bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical properties of an ideal nonparabolic Fermi gas
Authors: Beresford R.
Published: July 2002

Formulas are derived for treating the carrier density and energy in isotropic nonparabolic bands using the framework of the Joyce–Dixon approximation [W. B. Joyce and R. W. Dixon, Appl. Phys. Lett. 31, 354 (1977)]. The approximations are worked out as a function of spatial dimension, including the quasi-one-dimensional case of a bulk semiconductor in a homogeneous magnetic field. Example calculations illustrate the utility of the methods for the narrow-gap III-V compounds, such as InAs and InSb, which are now being developed for device applications.
Laser-projection-patterned etching of GaAs in a chlorine atmosphere
Authors: Foulon F. , Green Mino , Goodall F. N. , De Unamuno S.
Published: July 2002

Laser-projection-patterned etching of gallium arsenide in a chlorine atmosphere performed with a pulsed KrF excimer laser (λ=248 nm, τ=15 ns) and deep uv projection optics (resolution: 2 μm) is reported. The etching process carried out in a vacuum system having a base pressure of 10−6 mbar is shown to result from a purely thermochemical reaction. Local laser surface heating results in the desorption of the chlorination products (AsCln and GaCln; n=1–3) formed between laser pulses. The results of a thermal model that gives the temporal evolution of the surface temperature during the laser pulse are reported: They have been used to calculate the activation energy of the etching process namely, a ΔE=0.35 eV. The influence of the etching parameters (laser energy density, gas pressure, and pulse repetition rate) on the etch rate was investigated. The effect of the laser irradiation in vacuum, Ar, and Cl2 ambient on the surface morphology is also reported. The spatial resolution of the etching process is shown to be controlled by the heat spread in the semiconductor surface and by the nonlinear dependence of the etch rate on the surface temperature. As a result, etched features smaller or larger than the photomask features are achieved depending on the laser energy density. Resolution down to 4 μm with a system having an optical limit of 2 μm was achieved at low fluences.
The critical current density in high fields in epitaxial thin films of Y1Ba2Cu3O7−δ : Flux pinning and pair breaking
Authors: Hampshire D. P. , Chan Siu-Wai
Published: July 2002

The critical current density (Jc) of two epitaxial thin films of Y1Ba2Cu3O7−δ has been measured as a function of temperature in magnetic fields up to 27 T. A detailed study of one of the films shows that when the transport current is in the a-b plane and the magnetic field is orthogonal to the c axis, the functional form of Jc is consistent with both a polynomial (flux pinning) description and an exponential (pair breaking) description. In contrast, when the current is in the a-b plane and the applied field is parallel to the c axis of the film, Jc is unambiguously exponential of the form Jc=α(T)exp[−μ0H/β(T)]. These results explain why there are conflicting suggestions in the literature as to the mechanism that determines Jc in high magnetic fields in thin films for J⊥H⊥c axis and provide evidence that pair breaking operates for J⊥ H∥c axis. However, the standard phenomenological superconductor-normal-superconductor junction model derived in the clean limit for the normal metal cannot describe this pair-breaking mechanism.
Longitudinal Kerr effect enhancement of a 384 track head for high data rate readout
Authors: LeTexier F. , Maillot C.
Published: July 2002

In a multitrack readout device, a transducer using the longitudinal Kerr effect performs a high data rate and high resolution playback of a conventional magnetic tape. The magneto-optical transducer is described. The enhancement of the transducer figure of merit and efficiency in order to improve its data rate performance is discussed. The thicknesses of the magnetic and gap layers are deduced from a matrix calculation and give a factor of merit enhanced up to 3.9×10−3. The soft magnetic properties of a very thin (30 nm) Sendust layer are carefully controlled (Hc=2 Oe) and allow to achieve experimentally a transducer efficiency of 30% at 50 kBPI. This head efficiency and the enhanced figure of merit allow the magnetic tape readout at 500 Mb/s using a low cost light source.
Optical study of (AlxGa1−x)0.5In0.5P/GaAs semiconductor alloys by spectroscopic ellipsometry
Authors: Lee H. , Klein M. V. , Aspnes D. E. , Kuo C. P. , Peanasky M. , Craford M. G.
Published: July 2002

The critical points of (AlxGa1−x)0.5In0.5P semiconductor alloys grown by metal-organic chemical vapor deposition and lattice matched to GaAs have been measured at room temperature in the 1.5–6.2 eV spectral region using spectroscopic ellipsometry. We have performed standard lineshape analysis of the second derivatives of pseudodielectric function data to obtain the standard critical point parameters (peak position, broadening parameter, amplitude, and excitonic phase angle) as a function of x. The properties of the E1 and the E1+Δ1 spectral structures have been observed for the first time. The anomalously small amplitude of the contribution of E1+Δ1 spectral feature relative to that of E1 is attributed to the k-linear interaction between the Λ4,5v and Λ6v valence bands, which is large in III-P alloys due to the very small spin-orbit splitting. The bowing parameter of Δ1 is determined and discussed. The critical point parameters smoothly interpolate between those of the two ternary endpoints, Ga0.5In0.5P and Al0.5In0.5P. These parameters are discussed and compared with those of AlxGa1−xAs, (AlxGa1−x)0.5In0.5As/InP, and InxGa1−xAsyP1−y /InP.
On the question of intersubband electric quadrupole transitions in quantum well structures
Authors: Sa’ar A.
Published: July 2002

Intersubband electric dipole transitions between subbands of the same symmetry are not allowed in symmetric quantum well structures. When a small asymmetry is introduced this selection rule is broken, and the forbidden transitions become allowed. However, we have found that in many cases where the asymmetry of the structure is not too large, the intersubband electric quadrupole transitions dominate the optical transitions. For example, we show that, when the asymmetry is induced by an external dc-electric field, then, the electric quadrupole transitions dominate up to fields of the order of 10 kV/cm. It is also shown that for structures with a ‘‘built-in’’ asymmetry, there is a wide range of structures where the electric quadrupole transitions dominate the optical transitions. The selection rules for these transitions differ from those of the electric dipole transitions. It is found that the maximum strength of the electric quadrupole transitions is achieved when the infrared light is linearly polarized in 45° relative to the growth direction (in the quantum well plan), and therefore, the 45° waveguide geometry is optimized for inducing these transitions.
Reply to ‘‘Comment on ‘Atomistic models of vacancy-mediated diffusion in silicon’ ’’ [J. Appl. Phys. 78, 2362 (1995)]
Authors: Dunham Scott T.
Published: July 2002

Antoncik has criticized both the conclusions and elements of the modeling approach in a previously published article on dopant diffusion in silicon (S. T. Dunham and C. D. Wu [J. Appl. Phys. 78, 2362 (1995)]). As is shown in this reply, those criticisms are without merit and the alternative explanation proposed to explain the rapid diffusivity increase observed at high donor concentrations is based on an analysis which contains a series of critical errors.
Sampling criteria for resonant antennas and scatterers
Authors: Yaghjian Arthur D.
Published: July 2002

From a knowledge of the characteristic constant of an antenna or scatterer, simple expressions are found for the modal bandlimits and near-field sample spacing in planar, cylindrical and spherical coordinates required to reconstruct the fields everywhere outside the antenna or scatterer (except possibly within a small fraction of a wavelength from discontinuities in the surface of the antenna or scatterer). For resonant antennas and scatterers the modal bandwidths can be much larger and the required sample spacings much smaller than for nonresonant antennas and scatterers. The modal bandwidths also determine upper bounds on the gain and effective area of antennas, and on the total and bistatic scattering cross sections of scatterers. The maximum possible radius of the significant reactive fields of an antenna or scatterer is shown to equal the radius of the maximum possible effective area of an antenna, or the radius of the maximum possible total scattering cross section of a scatterer. For resonant antennas and scatterers, this radius can be much larger than the radius of the sphere circumscribing the physical antenna or scatterer.
The optical processes in AlInP/GaInP/AlInP quantum wells
Authors: Ishitani Yoshihiro , Minagawa Shigekazu , Kita Takashi , Nishino Taneo , Yaguchi Hiroyuki , Shiraki Yasuhiro
Published: July 2002

The optical processes in AlInP/GaInP/AlInP quantum wells free from long-range ordering are examined by photoluminescence (PL), photoluminescence excitation (PLE), and photoreflectance (PR) measurements. The PL method observes lower transition energy than the PLE and the PR methods which observe the space-averaged transition energy. This is because PL originates from localized lower-energy states to which an exciton state relaxes. By analyzing these measurements carried on 20- and 60-Å-wide wells, the reason for this deviation is ascribed to the fluctuation of the transition energy due to the local variation of the well width by one molecular layer. The plausible share of the band offset for the conduction band against the energy gap difference at the Γ point is obtained by comparing the experimentally obtained relative position of the energy levels in AlInP barriers and the 20 Å GaInP well with the calculated ones. This is found to be 0.75 (±0.06).
New neutron magnetic low index leaky waveguide coupler
Authors: Pogossian S. P. , Ben-Youssef J. , Le Gall H. , Desvignes J. M. , Menelle A.
Published: July 2002

In the present paper a new kind of neutron De Broglie wave leaky coupler-guide structure is proposed. This new magnetic coupler allows only one neutron polarization to be coupled into a magnetic neutron thin film waveguide. It is shown theoretically that this new thin film coupler decrease considerably the losses due to the roughness of the interfaces of the multilayer structure. Moreover a transmitted resonance peak is observed in a wavelength region accessible only for guided neutron waves. Thus a magnetic neutron coupler is proposed and a transmitted polarized neutron guided wave is observed experimentally.
Bulk damage effects in irradiated silicon detectors due to clustered divacancies
Authors: Gill K. , Hall G. , MacEvoy B.
Published: July 2002

High resistivity silicon particle detectors will be used extensively in experiments at the future CERN Large Hadron Collider. The detectors will be exposed to particle fluences equivalent to ∼1014 (1 MeV neutrons)/cm2, causing significant atomic displacement damage. A model has been developed to estimate the evolution of defect concentrations and the electrical behavior of irradiated silicon detectors using Shockley–Read–Hall (SRH) semiconductor statistics. The observed increases in leakage current and doping concentration changes can be described well after Co60-gamma irradiation but less well after fast neutron irradiation. A possible non-SRH mechanism is considered, based on the hypothesis of charge transfer between clustered divacancy defects in neutron damaged silicon detectors. This leads to a large enhancement over the SRH prediction for V2 acceptor state occupancy and carrier generation rate which may resolve the discrepancy between the model and neutron damage data.
Short wavelength (1–4 μm) infrared detectors using intersubband transitions in GaAs-based quantum wells
Authors: Liu H. C. , Buchanan M. , Wasilewski Z. R.
Published: July 2002

We explore the possibility of covering the short wavelength infrared region using intersubband transitions in GaAs-based quantum wells. We investigate InGaAs wells with AlAs thin confining barriers. For this type of double-barrier resonant-final-state detectors, the dark current decreases with increasing detection wavelength. Photocurrents due to intersubband transition are observed down to a wavelength of about 1 μm. The spectra also reveal interesting physical effects apparently related to the indirect band minima at the X point. The responsivity for the 3–4 μmm detectors reaches up to 0.01 A/W; and the background limited temperature is in the range of 80–100 K.
Systematic study of basis set superposition errors in the calculated interaction energy of two HF molecules
Authors: Schwenke David W. , Truhlar Donald G.
Published: July 2002

We have calculated the interaction energy of two HF molecules at the single-configuration Hartree–Fock level using 34 different basis sets in an effort to assess the reliability and usefulness of the counterpoise correction to account for basis set incompleteness. We find large counterpoise corrections for all configurations studied, and we show that using a large enough basis set so that the counterpoise correction is small does not guarantee accurate results. Furthermore even for smaller basis sets the inclusion of counterpoise corrections does not systematically improve the accuracy of the calculations.
Nascent PO(X 2Π) E,V,R,T excitations from collision-free IR laser photolysis: Specificity toward the PO(X 2Π1/2) spin-orbit state
Authors: Chou Jim-Son , Sumida David S. , Wittig C.
Published: July 2002

PO (X 2Π) is produced via the collision-free infrared multiple photon dissociation (IRMPD) of volatile organophosphorous molecules, and is detected by two-frequency two-photon ionization, using the B 2Σ+ state to provide a spectral signature from which X 2Π populations are obtained. Sequential dissociations occur during the IR laser photolysis, in which nascent fragments continue to undergo IRMPD, and PO (X 2Π) accrues from a series of bond fission reactions. Nascent vibrational, rotational, and translational excitations are in sensible accord with this mechanism, except for a few rotational states near J=19.5. Unlike the nuclear degrees of freedom, the PO (X 2Π) spin-orbit states are populated quite selectively. The 2Π3/2 state, lying only 224 cm−1 above the 2Π1/2 ground state, contains only ∼11% of the population, compared to 34% for a 300 K sample. This result is unambiguous; it persists with all precursors, laser fluences, etc., and is verified by comparisons to spectra obtained using a microwave discharge, a flame, and when thermalizing nascent excitations with an inert diluent. This result underscores the importance of the separate potential surfaces which correlate to the product spin-orbit states, and the small amount of 2Π3/2 population can be accounted for by nonadiabatic coupling during dissociation, and/or ‘‘freezing’’ the amount of S1 character in an excited precursor in which S0 and S1 are coupled nonradiatively. We note that such electronic specificity should be dealt with in the analogous recombination reactions.
Localized orbital/local origin method for calculation and analysis of NMR shieldings. Applications to 13C shielding tensors
Authors: Hansen Aage E. , Bouman Thomas D.
Published: July 2002

A theory of NMR shielding tensors is derived from Ramsey’s expressions, using the framework of the random phase approximation (RPA) and localized molecular orbitals. By expanding angular momentum terms relative to a local origin for each orbital and using properties of the RPA solutions, we arrive at shielding expressions that contain no reference to an overall gauge origin and that lead to appropriate damping of basis set errors in contributions from distant groups. The expressions allow an analysis of the shielding into intrinsic bond and bond–bond coupling contributions. The resulting method is a variant of the coupled-Hartree–Fock approach. Ab initio results are presented for 13C isotropic shieldings and shielding tensors in a number of organic molecules ranging in size up to benzene. These results agree very well with experiment, for both isotropic shieldings and principal tensor components, even in double-zeta basis sets. For some low symmetry molecules we study the asymmetry of the shielding tensor and show that the antisymmetry can be quite large even if the anitsotropy is moderate. The bond analyses shed light on the effects of molecular structural features on the shieldings and their anisotropies, and emphasize in particular the conformational effects in bond–bond coupling contributions.
A weakly bound metastable state of Cr2: Possible evidence for a double minimum ground state
Authors: Moskovits M. , Limm W. , Mejean T.
Published: July 2002

When matrix isolated Cr2 molecules are pumped with radiation corresponding to the x 1Σg → A1Σu absorption a new absorption appears belonging to a metastable species of Cr2, whose lifetime ranges between 0.2 to 0.7 s according to the matrix used and the matrix site pumped. A two-color experiment produced the resonance Raman spectrum of this state which in solid argon has vibrational constants ωe=78.6 and ωeXe=0.4 cm−1. The new state is interpreted as being either a member of the ground state manifold of states with high spin multiplicity, or more likely a long-bond form of Cr2 corresponding to the molecule trapped in the outer minimum of a double minimum ground state potential. If so the lifetime observed is the inverse rate of photon-assisted tunneling from the outer to the inner minimum.
Rotational analysis of the Ã ← X̃ system of C2H
Authors: Curl R. F. , Carrick P. G. , Merer A. J.
Published: July 2002

Precise frequencies for the observed lines of five bands of the Ã 2Π ← X̃ 2Σ system of C2H are reported. These frequencies have been least-squares fitted with an effective rotational Hamiltonian and the resulting parameters are given.
The mechanism of orientation of acridine in oriented polyethylene
Authors: Parikh D. , Phillips P. J.
Published: July 2002

Spectroscopic and thermal analytical techniques have been used to study the system polyethylene-acridine at solute concentrations of up to a few percent. All data are consistent with the epitaxial growth of acridine microcrystals on the surfaces of the crystalline microfibrils of oriented polyethylene. Acridine microcrystals could not be detected in an unoriented polymer suggesting that the critical lattice match needed for epitaxial growth occurs only when the (110) planes of polyethylene are exposed due to slip during mechanical deformation. Polyethylene is known to crystallize epitaxially on crystals of anthracene and other fused ring compounds providing corroboration of the basic mechanism. It is suggested that specific adsorption related to the mechanism of epitaxial growth of microcrystals may be the major orienting mechanism for, at least, symmetrical fused ring compounds.
Molecular dynamics of narrow, liquid-filled pores
Authors: Magda J. J. , Tirrell M. , Davis H. T.
Published: July 2002

Molecular dynamics studies are reported for a 6-12 Lennard–Jones liquid in pore channels ranging from about 2–12 molecules wide. The pore walls are modeled as flat surfaces interacting with the fluid molecules via a continuous potential varying only with perpendicular distance from the wall. Liquid density profiles, solvation forces, interfacial tensions, and self-diffusion coefficients along the pore axis were computed. The density profiles indicate multilayer adsorption in the pore, whereas the locally defined diffusion coefficients do not vary significantly across the pore. The pore-averaged diffusivity as well as the solvation force oscillate with varying pore width at constant chemical potential. For pore widths greater than ten molecular diameters, the average diffusion coefficient is almost equal to its bulk value, and the solvation force equals the bulk pressure. In the smaller pores the mean square displacement normal to the pore walls never achieves linearity in time, and thus does not reach a diffusive limit. Thermodynamic equations relating the solvation force to the interfacial tension are derived, and the appropriate mechanical expressions for these quantities are identified. Simulation results are shown to be consistent with these thermodynamic equations. The simulations presented here will be useful in the development of the theory of fluid structure and transport in the tight pores occurring in such materials as vicor glass, clay dispersions, and biological pores and membranes.
Photoionization of single rotational levels in excited B 2Π, C 2Π, and D 2Σ+ states of 14N16O
Authors: Rottke H. , Zacharias H.
Published: July 2002

Rotational state selective fluorescence excitation and two-color resonantly enhanced ionization of nitric oxide has been investigated under collision free conditions. Single photon absorption from a tunable VUV laser source prepared the molecules in the intermediate B 2Π, C 2Π, and D 2Σ+ states energetically close to the dissociation limit of NO. The dissociation of C 2Π(v′=0) at J′=2.5(2Π3/2) and J′=3.5(2Π1/2) as well as B 2Π(v′=7) at J′=4.5(2Π3/2) and J′=7.5 (2Π1/2) was clearly established. Time delayed photoionization was used to measure the lifetimes of stable and weakly predissociative states. Photoionization cross sections of single rotational levels in the electronically excited states have been determined by saturating the ionization step.
Rotational structure of ammonia N–H stretch overtones: Five and six quanta bands
Authors: Coy Stephen L. , Lehmann Kevin K.
Published: July 2002

The five and six quanta bands of the N–H overtone bands of NH3, with origins near 15 450 and 18 110 cm−1, respectively, have been rotationally analyzed. Assignments of 318 and 105 lines in the two regions have been made on the basis of microwave-detected microwave-optical double resonance spectra and of photoacoustic absorption spectra. Both regions contain hybrid bands with parallel and perpendicular components which show Coriolis interactions between the optically bright components, and frequent, generally patternless, interactions with background dark vibrational states. These interactions cause almost all lines to be broken into several components. The five quanta region also shows a strong Fermi interaction, suggested due to 4νN–H+3νa2, which causes a negative inversion splitting in the upper states. The relative strengths of the parallel and perpendicular bands are (0.3:0.7) and (0.35:0.65) for the five and six quanta regions, in contrast to lower energies where parallel bands are weak or absent. The band structure can be qualitatively explained on the basis of a local mode description as the A and E states with five and with six quanta of stretch in one of the N–H bonds. Rotational constants of these bands can be well predicted from projections based on the single quanta excitations in contrast to the bands lower in energy.
Dissociation dynamics of H2 on Ni(100), Ni(110), and Ni(111) surfaces
Authors: Lee Chyuan-Yih , DePristo Andrew E.
Published: July 2002

A realistic potential energy surface (PES) for the interaction between H2 and Ni surfaces was obtained using a many body expansion approach. Its analytical form is suited for any surface of the Ni crystal so that the parameter values obtained from experimental results or ab initio calculations on one surface can also be used for other faces. We have estimated the parameter values from ab initio cluster calculations on the H2–Ni (100) system. The PES for (100), (110), and (111) surfaces are presented. Using classical trajectories, we have calculated the dissociative sticking probabilities for all three surfaces at a variety of initial translational energies, vibrational states, rotational states, and incident angles. The dissociative sticking probability for the (110) surface is insensitive to the translational energy while that for the (111) surface increases with increasing translational energy. These features are in good agreement with experiments. We also predict that the (100) surface has a dissociative sticking probability which is insensitive to translational energy and is smaller than the (110) surface. The former feature is not in accord with experiment. The reflected molecules are shown to have low rotational energy even for high initial rotational states. Attributes of both the dissociative sticking probabilities and the reflected molecules are correlated with features of the PES. Most notably, we show that very good scaling of the dissociation probability with normal kinetic energy can arise even when the barrier to dissociation is not in the entrance channel (i.e., translational motion perpendicular to the surface) but in the exit channel (i.e., bond stretching of the H2).
Tunneling conduction in disordered dissipative systems
Authors: Phillips Philip
Published: July 2002

We consider in this paper the incoherent transport via tunneling of an electron among randomly distributed impurity centers in a heat bath. The heat bath is modeled as a collection of harmonic oscillators. A general expression is first derived using standard instanton methods for the distance dependence of the rate of tunneling between two spatially separated impurity centers coupled to a heat bath. We find that there are two leading terms in the tunneling rate: (1) the standard e−r/r0 from the wave function overlap and (2) the dissipation correction e−ηr2,η the friction of the medium. Using this rate, we solve the pair approximation to the master equation for incoherent tunneling transport among N randomly distributed impurity sites and obtain the time dependent diffusion coefficient and the site return Green function. From the long-time limit of the return Green function, we show that as a result of dissipation, there is at long times strictly no diffusion when d=1, 2 at low impurity concentrations. A crossover region from nondiffusive to diffusive transport is shown to exist when d=3 that is determined by the magnitude of the friction, η. We discuss the relationship between these results and classical percolation.
Applications of the generator coordinate approximation to diatomic systems. I. The hydrogen molecular ion
Authors: Deumens E. , Ohrn Y. , Lathouwers L. , Van Leuven P.
Published: July 2002

Calculations in the generator coordinate approximation are presented for vibration–rotation levels of the hydrogen molecular ion in its ground electronic state. The numerical results confirm general theoretical predictions made earlier and demonstrate that the generator coordinate approximation is a workable nonadiabatic approach to molecules.
Hyperfine structure of the BaI X 2Σ+ and C 2Π states
Authors: Ernst W. E. , Kändler J. , Noda C. , McKillop J. S. , Zare R. N.
Published: July 2002

Optical-microwave double resonance measurements were carried out to find the hyperfine structure constants of the v=0 level of the BaI X 2Σ+ state. These were combined with sub-Doppler optical measurements of the BaI C 2Π–X 2Σ+(0,0) band in order to derive the hyperfine structure constants of the excited state. We have determined the following molecular constants (in MHz) where the numbers in parentheses represent one standard deviation in a least squares fit: for the BaI X 2Σ+ state, γ″=75.8501(33), b″=93.117(19), c″=52.170(54), and eQq″=−33.62(12), and for the BaI C 2Π state, a′=263(53), b′+c′=−430(212), d′=−66.7(1.4), and eQq′=−214(11). The Fermi contact interaction and the electric quadrupole coupling constants for both the BaI X and C states appear to arise from the distortion of closed-shell I− orbitals by the field of the Ba+ ion. In the BaI X state, the charge distribution on the Ba+ center is directed away from I− while in the C state toward I−.
Energy diffusion in many-dimensional Markovian systems: The consequences of competition between inter- and intramolecular vibrational energy transfer
Authors: Straub John E. , Berne Bruce J.
Published: July 2002

We examine the dynamics of barrier crossing using a two-dimensional Langevin equation in the low friction regime. We find that as the friction on the nonreactive mode is increased there is a transition from two-dimensional Markovian dynamics to one-dimensional non-Markovian dynamics resulting in a qualitative change in the behavior of the rate constant as a function of friction. This result supports the conclusion that existing theories which predict rates for energy activation in Markovian and non-Markovian systems have limited ranges of validity and are not as general as had previously been supposed. A method is proposed which analyzes the eigenvalues of the full system and provides a criterion for the validity of the various theories. These ideas are then used to interpret the results of a recent experimental study. In the Appendix we discuss, in some detail, the role of a random force in aiding intramolecular energy transfer.
Photodissociation of methylnitrite: State distributions, recoil velocity distribution, and alignment effects of the NO(X 2Π) photofragment
Authors: Brühlmann U. , Dubs M. , Huber J. Robert
Published: July 2002

The photodissociation of CH3ONO→CH3O+NO at 363.9 nm generates vibrationally, rotationally, and translationally excited fragments (fNOvib =3%, fNOrot =15%, fNOtrans =33%). By means of two-photon LIF and sub-Doppler spectroscopy in combination with various polarization schemes of dissociation and probe lasers, the nascent NO(X 2Π) photofragment was characterized with respect to state distributions and three-dimensional recoil velocity distribution. Furthermore, the rotational alignment and the Λ-state populations were determined. Through the rotational alignment dependence of Doppler profiles probed by differently polarized transitions (R and S), the J–v vector correlation was assessed. Based on these results, stereochemical and dynamical information about the dissociation was obtained which shows that the fragmentation process is planar and takes place within 210 fs.
Quantum mechanical rate constants in the presence of linear dissipative mechanism
Authors: Mohanty Udayan
Published: July 2002

The path integral representation of the flux–flux autocorrelation function, whose time integral yields the quantum mechanical rate constant, is analyzed. It is shown that if (a) the coupling between the system and the bath is linear in the system coordinates, (b) β=∞, and (c) the classical equation of motion is, in Fourier-transformed form, K̂(ω)q(ω)=[−∂V̂/∂q(q)](ω), where V̂(q) is a conserved potential, then there is a simple algorithm for writing down the imaginary time effective action functional which determines the flux–flux correlation function.
Multipole moments, polarizabilities, and hyperpolarizabilities for N2 from fourth-order many-body perturbation theory calculations
Authors: Maroulis George , Thakkar Ajit J.
Published: July 2002

All multipole moment, polarizability, and hyperpolarizability tensors up to the fourth rank are calculated for the ground 1Σ+g state of N2 at its equilibrium bond length. These properties are obtained from fourth-order Mo/ller–Plesset perturbation theory energies of N2 in the presence of various configurations of point charges. Electron correlation was found to affect the longitudinal components the most. Some of the anisotropies of these tensors change by as much as 105% upon inclusion of electron correlation. The results are in good agreement with all previous reliable theoretical and experimental values. The calculated values of the quadrupole–quadrupole (C) and dipole–octopole polarizabilities, and the dipole–dipole–quadrupole (B) and dipole–dipole–dipole–dipole (γ) hyperpolarizabilities are the most accurate ones available. Our best vibrationless estimates of the isotropic averages of these quantities are C̄=40.371 e2 a40 E−1h, B̄=−149 e3 a40 E−2h, and γ̄=830e4 a40 E−3h.
Rotational temperature dependences of gas phase ion–molecule reactions
Authors: Viggiano A. A. , Morris Robert A. , Paulson John F.
Published: July 2002

A technique for measuring the rotational temperature dependences of gas phase ion–molecule rate constants is presented. The technique involves measuring the kinetic energy dependences of the rate constants at several temperatures in a variable temperature selected ion flow drift tube. For a monatomic ion, comparing the rate constants at the same center of mass kinetic energy at different temperatures yields the dependence of the rate constant on the internal temperature of the reactant neutral. For neutrals in which the vibrational modes are inactive at the temperatures of the experiment, the internal energy dependence is the rotational temperature dependence. Two examples are presented here, one in which rotational energy significantly influences the rate constants, approximately T−0.5, and one in which it does not. Implications for past drift tube experiments are discussed.
Scattering from a classically chaotic repellor
Authors: Gaspard Pierre , Rice Stuart A.
Published: July 2002

We report a study of the classical scattering of a point particle from three hard circular discs in a plane, which we propose as a model of an idealized unimolecular fragmentation. The system possesses a fractal and chaotic metastable classical state. On the basis of a coding of the system dynamics, we develop a method to construct the invariant probability measure and to calculate the particle escape rate, the Hausdorff dimension, the Kolmogorov–Sinai entropy per unit time and the mean largest Lyapunov exponent of the repellor. The relations between these characteristics of the system dynamics are discussed. In particular, we show that, in general, chaos inhibits escaping from the metastable state. The theory is compared with numerical simulations. We also introduce the classical tools necessary for the semiclassical quantization of the dynamics; the latter is discussed in the following paper.
Elastic free energy in swollen polymer networks
Authors: Geissler E. , Horkay F. , Hecht A. M. , Zrinyi M.
Published: July 2002

Polymer networks with sparse chemical crosslinks are composed in general of deeply interpenetrating coils. The microscopic statistical features of such gels are defined by three characteristic lengths: Rt, the separation between the end (junction) points of each component coil; Rs, the distance between two spatially nearest neighbor junction points; and ξ, the polymer–polymer correlation length. These distances are related through R2t =AR3s/ξ, where A is a numerical constant that depends upon the network structure. While Rt varies extremely weakly with polymer volume fraction φ, and ξ depends strongly on φ as predicted by scaling theory, only Rs deforms affinely (i.e., as φ−1/3) upon swelling. The swelling pressure in such a system consists of two parts, mixing and elastic. The mixing part for each coil contains a factor (Rt0/Rt)2 to allow for correlations in the concentration, where Rt0 is the unperturbed end-to-end distance. This situation is the same as for polymer solutions. For the elastic part of the swelling pressure, experimental observations are described that prove the following equivalences, in lightly crosslinked polyvinyl acetate gels: (1) the shear modulus is equal to the volume elastic modulus; (2) the ratio of the shear modulus of a gel swollen in a theta diluent to that of the same gel swollen to the same degree in a good solvent is a constant equal to the ratio of the corresponding temperatures, T/θ. It is concluded that the individual component coils make no significant contribution to the network elasticity, the latter being provided only by the mutual folding of the interpenetrating coils.
The structure of the CO2–CO2–H2O van der Waals complex determined by microwave spectroscopy
Authors: Peterson K. I. , Suenram R. D. , Lovas F. J.
Published: July 2002

Rotational spectra of CO2 –CO2 –H2 O, CO2 –CO2 –D2 O, 13 CO2 –13 CO2 –H2 O and CO2 –CO2 –H2 18 O have been measured using a pulsed-molecular-beam Fabry–Perot Fourier-transform microwave spectrometer. An asymmetric top spectrum is observed with rotational constants, A=3313.411(5) MHz, B=1470.548(3) MHz, and C=1308.850(3) MHz for the normal species. The dipole moment obtained is μT =μb =1.989(2) D. Only b-type transitions are observed with the transitions showing a 3 to 1 intensity alternation depending on whether Ka +Kc is odd or even, respectively. This indicates a structure with twofold symmetry with the C2v axis of the water subunit aligned with the C2 axis of the complex. The CO2 subunits lie in a plane which is perpendicular to the C2 axis and located 2.47 Å below the oxygen atom of the water subunit; the C–C bond length is 3.413(2) Å. The orientation of the CO2 subunits in CO2 –CO2 –H2 O is very similar to that observed in CO2 –CO2 although the C–C bond length is 0.19 Å shorter in the trimer. The C–O bond distances between the H2 O and two CO2 subunits are both 3.00(2) Å which is 0.16 Å longer than that found in the CO2 –H2 O dimer. The hydrogens of the H2 O subunit are directed away from the CO2 –CO2 plane although their angular orientation around the b axis is not well determined.
Nondipole light scattering by partially oriented ensembles. III. The Müller pattern for achiral macromolecules
Authors: Tian Duan , McClain W. M.
Published: July 2002

The Müller matrix representing elastic scattering of light by an isotropic suspension of particles has long been known to contain eight vanishing elements among its total of sixteen elements, provided that the particles have no handedness. We generalize the proof of this property to the case of anisotropic suspensions of particles, so that it may apply to symmetric biostructures such as viruses, undergoing electrophoresis. The matrix for the partially oriented case generally contains four crucial pieces of structural information missing from the randomly oriented case. In particular, the circular intensity difference scattering (CIDS) element M14 may be nonzero for an oriented ensemble of particles that have no helicity at all.
Solvent effects in weak electrolytes. II. Dipolar hard sphere solvent and the sticky electrolyte model with L=σ
Authors: Zhu Jianjun , Rasaiah Jayendran C.
Published: July 2002

The sticky electrolyte model in a dipolar solvent is studied in this paper. A detailed separation of the Ornstein–Zernike (OZ) equations and their solution in the mean spherical (MS) approximation for binding (or stickiness) at L=σ are given. The results derived earlier by Adelman and Deutch, Blum et al. and by Ho/ye et al. in this approximation are reproduced when the stickiness is switched off. Also when the density of the solvent goes to zero, the results reduce to those of the sticky electrolyte model (SEM) in a continuum solvent. It is found that the PY/MS approximation gives negative solutions for the association parameter λ, while the HNC/MS approximation works in a narrow interval of the sticky potential well depth ε2 between the positive and negative ions. As expected, the ion association increases when sticky potential well becomes deeper, but the dipole moment of the solvent is found to have a strong screening effect on this association. The study of the radial distribution functions of this system shows that the probability of a free ion appearing near a counter ion is greatly decreased when binding occurs between the oppositely charged ions at the contact; the opposite happens for ions of the same sign. The absolute value of the ion solvation energy becomes smaller as the electrolyte concentration increases and when stickiness between oppositely charged ions is introduced.
Kinetic isotope effects for hydrogen diffusion in bulk nickel and on nickel surfaces
Authors: Rice Betsy M. , Garrett Bruce C. , Koszykowski Michael L. , Foiles Stephen M. , Daw Murray S.
Published: July 2002

Diffusion coefficients for H, D, and T on a Ni(100) surface and in bulk Ni are calculated using variational transition state theory with semiclassical ground-state transmission coefficients using two potential energy surfaces obtained by the embedded atom method (EAM). The original EAM potential reproduces experimental bulk diffusion coefficients, but greatly overestimates the diffusion coefficients for H and D on Ni(100). By refining the empirical potential parameters, a new EAM potential is developed that accurately reproduces both the bulk and surface diffusion coefficients. The variational transition state theory calculations are used to analyze the unusually low (compared to gas phase) H/D kinetic isotope effects for diffusion in bulk and on the Ni(100) surface. For the temperature range for which experiments have been carried out, quantum mechanical tunneling contributes negligibly to the diffusion and, in these cases, the kinetic isotope effect is determined largely by the change in zero-point energy between the reactant equilibrium binding sites and the transition state. A near equality of the reactant and transition state zero-point energies leads to the unusually low kinetic isotope effects. Using the same refined EAM potential energy surface, the energetics of diffusion on the Ni(111) and Ni(110) surfaces are also presented. The barriers for diffusion on these two surfaces are sufficiently low, about 1.0 kcal/mol, that the approximation of uncorrelated hops needed to relate the computed hopping rate to the diffusion coefficient is suspect. Although diffusion coefficients were not computed, based upon an analysis of the zero-point energies at the reactants and saddle points, we predict that the H/D kinetic isotope effects for diffusion on these two surfaces will also be close to unity.
The intramultiplet mixing of Zn(43P
J) by collisions with 4He and 3He
Authors: Umemoto Hironobu , Masaki Akira , Ohnuma Toshiharu , Takayanagi Toshiyuki , Sato Shin , Misaizu Fuminori , Fuke Kiyokazu
Published: July 2002

The intramultiplet mixing processes of Zn(43PJ ) by collisions with 4He and 3He were studied by employing pulsed laser techniques. The following cross sections were obtained: Zn(43P1)+4He→Zn(43P2)+4He: 3.1; Zn(43P1)+4He→Zn(43P0)+4He: 1.5; Zn(43P1)+3He→Zn(43P2)+3He: 4.7; Zn(43P1)+3He→Zn(43P0)+3He: 1.8; in units of 10−17 cm2. A quantum close-coupling calculation was carried out by assuming various interaction potentials. The cross sections for the production of Zn(43P2) from Zn(43P1) could be well reproduced by the calculation for both 4He and 3He. On the other hand, the calculated cross sections for the production of Zn(43P0) were found to be always smaller than the experimental ones.
Ab initio transition state theory calculations of the reaction rate for OH+CH4→H2O+CH3
Authors: Truong Thanh N. , Truhlar Donald G.
Published: July 2002

We have carried out ab initio calculations using Mo/ller–Plesset perturbation theory, scaling all correlation energy in second order (MP-SAC2) with several large basis sets, for the reaction OH+CH4→H2O+CH3. We found that correlation has a large effect on the geometry, barrier height, and vibrational frequencies of the transition state. The final calculated values, obtained with a correlation-balanced basis set, for the forward and reverse classical barrier heights are 7.9 and 21.2 kcal/mol, respectively. We have used these with transition state theory and an Eckart model for semiclassical tunneling calculations of the rate constants for the above reaction in the temperature range from 200 to 2000 K. We found that the present model, which requires information only at the reactants, transition state, and products, predicts rate constants of the same order of magnitude as the experimental data for this wide temperature range.
Theoretical characterization of the potential energy surface for H+O2 = HO*2 = OH+O. III. Computed points to define a global potential energy surface
Authors: Walch Stephen P. , Duchovic Ronald J.
Published: July 2002

Recent ab initio calculations have focused on the minimum energy path region of this surface [J. Chem. Phys. 88, 6273 (1988), Paper I] and on the saddle point region for H atom exchange via a T-shaped HO2 complex (J. Chem. Phys. 91, 2373 (1989), Paper II). In this paper, the results of additional calculations are discussed that, combined with the previously reported results, provide a global representation of the potential energy surface for this reaction. Complete active space SCF/externally contracted configuration interaction calculations (CASSCF/CCI) were carried out using the same wave function employed in II. The new calculations presented here characterize the potential energy surface for a variety of H atom approach angles, ranging from perpendicular to collinear (measured with respect to the O2 bond) and for a variety of H atom to O2 center-of-mass distances. Additionally, a new collinear exchange saddle point is reported. Using the ab initio results from these new calculations, optimal geometries, and harmonic frequencies based on local polynomial representations of the potential energy surface are reported along the constrained energy minimum path, while anharmonic frequencies are calculated at both the O2 and OH asymptotes and at the HO2 intermediate.
Many-body effects in molecular dynamics simulations of Na+(H2O)n and Cl−(H2O)n clusters
Authors: Perera Lalith , Berkowitz Max L.
Published: July 2002

Many-body effects were examined in a series of molecular dynamics computer simulations on the ionic aqueous clusters Na+(H2O)n (n=4,5,6,14) and Cl−(H2O)n (n=4,5,6,7,8,14). Two potential models were used in the simulations. In one model (TIP4P) the potential was pairwise additive, while in the second model (SPCE/POL) the many body effects were explicitly included through a self-consistent polarization routine. The two models produce equilibrium structures which are significantly different in energy and geometry. The SPCE/POL model consistently predicts energetically more stable products. In addition, for the anion cluster systems the SPCE/POL model places the Cl− on the surface of the water cluster.
Tertiary interactions in a model of the helix–coil transition
Authors: Ebeling Martin , Nadler Walter
Published: July 2002

Into the classical Lifson–Roig model of the helix–coil transition in homo(poly)amino acids, we introduce a simple long-range interaction which models hydrophobic contacts between secondary structure elements, yet at the same time, retains its essentially 1D character. Although its properties cannot be determined analytically anymore, the model can be studied readily by Monte Carlo computer simulations. We show that our model assumptions lead to a markedly changed behavior in the transition from random coil to ‘‘folded’’ states. Remarkably, even in a homopolymer, the additional long-range interactions stabilize structures resembling secondary structure elements of globular proteins in number and average size. We present results from our simulations which demonstrate a glasslike transition behavior of the homopolymer model as well as an on-site construction mechanism for the formation of α helices.
Coupled cluster theory for high spin, open shell reference wave functions
Authors: Knowles Peter J. , Hampel Claudia , Werner Hans-Joachim
Published: July 2002

The coupled cluster method restricted to single and double excitations (CCSD) is considered for the case of a spin restricted Hartree–Fock open shell reference determinant. A spin–orbital based formulation, in which the cluster operator spans exactly the minimal first order interacting space, is presented, and computationally optimal working equations are given. In the limit of a large number of closed shell orbitals, the cost is identical to that of an optimum treatment of an equivalent closed shell problem, which is obtained as a special case of the formulation presented. The theory is applied to the calculation of a number of diatomic potential energy functions and compared with spin-unrestricted theory.