohshima N.
Published: July 2002

We investigated the formation process of cross-hatch patterns (CHPs) and the lattice relaxation process in the growth of an (InAs)1(GaAs)4 strained short-period superlattice (SSPS) and an In0.2Ga0.8As alloy layer on GaAs(100) substrates. By using x-ray diffraction and cross-sectional transmission electron microscopy, it was found that the lattice relaxation in the (InAs)1(GaAs)4SSPS proceeded as fast as that in the In0.2Ga0.8As alloy layer. The surfaces of the grown layers showed CHPs, and the surface roughness increased by means of the evolution of the CHPs. The surface roughness of the SSPSs was larger than that of the alloy layers, whereas no apparent difference was observed in the lattice relaxation process between the SSPSs and the alloy layers. Additionally, the height of surface ridges parallel to the [01̄1] direction was higher than that parallel to the [011] direction. We observed the distribution of highly strained InAs and GaAs islands on the surfaces of strained InGaAs layers by using an atomic force microscope. As a result, it was clarified that the InAs islands were accumulated on top of the surface ridges, whereas the GaAs islands were distributed uniformly on the surface of strained InGaAs layer. It was considered that the nonuniform incorporation of In atoms during growth of layers contributes mainly to the evolution of CHPs in the InGaAs-on-GaAs heteroepitaxy. We propose a developmental model of CHPs based on the surface diffusion of the In atoms. In this model, the asymmetry of CHPs was well understood by anisotropy in surface diffusion length of In atoms. Additionally, it was clarified that the evolution of CHPs in the growth of InGaAs at a high temperature can be suppressed by growing a fully relaxed InGaAs layer at a low temperature before the high-temperature growth.
Optimum growth parameters for type-II infrared lasers
Authors: Yang M. J. , Moore W. J. , Bennett B. R. , Shanabrook B. V. , Cross J. O. , Bewley W. W. , Felix C. L. , Vurgaftman I. , Meyer J. R.
Published: July 2002

The surface, structural, and optical properties of InAs/InGaSb/AlSb mid-infrared lasers grown by molecular beam epitaxy have been systematically studied, respectively, by Nomarski differential interference contrast, high-resolution x-ray diffraction, and variable-temperature photoluminescence. It is found that the optimum growth temperature is between 400 and 450 °C, based on the calibrated transmission thermometry. In addition, the impact of interfacial bond type and Sb sources has been investigated. A 5.91 μm laser, grown with the optimal growth parameters, exhibits a maximum cw operating temperature of 210 K.
Two-step room temperature grain growth in electroplated copper
Authors: Brongersma S. H. , Richard E. , Vervoort I. , Bender H. , Vandervorst W. , Lagrange S. , Beyer G. , Maex K.
Published: July 2002

Electroplated copper exhibits some surprising changes at room temperature in sheet resistance, stress, and microstructure. This behavior, now known as self-annealing, is shown here to be intimately linked to the composition of the plating bath and the resulting incorporation of organic additives in the Cu layer. Their addition is a necessary condition for self-annealing to occur, but slows down the process for higher concentrations. The phenomenon also depends critically on film thickness, showing an accelerated transformation when film thickness increases. This dependence is explained in terms of a very rapid primary crystallization from the top surface down just after deposition, followed by a slower lateral recrystallization producing large secondary grains. The stress and sheet resistance during recrystallization are identified as two noncorrelated variables.
Magnetization loops of submicron ferromagnetic permalloy dot arrays
Authors: Schneider M. , Hoffmann Horst
Published: July 2002

Polycrystalline Permalloy films were deposited onto silicon substrates by rf sputtering. Arrays of cylindrical dots with diameter d between 150 and 550 nm were patterned by optical holographic lithography. The dots were arranged on a square lattice with period p, ranging from 310 to 1030 nm. The height h of dots, given by the thickness of the permalloy films, ranged between 10 and 80 nm. The shape of the magnetization loops of the dot arrays depends strongly on the aspect ratio r=d/h. The in-plane saturation field is given by the intrinsic demagnetization field of single, isolated dots. The dipolar interaction between the dots is negligible. For cylinders with elliptical base plane, the magnetization loop depends on the field direction. The measured magnetization loops of the arrays are the superposition of the magnetization loops of the single dots without dipolar interaction. A model of magnetization reversal in applied fields is given.
High resolution measurement of localized-state distributions from transient photoconductivity in amorphous and polymeric semiconductors
Authors: Nagase Takashi , Kishimoto Ko-hei , Naito Hiroyoshi
Published: July 2002

The energy resolution of a method for measuring localized-state distributions in amorphous and polymeric semiconductors has been improved in terms of Tikhonov regularization. The method is based on the analysis of transient photoconductivity data using Laplace transforms, and is valid for both pre- and postmonomolecular recombination regimes of the transient photoconductivity. The improvement of the energy resolution is shown using transient photoconductivity data numerically generated from model localized-state distributions appropriate for the materials. In addition, the measurement of localized-state distributions from time-of-flight transient photocurrents is examined. The applicability of the method with improved energy resolution is demonstrated for undoped hydrogenated amorphous silicon, poly(para-phenylene vinylene) and poly(methylphenylsilane).
Complete thermal characterization of film-on-substrate system by modulated thermoreflectance microscopy and multiparameter fitting
Authors: Li Bincheng , Roger J. P. , Pottier L. , Fournier D.
Published: July 2002

Modulated thermoreflectance microscopy is applied to a complete thermal characterization of a thin film of gold (78 nm) or YBaCuO (300 nm) on a LaAlO3 substrate. The phase profile, measured at several modulation frequencies covering an appropriate range, is fitted with a rigorous thermal diffusion model. This leads to a simultaneous estimation of the thermal diffusivities of the film and the substrate, as well as of the thermal film/substrate boundary resistance. The estimated values for the gold film sample are, respectively, 4.3×10−6 m2 s−1 (substrate diffusivity), 1.0×10−4 m2 s−1 (film diffusivity), and 1.0×10−8 m2 KW−1 (thermal boundary resistance), while for the thermally anisotropic YBaCuO film sample are, 4.1×10−6 m2 s−1, 3.5×10−6 m2 s−1 (in-plane diffusivity), and 8.0×10−8 m2 KW−1, respectively.
Effect of quantum well location on single quantum well p-i-n photodiode dark currents
Authors: Nelson Jenny , Ballard Ian , Barnham Keith , Connolly James P. , Roberts John S. , Pate Malcolm
Published: July 2002

The photocurrent available from a p-i-n solar cell can be increased by the addition of quantum wells (QWs) to the undoped region. At the same time the QWs reduce the open-circuit voltage by introducing areas of lower band gap where recombination is enhanced. This increase in recombination should be as small as possible for the most favorable effect on the photovoltaic efficiency of the device. Theoretical considerations indicate that nonradiative recombination, which is the dominant loss mechanism in AlxGa1−xAs/GaAs QW structures, may be reduced by positioning the QWs away from the point where the electron-hole product is a maximum. For p-i-n diodes, where recombination is greatest at or near the center of the space charge region, this means locating the QWs closer to the doped regions. Spectral response should not be affected so long as the QWs are still located within the field bearing region. Thus, improved photovoltaic performance may be expected through strategic location of the QWs. We report on measurements on a series of Al0.36Ga0.64As p-i-n photodiodes, three of which contained a single 87 Å GaAs QW within the i region, and one which was a control sample with no QW. The three QW samples were grown with the QW located nearer to the p-doped layer, centrally, and nearer to the n-doped layer, respectively. Spectral response measurements confirm that for good quality samples photocurrent is independent of QW location within the depleted region. Contrary to expectations, the dark current is highest for the sample with the QW located closer to the n region. We analyze these results in terms of structure and doping profile, and compare them with the predictions of a self-consistent model. The observed behavior is attributed to a relatively high unintentional background doping in the intrinsic region.
Application of orthoferrites for light spot position measurements
Authors: Didosyan Y. S. , Hauser H. , Nicolics J. , Haberl F.
Published: July 2002

Results of measurements of the light spot positions based on the spatial light modulation in an orthoferrite crystal are presented. The reproducibility of the measurements is 0.8 μm and the sampling rate is 600 kHz.
Difference frequency laser spectroscopy of the ν1 band of HOCO+
Authors: Amano T. , Tanaka Keiichi
Published: July 2002

The ν1 fundamental band of HOCO+ has been detected in absorption with a difference frequency laser spectrometer. The molecular constants have been determined for both the ground and excited states. The band origin is found to be 3375.374 26(25) cm−1.
CASSCF and CCI calculations of the vibrational band strengths of HCN
Authors: Jo/rgensen Uffe G. , Almlöf Jan , Gustafsson Bengt , Larsson Mats , Siegbahn Per
Published: July 2002

HCN has been observed to be a constituent of late type carbon-rich stellar atmospheres and has recently been suggested to be a very important opacity source in cooler carbon stars. The transitions giving rise to the combination vibration–rotation bands in the near infrared region may be expected to be the most important HCN transitions for the structure of the carbon-star atmospheres. In this paper CASSCF and CCI calculations of vibrational bands in the interval 3000–10 000 cm−1 are presented. Accounting for the HCN opacity extends the model atmosphere by a factor of 5, and lowers the gas pressure in the surface layers by one or two orders of magnitude. The structures, e.g., the gas pressures, in the upper layers of these models are uncertain by more than a factor of 3 as a result of the uncertainties in the potential surface of the molecule and in the method for handling the degeneracy for the vibrational bands. These uncertainties are, however, of minor importance in practical applications in present carbon-star work.
The dynamics of the dissociative adsorption of alkanes on Ir(110)
Authors: Hamza A. V. , Steinruck H.-P. , Madix R. J.
Published: July 2002

The dissociative adsorption of C1–C4 alkanes on Ir(110)–(1×2) was investigated using supersonic molecular beam techniques. Three regimes of reactivity were observed. At incident kinetic energies less than 100 kJ/mol the dissociative adsorption probability for propane and butane increased to about 0.6 with both decreasing surface temperature and decreasing translational energy of the incident molecule suggesting that activation occured via trapping of the alkane on the surface. At kinetic energies below 110 kJ/mol for butane, 90 kJ/mol for propane, 60 kJ/mol for ethane, 40 kJ/mol for methane, and at elevated surface temperatures the reaction probabilities for alkanes were independent of surface temperature and translational energy, but increased significantly with the molecular weight or carbon chain length of the incident alkane. This behavior indicates an unactivated reaction channel for each species except methane. At kinetic energies above 110 kJ/mol for butane, 90 kJ/mol for propane, 60 kJ/mol for ethane, and 40 kJ/mol for methane the initial dissociative sticking probability increased with increasing translational energy and was independent of surface temperature, indicating direct translational activation of the incident alkane. The translational energy required to activate the incident alkane via this channel increased with carbon number or molecular weight, indicative of energy transfer processes that dissipate energy in the reactive collision. The dissipative transfer of translational energy out of the reactive channel via a hard cube collision with the surface accounts qualitatively for the increase in the apparent activation barrier with increasing molecular weight in this activated channel.
Hartree–Fock calculation of the electronic structure of a Cu+ impurity in NaCl
Authors: Winter N. W. , Pitzer R. M. , Temple D. K.
Published: July 2002

Hartree–Fock cluster calculations have been carried out for the ground 3d10 and excited 3d94s configurations of the Cu+ ion in a NaCl host. Special emphasis has been given to providing an accurate representation of the Coulomb potential due to the remainder of the lattice. Configuration coordinate curves were determined for the symmetric displacement of the nearest-neighbor Cl− ions and are compared to recent Xα calculations. The Hartree–Fock equilibrium Cu–Cl distance was found to be 5.327 bohr, slightly shorter than the calculated nearest-neighbor distance of 5.353 bohr for the pure NaCl host. Comparison of the Hartree–Fock and Xα ground and excited state energies, shows that the Xα approximation reverses the ordering of the 3T2g and 1Eg states, overestimates the equilibrium nearest-neighbor distance, and predicts the a1g vibrational frequency to be about twice the Hartree–Fock value. Using the Franck–Condon factors found with the ab initio potential energy curves, the calculated bandwidths for the 1,3T2g states are found to be in excellent agreement with the low-temperature absorption spectra.
Optimal control of selective vibrational excitation in harmonic linear chain molecules
Authors: Shi Shenghua , Woody Andrea , Rabitz Herschel
Published: July 2002

A formalism for designing an optical field for selective vibrational excitation in linear harmonic chain molecules is presented based on optimal control theory. The optimizing functional producing the field designs is flexible to allow for the imposition of desirable laboratory and theoretical constraints. The designed optimal fields, which successfully lead to local bond excitations, exhibit complex structure on the time scale of 10 fs. Analysis of the optimal fields shows a high degree of cooperativety between the temporal structure of the fields and the dynamical capabilities of the molecules. It is generally impossible using only spectral information to devise the optical field needed to selectively excite a local bond in a polyatomic molecule. These results explain why the previous intuitively based laboratory attempts at site specific chemistry have yielded disappointing results.
Late stage spinodal decomposition in an oligomeric blend of polystyrene/polybutadiene: A test of the scaling law for the structure function
Authors: Tomlins P. E. , Higgins J. S.
Published: July 2002

The late stages of phase separation by spinodal decomposition have been studied by light scattering in an oligomeric blend of polystyrene (PS) with polybutadiene (PB) as a function of both quench depth and composition. The scattering function I(q,t) follows a dynamical scaling law of the form S(q,t)=qm(t)−3S̃[q/qm(t)] where the wave vector qm corresponds to the maximum in I(q,t). The profiles of the normalized scaled structure functions are compared with that predicted by Furukawa for critical mixtures (percolation regime) which is given by S̃(x)=4x2/(3+x8)[S̃(1)=1]. Apart from one exception, Furukawa’s function adequately describes the form of the normalized structure functions for a 60% PS mixture. However there are problems with some of the comparisons with the 50% and 70% PS blends. These disparities may in part be attributable to the fact that some of the data may lie outside the ‘‘true’’ late stages.
Resonance positions and widths by a similarity transformation of a Hermitian Hamiltonian matrix
Authors: Lipkin Nurit , Moiseyev Nimrod , Leforestier Claude
Published: July 2002

The resonances obtained by a method, which is based on the original work of Datta and Chu [Chem. Phys. Lett. 87, 357 (1982)] and of Schneider [Chem. Phys. Lett. 31, 237 (1975); Phys. Rev. A 11, 1957 (1975)], are studied for a one-dimensional model, for a two-dimensional model suggested by Chu, describing rotational predissociation in van der Waals complexes, and for the variational predissociation of Ne–ICl in the adiabatic approximation for a potential surface given by Delgado-Barrio and Villarreal (to be published). The resonance positions and widths are associated with the complex eigenvalues of a complex matrix obtained by a product of three matrices St H S, where H is the Hermitian Hamiltonian matrix, and S is an overlap matrix between complex scaled and unscaled basis functions. The method has the advantage of being applicable to potentials given numerically on the real axis. It also avoids the need to construct a new complex Hamiltonian matrix for each scaling angle, as is the case in the complex coordinate method.
Dynamics of the chemisorption of N2 on W(100): Precursor-mediated and activated dissociation
Authors: Rettner C. T. , Schweizer E. K. , Stein H.
Published: July 2002

The dissociative chemisorption probability of N2 on W(100) is found to proceed by way of two dynamically distinct channels. At low kinetic energies Ei, dissociation proceeds primarily by way of a precursor-mediated process, where the dissociation probability is found to fall with increasing Ei, reflecting the energy dependence of the trapping probability into this state. Dissociation at low energies is also strongly dependent on surface temperature Ts which effects the fraction of trapped species that desorb. For energies above about 0.45 eV, the dissociation probability is found to rise from a minimum of about 0.14 at Ts=800 K to over 0.45 at Ei=5 eV. Over this range we believe that kinetic energy enables the incident molecules to directly overcome a barrier in the reaction coordinate. Throughout the entire range of energies we observe only slight variations of the dissociation probability with the angle of incidence, with no discernible sensitivity for energies below ∼0.5 eV. For energies between 1 and 4 eV, associated with the ‘‘activated’’ channel, we observe a slight preference for non-normal incidence, with a clear preference for normal incidence only for Ei>5 eV.

While the ‘‘precursor-mediated’’ channel displays a considerable sensitively to surface temperature, results at high energy are found to be essentially independent of this parameter. Moreover, dissociation by way of the precursor-mediated channel is found to be insensitive to surface coverage, in contrast to a roughly linear decrease in the dissociation probability with surface coverage observed for dissociation via the activated process. In this latter case, we find that the saturation coverage remains approximately constant at about 0.6 atomic monolayers for all conditions, up to the highest incidence energies. This differs from previous observations for the dissociation of O2 and N2 on W(110), where the saturation coverage was found to rise with increasing Ei. Finally we find that the dissociation probability vs kinetic energy curve for the ‘‘direct’’ dissociation case is qualitatively similar to that for the N2/W(110) system, but with a threshold that is ∼0.4 eV lower. We argue that the ‘‘precursor-mediated’’ mechanism does not contribute significantly to dissociative chemisorption in the W(110) case and conclude that the primary difference between N2 dissociation on the W(110) and W(100) surfaces is that the barrier to dissociation is slightly higher in the W(110) case.
Steric effect in the scattering of hexapole-oriented beams of symmetric-top molecules by graphite(0001)
Authors: Curtiss Thomas J. , Mackay R. Scott , Bernstein Richard B.
Published: July 2002

The orientational dependence of the scattering of hexapole-focused, oriented molecule beams of six symmetric-top molecules (CH3Cl, CH3F, CHCl3, CHF3, t-BuCl, CH3CN) and CH3OH from the (0001) surface of a graphite crystal is reported. Experimental angular distribution data are well represented by a two-component model, consisting of a ‘‘trapped/desorbed’’ and a ‘‘direct’’ scattered contribution. The steric effect, defined as the difference (‘‘heads’’ vs ‘‘tails’’) divided by the average of the scattered signals, has been measured as a function of the scattering angle and the degree of orientation of the molecules. There is considerable diversity among the different molecules with respect to the direction and magnitude of the steric effect of the scattering (and trapping). In all cases, however, the magnitude of the steric effect is essentially a linear function of the degree of orientation. Limited data on the incident energy dependence of the angular distributions and the steric effect are also presented. A model which deconvolutes the steric effect for the trapped/desorbed and directly scattered components is introduced. It also provides an independent estimate of the trapping probability of the incident molecules.
Two-point probability function for distributions of oriented hard ellipsoids
Authors: Lado F. , Torquato S.
Published: July 2002

The macroscopic properties of two-phase random heterogeneous media depend upon an infinite sequence of n-point functions S(i)n(x1,x2,...,xn) giving the joint probability of finding n points with positions x1,x2,...,xn all in phase i. This paper reports the first study and calculation of the two-point probability function S(i)2 for distributions of oriented, hard spheroids with eccentricity ε in a matrix. This is a useful model of statistically anisotropic two-phase media, enabling one to examine the special limiting cases of oriented disks (ε=0), spheres (ε=1), and oriented needles (ε=∞).
Molecular beam studies of H2 and D2 dissociative chemisorption on Pt(111)
Authors: Luntz A. C. , Brown J. K. , Williams M. D.
Published: July 2002

Molecular beam techniques have been utilized to measure the dissociative chemisorption probability at zero surface coverage S0 for D2(H2) on Pt(111) as a function of initial energy Ei, angle of incidence θi, surface temperature Ts, isotopic mass and nozzle temperature Tn. S0 shows a large increase with translational energy, but no threshold in Ei, a peaking at θi=0°, and an independence with Ts, isotope and Tn. These results are interpreted in terms of direct dissociative chemisorption on the Pt(111) terraces. The dynamical picture that emerges is that although there is no significant barrier to dissociation along the minimum energy path, barriers do exist along nonoptimal reactive trajectories. Thus, the ‘‘translational activation’’ and other dynamical observations are intimately related to the multidimensional aspects of the dissociative potential energy surface. Some aspects of the dissociative chemisorption, however, still seem somewhat surprising within this general description.
Concentration fluctuation effects on disorder–order transitions in block copolymer melts
Authors: Mayes A. M. , de la Cruz M. Olvera
Published: July 2002

We study transitions to periodic structures in block copolymers melts, including concentration fluctuations self-consistently in the analysis. Phase diagrams are constructed for diblock and triblock copolymers of finite molecular weight. The concentration fluctuations increase the stability of the isotropic state, and induce a shift in k*, the wave vector of maximum scattered intensity, to lower values as the transition is approached. We calculate the shift in k* near the transition, and obtain the anisotropic scattering function in the ordered state.
High pressure solid phases of benzene. II. Calculations of the vibration frequencies and evolution of the bonds in C6H6 and C6D6 up to 20 GPa
Authors: Thiéry M. M. , Besson J. M. , Bribes J. L.
Published: July 2002

The complete vibrational spectra of crystalline C6H6 and C6D6 have been calculated for the different pressure-induced solid phases recently determined at 293 K up to 25 GPa, and compared to Raman scattering data. The normal coordinate analysis has been carried out by using intermolecular Buckingham-type atom–atom interactions and the intramolecular force field of the free molecule. Results of such frequency calculations are compared to experimental values at ambient pressure. The variation of the relevant crystalline parameters is discussed to construct a model and calculate the vibrational frequencies under pressure. The quantitative fit of the frequency shift of the Raman active modes under pressure demonstrates the necessity of including different C–C and C–H (C–D) bond compressibilities within the benzene molecule. Such intramolecular distance variations which allow to estimate the frequency corrections for the totally symmetric (a1g) breathing modes, have been determined from the observed pressure-frequency dependence of these internal modes. The behavior of other nonsymmetric (e2g) internal modes which become comparatively weak under pressure, suggests a charge delocalization within—and possibly out of—the benzene ring, eventually leading to irreversible opening of the hexagonal cycle. This can be directly related to the irreversible transformation of benzene to a polymer which is observed after pressurization above 20 GPa.
On the reactions of hydrated electrons with OH⋅ and H3O+. Analysis of photoionization experiments
Authors: Goulet T. , Jay-Gerin J.-P.
Published: July 2002

With the help of Monte Carlo simulation techniques, we study the recombination kinetics of hydrated electrons (e−aq) with H3O+ and OH⋅ which results from the photoionization of pure water with femtosecond pulsed lasers. A full description of the simulation procedure is given and various comparisons are made with analytical formulations of the reaction kinetics. Particular attention is given to the reaction of e−aq with H3O+, which is only partially diffusion controlled and which involves a Coulombic interaction with dielectric saturation effects. We find that the probability of reaction per e−aq –H3O+ encounter is small (∼6%) and that the encounter duration can be of the order of a few picoseconds. The competition between the reaction of e−aq with H3O+ and with OH⋅ is analyzed with the simulations and with the independent reaction times method. Both approaches indicate that the e−aq decay is largely dominated by the reaction of e−aq with OH⋅. The effect of neighboring ionization sites on the e−aq decay kinetics is also included in the simulations to account for different possible densities of ionization sites. The initial separation between the reactants is found to be about 1 nm, in agreement with previous determinations. The significance of this last value and the constraints that it puts on the initial kinetic energy of the photoelectrons is discussed.
A three-dimensional study of NeICl predissociation resonances by the complex scaled discrete variable representation method
Authors: Lipkin Nurit , Moiseyev Nimrod , Leforestier Claude
Published: July 2002

A study of the vibrational predissociation of the van der Waals complex NeICl on the B electronic surface, NeICl (B, v=2) to Ne+ICl (B, v=1, j), was conducted using the discrete variable representation (DVR) formulation of the complex coordinate method. The resonance positions, widths, and wave functions were inferred from the complex eigenvalues and eigenvectors of the complex scaled Hamiltonian matrix. Assignment of the various states was done by the natural expansion analysis. The natural expansion analysis provides a rigorous criterion as to the separability of the different modes in a given coordinate system, and allows for quantum number assignment to the vibrational, stretching, and bending modes of NeICl. The rotational distribution of the ICl fragment after dissociation was obtained by the asymptotic analysis of the tail of the complex scaled square-integrable resonance wave functions and was found to be in good agreement with theoretical results previously obtained for this system by Roncero et al. [J. Chem. Phys. 92, 3348 (1990)] and with experimental measurements by Skene, Drobits, and Lester [J. Chem. Phys. 85, 2329 (1986)].
The Stark effect in methane’s 3ν1+ν3 vibrational overtone band
Authors: Boraas Kirk , De Boer David F. , Lin Zhen , Reilly James P.
Published: July 2002

Methane’s 3ν1+ν3 vibrational overtone transition centered at 8871 Å is studied with a molecular beam/Stark apparatus having 7.5 MHz Doppler limited resolution. First-order Stark splitting is observed for transitions between levels of E-type symmetry. From the measured splittings, we estimate the magnitude of the vibrationally induced dipole moment of the 3ν1+ν3 vibrational state.