bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of melt depth in laser crystallized poly-Si thin film transistors
Authors: Brotherton S. D. , McCulloch D. J. , Gowers J. P. , Ayres J. R. , Trainor M. J.
Published: July 2002

The influence of film thickness and incident excimer laser energy density on the properties of poly-Si thin film transistors has been investigated and a coherent pattern of behavior has been identified which establishes controlled melt-through of the film as a key condition for achieving high quality devices. The conditions were correlated with the appearance of large grains and gave consistent results from both n- and p-channel devices, with carrier mobilities of more than 150 and 80 cm2/V s, respectively, and leakage currents of less than 2×10−14 A/μm. From a study of static irradiations, using a semi-Gaussian laser beam, the results are shown to be consistent with the super lateral grain growth (SLG) model. The trailing edge of the beam, when used in a swept mode, has been demonstrated to play an important role in extending the size of the energy window for this effect by re-setting the material into the SLG regime.
A systematic study and optimization of parameters affecting grain size and surface roughness in excimer laser annealed polysilicon thin films
Authors: Marmorstein Aaron , Voutsas Apostolos T. , Solanki Raj
Published: July 2002

A thorough investigation of polysilicon films crystallized from amorphous silicon using a XeCl excimer laser has been completed. Emphasis was placed on the development of large crystals, good uniformity, and low surface roughness, with an eye towards improving throughput. This was accomplished through use of SiO2 barrier layers, multiple passes, and careful consideration of annealing ambient. Thermal modeling was performed to help interpret the observed trends and aid in optimization. By controlling these process parameters, samples were produced with a high degree of uniformity and two-dimensional structural ordering, with grains nearly identical in size and stacked in rows and columns. However, a biproduct of the ordering was the creation of large topographical features at the intersection points of multiple grain boundaries, which lead to surface roughness in the range of 12 nm. By applying more shots to the same area, films with maximum grain sizes on the order of 3 μm and reduced surface roughness (6 nm) were realized.
Magnetic properties of single-domain SrFe12O19 particles synthesized by citrate precursor technique
Authors: Vijayalakshmi A. , Gajbhiye N. S.
Published: July 2002

Nanosize particles of SrFe12O19 have been synthesized by a citrate precursor technique with various Fe/Sr ratios of 12.0 to 9.0. X-ray diffraction (XRD) studies showed the formation of ultrafine, monophasic SrFe12O19 particles with comparable unit cell parameters. Magnetic clusters were observed by scanning electron microscopy and Branauer–Emmett–Teller surface area measurements. Mossbauer results reveal the ferrimagnetic and superparamagnetic nature and the change in hyperfine parameters with decline in particle size and Fe/Sr ratios. The magnetic properties of small SrFe12O19 particles (Fe/Sr=12.0) showed a considerable enhancement in TC from a bulk value of 750–883 K measured for 20–5.7 nm XRD particle size. The shift in Curie temperature is well defined by the finite size scaling formula [TC(D)−TC(∞)]TC(∞)−1=(D/D0)−1/ν with ν=0.75–0.02 and D0=1.52 nm. The change in Hic, σs, and σr values were observed with the decline in particle size and the Fe/Sr ratios where Hic of 6500 Oe is achieved for isotropic SrFe12O19 particles with a Fe/Sr ratio of 10.8. Such changes are explained by the additional Sr leading to the formation of a nonmagnetic layer around the particles which inhibits their growth, limiting them to single-domain size.
Scattering birefringence polarizers based on oriented blends of poly(ethylene terephthalate) and core–shell particles
Authors: Dirix Y. , Jagt H. , Hikmet R. , Bastiaansen C.
Published: July 2002

Here, we present the results on the optical characteristics of polymeric, linear polarizers produced from oriented, two-component, polymer blends. By a proper selection of materials and drawing conditions, the refractive index of the dispersed phase of the blend is matched with the ordinary refractive index of the birefringent continuous phase while, at the same time, a large refractive index mismatch is generated in the perpendicular direction. The films are, therefore, transparent or opaque, dependent on the polarization direction of the incident light and act as linear polarizers with a high contrast ratio (≈1000) between the transparent and opaque state. These polarizers are all-polymer-based linear polarizers that operate on the anisotropic scattering of light, instead of anisotropic absorption, which offers some unique features and advantages.
Absolute density and reaction kinetics of fluorine atoms in high-density c-C4F8 plasmas
Authors: Sasaki K. , Kawai Y. , Suzuki C. , Kadota K.
Published: July 2002

Absolute density and reaction kinetics of fluorine (F) atoms in high-density octafluorocyclobutane (c-C4F8) plasmas were examined using vacuum ultraviolet absorption spectroscopy. The F atom densities, corresponding to electron densities ranging from 1×1011 to 5×1012 cm−3, were 1×1012–5×1013 cm−3 for gas pressures of 2–7 mTorr and rf powers of 0.2–1.5 kW. The F atom density was linearly dependent on the electron density for ne<1.5×1012 cm−3. According to lifetime measurements in the afterglow, two decay processes were found in the F atom density: exponential (first-order kinetics) and linear (zero-order kinetics) decay components. The linear-decay component became significant at high gas pressures. The time constant of the exponential-decay component ranged from 5 to 100 ms, which corresponds to surface loss probabilities of 10−1–10−3. The surface loss probability varied inversely with the F atom density.
Structure of water molecules under 14 T magnetic field
Authors: Iwasaka M. , Ueno S.
Published: July 2002

In this study, we investigated the effects of strong magnetic fields of up to 14 T on the near-infrared spectrum of water molecules. We used a near-infrared spectrophotometer, which has an external optical cell box in a 14 T superconducting magnet. We measured the near-infrared spectrum of water in the range of 900–2000 nm by changing the optical path lengths from 0.1 to 100 mm. The peak wavelengths in the near-infrared spectrum in the range of 900–2000 nm of water, increased in length by 1–3 nm under a 14 T magnetic field. Also, we measured a near-infrared spectrum of glucose solutions under a magnetic field at 14 T to investigate the hydration of sugar. In contrast to the case of water, the peak wavelength of glucose solutions showed blueshift under a 14 T magnetic field. When the magnetic fields were changed from 0 to 14 T, the differential optical absorbance of the glucose solution at 958 nm increased compared to the controls. There is a possibility that the 14 T magnetic field affects the formation of hydrogen bonds of water molecules and the hydration of glucose molecules.
Calculated electromigration wind force in face-centered-cubic and body-centered-cubic metals
Authors: Dekker J. P. , Lodder A.
Published: July 2002

The wind force has been calculated for most of the face-centered-cubic and body-centered-cubic metals of the Periodic Table. Both self-electromigration and impurity migration have been considered. The migration has been treated as the motion of an atom to a neighboring vacancy. The calculations are done using a state-of-the-art code for the electronic structure of a dilute alloy.
Growth model for GaN with comparison to structural, optical, and electrical properties
Authors: Koleske D. D. , Wickenden A. E. , Henry R. L. , DeSisto W. J. , Gorman R. J.
Published: July 2002

A kinetic model is presented to explain the metal organic vapor phase epitaxy (MOVPE) growth of GaN. The model is based upon measured desorption rates and assumptions on the precursor dissociation and sticking probabilities. The model shows how the growth temperature and V/III ratio are linked for the growth of high quality GaN films. From a comparison of growth conditions cited in the literature to the quality of GaN produced, optimal film growth appears to occur when the V/III ratio is chosen to be slightly larger than the N to Ga desorption ratio. The relationship between the growth temperature, V/III ratio, and GaN quality are explained in terms of how the growth parameters influence the incorporation of Ga and N atoms into the growing film. The Ga and N diffusion lengths are estimated to be 2–20 nm and <1 nm at 1050 °C, respectively, for practical MOVPE growth rates. Growth conditions for smooth (0001) surface morphology are described in terms of the growth model, as well as possible origins for defect incorporation in GaN. As a result of the large N desorption rate, it is suggested that during growth N is incorporated into the GaN lattice via an adsorption/desorption cycle. Application of the growth model to establishing the growth process conditions and run-to-run reproducibility are also discussed.
The influence of “starving plasma” regime on carbon content and bonds in a-Si1−xCx:H thin films
Authors: Pereyra I. , Carreño M. N. P. , Tabacniks M. H. , Prado R. J. , Fantini M. C. A.
Published: July 2002

Differences on carbon content and chemical bonds in a-Si1−xCx:H were observed and analyzed in carbon rich and silicon rich films, deposited by plasma enhanced chemical vapor deposition from mixtures of silane and methane. The influence of the radio frequency low power density regime on the film’s properties was investigated. The content of Si, C, and H in the solid phase was obtained by Rutherford back scattering and forward recoil spectrometry. The bondings were analyzed by Fourier transform infrared spectroscopy. Quantitative analysis on the film’s chemical composition was performed combining the vibrational spectra with the stoichiometry data. The results showed that under “silane starving plasma” conditions, a carbon content as high as 70 at. % is achieved and the main carbon bonds are tetragonal C–H,  C–H2, and Si–C.
Modified compensation model of CdTe
Authors: Fiederle M. , Eiche C. , Salk M. , Schwarz R. , Benz K. W. , Stadler W. , Hofmann D. M. , Meyer B. K.
Published: July 2002

The traditional compensation model to explain the high resistivity properties of CdTe is based on the presence of a deep acceptor level of the cadmium vacancy in the middle of the band gap. A new compensation model based on a deep intrinsic donor level is presented. The compensation model is used together with an appropriate segregation model to calculate axial distributions of resistivity which are compared with spatially resolved resistivity measurements. The Te-antisite defect is discussed as a possible origin cause of this intrinsic defect, which is also supported by theoretical calculations.
Effects of frequency on the two-dimensional structure of capacitively coupled plasma in Ar
Authors: Kitajima T. , Takeo Y. , Nakano N. , Makabe T.
Published: July 2002

A time-modulated and time-averaged two-dimensional (2D) profile of the net excitation rate of Ar(3p5) and Ar+(4p4D7/2) was observed in capacitively coupled plasma (CCP) in Ar as a function of the driving frequency (1–100 MHz) through the use of 2D-t optical emission spectroscopy (OES). A significant improvement in the detection efficiency was achieved for the time-resolved OES. Results are presented for the basic characteristics of 2D and 2D-t profiles of the net excitation rate in Ar from 1 to 100 MHz at 1.0 Torr. For comparison, 2D profiles in CCP between high frequency (13.56 MHz) and very high frequency (100 MHz) at 0.1 Torr and 25 mTorr under the same power dissipation are discussed. The advantages of VHF operation are described and discussed based on the radial uniformity and high density in CCP at low pressure. The appearance of a local emission peak at the edge of the rf electrode is observed in CCP, which is caused by changes in the sustaining mechanism due to increases in the driving frequency.
Electronic properties of tetrahedral amorphous carbon investigated by scanning tunneling microscopy
Authors: Arena C. , Kleinsorge B. , Robertson J. , Milne W. I. , Welland M. E.
Published: July 2002

Scanning tunneling spectroscopy (STS) has been used to investigate the electronic structure of tetrahedral amorphous carbon (ta-C) films. The density of states (DOS) was derived from the normalized conductivity spectra at fixed tip heights. Band edges were defined by extrapolating the generalized conductivity (dI/dV) within ±2 eV of the Fermi level. The resulting band gap was found to be similar to that measured optically for ta-C films of different sp3 content. The Fermi level of undoped ta-C was found to lie below midgap, confirming ta-C to be a weakly p-type semiconductor. The valence band tail is found to be steeper than the conduction band tail, the opposite of the situation in a-Si:H. With nitrogen addition, the Fermi level is found to move above midgap, confirming that n-type doping is occurring. The band gap is found to decline at higher N contents. The DOS at larger tip heights and as a function of surface etching shows evidence of a surface layer with a narrower band gap, which we identify as the sp2-rich surface layer seen previously by cross-sectional electron energy loss spectroscopy. The STS DOS shows peaks which we attribute to states of the surface layer having more graphitic bonds. The scanning tunneling microscopy images show that ta-C deposited on Si has a root mean square surface roughness of 1.7 Å and this increases to about 10 Å for films deposited on metals such as Ti.
Oxidation-induced traps near SiO2/SiGe interface
Authors: Ahn C. G. , Kang H. S. , Kwon Y. K. , Lee S. M. , Ryum B. R. , Kang B. K.
Published: July 2002

Using an Al/SiO2(wet)/Si0.9Ge0.1/n–Si/Al capacitor structure, effects of oxidation on bulk trap and interface states near the SiO2/SiGe interface are investigated. Two peaks at the energy levels of 0.23 eV (D1) and 0.40 eV (D2) below the conduction band edge are observed with the capacitance deep level transient spectroscopy (DLTS) method. The DLTS measurement results show a characteristic feature of interface states for the D1 peak. The interface state distribution obtained by the capacitance–voltage method also has a high density (6.9×1012/cm2 eV) peak at an energy level of 0.23 eV below the conduction band edge. The Si–O– dangling bonds are thought to be the source of the D1 peak. The annealing behaviors of the D2 peak show that D2 is a divacancy related bulk trap. The capture cross section and the trap density for the bulk trap D2 are 2.06×10−15/cm2 and 1.8×1014/cm3, respectively. The density of D2 is significantly reduced after low temperature postmetallization annealing.
Impact of molecular orientation on thermal conduction in spin-coated polyimide films
Authors: Kurabayashi K. , Goodson K. E.
Published: July 2002

Polymer films are serving as passive regions in fast logic circuits and as active regions in organic optoelectronic devices, such as light-emitting diodes. Recent data illustrated the strong anisotropy in the thermal conductivity of polyimide films of thickness near 1 μm, with the in-plane value larger by a factor of approximately six. This manuscript extends previous theoretical work on heat conduction in stretched bulk polymers to model the conductivity anisotropy in spin-coated polymer films. Predictions are based on the standard deviation of the angle of molecular orientation with respect to the film in-plane direction, which can be investigated using birefringence data, and the expected conductivity anisotropy in a material with perfectly aligned chains. The modeling and previous data indicate that the anisotropy factor could increase to a value larger than 10 for polyimide films much thinner than 1 μm.
Deuterium transport through device structures
Authors: Chen P. J. , Wallace R. M.
Published: July 2002

We use secondary ion mass spectrometry to characterize the hydrogen/deuterium distribution and concentration on complimentary “metal” oxide silicon (CMOS) test structures subjected to molecular deuterium (D2) anneals. We examine the temperature dependence and the influence of doping on the transport of deuterium to the gate oxide interfaces resulting in interface passivation. We find that undoped polycrystalline silicon appears to be an efficient barrier for deuterium transport at typical postmetallization sintering temperatures. We also examine the permeability of device structures that include dielectric encapsulation layers after typical postmetal sintering conditions employed in a conventional CMOS process flow. It is found that typical low temperature deposited oxide dielectrics are quite permeable by molecular deuterium at typical sintering temperatures (435 °C). In contrast, chemical vapor deposited silicon nitride dielectrics appear to form a complete barrier to deuterium diffusion (even for layers as thin as 300 Å). We also find that nitrides which receive a high thermal budget exposure, such as the source/drain anneal, appears to regain permeability to deuterium diffusion/transport.
Excimer laser annealing of amorphous and solid-phase-crystallized silicon films
Authors: Miyasaka Mitsutoshi , Stoemenos John
Published: July 2002

Very thin (25–50-nm-thick) amorphous silicon (a-Si) films were crystallized into polycrystalline silicon (polysilicon) films by the combination of low temperature solid phase crystallization (SPC) and subsequent excimer laser annealing (ELA). These films are, then, subjected to a standard low temperature process (<600 °C) of thin film transistor (TFT) fabrication. The performance of resultant TFTs was compared to those fabricated on polysilicon films obtained by simple excimer laser annealing of amorphous silicon films. The electrical characteristics of the TFTs were correlated with the structural characteristics of the polysilicon films, using transmission electron microscopy and x-ray diffraction as analytical tools. The polysilicon films obtained by the SPC process consist of large and heavily defected crystalline grains. These defects, however, could be eliminated by melting and solidifying the polysilicon films during the ELA process. As a result, the electrical properties of the 50-nm-thick polysilicon film subjected to SPC+ELA process improved dramatically when the laser energy density was between 280 and 360 mJ cm−2. The SPC+ELA method did not work successfully for the 25-nm-thick films due to the instability of the laser energy density. Hillock formation at the grain boundaries during the ELA process is also discussed in this article.
Vacancy and self-interstitial concentration incorporated into growing silicon crystals
Authors: Voronkov V. V. , Falster R.
Published: July 2002

The concentration of vacancies (for the growth rate V over the critical value Vt) and of self-interstitials (for V<Vt) incorporated into growing silicon crystals is obtained numerically by solving the axial diffusion problem for fast-recombining point defects. An analytical solution is obtained for the important near-critical case. A simple but quite precise interpolation expression is found. The critical growth rate Vt for the changeover from interstitial to vacancy incorporation is proportional to the near-interface temperature gradient G0 even if the axial profile of the gradient, G(z), is of considerable nonuniformity. The experimental critical ratio V/G0, together with the observed amount of vacancies in voids, provides a good estimate for the interstitial self-diffusional product at the melting point, 3×1011 cm−1 s−1, in accord with reported data.
Lifetime of small polarons in iron-doped lithium–niobate crystals
Authors: Berben D. , Buse K. , Wevering S. , Herth P. , Imlau M. , Woike Th.
Published: July 2002

Pulsed illumination of lithium–niobate crystals with green light excites electrons from deep traps into the intrinsic defect NbLi5+ (Nb on Li site in the valence state 5+) and creates NbLi4+ centers (small polarons). The electrons trapped in this more shallow center increase the light absorption in the red and near infrared. The dark decay of the polaron concentration is observed by monitoring the relaxation of these absorption changes. Iron-doped lithium–niobate crystals with different concentrations of NbLi are investigated for various illumination conditions and temperatures. The relaxation shows a stretched-exponential behavior which is in disagreement with the predictions of the standard rate-equation-based model. The observed lifetimes of the polarons range from tens of nanoseconds to some milliseconds. Computer simulations reveal that all results can be explained considering distance-dependent excitation and recombination rates, i.e., the lifetime of an individual polaron depends on the distance to the next available deep electron trap. Based on the new insights, tailoring of lithium–niobate crystals for nonvolatile holographic storage becomes possible.
Nonadiabatic molecular collisions. II. A further trajectory-surface-hopping study of the ArH+2 system
Authors: Chapman Sally
Published: July 2002

Both charge transfer and chemical reaction are studied for the reactants Ar++H2, Ar+H+2, and Ar+D+2, using the trajectory-surface-hopping model with diatomics-in-molecules 2A′ surfaces for ArH+2. Results are compared with a number of recent experiments. Agreement with experiment is generally satisfactory. The reactions are direct. The Ar++H2 → ArH++H reaction is well characterized as a stripping process. Charge transfer occurs predominantly by long-range electron jump. The Ar+H+2 and Ar+D+2 cross sections depend sensitively on reactant vibration, rising sharply from v=0 to v=1, and falling gradually for v≥2. The ArH+ product is rotationally hot. Points of disagreement with experiment are discussed in the light of the approximations in the surface and the TSH model.
The infrared predissociation spectra of water clusters
Authors: Coker D. F. , Miller R. E. , Watts R. O.
Published: July 2002

Infrared predissociation spectra of water clusters have been measured in the frequency range 3000–3800 cm−1 using a molecular beam–color center laser apparatus. The transition from a spectrum resembling that of liquid water to that of the dimer is clearly seen. Detailed theoretical analyses using normal mode theory, local mode theory, and a quantum simulation method are used to interpret the spectrum in terms of a potential surface that includes both intramolecular and intermolecular degrees of freedom.
Theory of Ostwald ripening for open systems
Authors: Beenakker C. W. J. , Ross John
Published: July 2002

Diffusion-controlled coarsening (Ostwald ripening) of a precipitate is analyzed for the case of an open system, immersed in a reservoir of constant solute concentration. Equivalence of the evolution of such open systems and closed systems studied previously is established in the limit of infinite systems. The cause of this equivalence is screening of the bulk of the system from the reservoir by diffusive interactions between the precipitated particles. The applicability to large, but finite systems is discussed.
Short range effective potentials for ionic fluids
Authors: Clarke J. H. R. , Smith W. , Woodcock L. V.
Published: July 2002

It is shown that the structure of a simple ionic liquid, potassium chloride, can be reproduced in computer simulations using short range effective pair (SHREP) potentials of a simple form. Aside from the balance between like and unlike particle interactions, the important parameters determining the structure are the depth ε and the position r0 of the unlike particle pair energy minimum. The results demonstrate that the long range ordering characteristic of ionic liquids is not a consequence of the long range of Coulomb interactions. It is further shown that first order perturbation theory can be used accurately to calculate the thermodynamic properties of an ionic liquid from a corresponding reference liquid generated using a SHREP potential. These results can be generalized to explain deviations from the Reiss–Mayer–Katz corresponding states law for alkali halides and suggest an alternative scheme, effective depth reduction (EDR), based on values of ε for the gas phase ion pairs.
A basis set study of the NMR chemical shift in PH3
Authors: Chesnut D. B. , Foley C. K.
Published: July 2002

Chemical shift calculations varying the heavy atom valence shell basis functions have been carried out for PH3 as a prototype molecule containing second row atoms. A triple valence split with two sets of d polarization functions, (66211/6211/11)=[5s,4p,2d], appears to be adequate for second-row atoms in the GIAO-SCF approach. Chemical shift calculations on SiH4, H2S, and HCl as well as PH3 employing this basis yield results in good agreement with experiment and with calculations of others involving larger bases. Bond length and bond angle shift derivatives in these hydrides are negative, continuing the general trend observed before for molecules containing first-row atoms.
The wavelength dependence of excimer laser photolysis of Fe(CO)5 in the gas phase. Transient infrared spectroscopy and kinetics of the Fe(CO)x (x=4,3,2) photofragments
Authors: Seder T. A. , Ouderkirk A. J. , Weitz Eric
Published: July 2002

The transient infrared absorption spectra of the coordinatively unsaturated Fe(CO)x species generated via excimer laser photolysis of gas phase Fe(CO)5 are presented and discussed. The photofragments produced upon 351, 248, and 193 nm photolysis are characterized. Fe(CO)3 and Fe(CO)4 are produced upon 351 nm photolysis. In addition to these two fragments, Fe(CO)2 is produced on 248 nm photolysis. The gas phase structures of these Fe(CO)x fragments are observed to be compatible with those determined from condensed phase experiments. The coordinatively unsaturated photofragments are typically formed with significant amounts of internal excitation. The rate constants for reaction of Fe(CO)4, Fe(CO)3, and Fe(CO)2 with CO are (3.5±0.9)×1010, (1.3±0.2)×1013, and (1.8±0.3)×1013 cm3 mol−1 s−1, respectively. The large difference in the magnitude of the rate constant for reaction of Fe(CO)4 vs Fe(CO)3 and Fe(CO)2 with CO is rationalized in terms of the spin states of the reactants and products. Following 193 nm photolysis, a new product is observed which is tentatively assigned as an excited electronic state of Fe(CO)3. A photochemical scheme which accounts for all observed products is presented.
Infrared spectra of matrix-isolated monomeric and dimeric hydrogen sulfide in solid O2
Authors: Woodbridge Eric L. , Tso Tai-Ly , McGrath Mark P. , Hehre Warren J. , Lee Edward K. C.
Published: July 2002

FTIR spectra of the matrix-isolated monomeric and complexed H2S in solid O2 at 13 K have been obtained at various matrix: solute ratios (M/S) in order to identify the ν3 absorption of the monomer. The ν1 absorption of the monomer was too weak to be seen at high dilutions, M/S∼4000 and 8000. The previous assignments of the monomer ν1 absorption in Ar and N2 are questioned. The satellite peaks appearing at low values of (M/S) are assigned to (H2S)2. The observed H-bonding dimer frequency, −50 cm−1 from v3, is comparable to the value calculated using second-order Mo/ller-Plesset perturbation theory. Frequencies calculated at the Hartree–Fock level for H2S dimer are essentially unchanged from those in the monomer.
Free energy of hydrophobic hydration: A molecular dynamics study of noble gases in water
Authors: Straatsma T. P. , Berendsen H. J. C. , Postma J. P. M.
Published: July 2002

The potential utility and limitations of two methods to determine free energy differences from molecular dynamics simulations (MD) are studied. The computation of the free energy of hydration of the inert gases serves as a simple but illustrative example. Good results are obtained for the inert gases from a perturbation treatment, using a reference ensemble obtained from a MD simulation of a cavity in water, if these atoms are comparable in size to the cavity and the calculated free energy differences are small. This limits the applicability of the perturbation treatment of a small number of cases. Larger free energy differences can be obtained with reasonable accuracy from MD simulations with continuously changing interaction parameters. This integration method is more generally applicable, but makes an additional simulation necessary.
State-resolved photofragmentation dynamics of Fe(CO)5 at 193, 248, 266, and 351 nm
Authors: Waller I. M. , Hepburn J. W.
Published: July 2002

The photofragmentation dynamics of Fe(CO)5 in a supersonic molecular beam have been studied for photolysis wavelengths of 193, 248, 266, and 351 nm. The CO photofragments formed under these collision-free conditions were detected by vacuum ultraviolet laser-induced fluorescence (VUV LIF). This allowed for the determination of the rotational and vibrational distributions for the CO products and, by using Doppler spectroscopy, the translational energy distributions were also determined. These data are presented along with the details of a statistical model calculation which reproduces the experimental data very precisely. The statistical model for the photodissociation dynamics, which works at all photolysis wavelengths studied, is based on sequential elimination of CO ligands, with complete energy randomization in the intermediate ground state Fe(CO)n fragments between elimination steps. The detailed mechanism for the ultraviolet photochemistry of Fe(CO)5 is discussed in light of these results and previous experimental and theoretical work.
A molecular-beam-optical and radio frequency-optical double-resonance study of the A 2Πr–X 2Σ+ band system of scandium monoxide
Authors: Childs W. J. , Steimle T. C.
Published: July 2002

A molecular-beam-optical and rf-optical double-resonance study of the A 2 Π (v′=0–2) –X 2Σ+ (v″=0–2) band systems of gas phase scandium monoxide has been performed. No localized perturbations in the X 2Σ+ state have been observed but strong perturbations in the A 2 Πr (v=1) state were detected. Quantum numbers for the optical spectrum could only be assigned assuming a negative value for the excited state Λ-doubling-type magnetic hyperfine parameter, contrary to current theoretical understanding of this interaction. The ground state magnetic hyperfine parameters can be interpreted in terms of ab initio models for the electronic states of ScO whereas the spin-rotation parameters cannot.
Order–disorder transition of starblock copolymers
Authors: Hashimoto Takeji , Ijichi Yasuhito , Fetters Lewis J.
Published: July 2002

Small-angle x-ray scattering from the segmental density fluctuations of a starblock copolymer with a given arm number n of 6 as well as that of a linear diblock copolymer (corresponding to the starblock copolymer with n=1) were studied in both ordered and disordered states. In the ordered state the characteristic wavelength (D) of the fluctuations for the copolymer with n=6 was found to have the same temperature (T) dependence as that for the diblock copolymer, where n=1, i.e., D∼T−1/3. In the disordered state, the theoretical spectrum of the fluctuations was found to essentially agree with the experimental results. The thermodynamic interaction parameter (χ) between the dissimilar monomers was also determined as a function of T from the fluctuation spectrum in the disordered state. The results show a significant n dependence.
Structural and dynamical correlations in Ag2Se: A molecular dynamics study of superionic and molten phases
Authors: Rino José P. , Hornos Yvone M. M. , Antonio Giomal A. , Ebbsjö Ingvar , Kalia Rajiv K. , Vashishta P.
Published: July 2002

We study the structural properties, single-particle dynamics, and the charge transport in superionic and molten Ag2Se using the method of molecular dynamics. The calculations are based on a model of interionic potentials in which the ions interact through Coulomb interaction, steric repulsion, and charge–dipole interaction due to the large electronic polarizability of the selenium ions. In the superionic phase the Ag ions diffuse through a stable bcc lattice of Se atoms. Structural and dynamical correlations are studied at five temperatures in the superionic phase and three temperatures in the molten phase. Among the structural correlations the results are presented for partial pair distribution functions, coordination numbers, partial structure factors, bond angle distributions, and the wave vector and temperature dependence of the Bragg intensities. Detailed comparison with the neutron and x-ray single crystal diffraction results are made whenever possible. Diffuse neutron and x-ray scattering is calculated and investigated in detail in the vicinity of q0=(1.6,1,0). It is shown that the anisotropic disks of intensity arise entirely due to the collective motions of silver ions and that these correlations manifest in the q space at a point where the Se–Ag partial structure factor is nearly zero. The calculated temperature dependence of the self-diffusion constant of silver is in good agreement with the tracer diffusion measurements. The spectra of velocity autocorrelation functions and the frequency dependent ionic conductivity are calculated. The Haven’s ratio, derived both from the calculated self-diffusion and zero frequency limit of the ionic conductivity, is in good agreement with the experimental results of Okazaki. In the molten phase the calculated neutron structure factor is compared directly with the neutron diffraction experiments of Susman et al. The results for self-diffusion of silver and selenium ions in the molten phase and the frequency dependent ionic conductivity are also discussed.
Quantum reactive scattering via the S-matrix version of the Kohn variational principle: Differential and integral cross sections for D+H2 →HD+H
Authors: Zhang John Z. H. , Miller William H.
Published: July 2002

A comprehensive survey of the quantum scattering methodology that results from applying the S-matrix version of the Kohn variational principle to the reactive scattering formulation given by Miller [J. Chem. Phys. 50, 407 (1969)] is presented. Results of calculations using this approach are reported for the reaction D+H2 →HD+H. The 3-d calculations include total angular momentum values from J=0 up to 31 in order to obtain converged integral and differential cross sections over a wide range of energy (0.4–1.35 eV total energy). Results are given for reaction probabilities for individual values of J, integral and differential cross sections for a number of energies, and state-to-state rate constants (i.e., a Boltzmann average over translational energy), and comparisons are made to a variety of different experimental results. A particularly interesting qualitative feature which is observed in the calculations is that the energy dependence of the differential cross section in the backward direction (θ=180°) shows a resonance structure (due to a short-lived DH2 collision complex) which is very similar to that in the J=0 reaction probability. This resonance structure does not appear in the energy dependence of the integral cross section, being averaged out by the sum over J.
Pseudospectral generalized valence-bond calculations: Application to methylene, ethylene, and silylene
Authors: Langlois Jean-Marc , Muller Richard P. , Coley Terry R. , Goddard William A. , Ringnalda Murco N. , Won Youngdo , Friesner Richard A.
Published: July 2002

The pseudospectral (PS) method for self-consistent-field calculations is extended for use in generalized valence-bond calculations and is used to calculate singlet–triplet excitation energies in methylene, silylene, and ethylene molecules and bond dissociation and twisting energies in ethylene. We find that the PS calculations lead to an accuracy in total energies of ≤0.1 kcal/mol and excitation energies to ≤0.01 kcal/mol for all systems. With effective core potentials on Si, we find greatly improved accuracy for PS.
Walking on potential energy surfaces
Authors: Nichols Jeff , Taylor Hugh , Schmidt Peter , Simons Jack
Published: July 2002

An algorithm for locating stationary points corresponding to local minima and transition states on potential energy surfaces is developed and analyzed. This method, which represents a substantial extension of an earlier algorithm, utilizes local gradient and Hessian (i.e., first and second energy derivative) information to generate a series of ‘‘steps’’ that are followed to the desired stationary point. By designing the step sequence to move energetically downhill in all coordinates, local minima can be found. By stepping uphill along one local eigenmode of the Hessian while minimizing the energy along all other modes, one locates transition states. A key element of this development is a more efficient parametrization of the step vector in terms of quantities that permit the direction (i.e., uphill or downhill) and length of the step to be carefully controlled. This, in turn, allows ‘‘walks’’ that trace streambeds connecting local minima to transition states and to neighboring local minima more closely than has been found using the earlier methods. Such streambed walks provide information that can be used in subsequent reaction-path dynamics simulations.
Shock wave study of the reaction HO2+HO2→H2O2+O2 : Confirmation of a rate constant minimum near 700 K
Authors: Hippler H. , Troe J. , Willner J.
Published: July 2002

The HO2 self-reaction HO2+HO2→H2O2+O2 was studied in shock waves between 750 and 1120 K. Two different HO2 sources were used, the thermal dissociation of CH3O2CH3 in the presence of excess O2 and the thermal dissociation of H2O2. The reaction is characterized by a curved Arrhenius plot of the rate constant k. The combination with low temperature data shows a deep minimum of k near 700 K. The rate constant, at 1 bar gas pressure, between 300 and 1100 K can be represented as k/cm3 mol−1 s−1=4.2×1014 exp(−6030 K/T) +1.3×1011  exp(+820 K/T). The first term in the present work was derived from measured ratios of k/ε and was evaluated with ε (HO2, 230 nm, 1000 K)=670 1 mol−1 cm−1. Furthermore, a value of the rate constant of 3×1012 cm3 mol−1 s−1 was derived near 1100 K for the reaction HO+H2O2→HO2+H2O; for the reaction HO+HO2→H2O+O2 a value of 3×1013 cm3 mol−1 s−1 was obtained.
Laser spectroscopy of low-lying excited states in YbO: Linkage of the Yb2+ f 13s and f 14 configurations
Authors: McDonald Stephen A. , Rice Steven F. , Field Robert W. , Linton Colan
Published: July 2002

Dye laser excitation and fluorescence spectra have been used to provide information on rotational structure and energy linkages between several low-lying electronic states of YbO. Configurational assignments of the observed low-lying states have been made on the basis of vibrational frequencies. The X 1Σ+ ground state, with ΔG1/2=683 cm−1, is assigned to the Yb2+ 4f14 configuration, whereas five states with Ω=0−, 1, 2, 1, and 3 and ΔG1/2∼820–830 cm−1 are assigned to Yb2+ 4f136s. The lowest 4f136s(Ω=0−) state is found to be 910 cm−1 above the 4f14, X 1Σ+ state, thus establishing the linkage between the  f14 and  f13s configurations. The observed electronic states are discussed in relation to the ligand field model and the electronic state energies are fitted to determine the ligand field parameters. The configurational assignment for the YbO ground state proposed by M. Dolg and H. Stoll [Theor. Chim. Acta 75, 369 (1989)] is shown conclusively to be incorrect.
Investigations of the dynamic behavior of fatty acid monolayers at the air–water interface using a displacement current-measuring technique coupled with the Langmuir-film technique
Authors: Iwamoto Mitsumasa , Majima Yutaka
Published: July 2002

The dynamic behavior of fatty acid monolayers at the air–water interface was investigated using a displacement current-measuring technique coupled with the so-called Langmuir-film technique and also the dipole moment of the acids was determined. The displacement current flowing though a short circuit was generated only when induced charges on an electrode suspended in air was changed by monolayer compression. The displacement current measurement was found to be a very sensitive method used for a better understanding of the relationship between the structure and function of the monolayers placed on the water surface and it was also found to be a very useful method for detecting the dynamic motion of molecules in the entire range from the so-called gaseous state to the solid state at the same time. Finally, the generalized displacement current was theoretically analyzed.
The multiplet state distribution of O(3P
J) produced in the photodissociation of O2 at 157 nm
Authors: Huang Yu-Lin , Gordon Robert J.
Published: July 2002

The fine structure population of O(3P) produced in the photodissociation of O2 at 157 nm was measured in a pump-and-probe experiment. The population of the ground state (J=2) was found to be 93%, with only 6% in the J=1 level and 1% in J=0. The predominance of J=2 is consistent with direct dissociation on the B 3Σ−u potential energy surface. Based on available information in the literature we rule out excitation of the 3Πu state and curve crossing to the 5Πu state as possible origins of the population with J<2. We infer that the populations of J<2 is due to nonadiabatic transitions at large distances. Two mechanisms consistent with the data are a Demkov-type interaction, which is caused by coupling by the radial kinetic energy operator.
Photodesorption of NO from Ag(111) and Cu(111)
Authors: So S. K. , Franchy R. , Ho W.
Published: July 2002

The adsorption, thermal reactions, and photoreactions of NO on Ag(111) and Cu(111) at 80–85 K have been studied by thermal-desorption spectroscopy (TDS), high-resolution electron-energy-loss spectroscopy (HREELS), and photon-induced desorption. Adsorption of NO on both surfaces is quite complicated. At saturation coverage, a number of chemical species are present, including atop and bridge-bonded NO, atomic N and O, and N2O. Photodesorption of NO, N2, and N2O is observed simultaneously under low-power photon irradiation in the wavelength range for 260–600 nm. From TD and HREEL spectra before and after photon irradiation, it is established that on both surfaces the atop NO is photoactive. Photon polarization, power-, and wavelength-dependences studies indicate that the mechanisms for photodesorption are nonthermal. A substrate-mediated mechanism involving photogenerated carriers at low photon energies (<3 eV) and a direct excitation mechanism of the adsorbate-surface complex at high photon energies are used to explain the observed photodesorption.
Absorption and fluorescence properties of Cr3+ doped nonstoichiometric green spinel
Authors: Garapon C. , Manaa H. , Moncorgé R.
Published: July 2002

The ground-state as well as the excited-state absorption and fluorescence properties of single crystals of Cr3+ doped nonstoichiometric green spinels MgO–2.6Al2O3 have been studied at various temperatures and in a broad spectral range (300–1700 nm). Evidence is found of the existence of four distinct classes of Cr3+ sites associated with weak, medium, and strong crystal fields with very specific absorption and fluorescence characteristics. The medium and strong field Cr3+ sites are associated with Cr3+ ions having more or less complex environments of Al3+ and Mg2+ ions and of vacancies due to the site inversions which characterize these spinel crystals and to their nonstoichiometry. They give rise to long-lived fluorescences between about 700 and 780 nm typical of (2E,4T2)→4A2 optical transitions. The weak field Cr3+ sites are attributed to Cr3+ ions having a nearest-neighbor cation vacancy in a [011] direction, thus to Cr3+ centers having a well-defined rhombic site symmetry. They give rise to a very interesting broadband infrared fluorescence peaking around 875 nm with a 23 μs lifetime that does not depend on the temperature (at least up to ∼300 K) that we examine in detail.
Rate coefficient for the reaction H+O2→OH+O: Results at high temperatures, 2000 to 5300 K
Authors: Du Hong , Hessler Jan P.
Published: July 2002

The tunable-laser flash-absorption technique has been used to study the high-temperature behavior of the reaction H+O2→OH+O by monitoring the absorption of the hydroxyl radical. Sensitivity analysis of a detailed reaction mechanism shows that for fuel rich mixtures only two reactions are sensitive when hydroxyl is monitored: H2+M→2H+M and H+O2→OH+O. Rate coefficients for these reactions have been determined by least-squares analysis of measured absorption profiles. For the rate of dissociation of H2 in krypton we obtain k1(T)=(8.86±0.88)×10−10 exp[−48321/T(K)] cm3 s−1 from 3450 to 5300 K. For the H+O2 reaction we combined our results with previous low temperature measurements and recommend k2(T)=(1.62±0.12)×10−10 exp[−(7474±122)/T(K)] cm3 s−1 from 960 to 5300 K. The uncertainties are at the 95% confidence level. Measured cross sections for rotational and vibrational energy transfer in O2 and OH have been used to show that relaxation effects do not influence the results. We compare our results to recent trajectory calculations. In addition, we calculate the rate of the reverse reaction, OH+O→H+O2, and compare it to trajectory and statistical adiabatic channel calculations. Finally, we point out that the first excited surface of the hydroperoxyl radical, 2A′, which correlates with H(2S)+O2(1Δg), may be needed to explain very high temperature results.
Stress relaxation in living polymers: Results from a Poisson renewal model
Authors: Granek R. , Cates M. E.
Published: July 2002

The problem of stress relaxation in entangled, reversibly breakable polymers (e.g., wormlike micelles) is considered. In the case where the dominant diffusive mode for the polymers is reptation, this problem has been treated in earlier numerical work by coupling the full reaction kinetics of scissions and recombinations to the dynamics of reptation (represented by a one-dimensional stochastic process). Here we study a simplified renewal model, which replaces the exact reaction kinetics by a Poisson jump process that neglects temporal correlations in the chain length experienced by a particular monomer or tube segment. Between jumps in chain length, the stress relaxation is presumed to follow that of an equivalent unbreakable chain. We apply the solution to the case of reptating flexible polymers and compare the resulting complex modulus with the earlier numerical treatments. It is found that agreement is very good. The renewal model is then used to analyze in detail, for the first time, the crossover to a rapid-scission regime in which chain diffusion between scission events is dominated by breathing modes. A third regime, in which the motion between scission events is Rouse-like, remains unsuitable for study with this model, for reasons that we explain. Various implications of the renewal model for the interpretation of experimental results are discussed. We also provide explicit estimates for chain lengths in CTAC/NaSal/NaCl systems using experimental Cole–Cole plots.
The structure of the water trimer from ab initio calculations
Authors: Xantheas Sotiris S. , Dunning Thom H.
Published: July 2002

The first fully optimized structure of the water trimer at the MP2 level of theory is reported. It corresponds to a cyclic chiral structure in which all O–O separations are equal to 2.80 Å, the OαH...Oβ hydrogen bonds are nonlinear, and two of the terminal hydrogens lie on one side of the O–O–O plane and the third lies on the other. This structure is in qualitative agreement with that reported recently by Pugliano and Saykally [Science 257, 1937 (1992)]. However, the calculations predict the O–O separations to be substantially shorter than those used to fit the far-infrared vibration–rotation–tunneling spectrum. Nonetheless, the computed structure reproduces the measured rotational constants of (D2O)3 ; the errors are ≤1% for A and B and 6% for C. An energy analysis yields a three-body term of 2.3 kcal/mol (∼15% of De with respect to three isolated water molecules).