bluuebunny/crossref_metadata_2025 huggingface dataset:
Heterostructures in GaInP grown using a change in Te doping
Authors: Hsu Y. , Fetzer C. M. , Stringfellow G. B. , Shurtleff J. K. , Choi C. J. , Seong T. Y.
Published: July 2002

In organometallic vapor phase epitaxy, changes in growth conditions can be used to modulate the extent of CuPt ordering and, hence, the band gap energy of GaInP. One method is to add Te during growth. An increase in the band gap energy of 0.1 eV due to a decrease in ordering has been obtained by increasing the input pressure of diethyltelluride from 0 to 8×10−6 Torr, which corresponds to a doping concentration of 6×1017 cm−3. This simple procedure offers an attractive method to grow quantum wells (QWs) and superlattices, which are useful for band gap engineering, by modulating the input pressure of the Te precursor. Various heterostructures with abrupt interfaces were successfully grown with interruptions at the interfaces between the Te-doped and undoped GaInP layers. QWs as thin as 10 nm can be clearly seen from transmission electron microscope images.
Microstructural and infrared optical properties of electrochemically etched highly doped 4H–SiC
Authors: Zangooie S. , Persson P. O. A. , Hilfiker J. N. , Hultman L. , Arwin H.
Published: July 2002

Pores in porous 4H–SiC are found to propagate first nearly parallel with the basal plane and then gradually change plane of propagation towards the direction of the c axis. A similar anisotropy in pore propagation is found in porous 6H–SiC. A disordered phase is encountered at the interface between crystalline SiC and the pores. Formation of this phase was attributed to the etching conditions. Characterization of the material with nondestructive infrared spectroscopic ellipsometry in the photon energy range 0.062–0.62 eV provides average thickness and porosity in good agreement with electron microscopy observations. Anodization of SiC introduces remarkable changes to the reststrahlen band. A shallow minimum at 0.113 eV is attributed to the Berreman effect. In addition, a sharp peak at 0.126 eV is discussed to be related to the in-depth inhomogeneity and particle shape effects in the material.
Optical properties of GaN epilayers and GaN/AlGaN quantum wells grown by molecular beam epitaxy on GaN(0001) single crystal substrate
Authors: Grandjean N. , Damilano B. , Massies J. , Neu G. , Teissere M. , Grzegory I. , Porowski S. , Gallart M. , Lefebvre P. , Gil B. , Albrecht M.
Published: July 2002

GaN epilayers and GaN/AlGaN quantum wells (QWs) were grown by molecular beam epitaxy on GaN(0001) single crystal substrates. Transmission electron microscopy (TEM) was used to assess the crystal quality of the homoepitaxial layers. A dislocation density of less than 105 cm−2 is deduced from TEM imaging. Low temperature (1.8 K) photoluminescence (PL) of homoepitaxial GaN reveals PL linewidths as low as 0.3 meV for bound excitons. The PL integrated intensity variation between 10 and 300 K is compared to that observed on a typical heteroepitaxial GaN/Al2O3 layer. A 2 nm thick GaN/Al0.1Ga0.9N QW has been studied by time-resolved and continuous wave PL. The decay time is close to a purely radiative decay, as expected for a low defect density. Finally, the built-in polarization field measured in a homoepitaxial QW is shown to be comparable to that measured on heteroepitaxial QWs grown either on sapphire or silicon substrates.
Absorption and photoluminescence spectra of the diluted magnetic semiconductor Ga1−xFexSb
Authors: Karar N. , Basu S. , Venkataraghavan R. , Arora B. M.
Published: July 2002

Bulk growth of the III–V diluted magnetic semiconductor, Ga1−xFexSb is reported for different iron concentrations. Room-temperature optical absorption coefficient measurements were performed. The dependence of the band gap Eg on the Fe concentration was studied. The band gap decreased with increasing x, and the variation was observed to follow a nonlinear trend. Photoluminescence (PL) measurements at 18 K were also carried out. The PL spectra showed a clear peak shift due to incorporation of Fe in the lattice, thus agreeing with the optical absorption results.
Self-affine roughness effects on electron transmission and electric current in tunnel junctions
Authors: Palasantzas G. , Barnas J. , De Hosson Th. M.
Published: July 2002

Interface roughness effects on electron transmission in tunnel junctions are investigated theoretically in the limit of thick barriers. The barrier roughness is described in terms of self-affine fractal scaling by the roughness exponent H, rms roughness amplitude w, and correlation length ξ. For realistic parameters diffuse transmission usually exceeds specular transmission. It is shown that for small roughness exponents (H<0.5) the transmission coefficient increases with decreasing ratio w/ξ. For large roughness exponents (or smoother interfaces at short wavelengths) the transmission coefficient has a maximum at a certain value of the ratio w/ξ. With increasing w/ξ the tunneling current behaves similarly as the transmission coefficient.
Mechanisms of external heavy atom effects on the lowest excited triplet states: Naphthalene and biphenyl X traps
Authors: Komada Yasuo , Yamauchi Seigo , Hirota Noboru
Published: July 2002

An external heavy atom effect was investigated on the phosphorescent states of naphthalene X traps and biphenyl X traps using a variety of heavy atom perturbers at liquid helium temperatures. Phosphorescence spectra, triplet lifetimes, sublevel properties, and detrapping rates were measured. The well-resolved phosphorescence spectra were obtained in almost all cases. The intensities of the vibronic bands were extremely weak in the heavy atom systems compared with those of the light atom systems. The triplet lifetimes range from 10 to 2300 ms in the naphthalene X traps and from 240 to 4200 ms in the biphenyl X traps. Selective enhancements were observed in the radiative decay rate constants of the triplet sublevels. Correlation between the intensities of phonon bands and the magnitudes of detrapping rates was found in the naphthalene X traps. From these results we conclude that: (i) The shortening of the triplet lifetimes is mostly due to the radiative enhancements of the T1→S0 transitions; (ii) the molecular states of the perturber, especially the lowest excited triplet states, are the most important perturbing states among the three possible states (atomic states, CT states, and molecular states); and (iii) the magnitudes of the intermolecular interactions are also enhanced in the heavy atom systems, which is partly due to those of electron–phonon interactions.
Vacuum ultraviolet laser spectroscopy. II. Spectra of Xe2 and excited state constants
Authors: Lipson R. H. , LaRocque P. E. , Stoicheff B. P.
Published: July 2002

Vibrationally and isotopically resolved fluorescence excitation spectra of Xe2 at 1300, 1485, and 1500 Å have been obtained and analyzed. They correspond to electronic transitions from the van der Waals ground state to the three lowest excited states. Molecules of Xe2 were formed in the v″=0 level of the ground state using a pulsed supersonic jet, and fluorescence emission was excited by a tunable, coherent, and monochromatic source of vacuum ultraviolet radiation generated by four-wave-sum-mixing in Mg and Zn vapors. For each of the three band systems, unambiguous assignment of upper-state vibrational quantum numbering was possible, resulting in the determination of accurate upper-state spectroscopic constants for the first time. From system I (at ∼1500 Å) with bands v′=36 to 43, constants for the A1u state of 129,132Xe2 were found to be T′e =63 089.9(6.9), ωe =137.48(34), ωex′e =1.1668(43), and De =4174.4(7.5) cm−1. Band system II (at ∼1485 Å) with bands v′=35 to 46, gave constants for the B0+u state: T′e =63 795.6(4.2), ωe =124.85(20), ωex′e =0.9372(24), and De =4446.3(5.2) cm−1. Finally, system III (at ∼1300 Å) and bands v′=16 to 27 yielded the following constants for the C0+u state: T′e =75 881.7(1.5), ωe =49.71(10), ωex′e =0.4222(41), and De =1500.0 (3.4) cm−1. Potential curves based on these constants were calculated for all three excited states.
Dielectric friction and ionic hydration near boundaries: Image charge effects
Authors: van der Zwan G. , Mazo R. M.
Published: July 2002

The effects of image charges on ions and polar particles near boundaries are calculated for several simple models. These are: dielectric friction of a translating charge and of a rotating dipole, and the electrostatic contribution to the hydration energy of an ion, all near a boundary. Image charge induced changes in dielectric friction and electrostatic free energy are found to be minor for molecular size particles, but appreciable for larger particles, e.g., macromolecules near membranes.
Spin polarization in SO photochemically generated from SO2
Authors: Kanamori Hideto , Butler James E. , Kawaguchi Kentarou , Yamada Chikashi , Hirota Eizi
Published: July 2002

The photochemical processes induced in sulfur dioxide by 193 nm excimer laser excitation have been investigated by examining vibration-rotation transitions of the reaction product SO with the use of tunable infrared diode-laser spectroscopy. Only X 3Σ− ground-state SO molecules were observed. About 70% of the nascent SO molecules were found to be in the v=2 state, about 20% in v=1, and some in v=5. The rotational distribution in each vibrational state differed significantly from the thermal distribution, and was shifted towards higher rotational levels in lower vibrational states. The electron spin was observed to be polarized in the v=1 and 2 states in a direction either parallel or antiparallel to the rotational angular momentum, i.e., the F1 and F3 spin sublevels are more populated than the F2 level in these states. This selective population was not observed for v=5. The origin of the spin polarization is discussed in terms of spin-orbit mixing between the C̃ state and a repulsive triplet state of SO2. The results obtained here emphasize the usefulness of high resolution, high sensitivity infrared laser spectroscopy in the study of primary processes in photochemistry.
Effect of lattice potential upon the surface diffusion of Si on Si(100)
Authors: NoorBatcha I. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

The surface diffusion of Si atoms on Si(100) is examined using three different lattice potentials suggested by Keating, by Weber, and by Baraff et al. Jump frequencies of Si atoms between adjacent adsorbtion sites are computed on each potential surface at 800, 1000, 1200, and 1500 K using classical trajectory methods. Diffusion coefficients are obtained directly from the jump frequencies and activation parameters from Arrhenius plots of the diffusion coefficients. The activation energy obtained in our previous calculations using Weber’s lattice potential is shown to be too small due to a lattice force field whose force constants are too large by about 25%. The best results are obtained with the Keating potential. This surface gives an activation energy of 7.47 kcal/mol for Si diffusion on Si(100). This result, coupled with the ratio of activation energies for Si(100) and Si(111) surfaces obtained in our previous studies, suggests an activation energy of 5.25 kcal/mol for Si diffusion on Si(111). This is in very good accord with the measured value of 4.6 kcal/mol obtained in ultrahigh vacuum deposition of silicon.
The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4
Authors: White Mary Anne , Wagner Brian D.
Published: July 2002

In order to clarify previous conflicting reports concerning the existence of an order–disorder phase transition in NH4AlF4 at T∼150 K, the heat capacity of this compound has been measured from T=15 to 305 K, by adiabatic calorimetry. The calorimetric results show a small anomaly in Cp centered at T=157 K (ΔS=0.02 R), and this is attributed to a soft mode phase transition associated with slight changes in the (AlF4)n−n layers. Analysis of the contribution of ammonium ion disorder to the heat capacity indicates that the ammonium ion disorder is gradually thermally excited, and can be adequately described by a three-dimensional Einstein oscillator for which θE ∼400 K. There is no evidence of an order–disorder phase transition association with the ammonium ions.
Semiclassical analysis of rotational state distributions in the photolysis of triatomic molecules: Mapping of ground state wave function and potential anisotropy
Authors: Schinke Reinhard , Engel Volker
Published: July 2002

We investigate the influence of final state interaction on rotational state distributions in the photolysis of (model) triatomic molecules. All calculations are performed within the infinite-order-sudden approximation (IOSA). We find distinct modal structures, which depend systematically on the parameters of the system such as anisotropy, energy, mass, etc. Most of the observations can be directly explained in the semiclassical limit of the IOSA. The key quantities are the classical excitation function and an amplitude, which represents the angular variation of the ground state wave function and the anisotropy of the excited state potential. Assuming hard core potentials analytical expressions for the excitation function can be derived which explain all the trends found in the numerical calculations. The observed structures are believed to exist whenever the dissociation process is impulsive and many rotational states are involved. They are closely related to rotational rainbows in normal scattering. We believe that recently reported experimental distributions for ICN at long laser wavelengths can be qualitatively explained by the structures investigated in this article.
Coupled-channel study of rotational excitation of an electronically excited diatomic molecule by atom impact: He(1S)+H2 (B 1Σ+u)
Authors: Grimes Randall M. , Lester William A. , Dupuis Michel
Published: July 2002

Rotational energy transfer cross sections have been computed in the rigid rotor model, for the energy range 0.015–0.150 eV, using an analytic fit to ab initio interaction energies determined by others and the authors with large basis set CI approaches. Long-range interactions are described by a multipole expansion. The potential energy surface is highly anisotropic and leads to integral inelastic cross sections, typically larger than those involving ground state systems, in fair agreement with results obtained using a previously determined fit.
Monte Carlo simulation of adhesive disks
Authors: Seaton N. A. , Glandt E. D.
Published: July 2002

A new method has been developed that permits the Monte Carlo simulation of systems in which the intermolecular potential contains a well which is both infinitely deep and infinitesimally wide. Adhesive potentials of this type are analytically tractable and have been used in a number of applications. The simulation algorithm combines the generation and acceptance steps of the conventional Metropolis method to overcome the effect of the singularity in the potential. The method is applied to a two-dimensional system of adhesive disks. Results are reported for the equation of state and the radial distribution function, which has delta function peaks not present in the three-dimensional Percus–Yevick solution. We also present an exact solution for one-dimensional adhesive rods.
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o study of the hydrogen bonding interactions of formamide with water and methanol
Authors: Jasien Paul G. , Stevens Walter J.
Published: July 2002

A b initio calculations of hydrogen bond energies for a number of water–formamide and methanol–formamide complexes are reported at both the SCF and correlated levels. Full gradient optimizations of these structures have been performed for basis sets of double zeta and double zeta plus polarization quality. For both water and methanol, the most stable 1:1 complex is found to be a cyclic double hydrogen bonded structure. Basis set effects on the calculated hydrogen bond energies were investigated as was the magnitude of the basis set superposition error. In all cases investigated, the addition of polarization functions to the basis set is found to decrease the calculated binding energy by approximately 2–4 kcal/mol, while correlation is found to increase the binding energy by ≂1 kcal/mol. Calculations on dihydrated formamide indicate a small three-body contribution to the total binding energy.
Stability and structure of cluster ions in the gas phase: Carbon dioxide with Cl−, H3O+, HCO+2, and HCO+
Authors: Hiraoka K. , Shoda T. , Morise K. , Yamabe S. , Kawai E. , Hirao K.
Published: July 2002

Thermodynamic values for the clustering reactions of carbon dioxide with Cl−, H3O+, HCO+2 , and HCO+ were measured with a pulsed electron-beam high-pressure mass spectrometer. The measured enthalpy changes are reproduced satisfactorily by the ab initio molecular orbital calculations. While Cl−(CO2)n is of the symmetric geometry, H3O+(CO2)n, HCO+2(CO2)n, and HCO+(CO2)n are of the form of the shell plus surroundings. The small fall-off of the bonding energy for the cluster growth is ascribed to the large quadrupole moment of CO2 as well as those on the shell.
An amplified Oregonator model simulating alternative excitabilities, transitions in types of oscillations, and temporary bistability in a closed system
Authors: Ruoff Peter , Noyes Richard M.
Published: July 2002

The original Oregonator model of five irreversible steps and one stoichiometric parameter has been amplified to seven irreversible steps. The amplified model simulates excitability of a reduced steady state induced by silver ions and of an oxidized steady state induced by bromide ions. It also simulates how oscillations during a single run might change from reduction pulses to oxidation pulses. Finally, it simulates a temporary bistability persisting for over two hours in a closed system. If independent measurements determine the rate of enolization of the organic substrate and the rate of reaction of that substrate with the oxidized form of the catalyst, this model may be able to simulate semiquantitatively a large fraction of reported behaviors of catalyzed homogeneous Belousov–Zhabotinsky systems. It will not model systems in which Br2 accumulates or in which there are high-frequency small-amplitude oscillations around an oxidized steady state. Procedures are suggested for simulating such behaviors.
Effect of translational energy on the molecular chemisorption of CO on Ni(111): Implications for the dynamics of the chemisorption process
Authors: Tang S. L. , Beckerle J. D. , Lee M. B. , Ceyer S. T.
Published: July 2002

The effect of translational energy on the molecular chemisorption of CO on a Ni(111) surface is used as a probe of the dynamics of the adsorption process. Initial adsorption probabilities, apparent saturation coverages, spatially resolved Auger coverage profiles, and high resolution electron energy loss spectra of CO deposited on the Ni surface from a supersonic molecular beam are measured as a function of translational energy. It is found that the initial adsorption process for CO molecules incident with energies less than 4 kcal/mol differs from that for molecules incident with higher energies. Molecules with kinetic energies below 4 kcal/mol adsorb with an initial adsorption probability of 0.85±0.04 and a high apparent saturation coverage. Molecules with translational energies between 7 to 30 kcal/mol have an initial adsorption probability of 0.46±0.03, and an apparent saturation coverage approximately half that of the low energy molecules. Since the CO packing density and the final chemisorption states are shown to be independent of incident energy, the two apparent saturation coverages are the result of a difference in the surface area over which the CO molecules are spread. This is verified by spatially resolved Auger coverage profile measurements. Molecules at low energies are initially adsorbed with higher mobility than those incident with larger translational energies. High resolution electron energy loss spectra and thermal desorption spectra show no translational energy-induced dissociation. The frequency shift of the bridge-bonded CO stretching mode measured at the periphery of the molecular beam image shows that the energy-induced difference in the CO mobility during the chemisorption process is qualitatively similar on both the clean and partially CO-covered surface. These results are interpreted as evidence for two adsorption pathways into the molecular chemisorption state. Molecules incident on the surface with low energies are identified as mobile precursor molecules to the molecularly chemisorbed molecules. The precursor molecules have access to the molecular chemisorption state via a low energy pathway. As the incident translational energy is increased beyond the effective 4–7 kcal/mol energy barrier, a new adsorption pathway directly into the less

mobile chemisorption state becomes accessible. The natures of the precursor molecule, the effective energy barrier and the low energy pathway are discussed.
Rotationally resolved hot atom collisional excitation of CO2(001) by time-resolved diode laser spectroscopy
Authors: O’Neill James A. , Wang Chen Xi , Cai Ji Ye , Flynn George W. , Weston Ralph E.
Published: July 2002

A tunable infrared diode laser was used to study the nascent rotationa distribution of CO2 molecules produced directly in the 0001 excited state as a result of collisions with  hot  hydrogen atoms formed in the UV photolysis of H2S.(AIP)
Pulsed-nozzle, Fourier-transform microwave spectroscopy of the methyl cyanide–acetylene dimer
Authors: Howard N. W. , Legon A. C.
Published: July 2002

The ground-state rotational spectra of five isotopic species of a weakly bound dimer formed between methyl cyanide and acetylene have been investigated by pulsed-nozzle, Fourier-transform microwave spectroscopy. The rotational constant B0, the centrifugal distortion constants DJ and DJK, and the 14N–nuclear quadrupole coupling constant χ(14N) have been determined for the symmetric top species CH3C14N⋅⋅⋅HCCH, CH3C14N⋅⋅⋅DCCH, CH3C14N⋅⋅⋅HCCD, CH3C14N⋅⋅⋅DCCD, and CH3C15N⋅⋅⋅HCCH. The values for the parent isotopic species are B0=977.4659(1) MHz, DJ=0.718(1) kHz, DJK=139.36(7) kHz, and χ(14N)=−3.95(6) MHz. The nature of the observed spectra and the magnitudes of the B0 values and DJ values are interpreted in terms of a hydrogen-bonded geometry having C3v symmetry, with the nuclei in the order H3CCN⋅⋅⋅HCCH, with r(N⋅⋅⋅C)=3.425(1) Å and with the hydrogen bond stretching force constant kσ=4.7(1) Nm−1.
Spin uncoupling in the 6s Rydberg states of methyl iodide. Rotational subband structure in two-photon resonant absorption
Authors: Dagata J. A. , Scott M. A. , McGlynn S. P.
Published: July 2002

Rotational envelopes of some of the lowest-energy (R6s) Rydberg transitions [i.e., the two electronically degenerate B3E(1) and C1E(1) band systems and the totally symmetric A1(0+) band system] of methyl iodide are calculated for a two-photon absorption process and compared to the experimental spectra obtained using the technique of two-photon resonant multiphoton ionization (MPI). The rotational band shapes, which were computed with molecular constants extracted from a simulation of high-resolution, one-photon absorption data, reproduce all significant features of the experimental MPI spectra, including temperature and polarization dependencies. Since one- and two-photon processes are governed by different selection rules, the present results are an independent verification of spin uncoupling in these R6s states. These results, therefore substantiate the conclusion that the R6s electron uncouples from the molecular frame during the evolution of the Rydberg state and that, consequently, the molecular constants extracted from experimental band shape analysis pertain to the freely rotating, electronically degenerate, molecular–ion core.
The influence of intramolecular vibrational relaxation on the pressure dependence of unimolecular rate constants
Authors: Borkovec Michal , Straub John E. , Berne Bruce J.
Published: July 2002

We study the rate constant in a bistable two degree of freedom system subject to impulsive BGK collisions. We find that the rate constant is very sensitive to the coupling of the degrees of freedom in the low collision regime. RRKM theory often overestimates the low collision rise—especially at higher collision rates, where the rate constant rises much more slowly making the molecule appear to have fewer degrees of freedom. We argue that such effects should be seen in experiments extending over a very wide range of pressures.
Dynamics of H2O2 photodissociation: OH product state and momentum distribution characterized by sub-Doppler and polarization spectroscopy
Authors: Gericke Karl-Heinz , Klee Stefan , Comes Franz Josef , Dixon Richard N.
Published: July 2002

Hydrogen peroxide has been optically excited at a wavelength of 266 nm and the OH photofragment completely characterized by Doppler and polarization spectroscopy using the laser-induced fluorescence technique. The entire internal state distribution (vibration, rotation, spin, and Λ components), translational energy, angular distribution, rotational alignment, and vector correlations between rotational and translation motions of OH products is measured. The hydroxyl radicals are formed in the X 2Π3/2,1/2 ground state with 90% of the available energy (248 kJ/mol) being released as OH recoil translation. The angular distribution is nearly a sin2 θ distribution about the electric vector of the photolysis laser. The internal motion of OH is vibrationally cold (no vibrationally excited OH was found) while the rotational excitation in v″=0 can be described by a Boltzmann distribution with a temperature parameter of Trot=(1530±150) K. The two spin states are found to be populated nearly statistically, in contrast to the Λ components which show an increasing inversion with increasing OH rotation. The observed profiles of recoil Doppler broadened spectral lines are strongly dependent on the nature of the transition, the excitation–detection geometry, and the relative polarizations of the dissociating and analyzing laser light. However, the line intensities show only a minor dependence on geometry and polarization indicating a low alignment of OH photofragments (A(2)0≤0.1). For the first time the vector correlation between product rotational and translational motions was analyzed and evaluated in terms of the three bipolar moments β20(20), β00(22), and β20 (22). The bipolar moment β20(20) corresponds to the conventionally defined anisotropy parameter β=2β20(20) =−0.71. The angular distribution peaks in the direction perpendicular to the electric vector of the dissociating laser light, indicating the predominant electronic excited state in H2O2 being of 1A symmetry. The moment β00(22) increases

with JOH showing a bias towards vOH and JOH being parallel to one another. The moment β20(22) is a measure of the mutual correlation of the fragment translational and rotational vectors and the transition dipole vector μ in the parent molecule. The positive value of this moment [β20(22)=0.11] indicates that the expectation value of 〈J2y〉 should be very small when μ is parallel to the z axis and vOH perpendicular to μ (x axis). 〈J2x〉 originates in the torsional motion in the H2O2 parent molecule, while 〈J2z〉 reflects the bending vibration of nearly planar H2O2 where the H atoms are in the trans position.
Tricritical phenomena in quasi-binary mixtures. IV. Ternary ethane systems
Authors: Goh M. Cynthia , Specovius Jürgen , Scott Robert L. , Knobler Charles M.
Published: July 2002

Phase equilibria have been studied in binary and quasi-binary mixtures of ethane with higher alkanes (n-hexadecane to n-eicosane) near the tricritical temperature. We report the temperatures and pressures corresponding to the lower and upper critical end points of various mixtures. The compositions of the coexisting phases in the three-phase region of six mixtures were also studied. The shrinkage of the width of the three-phase region is consistent with the asymptotic classical theory of tricritical points, while an extended classical theory is necessary to account for the details of the coexistence curves. An examination of the deviations from the quasi-binary approximation shows that it is a very good approximation for the ethane+(n-heptadecane+n-octadecane) system.
Activated barrier crossing for many degrees of freedom: Corrections to the low friction Kramers result
Authors: Borkovec Michal , Berne Bruce J.
Published: July 2002

We evaluate corrections to the activated barrier crossing rate constant for the Kramers model at low friction γ for n strongly coupled degrees of freedom. The rate constant behaves as γ(1−const γ1/(n+1)+⋅⋅⋅) at low γ. As n increases the correction terms become increasingly important and they will substantially diminish the rate constant in a physically accessible low friction regime.
Local space approximation for treatment of impurities in polymers. Solitons in polyacetylene
Authors: Kirtman Bernard , DeMelo Celso P.
Published: July 2002

The local space approximation (LSA) is applied to neutral and positive solitons, S0 and S+, in polyacetylene. In the treatment presented here a solitonic fragment is embedded between two finite polyene side chains. On the basis of INDO model calculations we conclude that electronic structure properties of the infinite system can be represented with chemical accuracy by a C9 solitonic fragment bonded to C4 or C6 side chains in S0 and C10 side chains in S+. The local space for each bond is defined by a C6H4 interaction complex augmented, for S+, with the π orbital of one additional carbon on the solitonic fragment and three additional carbons on the side chain. These results establish the feasibility of using the LSA method to carry out accurate ab initio Hartree–Fock and CI calculations based on a finite chain model for the soliton.
Density functional theory of soft sphere freezing
Authors: Barrat J. L. , Hansen J. P. , Pastore G. , Waisman E. M.
Published: July 2002

The freezing transition of ‘‘soft spheres,’’ which interact via inverse-power potentials v(r)∼r−n, is systematically investigated for various values of the steepness n, using density functional theory. Two ‘‘second-order’’ theories are considered, which require as only input the static pair structure of the homogeneous fluid. Both theories overestimate the densities of the coexisting fluid and solid phases, and the disagreement with computer simulation data worsens as the steepness n decreases. The two theories also predict that the fcc phase is more stable than the bcc phase for all n, which contradicts the behavior expected for soft repulsions.
Polymer fracture in steady and transient elongational flows
Authors: Rabin Yitzhak
Published: July 2002

The scaling of the critical strain rate for fracture of polymers, observed in cross-slot-type elongational flow devices and predicted by recent models, differs from that obtained in flows through sudden contractions. We argue that while the former experiments are performed under quasi-steady-state conditions that lead to full stretching of the polymers, the transient nature of the latter results in a partial, nonuniform stretching of the chains. An expression for the scaling of the critical strain-rate for fracture of such partially stretched chains is derived and is shown to agree with experiments on elongational flows through contractions. Further experimental tests of the above model are proposed.
Burn–Green–Yvon (BGY) equation for the electric double layer in the dilute solution regime: A ‘‘nonlocal’’ closure
Authors: Bruno E. , Caccamo C. , Pizzimenti G.
Published: July 2002

By inspiring to some recent developments of density functional theory for inhomogeneous classical fluids we present a closure of the Born–Green–Yvon (BGY) equation for the electric double layer, which allows us to take into account nonlocality effects in the approximated form of the total inhomogeneous pair correlation function hi j (r1, r2). The proposed approach improves previously published (Ref. 1) BGY results for the electric double layer in the low concentrated electrolyte solution regime.
A comparison of two classical trajectory surface hopping methods for Na(2P)+H2,D2 quenching
Authors: Eaker Charles W.
Published: July 2002

Trajectory surface hopping calculations were performed on optimized diatomics-in-molecules surfaces to study Na(2P) collisions with H2 and D2 molecules (v=0, j=1) at four different translational energies (0.039, 0.062, 0.101, and 0.140 eV). Two methods were used to predict surface hopping: (1) transformation of the multidimensional surface intersection to a local one-dimensional curve crossing and calculation of the Landau–Zener transition probability, and (2) integration of the coefficients of the adiabatic electronic states to determine transition probability. For all initial conditions used in this work, we found that method (2) gave significantly larger quenching cross sections. Also in this paper we present results that show nonadiabatic coupling terms calculated by the diatomics-in-molecules method are in good agreement with ab initio values.
Pressure effects on kinetics and decay processes in argon under selective photoexcitation
Authors: Moutard P. , Laporte P. , Subtil J.-L. , Damany N. , Damany H.
Published: July 2002

Kinetics associated with the decay of low lying excited states of argon is investigated in a pressure range up to 4×106 Pa. Selective pulsed vacuum-ultraviolet excitation is used for the first time for a kinetic study of argon allowing a rather detailed analysis. The results are deduced from time resolved total luminescence of the well-known second continuum which is associated with the lowest excimer states 0+u(3P1) and 1u, 0−u(3P2), having fast and slow radiative decays, respectively. A different ratio of the fast to the slow component is observed when exciting in the high or in the low energy side of the first self-broadened line, the 1S0→3P1 transition centered at 106.6 nm, quite similar to what has been reported for xenon. The kinetic model introduced by Wenck et al. for xenon [Chem. Phys. Lett. 66, 138 (1979)] is the only one that agrees with the observed time and pressure dependences. In this model, the decay of the atomic state 3P1 leads to efficient population of 1u, 0−u(3P2), via 0+u(3P1), and 1g, 0−g(3P2), molecular states and the atomic state 3P2. The radiative lifetime of the (1u, 0−u) excimer state is presently determined to be 3.15(5) μs. The decay associated with 1P1 excitation is more complicated since it involves more intermediate steps. No difference is detected when exciting in the low or in the high energy side of the corresponding self-broadened line. That result, together with the time behavior, allows one to conclude that the atomic state 3P0 is efficiently populated in the decay sequence, thus introducing a rather long lived reservoir effect. Such an effect is totally different from what was reported for xenon in which collision coupling with the nearby 6p levels plays a major role.
Vibrational contributions to the second hyperpolarizability of CF4
Authors: Lu Z. , Shelton D. P.
Published: July 2002

The frequency dependence of the second hyperpolarizability (γ) of CF4 has been measured in the visible (488 nm<λ<660 nm) by means of gas-phase electric-field-induced second harmonic generation (ESHG). The results of these experiments are compared with the results of other nonlinear optics experiments which have measured γ by third harmonic generation (THG), ESHG, and the dc Kerr effect. In order to make this comparison, expressions for the vibrational contributions to γCF4 have been derived and numerically evaluated for each of these optical processes. This comparison indicates that vibrational contributions to γCF4 are significant for both the dc Kerr effect and ESHG.
Aggregation of anisotropic particles
Authors: Meakin Paul , Chen Zhong-Ying , Evesque Pierre
Published: July 2002

The diffusion limited aggregation of particles with anisotropic sticking probabilities has been investigated using computer models. All of our simulations have been carried out using 2d square lattices with square ‘‘particles’’ having two more sticky and two less sticky sides with sides of different kinds adjacent to each other. In both the limits of fast and slow particle rotation the anisotropy of the particles enhances the anisotropy of the square lattice and cross-shaped clusters (with side branches) are formed which resemble those generated by very much larger scale simulations of the regular DLA process. In the slow rotation limit a bias in the number of particles launched with sticky sides facing in the X or Y directions on the lattice leads to the formation of needle-shaped clusters whose radius of gyration (Rg) increases with cluster mass (M) according to Rg ∼M2/3.
Analytical representation of the excess thermodynamic equation of state for classical fluid mixtures of molecules interacting with α-exponential-six pair potentials up to high densities
Authors: Brown W. Byers
Published: July 2002

A method for generating analytical representations of the thermodynamic equation of state for classical fluids of molecules interacting with the α-exponential-6 potential is presented. The key step is the introduction in place of the three original variables α, β* =ε/kT, ρ* =Nr3m/V, of a new set of related variables αβ, βα, z=ρ* (αβ/ α)3 in terms of which the excess thermodynamic functions behave more smoothly. A particular 4×4×4=64 term representation valid up to the high densities and temperatures reached in shocks and detonations is presented, and is in excellent agreement with Monte Carlo simulations. It is 10–40 times more accurate than the corresponding representations in terms of the original variables, and is nearly 100 times faster to compute than the simplest of the statistical mechanical perturbation theories on which it is based.
Molecular dynamics study of binary soft-sphere mixtures: Jump motions of atoms in the glassy state
Authors: Miyagawa H. , Hiwatari Y. , Bernu B. , Hansen J. P.
Published: July 2002

Three very long runs of constant temperature molecular dynamics (MD) simulations, extending over typically 2×105 time steps, have been carried out for a state just above the glass transition of equimolar soft-sphere mixtures, with 500 atoms interacting through vαβ(r) =ε(σαβ /r)12, where σαβ =(σα +σβ )/2, and α,β (species indices)=1, 2. The ratio of diameters σ2/σ1 was chosen to be 1.2. Jump-type atomic motions are clearly found to occur in the glassy states. A coordinated (strongly correlated) jump motion was observed, where a cluster of several atoms dynamically linked at nearest-neighbor distances jump at successive close times. These simulations demonstrate that the characteristic time scale of structural relaxation becomes very long near the glass transition, and calculated mean square displacements exhibit complicated behaviors. Thermodynamic quantities, however, such as the mean pressure, energy, and pair distribution functions, appear to behave normaly, with no significant observable relaxations for increasing time, so that their averages are well defined.
Comparison of experiment and theory for the resonance Raman spectrum of I2 in solution. IV. Band shapes and hot bands
Authors: Sension Roseanne J. , Strauss Herbert L.
Published: July 2002

Each observed Raman transition of I2 in solution is made up of an overlapping series consisting of the fundamental, or an overtone, and its associated hot bands. As the excitation frequency is changed, these various components change their relative intensities, making it appear that the Raman transition changes frequency. The Raman excitation profiles of the separate transitions have been measured and the results compared to calculations. The results suggest the presence of heretofore uncharacterized I2 states—perhaps charge transfer states—that contribute to the Raman intensity. However, we cannot suggest a specific state that would be consistent with all the experimental results presented in this series of papers.
Solvent perturbations on the excited state symmetry of randomly oriented molecules by two-photon absorption
Authors: Mohler C. E. , Wirth M. J.
Published: July 2002

The polarization dependence of two-photon absorption is used to determine the excited state symmetry of solute molecules dissolved in room temperature liquids. Perturbations of excited state symmetry are interpreted as solvent induced state mixing in the solute. Using a first-order expansion in the zero-order system wave functions, an expression is derived which describes the effects of solute–solvent electric dipole interactions on the symmetry of the solute excited state. Measurements on the S0→S1 0–0 band of fluorene in nonassociating polar and nonpolar liquids are fit well by the derived expression, using continuum reaction field models. Deviations from the derived expression are found when fluorene is dissolved in liquids capable of self-association or complexation with aromatic solutes. In n-alcohol solvents, perturbations on the excited state symmetry of fluorene are adequately accounted for using Kirkwood g factors for the solvent.
Theoretical studies of oxygen rings: Cyclotetraoxygen, O4
Authors: Seidl Edward T. , Schaefer Henry F.
Published: July 2002

An analogy is constructed between the known composition of elemental sulfur (principally S8 rings) and the unknown oxygen rings. Due to the weakness of O–O single bonds, as in hydrogen peroxide, it is hypothesized that oxygen rings are potential high energy density materials. A particularly attractive candidate is the O4 molecule, for which ring strain is expected to provide further destabilization relative to two separated O2 molecules. To pursue these qualitative suggestions, ab initio molecular quantum mechanics has been employed. Both self-consistent field (SCF) and configuration interaction including single and double excitations (CISD) methods have been employed in conjunction with double zeta plus polarization basis sets. At the highest level of theory the nonplanar (D2d point group, O–O–O–O torsional angle 25°) equilibrium structure is predicted to lie 2.9 kcal below the planar D4h structure, which is a transition state. The infrared spectrum is predicted at the DZ+P CISD level, as well as lower levels of theory. The O4 minimum is predicted to lie ∼100 kcal/mol above the asymptotic limit of two O2 molecules.
The statistical mechanics of the ionic equilibrium of water: A computer simulation study
Authors: Guissani Y. , Guillot B. , Bratos S.
Published: July 2002

A theory is presented to study the ionic equilibrium of water. Its equilibrium constant is expressed in terms of the chemical potentials of H2O, H3O+, and HO− and is calculated by combining molecular dynamics and test particle methods. Intermolecular potentials are studied in detail. The theory predicts a pH of a correct order of magnitude. Polarization effects are surprisingly large; both the polarization of water molecules by the ion and the backpolarization of strong H bonds around the ion by water molecules play an important role and dominate the process under investigation.
Electronic states and potential energy surfaces of RhH
Authors: Balasubramanian K. , Liao Dai-Wei
Published: July 2002

Complete active space MCSCF (CASSCF) followed by first-order configuration interaction (FOCI) calculations are carried out on nine λ–s states of RhH. In addition, relativistic configuration interaction (RCI) calculations which include spin–orbit interaction are carried out on 19 ω–ω states of RhH. The potential energy surfaces and spectroscopic properties (Re,Te,ωe,De,μ) are computed for the electronic states of RhH.The calculated CASSCF/FOCI dipole moments of both the ground and excited states reveal considerable ionic character (Rh+H−) with the exception of the 1Δ state which is somewhat less ionic. The spin–orbit effects are found to be quite significant for the electronic states of RhH. The spin–orbit contaminations of the low-Ω states [1,1(II),1(III),1(IV),0+] are found to be appreciable. The spin–orbit splittings of low-lying electronic states were found to be 1200–3200 cm−1.
The torsional spectra of jet-cooled methyl substituted indoles in the ground and first excited states
Authors: Bickel Grant A. , Leach Gary W. , Demmer David R. , Hager James W. , Wallace Stephen C.
Published: July 2002

We have analyzed the jet-cooled torsional spectra of the S1–S0 transitions for the seven monomethyl indoles. The potential curves which describe the hindered internal rotation in both the ground and excited states are modeled with the form: V(φ)= 1/2  ∑n Vn(1−cos nφ). The changes in the torsional potential upon electronic excitation can be interpreted as arising from changes in the electron density adjacent to the methyl rotor. We present evidence that the methyl rotor is then a unique probe with which to obtain information about the detailed electronic structure of the parent indole molecule. This approach provides new evidence for the assignment of the first excited electronic state of indole observed in the vapor phase to 1Lb.
Photoion–photoion coincidence study of the fragmentation of the doubly charged N2+2 cation
Authors: Besnard M. J. , Hellner L. , Dujardin G. , Winkoun D.
Published: July 2002

Formation and fragmentation of the doubly charged N2+2 cation were studied by using the photoion–photoion coincidence (PIPICO) method. Six electronic states of N2+2 were observed. Their ionization energies were measured with uncertainties of about 0.3 eV, which allowed a tentative assignment of these states. It was furthermore demonstrated that N2+2 dissociates into N+(3P)+N+(3P) whichever initial electronic state is populated.
Dissociation dynamics of FCO and HCO radicals
Authors: Francisco J. S. , Goldstein A. N. , Williams I. H.
Published: July 2002

Dissociation energies and barriers to dissociation for XCO→X+CO have been calculated for X̃ 2A′ and Ã 2π states of FCO and HCO by ab initio molecular orbital methods. At the PUMP4//UMP2/6-311G* level, D○298 (F-CO)=22.3 kcal mol−1 and ΔH298=24.2 kcal mol−1 for dissociation of ground-state FCO; these values are much higher than the corresponding bond energy and activation enthalpy for HCO dissociation. Calculated RRKM rate constants suggests that the lifetime of FCO under stratospheric conditions is sufficient to allow bimolecular reactions to compete with dissociation. Reaction with O2 may provide an in situ source of stratospheric CO2.
High density Monte Carlo simulations of chain molecules: Bulk equation of state and density profile near walls
Authors: Dickman Ronald , Hall Carol K.
Published: July 2002

We introduce a new Monte Carlo method suitable for simulations of chain molecules over a wide range of densities. Results for the equation of state of chains composed of 4, 8, and 16 freely joined hard spheres are compared with the predictions of several theories. The density profile of the fluid in the vicinity of the wall, and the scaling of the pressure with chain length are also discussed.
The vibrational predissociation dynamics of ArCl2: Intramolecular vibrational relaxation in a triatomic van der Waals molecule?
Authors: Evard Dwight D. , Bieler Craig R. , Cline Joseph I. , Sivakumar N. , Janda Kenneth C.
Published: July 2002

Rotational–vibrational distributions are reported for Cl2 product states of the vibrational predissociation of the v=6 through v=12 levels of the ArCl2 molecule in the B electronic state. For v=6 and 7, the Δv=−1 dissociation channel dominates the predissociation dynamics, and the observed rotational distribution appears to be slightly inverted. This inversion does not appear to be due to ‘‘momentum gap’’ constraints but may be due to kinematic symmetry selection rules. Above v=7 the molecule decays via the Δv=−2 channel. Unlike the rotational distributions observed for HeCl2 and NeCl2, those of the ArCl2 Δv=−2 channel are highly structured and have a strong dependence on the initially excited vibrational level. It is argued that the dependence of the observed rotational distribution on the initial vibrational state (for the dissociation of ArCl2) is due to intermediate resonances in a sequential, IVR mechanism, whereas the dissociation of HeCl2 and NeCl2 occurs by a direct mechanism. Although this conclusion cannot be proved at this time, experiments and calculations are suggested that would directly measure the participation of intermediate levels.