bluuebunny/crossref_metadata_2025 huggingface dataset:
Atomistic simulation study of the interactions of SiH3 radicals with silicon surfaces
Authors: Ramalingam Shyam , Maroudas Dimitrios , Aydil Eray S.
Published: July 2002

SiH 3 radicals created by electron impact dissociation of SiH4 in reactive gas discharges are widely believed to be the dominant precursor for plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH3 radicals with a variety of crystalline and amorphous silicon surfaces through atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds has the strongest influence on the radical–surface reaction mechanisms and the corresponding reaction probabilities. The SiH3 radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si–Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is much less reactive; the SiH3 radical was reflected back into the gas phase in all but two of the 16 simulations of radical impingement designed to sample the high-symmetry adsorption sites on the surface. When SiH3 reacts on the H-terminated surface, it either inserts into the Si–Si dimer bond or returns to the gas phase as SiH4 after abstracting H from the surface. The insertion into the Si–Si bond occurs through a dissociative adsorption reaction mechanism that produces two surface SiH2 species after transfer of one of the H atoms from SiH3 to one of the dimer Si atoms. The energetics and dynamics of the surface reactions are analyzed in detail. During simulations of a-Si:H film growth, adsorption onto a dangling bond, dissociative insertion, and H abstraction reactions also were observed to occur with similar energetics as the corresponding reactions on crystalline surfaces. The radical is much more mobile on surfaces of a-Si:H films than crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.
Effect of ultrathin Mo and MoSix layer on Ti silicide reaction
Authors: Ohmi S. , Tung R. T.
Published: July 2002

The presence of a small amount of Mo was shown to have a significant effect on the C49–TiSi2 to C54–TiSi2 phase transformation in the Ti–Si reaction. The formation of the C54–TiSi2 phase was facilitated when an ultrathin layer of either Mo or α-MoSix was inserted at or close to the Ti/Si interface. Mo deposited on the surface of Ti had no beneficial effect on the Ti–silicide reaction; neither did Mo implanted into preexisting C49–TiSi2 films. The optimum thickness of interfacial Mo and α-MoSix layers was found to be less than 0.3 nm. Transmission electron microscopy and diffraction investigations demonstrated that Mo did not alter the sequence of the Ti–Si reaction, as proposed in recent studies. Rather, the most obvious effect of molybdenum was a reduction of the grain size of the C49–TiSi2 phase, which could lead to an increase in the nucleation density of the desired C54–TiSi2 phase and account for the observed reduction in formation temperature.
Effect of working on the aligning capability of liquid crystals on the rubbed polymer surfaces
Authors: Seo Dae-Shik , Kobayashi Shunsuke
Published: July 2002

The washing effects on the liquid crystal (LC) alignment capability on the rubbed polyimide PI surface were investigated. The induced optical retardation from the nonwashing process on the rubbed PI surface is larger when compared with the washing processes; it is attributed to the removal of dust in the washing process. The generated pretilt angles in 4-n-pentyl-4′-cyanobiphenyl (5CB) from the nonwashing process on the rubbed PI surface are almost the same as that of the rubbed PI surface after washing process. The polar anchoring energy of the weakly rubbed PI surfaces is increased by the washing process; the washing effects are clearly observed. The surface order parameter SS of 5CB for all washing processes is larger than that with the nonwashing process; it is attributed to the washing process. Consequently, we suggest that the LC aligning capability is largely attributed to the washing effects.
Dislocation core reconstruction and its effect on dislocation mobility in silicon
Authors: Justo João F. , Bulatov Vasily V. , Yip Sidney
Published: July 2002

Through atomistic calculations of kink nucleation and migration in the core of partial dislocations in silicon we demonstrate that symmetry-breaking structural reconstructions will strongly affect dislocation mobility. Core reconstructions give rise to multiple kink species, and, relative to kinks in an unreconstructed dislocation, an increase in kink formation and migration energies. These factors provide additional resistance to dislocation motion which scales with the energy reconstruction. Our results indicate that the observed variations of dislocation mobility in going from elemental to IV–IV, and further to III–V and II–VI zinc-blende semiconductors, can be attributed in part to the weakening of core reconstruction across the series.
Thickness dependence of C-54 TiSi2 phase formation in TiN/Ti/Si(100) thin film structures annealed in nitrogen ambient
Authors: Santucci S. , Phani A. R. , De Biase M. , Alfonsetti R. , Moccia G. , Terracciano A. , Missori M.
Published: July 2002

One of the most used metallization schemes on silicon is the TiN/TiSi2/Si structure, since it has taken advantage of good electrical contact between Si and TiSi2 and the property as a diffusion barrier of TiN. In the present study, this structure has been realized by sputter depositing different layers of TiN/Ti onto silicon (100) substrate and annealing them in nitrogen ambient at different temperatures. Transmission electron microscopy as well as grazing incidence x-ray diffraction have been employed to study the morphology and the crystallographic properties of the formed layers. Initial stage of the C-49 TiSi2 formation has been investigated at 590 °C and complete formation of C-54 TiSi2 phase observed at 710 °C for higher thickness samples. Our attention has been focused on the TiN/TiSi2/Si structures obtained after annealing the deposited films at 710 and 850 °C. We demonstrate that at the annealing temperature of 850 °C as the thickness of the TiN/Ti deposited films is decreased from 600/560 to 175/30 Å, the formation of TiSi2 (C-54) phase is almost inhibited due to a faster consumption of Ti in the formation of TiN.
Hole initiated impact ionization in wide band gap semiconductors
Authors: Reigrotzki Martin , Redmer Ronald , Fitzer Niels , Goodnick Stephen M. , Dür Manfred , Schattke Wolfgang
Published: July 2002

Band-to-band impact ionization by hot electrons and holes is an important process in high-field transport in semiconductors, leading to carrier multiplication and avalanche breakdown. Here we perform first principles calculations for the respective microscopic scattering rates of both electrons and holes in various wide band gap semiconductors. The impact ionization rates themselves are calculated directly from the electronic band structure derived from empirical pseudopotential calculations for cubic GaN, ZnS, and SrS. In comparison with the electron rates, a cutoff in the hole rate is found due to the relatively narrow valence bandwidths in these wide band gap semiconductors, which correspondingly reduces hole initiated carrier multiplication.
Response to “Comment on ‘Monte Carlo simulation of the laser-induced plasma plume expansion under vacuum: Comparison with experiments’ ” [J. Appl. Phys. 86, 4709 (1999)]
Authors: Garrelie F. , Aubreton J. , Catherinot A.
Published: July 2002

This response recalls the bases of the Monte Carlo methods of gas flow simulations. Basic points and explicit parameters used in both the DSMC method and the method of random trajectories are underlined. Some clarifications are made on the confusion made by the author of the comment on this subject. The justification of the validity of the method is recalled, as already made in the original article.
Photoluminescence of hydrogenated amorphous carbon nitride films after ultraviolet light irradiation and thermal annealing
Authors: Zhang Mei , Nakayama Yoshikazu , Harada Susumu
Published: July 2002

The photoluminescence (PL) of hydrogenated amorphous carbon nitride films and its behaviors due to ultraviolet (UV) light irradiation and thermal annealing have been investigated. The films were produced using an electron cyclotron resonance plasma with various mixtures of N2 and CH4 gases. The broad PL spectra centered around 2.3 eV have been observed in air at room temperature and the PL intensity is inversely proportional to the concentration of nitrogen in the film. The UV irradiation results in the modification of configuration and increase of the PL intensity. Moreover, a new luminescence center around 2.76 eV occurs in the film with higher nitrogen content. The changes in film properties due to UV irradiation correlate with the incorporation of nitrogen in the film. Different from the effects of UV irradiation, thermal annealing promotes graphitization of the film. The PL intensity decreases with red shift as annealing temperature increases and is completely quenched when the temperature is over 400 °C. The changes of PL spectra are discussed on the basis of a model in which the sp2 clusters are considered as luminescence centers.
Approximate methods for the solution of quantum wires and dots: Connection rules between pyramidal, cuboidal, and cubic dots
Authors: Califano M. , Harrison P.
Published: July 2002

Energy eigenvalues of the electronic ground state are calculated for rectangular and triangular GaAs/Ga0.6Al0.4As quantum wires as well as for cuboidal and pyramidal quantum dots of the same material. The wire (dot) geometries are approximated by a superposition of perpendicular independent finite one-dimensional potential wells. A perturbation is added to the system to improve the approximation. Excellent agreement with more complex treatments is obtained. The method is applied to investigate the ground state energy dependence on volume and aspect ratio for finite barrier cubic, cuboidal, and pyramidal quantum dots. It is shown that the energy eigenvalues of cubes are equal to those of cuboids of the same volume and aspect ratio similar to one. In addition, a relationship has been found between the volumes of pyramidal quantum dots (often the result of self-assembling in strain layered epitaxy) and cuboidal dots with the same ground state energy and aspect ratios close to one.
Structure and magnetic properties of Pr2Fe17−xMnx (x=0–9) compounds
Authors: Sun Zhi-gang , Zhang Hong-wei , Wang Jing-yun , Shen Bao-gen
Published: July 2002

The influence of Mn substitution for Fe on the structure and magnetic properties of Pr2Fe17−xMnx compounds with 0⩽x⩽9 was investigated. X-ray diffraction indicates that all samples are single phase with the rhombohedral Th2Zn17-type structure. The unit cell volume V remains nearly constant with increasing manganese content up to x=2, then increases linearly at a rate of about 2.5 Å3 per Mn atom up to x=8 and there is a large increase of cell volume at x=9. The Curie temperature increases a little from 298 K at x=0 to 308 K at x=1, and then decreases to 37 K at x=9 quickly. The saturation magnetization at 5 K was found to decrease monotonously with increasing Mn concentration. The thermal magnetization curves M(T) of the compounds with x=1–9 show a strong decrease below TC with decreasing temperature. Also it presents considerable magnetic hardness at low temperature. This can be attributed to the presence of narrow Bloch walls.
Improvement in the electrical properties in Pt/Pb(Zr0.52Ti0.48)O3/Pt ferroelectric capacitors using a wet cleaning method
Authors: Lee June Key , Park Youngsoo , Chung Ilsub , Oh Sang Jeong , Jung Dong Jin , Song Yoon Jong , Koo Bon Jae , Lee Sung Yung , Kim Kinam , Desu Seshu B.
Published: July 2002

A wet cleaning solution was designed to specifically eliminate nonferroelectric phases, such as pyrochlore, PbO, and the etching damaged layer. Scanning electron microscopy pictures clearly showed that treatment with the cleaning solution completely removed these nonferroelectric phases. After removing the nonferroelectric phases, ferroelectric properties such as remnant polarization, coercive voltage, and leakage current, were remarkably improved. In addition, the wet cleaned ferroelectric capacitors yielded superior endurance against hydrogen-induced damage compared to those of the noncleaned capacitors.
Return current in hysteretic Josephson junctions: Experimental distribution in the thermal activation regime
Authors: Castellano M. G. , Torrioli G. , Chiarello F. , Cosmelli C. , Carelli P.
Published: July 2002

We present an experimental study on the retrapping process of a hysteretic, high-quality Josephson junction; namely, we have measured the distribution of the values at which the junction switches back from the voltage state to the zero-voltage state, as a function of the applied magnetic field. While the opposite process (escape from the zero-voltage state) has been extensively studied in the past, both from the theoretical and the experimental point of view, little is found in the literature on the retrapping process. In terms of the tilted washboard potential, the process corresponds to the retrapping from the running state to a locked state in a potential well. The interest of the measurements is in the fact that the value of the return current can be directly related to the dissipation in the junction. While the deterministic behavior, experimentally measured through the I–V curve, appears to be in agreement with the theoretical predictions, even in minor details, the statistical behavior is strongly different from what is expected. The disagreement is found even in zero-applied magnetic field and it cannot be attributed to external noise in the system. From the experimental statistical properties, we find values for the effective dissipation much lower than those obtained from the deterministic curves, a result which could be of interest in experiments on the observation of macroscopic quantum phenomena.
Sensing magnetic fields using superparamagnetic nanomagnets
Authors: Cowburn R. P. , Koltsov D. K. , Adeyeye A. O. , Welland M. E.
Published: July 2002

An experimental investigation using magnetooptical magnetometry is described into the dependence of susceptibility and hysteresis on the lateral size (30–500 nm), thickness (3–7.5 nm), and geometric shape (triangular, square, and pentagonal) of supermalloy nanomagnets made by electron beam lithography. We show that as the lateral size of the nanomagnets is reduced, magnetic softness is at first reduced and then increases abruptly. We show that this increase in softness is due to thermal fluctuations overcoming the anisotropy imposed by the geometric shape of the nanomagnets, leading to superparamagnetism. Nanometer scale magnetic elements with high susceptibility, zero hysteresis, and saturation fields of a few oersteds can thus be made. Implications for field sensing technology are discussed.
Populations of excited atomic states along argon surface-wave plasma columns at low and intermediate pressures
Authors: Lao C. , Gamero A. , Sola A. , Petrova Ts. , Benova E. , Petrov G. M. , Zhelyazkov I.
Published: July 2002

The axial distributions of the electrons and 3p54s and 3p54p excited atoms in argon plasma columns sustained by traveling electromagnetic waves have been studied both experimentally and theoretically in the gas pressure range of 0.2–2.8 Torr. Various diagnostic methods (surface-wave interferometry, emission and absorption spectroscopy) have been used in data gathering. The theoretical model includes a self-consistent solution to the electron Boltzmann equation, electron energy balance equation, a set of balance equations for excited atoms and charged particles, the gas thermal balance equation, the wave dispersion relation, and the wave energy balance equation. The agreement between experimental data and theoretical results is very good.
Electroluminescence mechanisms in organic light emitting devices employing a europium chelate doped in a wide energy gap bipolar conducting host
Authors: Adachi Chihaya , Baldo Marc A. , Forrest Stephen R.
Published: July 2002

The mechanism for energy transfer leading to electroluminescence (EL) of a lanthanide complex, Eu(TTA)3phen (TTA=thenoyltrifluoroacetone,phen=1,10-phenanthroline), doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host is investigated. With the device structure of anode/hole transport layer/Eu(TTA)3phen(1%):CPB/electron transport layer/cathode, we achieve a maximum external EL quantum efficiency (η) of 1.4% at a current density of 0.4 mA/cm2. Saturated red Eu3+ emission based on D5x–7Fx transitions is centered at a wavelength of 612 nm with a full width at half maximum of 3 nm. From analysis of the electroluminescent and photoluminescent spectra, and the current density–voltage characteristics, we conclude that direct trapping of holes and electrons and subsequent formation of the excitons occurs on the dopant, leading to high quantum efficiencies at low current densities. With increasing current between 1 and 100 mA/cm2, however, a significant decrease of η along with an increase in CBP host emission is observed. We demonstrate that the decrease in η at high current densities can be explained by triplet–triplet annihilation.
Dissociation of Al2O3(0001) substrates and the roles of silicon and oxygen in n-type GaN thin solid films grown by gas-source molecular beam epitaxy
Authors: Van Nostrand J. E. , Solomon J. , Saxler A. , Xie Q.-H. , Reynolds D. C. , Look D. C.
Published: July 2002

Unintentionally doped and silicon doped GaN films prepared by molecular beam epitaxy using ammonia are investigated. Hall, secondary ion mass spectroscopy (SIMS), photoluminescence, and x-ray data are utilized for analysis of sources of autodoping of GaN epitaxial films in an effort to identify whether the n-type background electron concentration is of impurity origin or native defect origin. We identify and quantify an anomalous relationship between the Si doping concentration and free carrier concentration and mobility using temperature dependent Hall measurements on a series of 2.0-μm-thick GaN(0001) films grown on sapphire with various Si doping concentrations. SIMS is used to identify oxygen as the origin of the excess free carriers in lightly doped and undoped GaN films. Further, the source of the oxygen is positively identified to be dissociation of the sapphire substrate at the nitride-sapphire interface. Dissociation of SiC at the nitride-carbide interface is also observed. Finally, SIMS is again utilized to show how Si doping can be utilized to suppress the diffusion of the oxygen into the GaN layer from the sapphire substrate. The mechanism of suppression is believed to be formation of a Si–O bond and a greatly reduced diffusion coefficient of the subsequent Si–O complex in GaN.
Anisotropic shock sensitivity and detonation temperature of pentaerythritol tetranitrate single crystal
Authors: Yoo C. S. , Holmes N. C. , Souers P. C. , Wu C. J. , Ree F. H. , Dick J. J.
Published: July 2002

Shock temperatures of pentaerythritol tetranitrate (PETN) single crystals have been measured by using a nanosecond time-resolved spectropyrometric system operated at six discrete wavelengths between 350 and 700 nm. The results show that the shock sensitivity of PETN is strongly dependent on the crystal orientation: Sensitive along the shock propagation normal to the (110) plane, but highly insensitive normal to the (100) plane. The detonation temperature of PETN is, however, independent from the crystal orientation and is determined to be 4140 (±70) K. The time-resolved data yielding the detonation velocity 8.28 (±0.10) mm/μs can be interpreted in the context of a modified thermal explosion model.
Concentration fluctuations in reaction kinetics
Authors: Zumofen G. , Blumen A. , Klafter J.
Published: July 2002

We study the kinetics of the A+A → P and the A+B → P diffusion-limited reactions by modeling the dynamics through random walks on regular and on fractal lattices. For the pseudounimolecular reactions A(0)≪B(0) we present exact analytical expressions for the target (DA=0) and the trapping (DB=0) problems. The kinetics of trapping as well as of the bimolecular reactions are then determined through simulation calculations. In all cases, we relate the results to Smoluchowski-type forms. Whereas the target and the A+A → P problems are well described by such forms, the trapping and the strictly bimolecular A+B → P reaction show different behaviors at longer times, due to fluctuations in the particle densities.
Electron impact spectroscopy of silane and germane
Authors: Dillon Michael A. , Wang R.-G. , Wang Z.-W. , Spence David
Published: July 2002

Electron impact spectra of silane and germane have been recorded using electrons of 200 eV incidence at scattering angles of ∼2°–9°. The range of energy losses covers the entire region of single electron excitation from the valence shell. In the bound state spectrum evidence is presented supporting the existence of two valence transitions in addition to anticipated s, p, and d Rydberg transitions. The 9° spectra reveal much structural detail in the ∼4 eV energy range preceding the 2A1 ion state. The most prominent band consists of a superposition of an optically allowed valence excitation and a symmetry forbidden Rydberg transition. The remaining peaks may be members of vibrational progressions belonging to either [d(t2), a1], [p, a1] ← (a1)2 dipole allowed or [d(e), a1], [s,a1] ← (a1)2 dipole forbidden channels. Of these two possibilities the evidence presented here favors the latter choice.
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o prediction of electronic and vibrational spectra: Application to the A(2Π) ← X(2Σ+) absorption of C2H at 3000 K
Authors: Reimers Jeffrey R. , Wilson Kent R. , Heller Eric J. , Langhoff Stephen R.
Published: July 2002

The combination of ab initio calculation of electronic wave functions with a wave packet calculation of the nuclear motion is used, within the Born–Oppenheimer approximation to compute the vibrational and electronic absorption of a polyatomic molecule. A particular virtue of this approach is that high as well as low temperature spectra are both calculable. This method is applied to C2H, for which the complete active space self-consistent field (CASSCF) method is used to determine full Born–Oppenheimer potential surfaces. Using the assumption that the A(2Π) ← X(2Σ+) absorption can be written as the sum of the A(2A′) ← X and A(2A″) ← X absorptions, the spectra are determined to 60 cm−1 resolution at a temperature of 3000 K. As a result of the large thermal bending amplitude at 3000 K, the calculated spectra are broad and have little resolved structure. Two bands are resolvable, one is due to the A(2A″) ← X absorption and is centered at 5500 cm−1, while the other is due to A(2A′) ← X absorption and is centered at 9500 cm−1. The dramatic blue shift of the A(2A′) ← X band results from the combination of the large X state thermal bending amplitude and high bending frequency of the A(2A′) state. We also determine the X state pure vibrational absorption spectrum and show it to be of much lower intensity than the pure electronic spectrum.
Vibrational relaxation of CO (n=1) in collisions with H2. II. Influence of H2 rotation
Authors: Bačić Z. , Schinke R. , Diercksen G. H. F.
Published: July 2002

Vibrational relaxation of CO (n1=1) in collision with ortho, para H2 is studied quantum mechanically, with the rotational degree of freedom of H2 included. This represents extension of our earlier study [Ref. 11, J. Chem. Phys. 81, 0000 (1984)], where H2 was treated as a structureless particle. The potential surface, described in detail in Ref. 11, consists of a SCF part, which includes explicitly the variation with the CO bond distance, and a damped long range dispersion contribution. The relaxation cross sections are calculated within the infinite order sudden approximation (IOSA) for CO rotation and within the coupled states approximation (CSA) for H2 rotation. Many of the trends clearly present in the cross sections can be understood in terms of the distorted wave approximation (DWA). The calculated relaxation rates in para H2 agree well (within a factor of 2) with the experimental results. However, judging from a limited number of calculated cross sections for relaxation in ortho H2, the ortho H2 relaxation rates would be comparable in magnitude to the para rates, in disagreement with experiment. Extensive comparison is made with the work of Poulsen and Billing (Ref. 12) and a number of significant, even qualitative differences regarding magnitude of some simultaneous CO vibrational and H2 rotational transitions are discussed.
Potentials for weakly bound states in I2 from diffuse spectra and predissociation data
Authors: Tellinghuisen Joel
Published: July 2002

Potential curves are derived for the 2431 1Πu and 2341 a(1g 3Π) states of I2 using existing data from several sources. Compared with previous estimates, the new potentials are thought to be valid over a wider range of internuclear distance, spanning the shallow bound wells at large R (>4Å) and the repulsive regions where they cross the well-known B(O+u 3Π) state at small R (<3.3 Å). Seven of the ten Hund’s case (c) molecular states which correlate with ground-state I atoms are now known experimentally, including three which cross the B state. The role of these states in the collisional quenching of B and in the geminate recombination of I atoms is considered.
The nonempirical calculation of second-order molecular properties by means of effective states. III. Correlated dynamic polarizabilities and dispersion coefficients for He, Ne, H2, N2, and O2
Authors: Visser Foppe , Wormer Paul E. S. , Jacobs Wim P. J. H.
Published: July 2002

Intramolecular correlation contributions to long-range dispersion interactions between closed shell and open shell molecules have been studied by means of perturbation calculations in bases of configuration state functions. All necessary dynamic polarizabilities have been calculated in large bases for He, Ne, H2, N2, and O2. These calculations employ a modified version of an algorithm which was published recently. Basis sets consisting of single and double substitutions from the reference function have been employed. It is shown that in a basis of unperturbed SCF-MO’s the truncation of the configuration basis at the single and double CI level does not give a satisfactory description of the correlation effects. The inclusion of triply excited states is studied for the Ne atom. This improves the results considerably. The possible extension of the present approach to larger systems is discussed.
Effective adiabatic approximation for a two level system coupled to a bath
Authors: Carmeli Benny , Chandler David
Published: July 2002

We consider the general problem of a two level system coupled to a fluctuating bath. We present a practical and accurate approximate theory for this type of system. Our treatment is based upon a variational perturbation theory in which the reference system is a two level system coupled to an adiabatic fluctuating bath. The methodology is easily generalized to situations involving more than two levels.
Photofragmentation dynamics of formaldehyde: H2(v, J) distributions
Authors: Debarre D. , Lefebvre M. , Péalat M. , Taran J.-P. E. , Bamford D. J. , Moore C. B.
Published: July 2002

Coherent anti-Stokes Raman Scattering (CARS) is used to monitor the rovibrational population distribution of prompt H2 produced from H2CO+hν→H2+CO. Photolysis is performed on K=1 lines of the 2141 S1←S0 transition. Scanning CARS spectroscopy is employed, with suppression of the nonresonant background of the H2CO. The detection sensitivity is ∼1012 cm−3 per quantum state. Helium buffer is added to cool the product translation to near room temperature. States v=0–4, J=1–9 are detected. The vibrational distribution peaks at v=1. The rotational distributions in the vibrational states are somewhat Boltzmann with ‘‘temperatures’’ ranging from 1700 to 2500 K for v=3–0. The total energy, 29 500 cm−1, is chosen near the threshold for dissociation. The internal energy of H2(v, J) sums with that found earlier for CO(v, J) and for translation to this total. An average impact parameter of 0.9 Å with a spread of a few tenths Å is found. H2 pushes away from a point about 0.3 Å beyond the C nucleus of CO. The distributions are controlled by the dynamics of the fragments as they separate in the steep, repulsive exit valley of the potential energy surface; they are entirely different from the statistical distributions typical for bond fission without a barrier. Rotational relaxation of H2 in collisions with He occurs at 300 K with a rate constant of (9±3)×10−13 cm3 molecule−1 s−1 for J=3→J=1 and (2.6±1)×10−13 cm3 molecule−1 s−1 for J=5→J=3.
Vibrational relaxation of CO(n=1) in collisions with He
Authors: Schinke R. , Diercksen G. H. F.
Published: July 2002

We present a full ab initio study of vibrational relaxation of CO(n=1) in collisions with 3He and 4He. The vibrational coordinate dependence of the interaction potential is calculated directly. Dynamical calculations are performed within the IOS and the CS approximation. The IOS cross sections agree very well with the corresponding CS results for both He isotopes, however, the ratio IOS/CS shows a different energy behavior for 3He and 4He, respectively. The CS rates for 3He agree extremely well with the experimental relaxation rates. Although only the reduced mass is changed for 4He the agreement with experiment is less favorable in this case. It is explicitly demonstrated that the often used procedure to extract the vibrational coordinate dependence of the interaction potential from a dumbbell fit of the rigid-rotor surface is wrong for He–CO. For 3He the corresponding cross sections are 3–10 times smaller than those obtained form the ab initio potential although the vibrationally elastic parts of the interaction potential are identical.
Collisional effects in Stark spectroscopy of molecules in 1Π electronic states
Authors: Alexander Millard H.
Published: July 2002

We present the theoretical framework necessary to describe inelastic collisions between Stark mixed Λ-doublet levels of a molecule in a 1Π electronic state, and the subsequent effect on fluorescence intensities. It is convenient to work with the spherical tensor moments of the excited state density matrix. For weak fields and short times after excitation the ratio of the fluorescence intensities for emission on ‘‘forbidden’’ and allowed 1Σ+←1Π lines is predicted to vary linearly with time and quadratically with field strength. Simulation studies are presented, based on collisional parameters suggested by previous theoretical studies of inelastic collisions of molecules in Π electronic states. The present article provides the foundation for the accurate interpretation of laser diagnostic studies in plasma environments, such as those described by Mandich, Gaebe, and Gottscho in the accompanying article.
A high-barrier potential energy surface for F+H2→HF+H
Authors: Steckler Rozeanne , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

Frisch et al. have estimated, by comparing ab initio potential energy and dynamics calculations to experiment, that the potential energy barrier for the title reaction is 2.7 kcal/mol, which is considerably higher than the 0.7–0.9 kcal/mol barriers on recent semiempirical surfaces we have constructed. In the present article we report a global potential energy surface with a 2.7 kcal/mol barrier and stretch and bend potentials based on ab initio calculations. Variational transition state theory and cross section calculations, based on this surface and including anharmonicity and multidimensional tunneling, are in very poor agreement with experiment, e.g., the thermal rate constant at 190 K is 30 times lower than the experimental value for F+H2 and 35 times lower than the experimental value for F+D2. We also report additional model calculations that pinpoint the reasons for the difference between the results of Frisch et al. and ours. The two main reasons are (i) Frisch et al. approximate the vibrationally adiabatic barrier height by its value at the saddle point rather than by its higher variational value, and (ii) Frisch et al. neglect important anharmonic effects in the bending degrees of freedom. This analysis identifies those features of the potential energy surface which it would be most useful to address in any future ab initio calculations directed to the threshold behavior of the F+H2 reaction or its isotopic analogs.
Dynamics of hyperspherical and local mode resonance decay studied by time dependent wave packet propagation
Authors: Bisseling R. H. , Kosloff R. , Manz J.
Published: July 2002

Time dependent wave packet propagation of resonance states of ABA molecules is used to demonstrate the correlation between the directionality of the lobes of the wave functions and mode selectivity of the unimolecular decay. This correlation was inferred by Hose and Taylor. The molecule is modeled by the Thiele–Wilson coupled Morse oscillators. A near-degenerate pair of resonances with extreme motions is studied in detail: The local ‘‘bond’’ mode with lobes pointing towards the exit valleys of the potential decays about 30 times faster than the hyperspherical ‘‘restricted precession’’ mode with dominant lobe on the potential ridge. This is in close analogy to mode selectivity in the Hénon–Heiles system. The wave function propagation technique also yields detailed insight into the dissociation mechanism. Out of several choices, only a single lobe penetrates into the exit valley. For the local mode resonance vibrational predissociation starts out primarily from extended vibrationally excited diatomic configurations, A↔B(ν*=1)⋅ ⋅ ⋅A→AB(ν′=0)+A. However, the hyperspherical mode resonance prefers compressed diatomic geometry just before dissociation, AB(ν*=3)⋅ ⋅ ⋅A→AB(ν′=0)+A. The results imply some general criteria for mode selective unimolecular chemical reactions, as well as a successful numerical test of the preparation of resonance wave functions and their propagation by the Fourier method.
Susceptibility and polarizability of atoms and ions
Authors: Patil S. H.
Published: July 2002

With the help of some relations for the oscillator strengths and semiclassical relations, we have calculated diamagnetic susceptibilities and dipole polarizabilities for a large number of atoms and ions. We have also obtained simple expressions for susceptibility and polarizability in terms of ionization energy.
Facilitated kinetic Ising models and the glass transition
Authors: Fredrickson Glenn H. , Andersen Hans C.
Published: July 2002

The class of ‘‘facilitated’’ kinetic Ising models introduced in a recent letter is investigated in greater detail. An interpretation of the models in terms of relaxation processes in dense fluids is described. Various types of kinetic behavior are predicted for the different spin models: (A) Arrhenius temperature dependence of the average structural relaxation time, (B) non-Arrhenius temperature dependence with a divergence of the relaxation time at a nonzero temperature, and (C) non-Arrhenius temperature dependence with a divergent relaxation time only at zero temperature. All of the models show nonexponential decay of equilibrium time correlation functions, consistent with the Kohlrausch–Williams–Watts empirical form. The nature of the glass transitions exhibited by the various models is discussed.
Electron thermalization processes in rare gases with the Ramsauer minimum
Authors: Suzuki Etsuhito , Hatano Yoshihiko
Published: July 2002

Time evolutions of electron mean energy in Ar, Kr, and Xe are observed by means of a pulse-radiolysis microwave-conductivity method, assuming that electron energy distribution function is always Maxwellian in the energy range from about 500 meV to thermal. Excess electron temperature, Te−Tg, decays exponentially, whose decay time is defined as a thermalization time. Products of gas pressure and thermalization time are constant for each gas, and are 8×102 Torr μs for Ar, 1.8×102 Torr μs for Kr, and 2.0×102 Torr μs for Xe.
Lifetimes and quenching of B 2Σ+ PO by atmospheric gases
Authors: Wong Koon Ng , Anderson William R. , Kotlar Anthony J. , DeWilde Mark A. , Decker Leon J.
Published: July 2002

Pulsed laser excited fluorescence in the B 2Σ+ ← X 2π system of gas phase PO was used to measure the lifetime for v′=0 of the B state. Rotationally resolved measurements for a few selected J′ levels, at Ar or He carrier gas pressures of ∼2 Torr, reveal no dependence of the lifetime on the rotational level excited. Earlier measurements of relative fluorescence intensities in the v′=0 vibrational progression were reinterpreted to extract the dependence of the electronic transition moment on internuclear distance. Using this transition moment, no lifetime dependence on rotational level is to be expected, even at low pressures. Rate constants for quenching of the B state PO by N2, O2, CO2, and H2O, and upper limits thereof for He and Ar are reported. O2 was found to react with ground state PO. A crude measurement of the rate constant was performed. The result is compared to two other known measurements. The rate constant is in excellent agreement with the previous measurement, but in poor agreement with that of a concurrent study. Further work is required to determine the reason for the discrepancy. A cathode ray–vidicon type waveform digitizer was found to lend itself readily to the lifetime measurements, wherein a low repetition rate laser was used. Unfortunately, it was discovered that small nonlinearities (∼1% of full scale) in the digitizer response have a marked effect on the fits to the exponential lifetime decays and, especially, the quenching rate constants. We believe this problem is not commonly recognized. A simple method for calibration of this type of digitizer and removal of these effects is presented.
Studies of atomic oxygen in O2+CF4 rf discharges by two-photon laser-induced fluorescence and optical emission spectroscopy
Authors: Walkup R. E. , Saenger K. L. , Selwyn G. S.
Published: July 2002

We have used two-photon laser-induced fluorescence to obtain quantitative measurements of the concentration of ground state O atoms in O2+CF4 rf discharges. Absolute calibration was achieved by generating a known concentration of atomic oxygen by UV laser photolysis of O2. Trace amounts of Ar were added to serve as an inert reference gas for concurrent optical emission measurements, in which the plasma-induced optical emission intensities from O* and Ar* lines were recorded. Emission line shapes were measured using a Fabry–Perot interfermoter to gain information on the mechanisms for formation of excited oxygen atoms in the plasma. Two excitation mechanisms were found to be important: (1) electron impact excitation of ground state atoms, e+O → O*+e, and (2) dissociative excitation of O2, e+O2 → O*+O+e. Evidence for both excitation mechanisms was obtained for O* (8446 Å) emission, with atomic excitation being dominant, whereas dissociative excitation appeared to be the dominant mechanism for O* (7774 Å) emission. Argon actinometry for the determination of ground state oxygen was directly tested. Because of the contribution from dissociative excitation, a strict proportionality, O*/Ar*∝[O]/[Ar], was not satisfied where O* (Ar*) is the intensity of an atomic oxygen (argon) emission line, and [O] ([Ar]) is the oxygen (argon) atom concentration. However, within certain limitations, the O* (8446 Å)/Ar* emission intensity ratio gives the right qualitative trends for the O atom concentration.
An improved intermolecular potential for nitrogen
Authors: van der Avoird A. , Wormer P. E. S. , Jansen A. P. J.
Published: July 2002

Using new ab initio calculations for the multipole and short range interactions and the results for the dispersion interactions recently calculated in our institute, we have constructed a new intermolecular potential for nitrogen. Its distance and angular dependence is expressed analytically in a spherical expansion. The long range dispersion interactions have been damped for charge penetration and exchange effects via the parameter-free damping functions of Tang and Toennies, generalized to the case of an anisotropic potential, and we have introduced two scaling constants in the short range repulsion in order to obtain a second virial coefficient that lies within the experimental error in the entire temperature range. The use of the new potential in lattice dynamics calculations yields good results for several properties of solid nitrogen.
The photodissociation of ClCN: A theoretical determination of the rotational state distribution of the CN fragment
Authors: Waite Boyd A. , Dunlap Brett I.
Published: July 2002

A parameter-free theoretical determination of the rotational state distribution of the CN fragment from photodissociation of ClCN at 193 nm is presented. The ground- and excited-state potential energy surfaces are calculated at a grid of points corresponding to bent and linear geometries using density functional theory. The points are fit to a functional form similar to one used previously in model studies of the effects of bending forces during ICN photodissociation. Rotational state distributions are obtained by a modified quasiclassical method. Comparison of the results obtained at 0 and 300 K are in excellent agreement with experimental determination of the rotational state distribution of the CN fragment for ClCN photodissociation. The importance of the bending force in the excited state (which results in extensive population in the rotational states j=50–65 of the CN fragment) is discussed.
Collinear quasiclassical trajectory study of collision-induced dissociation on a model potential energy surface
Authors: Kaye Jack A. , Kuppermann Aron
Published: July 2002

Quasiclassical trajectory calculations have been carried out at energies above the threshold for collision-induced dissociation for a model symmetric collinear atom–diatomic molecule system. Exact quantum mechanical calculations have shown that quasiclassical trajectories give a qualitatively correct picture of the dynamics in this system, in so far as reaction and total dissociation probabilities are concerned. Trajectories leading to dissociation are found to lie almost entirely in well-defined reactivity bands, with the exception of a few occurring in a small chattering region in which the outcome of the trajectory is extremely sensitive to its initial conditions. The energy distribution functions of the dissociated atoms are obtained and shown to vary substantially with initial conditions (reagent vibrational and translation energy). The form of these distributions is, to a major extent, determined by the position and width of the reactivity bands. The different dissociation reactivity bands are shown to be associated with different types of trajectories. Part of the vibrational enhancement of dissociation arises from the fact that the simplest possible trajectory leading to dissociation (one which crosses the symmetric stretch line only once prior to the onset of dissociation) is not obtained with ground state reagents.
On the Percus–Yevick equation for a rigid disk fluid
Authors: Leutheusser E.
Published: July 2002

The Percus–Yevick integral equation for the pair correlation function of a hard disk fluid is solved approximately by a simple Ansatz which is exact in one as well as in three dimensions. The comparison with the numerical exact solution shows very good agreement in the whole density regime.
Test of Hubbard–Onsager’s dielectric friction theory for ion migration in mixed solvents. II. Dioxane–water systems
Authors: Ibuki K. , Nakahara M.
Published: July 2002

Hubbard–Onsager’s (HO) dielectric friction theory for the limiting ionic conductance is tested for various monovalent ions in solvent mixtures of dioxane and water by taking the residual friction coefficient which is defined as the overall friction coefficient subtracted by the Stokes friction coefficient for slip. The HO theory combined with the bulk solvent properties is successful in predicting the ion size and solvent composition dependencies of the residual friction coefficients Δζ for the alkali metal and halide ions except in the dioxane-rich region. The HO theory works much better than the early dielectric friction theory by Zwanzig because the effect of the coupling of the viscous and dielectric frictions on the velocity field is neglected in the latter theory. It turns out that the HO theory serves an important theoretical framework for the interpretation of the limiting ionic conductances experimentally obtained.
Two-dimensional Fourier transform ESR spectroscopy
Authors: Gorcester Jeff , Freed Jack H.
Published: July 2002

New Fourier transform ESR techniques, which permit rapid data acquisition of ESR spectra, now allow one to perform a variety of two-dimensional ESR experiments. Two prototype experiments on a fast motional nitroxide spectrum are described. The first is based upon detection of the Han echo and may be used for discriminating the homogeneous broadening of an inhomogeneous spectrum (and/or the echo envelope modulation). The second is based upon detection of the FID after the three-pulse Jeener–Ernst sequence, and it leads to cross peaks due to Heisenberg spin exhange among the hyperfine lines.
A measurement of the low energy stereostructure of protonated acetylene, C2H+3
Authors: Kanter E. P. , Vager Z. , Both G. , Zajfman D.
Published: July 2002

A new experimental method for obtaining detailed structural informatio on small polyatomic molecular ions is presented. The low−energy structure of protonated acetylene has been studied using this method.(AIP)
Activation of n-butane with translational energy on Ir(110)–(1×2)
Authors: Hamza A. V. , Steinrück H.-P. , Madix R. J.
Published: July 2002

The initial activation probability s0 for reactive dissociation of n-butane on Ir(110)–(1×2) exhibits at least two reaction channels. For one the direct collisional activation probability is at least 0.21 independent of incident kinetic energy, increasing to near 0.7 with decreasing surface temperature and kinetic energy of the incident beam of butane as a result of trapping of the species in the molecule–surface potential well. At kinetic energies above 120 kJ/mol the reaction probability again increases due to the onset of an activated route. Comparison to the results obtained with ethane suggests that energy transfer to the surface dissipates energy in the collision so that less energy is available to penetrate the longer range repulsive barrier in order to form carbon–metal and hydrogen–metal bonds.
Collision complexes in the reactions of CH+3 with C2H4 and C2H2
Authors: Sonnenfroh D. M. , Farrar J. M.
Published: July 2002

We present a crossed beam study of the reactions of CH+3 with ethylene and acetylene in the relative collision energy from 0.5 to 2.0 eV. Both systems involve condensation reactions proceeding through the formation of transient collision complexes living a fraction of a rotational period which decay through ejection of one or two molecules of H2. In addition, the ethylene system also yields CH4 + C2H+3 , nominally a hydride abstraction, but also proceeding through a long-lived collision complex. Isotopic scrambling in the CD+3 +C2H4 also confirms the presence of a transient collision complex in the reaction. The similarities accompanying the flux distributions for C2H+3 and C3H+5 production in the ethylene reaction suggest a common C3H+7 intermediate. Both the acetylene and ethylene reactions yield the condensation product C3H+3 , produced through transient intermediates. The flux distribution for the ethylene reaction suggests that C3H+3 formation proceeds by sequential ejection of two H2 molecules from the initial C3H+7 intermediate. The widths of the kinetic energy distributions for C3H+3 formation imply that at least 15% of the C3H+3 products must be the more stable cyclopropenium isomer.