bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and dynamical behavior of model fluids at high and low densities
Authors: Schaink H. , Hoheisel C.
Published: July 2002

The Lennard-Jones (LJ) and the ‘hard soft sphere’ (HSS) system are studied at high and very low density by molecular dynamics (MD) calculations. Comparsion of the various pair correlation functions shows the governing role of attractive forces at low densities and the dominating influence of repulsive forces at liquidlike densities. The results at low densities show clearly the ‘long-ranged’ structure of the LJ potential, which cannot be noticed at high densities. The well-known fact that the viscosity is proportional to the density at high density states and inversely proportional at low density is quantitatively studied. In particular, the relaxation times of the momentum and positional configurations are investigated by the momentum and force autocorrelation function as well as scattering functions. The results for the relaxation processes analyzed in terms of the momentum and force correlation functions differ completely from those studied by the collective scattering functions. Only the collective processes exhibit the density-dependent behavior assumed recently to explain the inversion of the density dependence of the viscosity constant by kinetic theorectical arguments.
Unoccupied electronic structure of phthalocyanine films
Authors: Rocco M. L. M. , Frank K.-H. , Yannoulis P. , Koch E.-E.
Published: July 2002

The unoccupied molecular orbitals of metal-free phthalocyanine (H2PC), nickel-phthalocyanine (NiPC), and copper-phthalocyanine (CuPC) adsorbed on single crystal surfaces have been studied by inverse photoemission spectroscopy (IPES) and near-edge x-ray absorption spectroscopy (NEXAFS). The features observed in the experimental data are assigned by reference to molecular orbital calculations. Carbon and nitrogen 1s NEXAFS measurements of NiPC adsorbed on Cu(110) surface indicates that the phthalocyanine molecular planes are parallel to the surface. X-ray absorption data recorded at the Ni LIII and LII edges show that the unoccupied Ni 3d level is located in the plane of the phthalocyanine rings.
Water vapor continuum in the millimeter spectral region
Authors: Ma Q. , Tipping R. H.
Published: July 2002

A theory is presented for the calculation of the continuous absorption of water molecules in the millimeter spectral region. The theory is based on a generalization of Fano’s theory in which the spectral density, the Fourier transform of the dipole-moment correlation function, is calculated for a system consisting of a pair of molecules. The internal states are written in terms of the line space of the system, and the resolvent operator is obtained using the well-known Lanczos algorithm. For the interaction between two water molecules, we include only the leading dipole–dipole term of the long-range anisotropic potential, and model the isotropic interaction, used to calculate the statistical weight within the quasi-static approximation, by a Lennard–Jones potential. Using reasonable values for the two Lennard–Jones potential parameters, and the known rotational constants and permanent dipole moment of a water molecule, we calculate the absorption coefficient for frequencies up to 450 GHz for temperatures between 282 and 315 K. The present results are in good agreement with an empirical model for the water continuum based on combined laboratory and atmospheric measurements. We conclude from our results that, contrary to some previous assertions, the strong negative temperature dependence as well as the magnitude of the continuum absorption, at least for the millimeter spectral region, can be explained in terms of the far-wings of allowed rotational transitions.
Rotational analysis of the (2000) stretching vibrational band system of 116SnH4
Authors: Halonen Marjo , Halonen Lauri , Bürger Hans , Sommer Silvia
Published: July 2002

A high resolution Fourier transform infrared spectrum, resolution (FWHM) 10.5×10−3 cm−1, of the (2000, A1/F2) stretching vibrational band system of 116SnH4 has been measured and analyzed in detail up to J=20. The rotational levels of the two vibrational states A1 and F2 are treated simultaneously by including vibration–rotation coupling terms in the Hamiltonian. 1290 infrared transitions from the ground vibrational state to the excited (2000) states have been included as data in the least squares optimization of 21 Hamiltonian parameters. The standard deviation of the fit obtained is 1.1×10−3 cm−1. The upper state vibrational energy level separation has been determined to be 0.0908 (5) cm−1 with the A1 state being higher in energy than the F2 state. The effective rovibrational parameters are found to satisfy closely the local mode constraints.
The effect of collisions in the multiphoton decomposition of 1-bromo 2-fluoroethane
Authors: McRae Glenn A. , Evans D. K. , Goodale J. W.
Published: July 2002

The multiphoton decomposition (MPD) of 1-bromo 2-fluoroethane, CH2BrCH2F, with a pulsed CO2 infrared laser is reported. The decomposition study was done at constant fluence and at pressures up to 2 kPa. Empirical representations of the pressure dependence of the MPD are reported. A collision-dependent reaction scheme is presented to explain the pressure dependence of the up-pumping through the vibrational manifold.
An experimental potential energy surface for internal rotation in glyoxal
Authors: Butz K. W. , Krajnovich D. J. , Parmenter C. S.
Published: July 2002

The torsional potential energy surface (TPES) for internal rotation of the CHO group in glyoxal CHOCHO has been derived experimentally by fitting observed energies for the torsional vibration ν7 in both trans- and cis-glyoxal to those calculated with the hindered rotation formalism of Lewis, Malloy, Chao, and Laane. The experimental energies were obtained from S1 → S0 single vibronic level fluorescence (SVLF) spectra of jet-cooled glyoxal. SVLF from the trans levels 73 and 74 plus the cis levels 00 and 51 yield the torsional vibrational energies of all 7n trans levels with n≤14 (except for 713) and that of the cis level 72. The energies of odd trans 7n levels with n≥7 as well as spectroscopic values of any cis 7n level were previously unknown. The best derived TPES fits the observed trans and cis levels to within 0.4 cm−1 except for two cases where the mismatch is 1.0 cm−1. The TPES is defined by the potential energy 2V=∑6n=1Vn (1−cos nφ) where φ is the torsional angle. For the best TPES, coefficients are (in cm−1) V1=1719.4, V2=1063.5, V3=−53.2, V4=−81.9, V5=21.3 and V6=2.9. For this TPES, the energy separation between the trans and cis potential wells is 1688 cm−1, the barrier to trans → cis internal rotation is 2077 cm−1 and the barrier to cis → trans rotation is 389 cm−1. As one moves from the trans well at φ=0°, the torsional barrier occurs at 110°.
Photodissociation near a rough metal surface: Effect of reaction fields
Authors: Das Purna C. , Puri Ashok , George Thomas F.
Published: July 2002

The modification of the photochemical dissociation rate of molecules in the presence of a rough metal surface is explored. Classical electromagnetic calculations are presented for the photodissociation rate of a point dipole near a rough surface modeled as a hemispheroidal bump on a semi-infinite flat plane. A correction is introduced by accounting for the reaction fields due to the dipole–substrate system radiating photons and coupling to delocalized surface plasmons. The effects of the shape and size of the bump, and the separation of the molecule from the bump on the rate of photodissociation of the molecule, are studied numerically.
A model of reactive dynamics in a detonation
Authors: Blais N. C. , Stine J. R.
Published: July 2002

Classical trajectories are used to examine the importance of many-body interactions in the chemical reactivity of condensed phase explosives under the high density conditions that are characteristic of a detonation wave. We have constructed a model based on the explosive liquid nitric oxide, and we examine how reactions occur when the system is compressed rapidly to about double liquid density. The probability of reaction is investigated with a realistic potential energy surface that is derived for six atoms but is equally applicable to four atoms. The model was found to have the proper energy characteristics to simulate an explosive material. We find that the probability of forming detonation products is higher for six atoms than for four atoms, ranging from a factor of 4 to a factor of 25 depending on the potential; more than can be accounted for on the basis of a statistical analysis. The details of the trajectories that lead to reaction products differ considerably between the four- and six-atom trials. We found that the four-atom system required higher compressions than the six-atom system for reactions to occur. This seems to indicate that the four-atom system needs a more symmetrical arrangement of bonds to react as compared to scarcely any symmetry requirements for six atoms.
Time scale dependence of diffusion in porous material: Dynamic light scattering and computer simulation
Authors: Guo Yihong , Langley Kenneth H. , Karasz Frank E.
Published: July 2002

Diffusion of polymer chains in liquid-filled pores of controlled pore size silica glasses was studied experimentally by using dynamic light scattering (DLS), and by a computer simulation based on a hydrodynamic theory of hard spheres in cylindrical pores. Dynamic light scattering at fixed scattering wave vector revealed faster apparent diffusion at short times (corresponding to diffusion within a single pore) followed by a slower relaxation which we attribute to macroscopic diffusion over distances large enough to average over the microscopic nonuniformities of the glass-pore matrix. The measured time at which the behavior crosses over from faster to slower diffusion was found to be independent of the light scattering wave vector, and is roughly equal to the time required for a polymer molecule to diffuse a distance comparable to the pore radius. At small ratio of polymer to pore radius, the crossover time was found to be proportional to the polymer radius, both in DLS measurements and in the computer simulation.
Polypeptide dynamics: Experimental tests of an optimized Rouse–Zimm type model
Authors: Hu Yi , MacInnis Jean M. , Cherayil Binny J. , Fleming Graham R. , Freed Karl F. , Perico Angelo
Published: July 2002

A theory for long time random coil peptide dynamics is developed based on a generalization of the optimized Rouse–Zimm model of Perico et al. [J. Chem. Phys. 87, 3677 (1987)] and Perico [J. Chem. Phys. 88, 3996 (1988) and Biopolymers 28, 1527 (1989)]. The generalized model employs the rotational potential energy for specific amino acid residues and amino acid friction coefficients to compute all input parameters in the model. Calculations of the fluorescence depolarization correlation function P2(t ) and of the local persistence length are found to be sensitive to the amino acid sequence, the length of the polypeptide chain, and the location of the probe. Model computations of P2(t ) are compared with new experimentally determined rotational correlation times (of the order of nanoseconds) from fluorescence depolarization measurements of three different synthetic 17-residue peptides, each containing a single tryptophan (TRP) residue as a probe. In addition, the previous anisotropy measurements on ACTH, glucagon, and their fragments are discussed and compared with the model calculations. Our results indicate that the theory gives a reasonable prediction for the fluorescence depolarization correlation times of random coil polypeptides, but the calculated rotational correlation function predicts a much faster initial decay and a slower final decay than is observed. Possible theoretical improvements are discussed.
Classical trajectory calculations of transport and relaxation properties for Ar–N2 mixtures
Authors: Gianturco F. A. , Venanzi M. , Dickinson A. S.
Published: July 2002

Classical trajectory calculations of transport and relaxation properties have been performed for Ar–N2 mixtures using the potential energy surface (PES) recently determined by Bowers et al. [J. Chem. Phys. 88, 5465 (1988)]. Generalized cross sections have been computed in the temperature range 77.3–1000 K. Extensive comparisons have been carried out with available measurements and with other calculations. The present system exhibits greater efficiency for rotational energy transfer (RET) processes and its interaction shows a deeper potential well than that of previously computed surfaces. A larger number of trajectories (up to 28 500 at the lowest total energy examined) has therefore been required to obtain converged results. The PES employed here shows impressive agreement with the available measurements for a wide variety of properties of the system and appears to be the most reliable currently available for Ar–N2 gaseous mixtures.
Infrared laser spectroscopy of H2 and D2 Rydberg states. II. Diode laser spectra and assignment of 5g–4f, 6h–5g, and 8i–6h systems
Authors: Davies P. B. , Guest M. A. , Stickland R. J.
Published: July 2002

Infrared diode laser absorption spectra of portions of the 5g–4f, 6h–5g, and 8i–6h Rydberg bands of H2 and D2 have been measured at Doppler limited resolution in low pressure A. C. discharges. The spectra, arising from L uncoupled states of H2 and D2, are assigned using an ab initio polarization model supported by intensity calculations. Details of the different implementations of this polarization model are given in the preceding paper. The most useful was the single channel vibrationally extended (1)/(2)  V6 model which became progressively better at higher n (and L). Results of multichannel calculations for a selected set of transitions are also reported.
Photodissociation dynamics and emission spectroscopy of H2S in its first absorption band: A time dependent quantum mechanical study
Authors: Dixon Richard N. , Marston C. Clay , Balint-Kurti Gabriel G.
Published: July 2002

A theoretical study of the photodissociation dynamics of H2S in its first absorption band is presented. The potential energy surfaces underlying the dynamics of the breakup process have been modeled so as to reproduce the principal features of all the available experimental data. The modeling is performed using time dependent quantum dynamical methods and involves the exact numerical solution of the time dependent Schrödinger equation. The fitting of the experimental observations requires the use of potential energy surfaces corresponding to two excited electronic states. We have been able to determine two such surfaces which reproduce the observed structure in the absorption spectrum, the main features of the emission spectrum of the dissociating molecule, and the vibrational distribution of the HS photofragments. The calculations utilize a recently developed method for analyzing the wave packet dynamics to extract the partial photodissociation cross sections. The photodissociation process is found to be consistent with an initial excitation to a single excited dissociative diabatic electronic state which is weakly coupled to another, whose principal role within the first absorption band is to introduce diffuse structure into the absorption spectrum and minor perturbations to the nuclear motion. The motion on the principal dissociative electronic surface is dominated by that in the two bond stretching coordinates, while that on the surface of the second perturbing state is a one dimensional vibrational motion which may correspond to either symmetric stretching or bending, but is most probably a complex combination of both. A new method is presented for performing the time dependent quantum mechanical calculation on two coupled potential energy surfaces when the motions on both surfaces are treated in a reduced dimensionality and the dynamically active coordinates on the two surfaces are different.
On the potential energy surface for collinear OH+2 (4Σ−)
Authors: Illas Francesc , Bagus Paul S. , Rubio Jaime , González Miguel
Published: July 2002

The potential energy surface for collinear OH+2 (4Σ−) has been investigated at the configuration interaction (CI) level using large primitive basis sets of Gaussian type orbitals (GTO) and the general atomic natural contraction (ANO). The CI calculations were carried out using the CIPSI algorithm and the accuracy established by comparison with full CI (FCI) test calibrations on the atoms and molecules of interest in the present work. For the reaction O+(4Su)+H2(1Σ+g)→OH+(3Σ−)+H(2Sg) the calculated value for the energy change of the reaction (ΔE) is −0.30 eV to be compared with an experimental value of −0.45 eV. The error in ΔE arises from the error in the ionization potential of oxygen and does only affect the reactants but not the products, while the discrepancy with the experiment is limited by the finite size of the basis set and not by truncations on the CI expansion. The collinear OH+2 minimum has been found to lie 0.338 eV below the products with an estimated accuracy of 0.01 eV.
Millimeter-wave spectrum of the aluminum monosulfide radical AlS (X 2Σ+)
Authors: Takano Shuro , Yamamoto Satoshi , Saito Shuji
Published: July 2002

The millimeter-wave rotational spectrum of the aluminum monosulfide radical AlS in its ground electronic state X 2Σ+ was observed with a source-modulated spectrometer combined with a high-temperature cell. The AlS radical was generated by a reaction between aluminum vapor and carbonyl sulfide. Eight rotational transitions, lowest N=4–3 and highest N=17–16, were measured in the frequency region of 67–284 GHz and 75 observed fine and hyperfine components were analyzed by least-squares methods. The analysis yielded the rotational and centrifugal distortion constants as well as the spin–rotation coupling constant with its centrifugal term and the hyperfine coupling constants including the nuclear quadrupole coupling term for the aluminum nucleus. The spin density of the unpaired electron was calculated by comparing the derived magnetic hyperfine coupling constants with the corresponding hyperfine coupling constants of the free aluminum atom. Applicability of this spin density calculation is discussed.
Effects of coordination environment on the Zr–F symmetric stretching frequency of fluorozirconate glasses, crystals, and melts
Authors: Phifer Carol C. , Gosztola David J. , Kieffer John , Austen Angell C.
Published: July 2002

A treatment based on Badger’s rule is used to estimate symmetric stretching frequencies νs of the nonbridging fluorides of various ZrFn groups in the presence of divalent and monovalent countercations. New vibrational spectra of MgZrF6, Ba2ZrF8, and β-BaZr2F10 are presented and incorporated into the method. The factors affecting νs are outlined and discussed in detail. Compilations of estimated frequencies are used to deduce the probable local structure around Zr ions in binary barium fluorozirconate glasses and in fluorozirconate melts containing lithium and sodium ions. It is concluded that the results of Raman spectroscopy indicate the presence of seven- and eight-coordinate Zr in some barium fluorozirconate glasses.
Vibrational energy level statistics of small molecules
Authors: Karrlein W.
Published: July 2002

A simple model Hamiltonian including kinetic coupling of adjacent bonds is chosen for investigations of the statistics of vibrational eigenenergies for a series of three- and four-atomic molecules. Depending on the coupling strength which is governed by structural characteristics of each molecule, the systems can be assigned to one of the two universal types of statistical behavior observed for energy levels: level clustering or level repulsion. Although the model is simple, the statistical measures are still sensitive enough to reveal significant differences between the molecules. These results are discussed in terms of the individual structural properties such as bond angles and atomic masses.
The torsional spectrum of CD3CD3
Authors: Moazzen-Ahmadi N. , McKellar A. R. W. , Johns J. W. C. , Ozier I.
Published: July 2002

The far-infrared spectrum of gaseous CD3CD3 has been measured in the region of the torsional fundamental under relatively large pressure-path length conditions. The observations were made using a modified Bomem spectrometer at a resolution of 0.016 cm−1, an absorption path of 20 m, and a gas temperature and pressure of 168 K and 80 Torr, respectively. The prominent features of the spectrum are the P, Q, and R branches of the torsional fundamental (v4=1←0) and the Q branch of the first torsional overtone (v4=2←1). The P and R branches of the fundamental consist of peaks about 0.2 cm−1 wide due to unresolved structure. In addition, a very high resolution (0.0014 cm−1) spectrum of the lowest lying degenerate fundamental ν9 of CD3CD3 was obtained. From the combined intensity and frequency analyses of the far-infrared spectrum and 442 lower state combination differences in ν9 and its associated hot band ν9+ν4−ν4, the effective leading coefficients Ṽ3=989.10(68) cm−1 and Ṽ6=9.47(29) cm−1 in the Fourier expansion of the hindering potential were determined. The rotational constant B was found to be 13 802.658(39) MHz and four distortion parameters were obtained. The torsional dipole constants μ⊥ and μ∥−μ⊥ were determined to be 7.41(74) and −19.9(2.0) μD, respectively. The experimental values of μ⊥ and μ∥−μ⊥ are shown to be in good agreement with values calculated from theoretical expressions. The shape of the torsional bands can be reproduced in detail with the model adopted here.
Spectroscopy of jet-cooled metal–monocyclopentadienyl complexes: Laser excitation spectra of calcium and cadmium cyclopentadienides
Authors: Ellis Andrew M. , Robles Eric S. J. , Miller Terry A.
Published: July 2002

A laser vaporization/photolysis technique for obtaining spectra of jet-cooled organometallic molecules is reported. It makes use of an excimer laser which vaporizes a metal sample and simultaneously photolyzes an organic precursor passing over the surface of the metal. The metal atoms and organic fragments then react to yield the organometallic species. In principle, this approach could be used to prepare a wide variety of gas-phase compounds of any metal for spectroscopic study. To illustrate its capabilities, laser excitation spectra of the monocyclopentadienide and monomethyl derivatives of Ca and Cd are presented. In the case of cadmium cyclopentadienide, this report constitutes the first spectroscopic observation of this molecule.
Hypermagnetizability anisotropy (Cotton–Mouton effect) for H2 and D2
Authors: Bishop David M. , Cybulski Sl/awomir M. , Pipin Janusz
Published: July 2002

Explicitly electron-correlated wave functions have been used to calculate the hypermagnetizability anisotropy (Δη) for H2 and D2. This property is the essential feature of the birefringence of a material in the presence of a magnetic field (the Cotton–Mouton effect). The calculations were carried out in the framework of perturbation theory and both dispersion and vibrational effects were fully taken into account. A detailed analysis of our results is made and it is concluded that electron correlation and ‘‘pure’’ vibrational effects are less important than vibrational averaging and dispersion. The experimental results are only in fair agreement with our theoretical ones.
A bond length reaction coordinate for unimolecular reactions. II. Microcanonical and canonical implementations with application to the dissociation of NCNO
Authors: Klippenstein Stephen J.
Published: July 2002

A method is described for implementing Rice–Ramsberger–Kassel–Marcus theory with the reaction coordinate chosen to be the bond length of the breaking bond. A consideration of both microcanonical and canonical calculations is given. The method described is similar to a previously described implementation in which the reaction coordinate was, instead, chosen to be the distance separating the centers of mass of the two dissociating fragments. This bond length based implementation is applied to the calculation of energy and angular momentum resolved rate constants and product vibrational distributions for the dissociation of NCNO. A comparison of the results of these calculations with the corresponding center-of-mass separation distance based results demonstrates that the bond length provides a considerably improved description of the reaction coordinate in the region of the inner transition state.
Modified treatment of heterogeneous nucleation rates
Authors: Fukaya T. , Tachiya M.
Published: July 2002

A modified theory is presented for the effect of multiple nucleation events on heterogeneous nucleation rates which was recently investigated by Weinberg in J. Chem. Phys. 92, 7515 (1990). A different and more reasonable expression is proposed for the intrinsic nucleation rate on an impurity particle as a function of the number of nuclei already present in the particle. By using it a simple analytical expression is derived for the overall heterogeneous nucleation rate and is compared with Weinberg’s result.
Some aspects of electron correlation, magnetism, and localization in spatially disordered systems
Authors: Logan David E.
Published: July 2002

We consider a disordered Hubbard model for a system characterized by quenched liquid-like disorder, with correlation treated at the generalized Hartree–Fock level and the possibility of local magnetic moments introduced from the outset. A simple theory based on averaged Green functions is used to describe the properties of the system in the local moment domains in particular, and their evolution with number density and both structural and electronic parameters of relevance. A probabilistically based mean-field theory is then developed to address the localization characteristics of the HF pseudoparticle states, and the consequent disorder-induced metal–insulator transition. Three principal density domains of interest are identified: a low density insulator with local magnetic moments, a metallic phase with local moments at intermediate densities, and a higher density nonmagnetic metallic state. The theory is used to provide an interpretation of bulk experiments on expanded fluid alkali elements, with particular emphasis on the insulating and ‘‘dirty’’ metallic domains.
A reduced dimension quantum wave packet study of photodissociation dynamics of diatomic molecules on surfaces
Authors: Guo Hua , Schatz George C.
Published: July 2002

Photodissociation dynamics of a model diatomic molecule adsorbed on an MgO (001) surface is studied using Hamiltonians having two actively treated mathematical dimensions. Dissociation/desorption at three orientations, i.e., θ=0°, 90°, and 180° (where θ is the angle between the molecular axis and the surface normal), is investigated. The dynamics is represented by quantum wave packets which provide detailed information about the time evolution of the system. Trapping of one or two dissociated atoms is observed and the trapped vibrational state distributions are calculated. When the light atom is between the surface and the heavy one, oscillation of this atom between the two heavy partners can cause energy transfer to the outer atom. This results in a broadened and upward shifted translational energy distribution of the desorbed heavy atom, compared to the gas phase result.
Hard star-shaped bodies and Monte Carlo simulations
Authors: Wojciechowski K. W.
Published: July 2002

A certain generalization of the hard convex body concept is indicated to be convenient in the Monte Carlo simulations. A two-dimensional system of highly anisotropic hard homonuclear dumbbells is considered as an example.
Softening of bulk modulus and negative Poisson’s ratio near the volume phase transition of polymer gels
Authors: Hirotsu Shunsuke
Published: July 2002

The static bulk and shear modulus of poly (N-isopropylacrylamide)/water gel (NIPA gel) have been obtained as a function of temperature by measuring the change of sample size induced by an applied tension. It is deduced that the method gives the static elastic moduli of gels to a good approximation except just above the discontinuous transition temperature T0, where the stress-induced shift of T0 made the estimation of elastic strain impossible. Two kinds of NIPA gels with different polymer concentration and crosslinking density were used as samples, of which one undergoes a discontinuous volume phase transition and the other a continuous one. In both gels, the bulk modulus softens drastically towards the transition showing that the soft mode of this transition is a uniform and isotropic volume-deformation mode. The shear modulus, on the other hand, shows only a small anomaly at the transition. The anomaly exhibited by the Poisson’s ratio σ is quite peculiar, i.e., it becomes negative and approaches −1 towards the critical point. It is shown that the results can be explained fairly well on the basis of the Flory-type phenomenological theory extended to take into account the concentration dependence of the interaction parameter.
Chemical waves in a continuously fed reactor: Numerical studies of the chemical pinwheel
Authors: Vasquez Desiderio A. , Horsthemke Werner , McCarty Peter
Published: July 2002

The Belousov–Zhabotinskii reaction in an annular gel reactor [Z. Noszticzius et al., Nature 329, 619 (1987)] is modeled by the reduced two-variable Oregonator with radial gradients in the concentration of bromate and malonic acid. This model is found to produce a wave shape and dispersion relation that are in satisfactory agreement with the experimental data. The numerical studies also reveal that the pinwheel dynamics can be understood in terms of the local chemistry in the reactor.
Electron attachment line shapes, cross sections, and rate constants at ultralow energies in CF3SO3H, (CF3SO2)2O, and CF3I
Authors: Alajajian S. H. , Man K-F. , Chutjian A.
Published: July 2002

Electron attachment cross sections are reported in the energy range 0–160 meV, and at resolutions of 6.0–6.5 meV (FWHM) for the molecules CF3SO3H (triflic acid), (CF3SO2)2O (triflic anhydride), and CF3I (methyl iodide). Use is made of the Kr photoionization method. Attachment line shapes are deconvoluted from the spectral slit (electron energy) function, and are converted to cross sections by normalization to thermal attachment rate constants at 300 K. Rate constants as a function of mean electron energy are calculated from the cross sections using a Maxwellian electron energy distribution function. Present data are compared with flowing-afterglow, Langmuir-probe (FALP) results in triflic acid and anhydride, and with high-Rydberg ionization results in CF3I.
A dynamic simulation method suppressing uninteresting degrees of freedom
Authors: Romiszowski Piotr , Yaris Robert
Published: July 2002

In order to be able to simulate slow or infrequent motions which require extremely long (or sometimes impossibly long) simulations, a method of simulation using an equation of motion with the uninteresting degrees of freedom suppressed—the generalized Langevin equation—has been used. This method obtains the memory function and effective potential used in the generalized Langevin equation by fitting to the fast motion behavior of a full simulation with all of the degrees of freedom retained. Hence, only a relatively short time full simulation is necessary. The method was tested on two model problems: a two-dimensional potential model designed to mimic conformational transitions and a two-dimensional array of ellipsoids (representing benzene rings) interacting with nearest-neighbor potentials. In both cases, the results obtained simulating with a reduced number of degrees of freedom are in good agreement with the results of the full simulation.
Photoionization mass spectrometric study of Si2H6
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The adiabatic I.P. of Si2H6 obtained by a photoionization mass spectrometric study at two temperatures is 9.74±0.02 eV. The first fragment, Si2H+4, initially appears with a shallow slope at ≤10.04±0.02 eV, and with a much steeper slope at ≤10.81±0.02 eV. It is argued that the initial onset corresponds to formation of H2SiSiH+2, while the steeper onset is attributed to formation of H3SiSiH+. The second fragment, Si2H5, has an appearance potential of ≤11.59±0.02 eV (11.41±0.03 is a probable value). Successive decomposition leads to Si2H+2 (from Si2H+4 ) and Si2H+3 (from Si2H+5 ). The photoion yield curve for Si2H+3 also displays shallow and steep onsets. Upper limits for the appearance potentials can be readily extracted, but the true thermochemical onsets are less well defined. Heats of formation (or upper limits) are presented for each of these species. For Si2H+6, Si2H+5, and Si2H+4, the experimental values are in good agreement with recent ab initio calculations. For the daughter species, the experimental values exceed the calculated ones, as expected.
Perturbative reactive scattering within a quasiadiabatic representation: Multichannel application
Authors: Peploski James C. , Eno Larry
Published: July 2002

Low energy reactive transition probabilities for a model multichannel collision problem, are determined within a so-called quasiadiabatic (QA) representation of the system electronic energy. The procedure involves setting up a set of coupled nonreactive surfaces (the QA representation) and then perturbatively mixing coupled-channel wave functions on the QA surfaces. It is applied to a hard-sphere-type model of the collinear A+BC reaction and for a relatively high system mass (5.0×104 a.u.). Optimization of the representation (which we have previously argued should temper maximization of the QA reactivity with a drive for balance between its diabatic and nonadiabatic components) yields results which are in very good agreement with exact ones (errors <10%) over a wide range of collision energies. At the same time, as the collision energy approaches the classical reactive threshold, we see evidence of QA failure; we trace this to difficulties with our particular optimization procedure when the diabatic contribution becomes dominant. ‘‘Conventional’’ perturbative results are generated for the same model problem and found to be poor in general (errors ≂40%–50%). It is demonstrated that the ineffectiveness of the conventional approach may be ascribed to the system’s high mass.
Multiconfiguration thermodynamic integration
Authors: Straatsma T. P. , McCammon J. A.
Published: July 2002

A modified thermodynamic integration technique is presented to obtain free energy differences from molecular dynamics simulations. In this multiconfiguration thermodynamic integration technique, the commonly employed single configuration (slow growth) approximation is not made. It is shown, by analysis of the sources of error, how the multiconfiguration variant of thermodynamic integration allows for a soundly based assessment of the statistical error in the evaluated free energy difference. Since ensembles of configurations are generated for each integration step, a statistical error can be evaluated for each integration step. By generating ensembles of different lengths, the statistical error can be equally distributed over the integration. This eliminates the difficult problem in single configuration thermodynamic integrations of determining the best rate of change of the Hamiltonian, which is usually based on equally distributing the free energy change. At the same time, this procedure leads to a more efficient use of computer time by providing the possibility of added accuracy from separate calculations of the same Hamiltonian change. The technique is applied to a simple but illustrative model system consisting of a monatomic solute in aqueous solution. In a second example, a combination of multiconfiguration thermodynamic integration and thermodynamic perturbation is used to obtain the potentials of mean force for rotation of the sidechain dihedral angles for valine and threonine dipeptides with restrained backbones in aqueous solution.
Visible-light-induced oxidation for O2 on GaAs(110): The role of hot electrons
Authors: Chen Y. , Seo J. M. , Stepniak F. , Weaver J. H.
Published: July 2002

Irradiation with 1.7 and 1.97 eV photons of O2 physisorbed on GaAs(110) at 25 K induced strong surface oxidation. The oxidation rate was dependent on photon energy as well as substrate doping type. The 1.97 eV photons induced reaction ∼30 times faster than did the 1.7 eV photons. For fixed photon energy, reaction on p-type substrates was ∼6 times faster than on n-type substrates. These results stand in contrast from those of room temperature experiments where reaction rate was independent of doping type and the dependence on photon energy reflected only the substrate photon absorption coefficient. We show that photoexcited hot electrons are responsible for photo-induced reactions at low temperature. Coupling between hot electrons and physisorbed O2 is via resonant tunneling involving the O2 electron affinity level.
Quantum calculations of the polarized emission spectrum of methyl iodide: The importance of interference effects
Authors: Wedlock Michael R. , Freed Karl F.
Published: July 2002

We present detailed quantum calculations of the polarized emission spectrum from methyl iodide after excitation at 266 nm. Methyl iodide has two dipole accessible and mutually coupled repulsive electronic surfaces at this excitation energy. Thus it is expected that there are interference contributions to the angularly resolved polarized emission spectrum. Using a one-dimensional model and time-independent quantum computations of the emission into ground state C–I overtone vibrations, these interference contributions are shown to significantly influence the polarized emission spectrum and, for example, to produce a counterintuitive dependence on final state overtone number. The calculations demonstrate that the influence of the interference terms is enhanced by the formation of polarized emission intensity ratios because of the differing signs of geometric coefficients associated with the interference terms. We also study the effect of varying the strength of the coupling potential and the range over which it acts. The nonintuitive nature of the interference terms and the amplification of their influence by the formation of polarized emission intensity ratios demonstrate why quantum computations are important for a complete understanding of the polarized emission spectroscopy experiment for methyl iodide.
The dynamics of surface rearrangements in Si adatom diffusion on the Si{100}(2×1) surface
Authors: Srivastava Deepak , Garrison Barbara J.
Published: July 2002

The Si adatom adsorption and diffusion on the fully relaxed Si{100}(2×1) surface is studied by a combination of molecular dynamics simulations with Tersoff’s potential for the Si interactions, a simplified transition state theory of Voter and lattice gas simulations. Six local minima for adsorption are found on the surface and the activation energies between each are determined. The Arrhenius behavior for the macroscopic diffusion is found to be D=5.67×10−3 exp(−0.75 eV/kT) cm2/s. In addition, it is found that the adatom diffusion is strongly anisotropic in nature and the direction of easy diffusion is perpendicular to the dimers (i.e., parallel to the dimer rows) of the original surface. The minimum energy path for the diffusion is found to be activated by the local unreconstruction (dimer opening) of the otherwise fully reconstructed surface.
Singlet molecular oxygen excitation of BrF B 3Π(0+)
Authors: Melton D. W. , Lowe B. F. , Perram G. P. , Roh W. B.
Published: July 2002

The excitation of BrF B 3Π(0+) by metastable singlet oxygen, O2(a 1Δ,b 1Σ), has been studied under steady-state conditions in a flow tube reactor. The chemiluminescence from BrF(B) indicates a nonthermal vibrational distribution that peaks at v′=3 and a ratio of BrF(B) to O2(b 1Σ+g) concentration of about 5×10−4. The emission is attributed to three-body recombination of bromine and fluorine atoms with a rate coefficient of approximately 7×10−31 cm6/molecule2 s and a single-step energy-transfer collision with O2(b 1Σ). The rate coefficients for quenching of O2(b 1Σ) by CO2, CF4, Br2, and Br have been determined from a linear Stern–Volmer analysis as 3.0, 0.027, 9.7, and 140×10−13 cm3/molecule s, respectively.
The influence of wave vector dependent dielectric properties on rotational friction. Rotational diffusion of phenoxazine dyes
Authors: Alavi D. S. , Hartman R. S. , Waldeck D. H.
Published: July 2002

The rotational diffusion of three mechanically similar phenoxazine dyes possessing distinct electrical properties was studied in isopropanol. The results, along with previously presented results from other polar solvents, were analyzed in terms of continuum theories for rotational dielectric friction. It was found that a continuum based theory can account for the observed rotational relaxation dynamics, but only with realistic modeling of the solute charge distribution (not a point dipole), and by accounting for both frequency and wave vector dependences of the solvent dielectric properties.
Observation of the pure rotational spectra of the ArOH and ArOD complexes by a Fourier-transform microwave spectrometer
Authors: Ohshima Yasuhiro , Iida Minoru , Endo Yasuki
Published: July 2002

Pure rotational spectra of open-shell van der Waals complexes, ArOH and ArOD, have been observed for the first time by a pulsed-nozzle Fourier-transform microwave spectrometer. The complexes were produced in a pulsed electronic discharge in a water/Ar mixture, with subsequent expansion of the discharge products into a supersonic jet. The observed J =5/2–3/2 transitions for both the species showed P-type doublings, as well as magnetic hyperfine splittings due to the proton or deuterium nuclear spin. The hyperfine coupling constants of the complexes are much smaller than those of free OH or OD, showing that the complex is undergoing a large amplitude motion in the ground vibronic state with an average amplitude exceeding 50°.
Observation of nonstatistical ortho–para ratio in hydrogen recombination at low temperatures
Authors: Xiao Y. M. , Buchman S. , Pollack L. , Kleppner D. , Greytak T. J.
Published: July 2002

We have demonstrated that the molecular hydrogen obtained from atomic recombination at 300 mK has nonstatistic ortho–para ratio. The ortho para conversion is less than 3% in the vibrational relaxation phase. We suggest that it is possible to carry out detailed comparison between calculations and measurements at conditions similar to our experiments.
Cage molecules containing elements of groups V and VI. II. Molecular dynamics study of P4S3 and P3−7
Authors: Seifert G. , Jones R. O.
Published: July 2002

Molecular dynamics (MD) simulations have been performed to determine the stability of the cagelike structure of the seven-membered systems P3−7 and P4S3. In both cases the cage structure remains intact during simulations of several picoseconds. However, fluxional behavior of the P3−7 trianion could be observed directly, whereas no isomerization took place in P4S3.
Microwave spectrum of alkali metal tetrahydroborate. II. Rotational transitions in the ground and excited vibrational states and molecular structure of LiBH4
Authors: Kawashima Yoshiyuki , Hirota Eizi
Published: July 2002

The rotational spectrum of LiBH4 was observed in the millimeter-wave region using a high- temperature absorption cell. The observed spectrum of LiBH4 showed the pattern of a symmetric top molecule: strong and weak for K=3n and 3n±1, respectively, because of the nuclear spin statistical weight for C3v symmetry. The rotational constants and centrifugal distortion constants for 7Li11BH4, 7Li10BH4, 6Li11BH4, and 6Li10BH4 species were determined. Four observed rotational constants gave rs (Li–B) to be 1.939 38±0.000 10 Å, which, combined with the assumption that θ(Hb–B–Ht)=113 °, led to r(B–Hb) and r(B–Ht) to be 1.256±0.015 Å and 1.216∓0.015 Å, respectively, where the signs should be taken in the stated order. The uncertainties of r(B–Hb) and r(B–Ht) are mainly due to that of θ (Hb–B–Ht), which is estimated to be ∓1.5 °. This bond length obtained for Li–B is much shorter than the reported value in crystal; 2.47 Å. The bond lengths derived indicate that LiBH4 has a tridentate molecular structure with three bridging hydrogen atoms as in the case of NaBH4, in agreement with predictions by ab initio calculations. Two sets of vibrational satellites were observed and analyzed for both 7Li11BH4 and 7Li10BH4. These satellites were assigned to the nondegenerate Li–B stretching and the doubly-degenerate Li–B–H bending mode.
Rotational analysis of the 0 band of the Ã 3E–X̃ 3A2 system of methylnitrene
Authors: Brazier C. R. , Carrick P. G. , Bernath P. F.
Published: July 2002

The optical emission spectrum of the methylnitrene radical has been observed at a resolution of 0.07 cm−1. Transitions to subbands with K up to 3 in the X̃ 3A2 ground state were observed and a full rotational analysis carried out. Comparison of the structural information with the highest level theoretical calculations shows agreement to within the estimated error.
A new diagrammatic summation for the effective dielectric response of composites
Authors: Barrera Rubén G. , Noguez Cecilia , Anda Enrique V.
Published: July 2002

We extended a previously developed diagrammatic formulation for the calculation of the effective dielectric response of composites prepared as a random, homogeneous, and isotropic distribution of small spherical inclusions in an otherwise homogeneous matrix. This is done within the long-wavelength, dipolar approximation in the low-density regime of inclusions. We propose a new diagrammatic summation and we compare our results with two recently reported computer simulations.
Layer interactions between dissimilar adsorbates. NH3 layers on chemisorbed CO on Ni(111): A reflection infrared study
Authors: Xu Zhi , Hanley Luke , Yates John T.
Published: July 2002

Fourier transform infrared reflection absorption spectroscopy (FT-IRAS) has been used to study the adsorption structure of NH3 on top of a saturated CO layer on Ni(111). The IRAS results show that NH3 is adsorbed on top of the saturated CO layer on Ni(111) with its C3v axis tilted away from the surface normal due to the interaction with underlayer CO molecules. Two types of overlayer NH3 species are observed, with one NH3 species exhibiting strong N-H–OC bonding. At the same time, the population of underlayer CO species (bridged and terminal) on different adsorption sites is strongly modified by the adsorption of NH3 molecules in the top layer, with terminal CO decreasing in coverage and bridged CO increasing in coverage. This is due to the influence of an extra (positive-outward) electric field imposed on the CO layer by the polarized NH3 overlayer.
Electronic structure of Ne2 below the first ionization limit
Authors: Kim S. B. , Herring C. M. , Eden J. G. , Ginter M. L.
Published: July 2002

Results from new laser excitation spectra of 20Ne2 have been combined with previously published experimental and theoretical data to provide a consistent picture describing many of the Rydberg states observed below the first ionization limit. These states are found to fall mainly into Rydberg series converging on the ...3σu X 2Σ+u ground core of Ne+2. Spectral data for the npσ 3Σ+g←a 3Σ+u and npπ 3Πg←a 3Σ+u series support the current assignments of the upper states. Although (0–0) bands dominate these series, other vibronic transitions are observed. The 6–23pσ 3Σ+g–a(0–0) bands are more perturbed than the npπ 3Πg–a(0–0) series; hence, data for 4–17pπ 3Π+g–a(0–0) and 4–6fλ(3Σ+g,3Π+g)–a(0–0) were used to determine a value of 34 409±2 cm−1 for the lowest ionization limit of 20Ne2. Rotational constants for the v=0 levels of 4–17pπ 3Πg and 10–23pσ 3Σ+g also are presented.