bluuebunny/crossref_metadata_2025 huggingface dataset:
Gas dynamics resulting from laser vaporization of metals in one dimension. I
Authors: Bellantone R. , Hahn Y.
Published: July 2002

A mathematical model for the transient gas dynamics in one dimension resulting from laser vaporization of metals in the presence of an ambient atmosphere (air) is presented. The gas dynamics is analyzed by considering four regions: a transition zone between the melt surface and the bulk vaporized metal (known as the Knudsen layer), the bulk vaporized metal, the disturbed, and the undisturbed air. The ratios of the temperature and density across the Knudsen layer are modeled by constructing a velocity distribution which is a sum of two variably weighted Maxwellian distributions. The formation of a shock front is considered, and where appropriate the shock front location and velocity are included in the boundary conditions. Profiles in space and time for the velocity, temperature, pressure, and density are determined in a mathematically consistent way by appropriate matching across the Knudsen layer, the vapor/air interface, and the interface between the disturbed and undisturbed air (which may or may not be a shock front). The specific case of homentropic flow, in which the specific entropy in each region is constant, is taken up. The gas dynamics was completely determined by the ambient atmospheric conditions and the velocity νK of the vapor exiting the Knudsen layer, and relations were obtained for thermodynamic parameters in terms of the velocity and specific entropy. It was found that the specific entropy varied with νK, but slowly enough so that the change was perturbative over a reasonably large range of exit velocities. Results of some calculations are presented for a commercially available material. It is estimated that the homentropic model is applicable to vaporization due to laser beams in which the change in intensity is ≲0.1 MW/cm2. The model can be applied to relatively low intensity irradiation such as occurs during welding and slow drilling, and is valid near the center line of a laser beam, or in deep holes being drilled in which the sidewalls prevent lateral flow of vaporized material.
Ultraviolet Raman spectroscopy characterizes chemical vapor deposition diamond film growth and oxidation
Authors: Bormett Richard W. , Asher Sanford A. , Witowski Robert E. , Partlow William D. , Lizewski Robert , Pettit Fred
Published: July 2002

The Raman spectra of diamond and chemical-vapor-deposition (CVD) diamond films in the UV have been excited within the diamond band gap at 228.9 nm for the first time. The lack of fluorescence in the UV-excited Raman spectrum of diamond and CVD diamond films allows Raman spectroscopy to monitor the carbon-hydrogen (C-H) stretching vibrations of the nondiamond components of the CVD film as well as the third-order phonon bands of diamond. The relative intensity of the C-H stretching bands at ∼2930 cm−1 to the diamond first-order phonon band at 1332 cm−1 is proportional to the atomic fraction of covalently bound hydrogen in the CVD diamond film. The third-order phonon band intensity and frequency maxima are very sensitive to the size of the diamond crystallite. Its intensity decreases, and the maximum shifts to lower frequency as the size of the diamond crystallite decreases. It is shown here that UV Raman diamond measurements have significantly greater information content than visible Raman measurements.
Analysis of infrared attenuated total reflection spectra from thin SiO2 films on Si
Authors: Bjorkman C. H. , Yamazaki T. , Miyazaki S. , Hirose M.
Published: July 2002

Infrared attenuated total reflection spectra from thin SiO2 films sandwiched between a Ge prism and a Si substrate were investigated. The measurements were performed in the range of Si-O-Si stretching vibrations and compared with calculated spectra using bulk values for the SiO2 dielectric function. This comparison enabled confirmation of the experimentally observed peak broadening and peak shift of the longitudinal-optical-phonon mode at ∼1250 cm−1 for films thicker than 30 Å by using the exact expression for calculating p-polarized spectra. It is also shown that the linear approximation for vibrational spectroscopy in this frequency range is only valid for thicknesses less than 15 Å.
Structural and optical properties of annealed proton-exchanged waveguides in z-cut LiTaO3
Authors: Åhlfeldt H. , Webjörn J. , Thomas P. A. , Teat S. J.
Published: July 2002

The structural properties of annealed proton-exchanged waveguides in z-cut LiTaO3 have been investigated. Observations made in a polarizing microscope together with infrared-absorption measurements indicate the formation of monoclinic Li(1−x)HxTaO3 after proton exchange in pure pyrophosphoric acid. This monoclinic phase disappears after annealing at 300 °C. High-resolution x-ray diffraction shows that the c lattice parameter is 0.57% larger in the exchanged layer than in the bulk. During annealing the c parameter relaxes and in addition there appears a second diffraction peak in the x-ray rocking curves. This peak is attributed to α-phase Li(1−x)HxTaO3. A model for the increase of the refractive index is suggested. It shows that the main reason for the difference in index increase between proton-exchanged waveguides in LiTaO3 and LiNbO3 is the stronger spontaneous polarization of LiNbO3. The model predicts an increase of the refractive index of LiTaO3 after annealing which is experimentally observed. A short-term postannealing index decrease is suggested to be related to a ferroelectric domain reversal.
Diffusing arsenic vacancies and their interaction with the native defect EL2 in GaAs
Authors: Luken K. M. , Morrow R. A.
Published: July 2002

We model the indiffusion of arsenic vacancies and their interaction with the midgap electron trap EL2 in GaAs samples that occurs during unprotected and proximity high-temperature anneals. From fits to existing data we find the diffusive capture of VAs by EL2 to be inhibited by a large (≳1 eV) repulsive barrier of unknown origin. In conjunction with other results from the literature we estimate the diffusivity of VAs to be 4×10−3 exp(−1.8 eV/kT) cm2/s, a value uncertain by at least an order of magnitude.
Submicrometer x-ray beam production by a thin film waveguide
Authors: Lagomarsino S. , Jark W. , Di Fonzo S. , Cedola A. , Mueller B. , Engström P. , Riekel C.
Published: July 2002

A hard x-ray waveguide capable of reducing a 13 keV monochromatic beam in one direction from 30 to only 0.1 μm has been characterized with synchrotron radiation. The guiding structure consisted of a carbon layer sandwiched between two Ni layers sputter-deposited onto float glass. A beam exiting at the end of the guide in guiding direction was observed when a resonance effect due to the formation of x-ray standing waves in the total reflection region takes place. The measured beam divergence (1.3 mrad) agrees with expectations. The total efficiency (exiting photon flux/incident photon flux) can ideally exceed 10−2 but is only 10−4 for this first prototype.
Chemical treatment effect of Si(111) surfaces in F-based aqueous solutions
Authors: Adachi Sadao , Arai Takehiko , Kobayashi Kazuyuki
Published: July 2002

Chemically treated Si(111) surfaces in aqueous HF (1.5%), NH4F (40%), and HBF4 (42%) solutions at 20 °C have been studied using spectroellipsometry (SE), ex situ atomic force microscopy (AFM), and contact-angle measurement techniques. The SE data clearly indicate that the solutions cause the removal of the native SiO2 film with etch rates of ∼72 (HF), ∼13 (NH4F), and ∼8 Å/min (HBF4), respectively. The SE data also indicate that when the native oxide film is completely etch removed, the resulting surface is slightly roughened. The roughened surface thicknesses are estimated to be ∼3 Å, in reasonable agreement with the AFM rms values. The measured contact angles suggest that the HF- and NH4F-treated surfaces are hydrophobic, while the HBF4-treated surface is, if anything, hydrophilic.
Temperature dependent space-charge-limited currents in amorphous and disordered semiconductors
Authors: Schauer František , Nešpůrek Stanislav , Valerián Hynek
Published: July 2002

The temperature dependent space-charge-limited currents (SCLCs), manifested by the dependence of the apparent pre-exponential factor of space-charge-limited (SCL) conductivity on the injection dependent activation energy, is examined. Starting from the exact theory of SCLC, the formula for the activation energy of SCL injection dependent conductivity was derived. The method of SCL currents seems to be a unique method to determine the true electrical conductivity pre-exponential factor of the electrical conductivity. Systematic determination of the relation of the pre-exponential factor of the conductivity on its activation energy during the measurements of voltage and temperature dependencies of SCL currents gives information on the reliability of the reconstruction of the density-of-electronic states.
Magnetic and structural properties of Nd2Fe17−xMnx solid solutions
Authors: Ezekwenna P. C. , Marasinghe G. K. , James W. J. , Pringle O. A. , Long Gary J. , Luo H. , Hu Z. , Yelon W. B. , l’Héritier Ph.
Published: July 2002

A series of Nd2Fe17−xMnx solid solutions with x values between 0 and 6 were prepared and analyzed using magnetic measurements, neutron diffraction, and Mössbauer spectroscopy. All of the Nd2Fe17−xMnx samples crystallized in the Th2Zn17−x-type rhombohedral structure. The lattice parameters and unit cell volumes decrease with increasing manganese content up to ∼x equal to 2, and then increase for higher manganese content. The magnetizations of Nd2Fe17−xMnx decrease with increasing manganese content and Nd2Fe17−xMnx is paramagnetic at room temperature for x greater than 3. The Curie temperature in Nd2Fe17−xMnx solid solutions is maximum for x equal to 0.5 and decreases at a rate of ∼10° per substituted manganese up to x equal to 3, after which it drops sharply. These results are discussed in terms of the manganese site occupancies in Nd2Fe17−xMnx.
Linear dichroism and birefringence effects in magnetic fluids
Authors: Xu M. , Ridler P. J.
Published: July 2002

The purpose of the paper is to explain the linear birefringence and dichroism observed in magnetic fluids when subjected to a magnetic field. Linear aggregates of nanosize near spherical particles, which are considered to be responsible for these phenomena, have been analyzed in terms of the classical oscillating dipole model where each particle is treated as a simple dipole under the action of the electric field of a light beam. The analysis is produced in terms of the number of particles in the aggregate for both spherical and slightly nonspherical particles. Optical anisotropy factors and expressions for the magnetic-field dependence of the birefringence and dichroism due to preexisting aggregates are given. Numerical calculations have been carried out for Fe3O4 particles. Finally, the formulae are applied to experimental data where a close fit is obtained.
Simulation of boron, phosphorus, and arsenic diffusion in silicon based on an integrated diffusion model, and the anomalous phosphorus diffusion mechanism
Authors: Uematsu Masashi
Published: July 2002

Boron (B), phosphorus (P), and arsenic (As) in-diffusion profiles were simulated based on an integrated diffusion model that takes into account the vacancy mechanism, the kick-out mechanism and the Frank–Turnbull mechanism. The simulations were done using just three parameters for B and P, and four parameters for As, each of which has a clear physical meaning and a physically reasonable value, with no additional ad hoc hypothesis. These parameters correspond to the diffusion of dopant species and of point defects that contribute to dopant diffusion. For the anomalous P diffusion profile, the vacancy mechanism governs the diffusion in the plateau region, while the kick-out mechanism governs it in the deeper region, where self-interstitials dominate in the kink region and P interstitials dominate in the tail region. This changeover from the vacancy contribution to the kick-out contribution is shown to be the mechanism for the appearance of the kink-and-tail profiles of P. Moreover, the comparison among B, P, and As diffusion is made to review the diffusion of these three dopants by means of a unified model.
An infinite order perturbation approach to gain calculation in injection semiconductor lasers
Authors: Ahmed Moustafa , Yamada Minoru
Published: July 2002

Nonlinear gain in injection semiconductor lasers is analyzed by applying an infinite order perturbation treatment to the density matrix analysis and taking in account the electron relaxation processes in single-mode operation. The infinitely expanded gain can be applied to future laser developments having much lower threshold current. Formulas for linear and higher orders of the expanded density matrix element and for their corresponding orders of the gain coefficient are investigated. The infinitely expanded gain is obtained in closed form which, in general, cannot be handled analytically. This closed form is simplified in the simple case of plane-wave fields and is approximated to the previously reported gain formulas within limits of perfectly homogeneous and inhomogeneous gain broadening. Numerical examples are given for the case of GaAs lasers having a specified intraband relaxation time. Based on these numerical results, simplified expressions are presented for linear and higher orders of the gain coefficient in terms of the injected carrier number. Furthermore, numerical criteria to truncate the infinite gain expansion for different higher orders are investigated based on the values of the lasing power. In the case of conventional semiconductor lasers, where the injection current is up to three times its threshold value, the third-order gain expansion corresponds to the infinitely expanded gain. However, the fifth-order expansion gives a more accurate description at higher current up to ten times the threshold. More exact gain analysis at higher ranges of the injection current requires higher orders in the gain expansion.
Mechanism of iron gettering in MeV Si ion implanted epitaxial silicon
Authors: Koveshnikov Sergei V. , Rozgonyi George A.
Published: July 2002

The gettering properties of the two damage regions, namely, a near-surface vacancy-rich region, Rp/2 and a buried layer of extended defects near the MeV Si ion projected range, Rp, have been quantitatively determined by means of secondary ion mass spectrometry and deep level transient spectroscopy techniques. To isolate the simultaneous and competitive gettering between iron and oxygen, the MeV defects were introduced into epitaxial silicon doped with boron. Czochralski Si samples were also used for comparison. Experiments on isochronal and isothermal annealing of Fe contaminated samples in conjunction with the rapid quenching procedure allowed us to establish the dominating gettering mechanisms in the two damage regions. Thus, gettering in the Rp region was driven by relaxation of supersaturated Fe, while the segregation-induced gettering mechanism was operative in the Rp/2 zone. As a result, the concentration of Fe, which was remained ungettered at elevated temperature, was well below the solid solubility limit. The major gettering parameters of MeV defects, such as gettering capacity, efficiency, and thermal stability have been determined.
Monodispersed Cr cluster formation by plasma-gas-condensation
Authors: Yamamuro S. , Sumiyama K. , Suzuki K.
Published: July 2002

Nanometer-sized Cr clusters in the size range of 7.6–13 nm have been produced by a plasma-gas-condensation-type cluster deposition apparatus, which combines a grow-discharge sputtering technique with an inert gas condensation technique. We have studied influences of the Ar gas pressure, PAr, and the Ar gas flow rate, VAr, on the size distribution of Cr clusters by transmission electron microscopy. Monodispersed Cr clusters are formed at both low PAr and low VAr. At low PAr, Cr clusters nucleate and grow only in the liquid-nitrogen-cooled growth region, and the deposition rate is rather low. At high PAr, on the other hand, a large amount of Cr clusters are formed even near the sputtering source, and the nucleation and growth occur over a wide region between the sputtering source and the growth region. Under this condition, the deposition rate is relatively high. Consequently, the formation mechanism of the monodispersed clusters is similar to that of monodispersed colloidal particles: The nucleation and growth processes are definitely separated and the coagulation of growing particles is prohibited. In the present experiments, these conditions are effectively attained by using a carrier gas flow and liquid-nitrogen-cooling of the cluster growth region.
Nanostructure magneto-optical thin films of rare earth (RE=Gd,Tb,Dy) doped cobalt spinel by sol–gel synthesis
Authors: Cheng Fuxiang , Liao Chunsheng , Kuang Junfeng , Xu Zhigang , Yan Chunhua , Chen Liangyao , Zhao Haibin , Liu Zhu
Published: July 2002

Nanocrystalline CoFe1.9RE0.1O4 [rare earth (RE=Gd,Tb,Dy)] films have been prepared by the sol–gel route, and their polar magneto-optical (MO) Kerr rotation and ellipticity have been determined in the spectral range of 4000–8000 Å. The films are composed of nanometer grains with the spinel structure. RE doped cobalt spinel ferrites are found to have dramatic changes in magnetic and MO properties, with increases in their coercive force and enhancement of the MO rotation in the Tb3+ doped sample.
Magnetic behavior of an array of cobalt nanowires
Authors: Garcı́a J. M. , Asenjo A. , Velázquez J. , Garcı́a D. , Vázquez M. , Aranda P. , Ruiz-Hitzky E.
Published: July 2002

Cobalt nanowires have been electrodeposited into the pores of Anodisc™ alumina membranes after placing on one side a layer of sputtered copper, which acts as electrode and substrate during the electrodeposition. Nanowires are 60 μm long, 170–220 nm in diameter depending on the size of the pores of the alumina membrane. This array of nanowires exhibits uniaxial magnetic anisotropy related to the particular shape of each individual nanowire. On the contrary to the expected behavior in a uniaxial magnetic system, the coercivity of the array exhibits a maximum when the applied field is in a perpendicular direction with respect to the easy axis. This magnetic behavior is analyzed considering dipolar interactions among nanowires, and the magnetization of the array is obtained as a function of the magnetic characteristics of each nanowire using an iterative method.
Coatings for optical pumping cells and extending the lifetime of hyperpolarized xenon
Authors: Breeze Steven R. , Lang Stephen , Moudrakovski Igor , Ratcliffe Chris I. , Ripmeester John A. , Simard Benoit , Santyr Giles
Published: July 2002

We propose evaporative induction heating as a method for the reliable production of coatings for glass cells suitable for the optical pumping and storage of hyperpolarized xenon. The long spin-lattice relaxation times of hyperpolarized xenon-129 contained in cells coated with polyethylene or dotriacontane showed that high quality coatings could be prepared this way. Measurements on cells coated with deuteriated versions of these compounds showed that the expected improvement in performance with isotopic substitution did not occur. This leads to some questions about the level of understanding of wall-induced relaxation in polarization cells.
z-scan measurement of the nonlinear absorption of a thin gold film
Authors: Smith David D. , Yoon Youngkwon , Boyd Robert W. , Campbell Joseph K. , Baker Lane A. , Crooks Richard M. , George Michael
Published: July 2002

We have used the z-scan technique at a wavelength (532 nm) near the transmission window of bulk gold to measure the nonlinear absorption coefficient of continuous approximately 50-Å-thick gold films, deposited onto surface-modified quartz substrates. For highly absorbing media such as metals, we demonstrate that determination of either the real or imaginary part of the third-order susceptibility requires a measurement of both nonlinear absorption and nonlinear refraction, i.e., both open- and closed-aperture z scans must be performed. Closed-aperture z scans did not yield a sufficient signal for the determination of the nonlinear refraction. However, open-aperture z scans yielded values ranging from β=1.9×10−3 to 5.3×10−3 cm/W in good agreement with predictions which ascribe the nonlinear response to a Fermi smearing mechanism. We note that the sign of the nonlinearity is reversed from that of gold nanoparticle composites, in accordance with the predictions of mean field theories.
Thermal effects in pulsed laser assisted scanning tunneling microscopy
Authors: Gerstner V. , Thon A. , Pfeiffer W.
Published: July 2002

The thermal response of a tunneling tip after illumination of the apex with an ultrashort laser pulse of 1 ps duration is investigated. The finite element method is applied to calculate the resulting time-dependent temperature distribution and the thermal expansion taking into account the elastic properties of the tip material. The calculation reveals the three-dimensional movement of the tip apex. The expansion of the tip occurs within a few nanoseconds and after 10 μs the tip has almost reached its original length again. The bending of the tip due to the asymmetric illumination of the tip occurs on the same time scale and is of the same order of magnitude as the axial expansion. Under tunneling conditions the absolute magnitude of the expansion can lead to the formation of nanocontacts. This accounts for the laser induced nanostructuring of surfaces that has been reported in literature. The application of the thermal expansion as a fast switch for the tunneling current is proposed.
A dynamical study of the H+CH3→CH4 recombination reaction
Authors: Duchovic Ronald J. , Hase William L.
Published: July 2002

The Monte Carlo quasiclassical trajectory method is used to study the molecular dynamics of the H+CH3→CH4 association reaction. The bimolecular rate constant is calculated for a temperature of 300 K and compared with experimental measurements. Both the nature of the reactive trajectories and the bimolecular rate constant are very sensitive to the shape of the C–H stretching potential energy curve.
Electronic structure of the MoFe3S4(SH)3−6 ion: A broken-symmetry metal–sulfur cluster
Authors: Cook Michael , Karplus Martin
Published: July 2002

A set of MS-Xα calculations on the MoFe3S4 (SH)3−6 ion is reported. Three levels of theoretical models are considered: spin-restricted, C3v symmetry, which is most easily compared with the traditional ligand-field picture; spin-polarized, C3v symmetry; and spin-polarized, Cs symmetry. An MS=3/2 ground state corresponding closely to the experimental spin-quartet ground state of a single-cube MoFe3S4 cluster is found at the level of the Cs broken-symmetry model. This state has two antiferromagnetically coupled pairs of metal ions: a net majority-spin iron pair and a net minority-spin Mo–Fe pair. The spin densities on the three iron atoms are nearly equal, with 3.06 electrons of spin density on each Fe; Mo carries only a very small net spin, 0.4 electrons. Calculated Mössbauer quadrupole splittings and isomer shifts agree well with experimental results. The model suggests that a quintet ground state should be produced on reduction, consistent with experiment. The present MoFe3S4 cluster calculations are compared extensively with corresponding results for the Fe4S4 cluster.
Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of CH−2 and CD−2
Authors: Leopold Doreen G. , Murray Kermit K. , Miller Amy E. Stevens , Lineberger W. C.
Published: July 2002

Photoelectron spectra are reported for the CH2(X̃ 3B1)+e−←CH−2 (X̃ 2B1) and CH2(ã 1A1)+e−←CH−2 (X̃ 2B1) transitions of the methylene and perdeuterated methylene anions, using a new flowing afterglow photoelectron spectrometer with improved energy resolution (11 meV). Rotational relaxation of the ions to ∼300 K and partial vibrational relaxation to <1000 K in the flowing afterglow negative ion source reveal richly structured photoelectron spectra. Detailed rotational band contour analyses yield an electron affinity of 0.652±0.006 eV and a singlet–triplet splitting of 9.00±0.09 kcal/mol for CH2. (See also the following paper by Bunker and Sears.) For CD2, results give an electron affinity of 0.645±0.006 eV and a singlet–triplet splitting of 8.98±0.09 kcal/mol. Deuterium shifts suggest a zero point vibrational contribution of 0.27±0.40 kcal/mol to the observed singlet–triplet splitting, implying a Te value of 8.7±0.5 kcal/mol. Vibrational and partially resolved rotational structure is observed up to ∼9000 cm−1 above the zero point vibrational level of the 3B1 states, revealing a previously unexplored region of the quasilinear potential surface of triplet methylene. Approximately 20 new vibration-rotation energy levels for CH2 and CD2 are measured to a precision of ∼30 cm−1 in the v2=2–7 region (bent molecule numbering). Bending vibrational frequencies in the methylene anions are determined to be 1230±30 cm−1 for CH− and 940±30 cm−1 for CD−2, and the ion equilibrium geometries are bracketed. The measured electron affinity also provides values for the bond strength and heat of formation of CH−2, and the gas phase acidity of CH3. A detailed description of the new flowing afterglow photoelectron spectrometer is given.
The higher order electron–electron coalescence condition for the intracule function for states of maximum spin multiplicity
Authors: Thakkar Ajit J.
Published: July 2002

The parallel spin components (hσσ) of the exact intracule function (h) are shown to satisfy the electron–electron coalescence condition (d3hσσ/du3)u=0 =(3/2)(d2hσσ/du2)u=0 for σ=α or β. This implies that the higher order coalescence condition (d3h/du3)u=0 =(3/2)(d2h/du2)u=0 (for S=‖MS‖=N/2) can be used as a nontrivial test of approximate wave functions for states of maximum spin multiplicity. This is particularly useful since the usual cusp condition h′(0)=h(0) is trivially satisfied with both sides equal to zero for such states. The higher order condition is used to test some explicitly correlated wave functions for the lowest triplet S and P states of the helium-like ions.
A stable linear reference potential algorithm for solution of the quantum close-coupled equations in molecular scattering theory
Authors: Alexander Millard H. , Manolopoulos David E.
Published: July 2002

We show how the linear reference potential method for solution of the close-coupled equations, which arise in inelastic scattering theory, can be reformulated in terms of an ‘‘imbedding-type’’ propagator. Explicit expressions are given for the blocks of the propagator matrix in terms of Airy functions. By representing these functions in terms of moduli and phases, in both classically allowed and classically forbidden regions, one can evaluate the propagator without any numerical difficulty. The resulting algorithm is tested on a highly pathological problem—the rotationally inelastic scattering of a polar molecule by a spherical ion at extremely low kinetic energy—and found to be completely stable.
Calculation of the tortuosity factor in single-phase transport through a structured medium
Authors: Lee Pil H. , Kozak John J.
Published: July 2002

In this paper we explore the interplay between the tortuosity factor and the porosity in influencing single-phase transport through porous media. We design a capillary model which incorporates features of both parallel and serial arrays of channels and calculate the first moment (the mean walklength) of the stochastic distribution function describing the passage of a permeant across the medium as a function of system size, boundary conditions, the surface density of entry and exit sites, and the internal structure of the medium. We show that for a porous medium with multiple entry/exit sites and a given specific permeability, the tortuosity factor increases monotonically with respect to the porosity (starting from a completely channeled flow), passes through a maximum, and then decreases with increase in porosity (to the limiting case of an entirely unrestricted flow). In the special case where the flow from the porous media is constricted (limited to a single exit channel), the tortuosity is found to be a strictly monotonically decreasing function of the porosity.
Analytic evaluation of energy gradients for the single and double excitation coupled cluster (CCSD) wave function: Theory and application
Authors: Scheiner Andrew C. , Scuseria Gustavo E. , Rice Julia E. , Lee Timothy J. , Schaefer Henry F.
Published: July 2002

The theory for the analytic evaluation of energy gradients for coupled cluster (CC) wave functions is presented. In particular, explicit expressions for the analytic energy gradient of the CC singles and doubles (CCSD) wave function for a closed-shell restricted Hartree–Fock reference determinant are presented and shown to scale as N6 where N is the one-electron number of atomic basis functions for the molecular system. Thus analytic CCSD gradients are found to be of the same magnitude in computational cost as is the evaluation of analytic gradients for the configuration interaction singles and doubles (CISD) wave function. Applications of this method are presented for the water molecule and the formaldehyde molecule using a double-ζ plus polarization (DZ+P) basis set. The CCSD equilibrium geometries, dipole moments, and, via finite differences of gradients, CCSD harmonic vibrational frequencies and infrared intensities are reported. For H2O these results are compared to analogous CISD, CISDT, CISDTQ, and experimental results, and it is found that the CCSD predictions are most comparable to those of CISDTQ for this particular system. For the case of H2CO, the CCSD results are compared to CISD and experimental predictions. In general, the CCSD results and timings are encouraging.
Semiclassical phase space evolution of Fermi resonance spectra
Authors: Kellman Michael E. , Lynch Eric D.
Published: July 2002

The evolution of the semiclassical phase space of a Fermi resonance spectrum is investigated as the strength of the resonance coupling is varied between zero and the strong coupling limit. The phase space evolution gives information beyond that contained in the phase space profile of the experimental spectrum alone. The zero-order phase space is found to be different in important respects from that of the pendulum model of a nonlinear resonance. In the weak coupling regime, the phase space evolution is essentially like that of a dynamical barrier picture. In the strong coupling regime of ‘‘intrinsic resonance,’’ the phase space structure is much different. Topology change appears to take place in a more discontinuous manner than in the weak coupling regime. The phase space evolution shows that some levels are problematic for an adiabatic switching treatment. The origin of some anomalous levels seen both in phase space profiles of experimental spectra and in semiclassical quantization studies is clarified.
Turing patterns in an open reactor
Authors: Vastano John A. , Pearson John E. , Horsthemke W. , Swinney Harry L.
Published: July 2002

Steady spatial chemical patterns have been found in model reaction–diffusion systems but have not yet been observed in any laboratory experiments. The reasons for this are discussed and the need for open reactors is stressed. A model open reactor is investigated in order to guide the experimental search for steady patterns. Specifically, Turing bifurcations in this reactor are studied for a simple autocatalytic chemistry (the Gray–Scott model) in order to determine the effects of varying diffusion coefficients, chemical time scales, and residence time. A description of all the steady-state bifurcations from an initially homogeneous state is obtained. The Liapunov–Schmidt reduction is used to determine the stability of the bifurcating solutions and a steady-state continuation technique is used to follow stable and unstable branches of bifurcating solutions.
Transport properties of ground state oxygen atoms
Authors: Holland Paul M. , Biolsi Louis
Published: July 2002

The transport properties of dilute monatomic gases depend on the two body interactions between like atoms. When two ground state oxygen atoms interact, they can follow any of 18 potential energy curves corresponding to O2, all of which contribute to the transport properties of the ground state atoms. Transport collision integrals have been calculated for those interactions with an attractive minimum in the potential by accurately representing ab initio quantum mechanical potential energy curves with the Hulburt–Hirschfelder potential. Repulsive ab initio potential energy curves have been accurately represented either with the exponential repulsive potential or with an exponential repulsive potential with an additional Gaussian term to model a shoulder-like feature on the repulsive wall. Results are given for viscosity, thermal conductivity, and diffusion and they are compared with previous theoretical calculations.
Monte Carlo lattice simulation of amphiphilic systems in two and three dimensions
Authors: Larson R. G.
Published: July 2002

Microstructures such as 2D micelles (in two dimensions) and lamellar arrays, cylinders, and spheres (in three dimensions) are allowed to self-assemble via Monte Carlo simulation of an idealized lattice model for amphiphile–oil–water systems. Energies, free energies, equilibrium phase diagrams, and solution microstructures are estimated by Monte Carlo sampling of configuration space. In two dimensions (2D) at a temperature at which oil units have only a 3% solubility in the water phase, amphiphile at 20% concentration solubilizes any ratio of oil and water into a single phase and at lower concentrations produces phases with ultralow interfacial tension between them. As expected, in 3D solubilization is weaker and interfacial tensions higher than in 2D at comparable temperatures. Nevertheless, 3D systems containing the longest amphiphile (four head and four tail units) shows ordered solution structures, such as micelles at modest amphiphile concentration and periodic lamellar and cylindrical structures at high amphiphile concentration that are analogous to those of diblock polymers mixed with low molecular weight homopolymers.
The melting line of the Weeks–Chandler–Anderson Lennard-Jones reference system
Authors: de Kuijper A. , Schouten J. A. , Michels J. P. J.
Published: July 2002

The purpose of this paper is to study the role of repulsive and attractive forces on the melting behavior of simple substances. The melting line of the purely repulsive Weeks–Chandler–Anderson Lennard-Jones reference system is calculated by Monte Carlo simulation. Comparison with the full Lennard-Jones system shows that attraction causes a constant shift of the melting line along the temperature axis. The pressure or temperature dependence of the calculated melting discontinuities ΔV and ΔH is compared with the equations recently obtained by van der Putten and Schouten.
Bound state eigenfunctions from wave packets: Time→energy resolution
Authors: Neuhauser Daniel
Published: July 2002

We present a method to obtain bound-state eigenfunctions in any arbitrary range of energies, by a Fourier resolution (from time to energy) of a real-time wave packet. The resolution is done simultaneously at a number of energies within the sought range, and the resulting vectors yield, after diagonalization, all bound-state eigenvalues and eigenfunctions within that range. The method is exemplified on a Morse potential: eigenfunctions for 18 high-lying states (n∼200) are obtained from resolution at 25 energies.
Intensities in local mode overtone spectra: Propane
Authors: Kjaergaard Henrik G. , Yu Hengtai , Schattka Bernhard J. , Henry Bryan R. , Tarr Allan W.
Published: July 2002

The gas phase vibrational overtone spectrum of propane is measured using conventional near infrared (NIR) spectroscopy for the ΔvCH=2–5 regions and intracavity dye laser photoacoustic spectroscopy (IDL-PAS) for the ΔvCH=5 and 6 regions. The peaks are assigned in terms of the local mode model. Experimental oscillator strengths are compared to values calculated for the CH-stretching components of the spectrum. The calculations use a harmonically coupled, anharmonic oscillator local mode model to obtain the vibrational wave functions, and ab initio MO calculations at the SCF level with a 6-31G* basis set to obtain the dipole moment function. The importance of intermanifold coupling is explored. The calculations can account for the fall-off in intensity with increasing v, and can give a reasonably quantitative account of the relative intensities of the individual peaks within a given vibrational manifold. The questions of the relative intensities of primary and secondary CH bonds, and of the relative intensities of different methyl CH bonds are also explored.
Kinetics of random sequential adsorption of rectangles and line segments
Authors: Vigil R. Dennis , Ziff Robert M.
Published: July 2002

Random sequential adsorption of nonoverlapping rectangles of arbitrary orientation onto a continuous plane was investigated by computer simulation. It is shown that the approach to the jamming limit of rectangles with various aspect ratios obeys the scaling law θJ−θ(τ)∼τ−1/3. Furthermore, the adsorption kinetics of rectangles approach those of line segments as the rectangle aspect ratio becomes large.
Enumeration of all compact conformations of copolymers with random sequence of links
Authors: Shakhnovich Eugene , Gutin Alexander
Published: July 2002

Exhaustive enumeration of all compact self-avoiding conformations of a chain of 27 monomers on the 3*3*3 fragment of a simple cubic lattice is given. Total number of conformations unrelated by symmetry is 103 346. This number is relatively small which makes it possible to make a numerically exact calculation of all thermodynamic functions this chain. Heteropolymers with random sequence of links were considered, and the freezing transition at finite temperature was observed. This transition is analogous to folding transition in proteins where unique structure is formed. The numeric results demonstrate the equivalence between random 3-dimensional heteropolymers and the random energy model found previously in analytical investigations. The possible application of these results to some problems of combinational optimization is discussed.
Quenching of angular momentum in the ground states of VC, NbC, VSi, and NbSi molecules
Authors: Hamrick Y. M. , Weltner W.
Published: July 2002

The electron spin resonance spectra of VC, VSi, NbC, and NbSi trapped in the solid rare gases (Ne, Ar, Kr) show, principally through the large negative values of Δg∥=g∥−ge, that the orbital angular moments of these molecules in their (proposed) ground 2Δ states are partially quenched. Observed metal hyperfine splittings also vary with the matrix gas. This unprecedented effect in rare-gas matrices is attributed to strong unsymmetrical interaction of these ionic molecules with the matrix such that each is situated in an orthorhombic crystal field. The crystal-field-splitting parameter δ is both molecule and matrix dependent, varying from approximately 1400 to 7800 cm−1.
The 3←0 CH stretch overtone of benzene: An optothermal study
Authors: Scotoni Mario , Boschetti Andrea , Oberhofer Nadia , Bassi Davide
Published: July 2002

The second CH stretch overtone of benzene at the rotational temperature of about 20 K has been measured in a supersonic molecular beam. Infrared absorption has been detected by means of the optothermal method. At least seven vibrational bands have been resolved in the wave numbers range from 8740 to 8870 cm−1. The main absorption peak is located around 8827 cm−1, but a significant absorption is observed also in ‘‘red’’ region of the spectrum, around 8770 cm−1. Experimental spectra have been compared with a theoretical calculation carried out following the method of Sibert et al. [J. Chem. Phys. 81, 1115 (1984)]. No free parameters have been included in the calculation in order to improve the fit with experimental data. Calculations do not provide a detailed quantitative description of experimental data. However the two spectra are qualitatively similar: (a) both are characterized by a main absorption structure located in the 8820–8840 cm−1 region and (b) calculations confirm the experimental observation that a significant part of the CH oscillator strength is borrowed by lines located on the ‘‘red’’ side of the main peak.
Molecular dynamics simulation of local chain motion in bulk amorphous polymers. I. Dynamics above the glass transition
Authors: Takeuchi Hisao , Roe Ryong-Joon
Published: July 2002

The local chain dynamics of bulk amorphous polymers above the glass transition temperature is studied by means of molecular dynamics simulations in the time scale up to 10−9 s. A model of polyethylene having a realistic density is used where a fully vibrational treatment is applied to the chains subject to nonbonded interactions between segments. Also used for comparison is a freely-rotating chain model with no torsional potential, but otherwise having the same features. Time-correlation functions of various vectors embedded in the polymer backbone are evaluated and the result shows a strong dependence of the relaxation on the direction of the vector in both models. The chain reorientation process is then analyzed in terms of the two separate components of the motion: one, the reorientation of the chain axis and the other, the rotation of the chain around it. On a short time scale, the reorientation process is affected directly by the conformational transitions. On the longer time scale, however, the relaxation of the chain axis is influenced significantly by the presence of surrounding chains. The surrounding chains act as a rigid ‘‘pipe’’ constraining the relaxation of the chain axis even after the correlation of the dihedral angles is lost. The chain is thus rapidly tumbling around the chain axis, while the chain axis itself slowly changes its direction according to the relaxation of the local structure.
Classical path surface-hopping dynamics. I. General theory and illustrative trajectories
Authors: Kuntz P. J.
Published: July 2002

A formulation of a trajectory surface-hopping method is presented which starts with some of the basic ideas of the standard method, extends these to include more than two states, and enlists the classical-path equations not only to propagate through the nonadiabatic region but also to effect the transitions, or surface hops, themselves. The latter is accomplished by letting the Hamiltonian matrix elements in the time-dependent Schrödinger equation become complex, allowing manipulation of the fluctuation in the various adiabatic state populations. The procedure automatically conserves total energy and angular momentum as well as probability, and ensures that energetically inaccessible states are not significantly populated. In regions of extended degeneracy, the method resembles the standard classical-path approach with no surface hopping. In fact, the evolution of the wave function can be controlled to behave either as in the surface-hopping extreme or in the pure semiclassical extreme, allowing the method to be tailored to suit individual systems. The procedure is illustrated by application to a well-studied collision induced predissociation, Ne+He+2→Ne++He+He, where vibration in the entrance channel, Ne+He+2, leads to the strong nonadiabatic behavior responsible for the large observed cross sections. A few preliminary calculations for singly charged argon trimer ions produced in the ionization process Arn→Ar+n→Ar+2+(n−2)Ar, demonstrate that the formulation can be easily extended to include many degrees of freedom.
Enhanced electron attachment to superexcited states of saturated tertiary amines
Authors: Pinnaduwage Lal A. , Christophorou Loucas G. , Bitouni Anna P.
Published: July 2002

Electron attachment measurements on excimer–laser-excited superexcited states (SES) of saturated amine compounds, and in particular on triethylamine (TEA), have been carried out employing a new experimental technique. A rate equation analysis based on a proposed model shows that the electron attachment rate constant for the SES is several orders of magnitude larger than that for the ground electronic state. The proposed mechanism for electron attachment to SES involves the capture of a near-zero-energy electron—(produced by the same laser pulse that produces the SES)—by a superexcited molecule to form a transient parent anion which subsequently dissociates producing a stable fragment anion. The similarity of the above mechanism to an electron-excited Feshbach resonance is indicated and a scheme for the identification of molecular systems that can be excited (via resonance-enhanced multiphoton excitation) to SES is outlined.
Laser induced dispersed fluorescence spectra of jet cooled NO2: The complete set of vibrational levels up to 10 000 cm−1 and the onset of the X̃ 2A1–Ã 2B2 vibronic interaction
Authors: Delon A. , Jost R.
Published: July 2002

The laser induced dispersed fluorescence spectra (L.I.D.F.S.) of jet cooled NO2 molecules in emission from 11 different vibronic levels located between 22 006 and 23 625 cm−1 was recorded. The corresponding variety of Franck–Condon accesses has allowed the observation of the complete set of the 191 lowest vibrational levels of the X̃ 2A1 ground state, located up to 10 000 cm−1. The vibrational band origins were measured to within typically 0.3 cm−1, assigned (n1, n2, n3), and fitted with a 24 coefficient Dunham expansion giving a typical residual error of 0.5 cm−1. Furthermore, the vibrationless Ã 2B2 level has probably also been observed at 9737 cm−1. A perturbative model of the X̃ 2A1–Ã 2B2 vibronic interaction has been used in order to calculate the energy shift of the high vibrational levels (around 10 000 cm−1) of the X̃ 2A1 state.