bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization and study of the thermodynamic equilibrium departure of an argon plasma flame produced by a surface-wave sustained discharge
Authors: Calzada M. D. , Sáez M. , Garcı́a M. C.
Published: July 2002

Surface wave induced plasma (SWP) is a particular class of microwave induced plasma. It is used in atomic emission and mass spectrometry as a source of excitation for the elemental analysis of samples in analytical chemistry. This discharge is mainly used at powers less than 300 W. At such low power, an efficient process of desolvation, atomization, excitation and ionization of the analyte can only be reached provided the density of power absorbed in the discharge is large. This requires a plasma whose volume increases very little with increasing absorbed power. At atmospheric pressure in the SWP, such a volume limitation can be achieved by having the plasma extending into ambient air in the form of a flame (open air discharge). This article presents the results of the experimental characterization and study of the degree of thermodynamic equilibrium of an argon plasma flame produced by a surfatron. The values of the plasma parameters and b(p) (departure from equilibrium) parameter for the ArI levels show that the discharge is a two-temperature plasma. By comparing the results with those obtained for a classical surface-wave plasma column, it can be concluded that the plasma in flame is more suited to be used as a source of excitation in analytical chemistry, due to its higher electronic density and temperatures values.
Density fluctuations, dynamic light scattering, longitudinal compliance, and stress modulus in a viscoelastic medium
Authors: Wang C. H. , Fischer E. W.
Published: July 2002

Correct relations between the dynamic light scattering spectrum and the mechanical creep compliance and longitudinal stress modulus of a viscoelastic liquid are provided. Analysis of the effect of temperature fluctuations on the longitudinal stress modulus is also given. The result provides a basis for determination of mechanical relaxation spectra using the dynamic light scattering technique.
Magnetic vibrational circular dichroism of transition metal hexacarbonyls
Authors: Devine T. R. , Keiderling T. A.
Published: July 2002

The magnetic vibrational circular dichroism (MVCD) of three neutral metal hexacarbonyls, M(CO)6(M=Cr, MO, W), is reported in the C≡O stretching and its first overtone regions. The spectra observed are dominated by large A terms in both regions. An attempt to correlate these results with those of calculations based on the Laux et al. empirical charge model was not successful. However, we have found that the A and B terms measured are consistent with a parametrized model based on vibronic coupling of the ground and lowest excited states.
Germanium L2,3M4,5M4,5 and L2,3M2,3M4,5 Auger spectra of volatile germanium compounds
Authors: Venezia Anna Maria , Cavell Ronald G.
Published: July 2002

Germanium L2,3M2,3M4,5 and L2,3M4,5M4,5 Auger spectra excited by Al Kα1,2 radiation were measured for GeH4, GeF4, GeCl4, Ge(CH3)4, and Ge(C2H5)4. Energies and intensities were calculated by theoretical models based on a mixed coupling scheme using jj coupling for the initial state and LS coupling for the final state. Relaxation contributions have been evaluated and shown to vary qualitatively with the polarizabilities of the molecule.
Charge-transfer theory of surface enhanced Raman spectroscopy: Herzberg–Teller contributions
Authors: Lombardi John R. , Birke Ronald L. , Lu Tianhong , Xu Jia
Published: July 2002

A comprehensive development of the charge-transfer theory of surface enhanced Raman scattering (SERS) is presented. We incorporate the Herzberg–Teller mixing of zero-order Born–Oppenheimer electronic states by means of vibronic interaction terms in the Hamiltonian. This is similar to the theory of Tang and Albrecht12 except that we include metal states as part of a molecule–metal system. When this is done we may no longer discard a term involving mixing of ground-state vibrations. The theory is comprehensive in that both molecule-to-metal and metal-to-molecule transfer is considered. Furthermore, both Franck–Condon and Herzberg–Teller contributions to the intensity are obtained. The former, however, contribute only to the intensity of totally symmetric vibrations, while the latter contribute to nontotally symmetric vibrations as well. Since overtones are observed in SERS only weakly if at all, the Herzberg–Teller terms are most consistent with experimental findings. The resulting formulas may be interpreted as a type of resonance Raman effect in which intensity for the charge transfer transitions is borrowed from an allowed molecular transition. We may also carry out the sum over metal states. This procedure predicts a logarithmic resonance at the Fermi level of the metal. We thus predict intensity vs voltage profiles such as I ∝ ‖ln(ωFI−ω+iΓ)‖2 which fits the experimental curves quite well.
Nonadiabatic interactions in the photodissociation of ICN
Authors: Goldfield E. M. , Houston P. L. , Ezra G. S.
Published: July 2002

Photodissociation of ICN in the Ã continuum has been modeled using classical trajectories assuming that all absorption from the linear ground state is to a single linear diabatic excited state which dissociates to form I*(2P1/2) and CN(2Σ+,v=0). It is also assumed that, in nonlinear excited state configurations, nonadiabatic transitions occur to a bent surface which correlates diabatically to ground state I(2P3/2) and CN(2Σ+,v=0). Empirical potential surfaces with frozen CN bond lengths are employed, while transitions between the surfaces are treated using either the Miller–Meyer classical electron model or a simple diabatic version of the Tully–Preston surface-hopping model. With the above assumptions, the Miller–Meyer method is found to give much better agreement with the experimental results. Theoretical results obtained with the Miller–Meyer method are compared with recent experimental data on the I*/I branching ratio, the average CN rotational energies, and the product CN rotational distributions as a function of photolysis wavelength for λ=248, 266, 280, 290, and 308 nm. Except for the branching ratio at 248 nm, we obtain satisfactory agreement with the experimental results.
Energy partitioning in O(1D2) reactions. II. O(1D2)+CH4 → OH(v′)+CH3
Authors: Aker P. M. , O’Brien J. J. A. , Sloan J. J.
Published: July 2002

The complete OH vibrational distribution created by the reaction of O(1D2) with CH4 has been recorded directly, using low pressure infrared emission spectroscopy. The result shows that the energy is partitioned nonstatistically into this degree of freedom. The population of OH(v′=2) is approximately equal to or slightly greater than that of OH(v′=1) under the conditions of this experiment and substantial populations are recorded from all levels up to OH(v′=4), the maximum allowed by the exoergicity of the reaction.
Monte Carlo simulation of electron thermalization in gases. VI. Microwave conductivities of He and Ar
Authors: Koura Katsuhisa
Published: July 2002

The transient microwave conductivity in the course of the electron thermalization in He and Ar is studied by the Monte Carlo simulation (MCS). Direct comparisons of the conductivity are made between the MCS results and available experimental results obtained by the microwave absorption measurement. The measured conductivity of He does not fit the MCS one but reasonably agrees with the theoretical one based on the assumption of the Maxwellian electron velocity distribution; the conductivity of Ar obtained by the MCS reveals a rather complicated peak pattern inconsistent with the measured one. Some possible reasons for these facts are briefly discussed. It is indicated that the difference between the Margenau and alternative conductivity formulas is small for He but significant for Ar with the Ramsauer minimum.
The influence of substrate motion on the self-diffusion of hydrogen and its isotopes on the copper (100) surface
Authors: Valone S. M. , Voter A. F. , Doll J. D.
Published: July 2002

Work is presented that examines the effect of substrate motion on the surface self-diffusion of hydrogen and its isotopes on the Cu(100) surface. Lattice motion, represented as a sum of Lennard-Jones interactions, is found to increase the diffusion constant of hydrogen and its isotopes at all temperatures examined. The increase varies from 1.3 to 4.0 over the temperature range from 1000 to 110 K. The results agree with the recent calculations of Lauderdale and Truhlar above 150 K. The quantum contribution to the isotope effect is enhanced relative to the values for the frozen substrate. These conclusions are based on approximate path integral calculations in which quantum-mechanical effects are treated in a semiclassical manner using temperature-dependent effective potentials. The differences between the present results and those of Lauderdale and Truhlar are attributed to a breakdown of these semiclassical approximations at low temperatures. In the temperature range considered, commonly accepted harmonic corrections to the classical results afford slightly poorer agreement with Lauderdale and Truhlar than the present results.
Comments on classical and nonclassical representations of critical demixing in liquid–liquid binary solutions
Authors: Singh Rajiv R. , Van Hook W. Alexander
Published: July 2002

Classical expressions for the free energy of binary mixtures do not agree with observed behavior near critical points (Tc,Xc) for liquid–liquid demixing. Very near (Tc,Xc) long-range concentration fluctuations produce logarithmic discontinuities in certain of the derivatives of the free energy and this necessitates an alternate description in terms of scaling laws. Arguments are presented that, for mixtures of small molecules, the range of such nonclassical effects is approximately (0<t<10−3);t=1−T/Tc. This is about the same as the range of nonclassical effects for other kinds of critical phenomena. Although it has been claimed that nonclassical effects in liquid–liquid equilibria extend to t=10−1, or even further, experiments in the region t>10−3 are as well or better described by classical thermodynamics as they are by scaling expressions. Classical thermodynamics also permits an accurate description of the pressure dependence of Tc and Xc, and suggests that the equal volume criterion used to locate Xc is not generally correct.
Structure and binding of Lennard-Jones clusters: 13≤N≤147
Authors: Northby J. A.
Published: July 2002

An efficient lattice based search and optimization procedure has been developed and used with various assumed pair potentials to find minimal energy structures on an icosahedrally derived lattice. These were then taken as initial configurations and allowed to relax freely under the Lennard-Jones pair potential to the adjacent energy minimum. The initial configurations and relaxed energies of the most tightly bound structures found for each N are presented for the range 13≤N≤147. While the energies obtained are rigorously only upper bounds to the absolute minimal energy of an N-atom Lennard-Jones cluster, they appear to be less than or equal to that of any other structures proposed previously. They are believed to be the most tightly bound structures of the multilayer icosahedral type, and to be reasonable candidates for the absolutely minimal energy structures in this size range. The most tightly bound configuration found was the truncated icosahedral structure at N=135.
UHF natural orbitals for defining and starting MC-SCF calculations
Authors: Pulay Peter , Hamilton Tracy P.
Published: July 2002

A simple way of generating starting orbitals for multiconfigurational SCF calculations (particularly of the CAS type) is to use the natural orbitals of the unrestricted Hartree–Fock wave function. Significant fractional occupancy of a UHF natural orbital indicates that the orbital should be included in the active space; this is illustrated for symmetrically stretched water, for NO, N2O4, and for the transition state on the Me3CO→Me2CO+CH3 reaction surface. Average natural orbitals should be used if there are several UHF solutions for the same state; this restores symmetry broken at the the UHF level. Configuration selection based on fractional occupation appears to be the basis for a reliable automated procedure. The UHF natural orbitals provide good starting orbitals for the two-configuration SCF and for the 4×4 CAS wave function in stretched F2, H2O2, and C2H6, methane, water, in twisted ethylene, in ozone, and for various CAS wave functions in the first asynchronous transition state of the HCNO+H2C2 1,3-cycloaddition. Comparison is made with the extended Hartree–Fock method.
Laser induced fluorescence study of the F+I2→IF+I reactive collision
Authors: Girard B. , Billy N. , Gouédard G. , Vigué J.
Published: July 2002

We have performed a crossed beam study of the F+I2→IF+I reaction. The IF product was detected by the LIF technique, using a cw single frequency dye laser. About 10 000 lines belonging to the B–X system of IF have been thus recorded. These lines have been assigned and the populations of about 1300 rovibrational levels of the X state were deduced from their intensities. This analysis takes into account the coherent saturation regime of the excitation process. In the present work, only the high vibrational levels of the X state (v″=8–20) have been studied. A striking result is the observation of bimodal rotational distributions for several vibrational levels: the rotational distribution presents a plateau and a peak for high rotational quantum numbers. The vibrational distribution presents a peak centered at v″=13, and as we have shown that the v″=0 peak observed by other authors is in fact an experimental artifact, the general features of the energy disposal in this reaction are now well known.
Calculation of the rate of entropy production for a model chemical reaction
Authors: Irvin Benjamin R. , Ross John
Published: July 2002

The rate of entropy production due to chemical reaction is calculated for a variety of parameter values in the reversible Oregonator model. The average values over cycles of oscillation are compared to those in the coexisting stationary states. The present work corrects an earlier calculation [A. K. Dutt, J. Chem. Phys. 86, 3959 (1987)]. Contrary to previous impressions and claims, there is no consistent relationship between the magnitudes of the entropy production in coexisting stationary and oscillatory states, in this case.
Atomic scale friction between the muscovite mica cleavage plane and a tungsten tip
Authors: Erlandsson R. , Hadziioannou G. , Mate C. M. , McClelland G. M. , Chiang S.
Published: July 2002

An atomic force microscope using optical lever detection has been used to study the friction between muscovite mica and a tungsten tip. The frictional force is shown to vary laterally with the periodicity of the hexagonal layer of SiO4 units that forms the cleavage plane of mica. The frictional force varies linearly with normal force, giving a coefficient of friction of 0.09.
Equilibrium structure and anharmonic force field of hypofluorous acid (HOF)
Authors: Halonen L. , Ha T.-K.
Published: July 2002

An equilibrium structure and an empirical anharmonic force field based on an ab initio surface are obtained using available experimental vibration–rotation data. Four of the six harmonic and four of the ten cubic force constants have been determined experimentally. The remaining values have been fixed at the ab initio values. A good fit to high resolution experimental data is obtained.
Ab initioconfiguration interaction study of the electronic and geometric structures of small sodium cationic clusters
Authors: Bonac̆ic-Koutecký Vlasta , Boustani Ihsan , Guest Martyn , Koutecký Jaroslav
Published: July 2002

The favorable geometries of small sodium clusters Na+n (n=3–9) are determined with the analytical gradient method in the framework of the ab initio SCF approximation. The transition from the planar towards three-dimensional cluster geometries is understood in terms of some basic quantum theoretical concepts. The binding energies per atom for Na+n clusters calculated with the MRD CI procedure increase, in general, as a function of n. Nevertheless, the atomization energy per atom as well as the ionization potential as functions of the nuclearity n exhibit well developed oscillations for even–odd n. Consequently, the fragmentation energy for the channel Na+n→Na+n−1+Na shows strong oscillations as well. This explains large abundances of cationic clusters with odd nuclearity found in some detection devices. The possible consequences for the fragmentation process of Na+n is discussed. It is found that the process Na+n→Na+n−2+Na2 is favorable for Na+5, Na+7, and Na+9.
Random sequential adsorption of unoriented rectangles onto a plane
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. The approach to the jamming limit was found to obey Feder’s law for a wide range of rectangle aspect ratios. The coverage fraction at the jamming limit was found to depend upon the aspect ratio of the adsorbed rectangles, with a maximum in the jamming coverage occurring at aspect ratios ≊2.
Close-coupled wave-packet calculations of the direct inelastic scattering of NO(X 2Π) from Ag(111)
Authors: Lemoine Didier , Corey Gregory C.
Published: July 2002

In this article we report an application of the time-dependent close-coupled wave-packet (CCWP) method to the rotationally inelastic scattering of NO(X 2Π) molecules from a rigid, flat Ag(111) surface. Previous applications of the CCWP method have been restricted to either direct scattering off purely repulsive potentials requiring short propagation times, or weakly physisorbed systems in which only a few internal states are coupled. The calculations reported here were performed for a molecule in an initial state with a momentum distribution peaked around E=6700 cm−1 scattering off a strongly anisotropic potential with a well depth of 4400 cm−1 and a long-range tail. Numerical procedures were introduced which enhance the efficiency of the CCWP method whenever a large number of internal states or a large number of grid points are needed to simulate the collision. For the current application to NO–Ag these techniques reduced the required CPU time by more than an order of magnitude. The resulting state–to–state transition probabilities are compared with previous time-independent close-coupled calculations, and with the semiclassical self-consistent eikonal method (SCEM). The agreement between the two quantum-mechanical methods is well within the accuracy of both numerical procedures. A comparison of the instantaneous transition probabilities calculated throughout the propagation shows good agreement between the CCWP and the SCEM calculations at high collision energies.
Optical absorption spectroscopy of sodium clusters as measured by collinear molecular beam photodepletion
Authors: Chris Wang C. R. , Pollack Stuart , Cameron Douglas , Kappes Manfred M.
Published: July 2002

Collinear molecular beam photodepletion was used to obtain particle specific electronic absorption information for Na3, Na4, and Na8 in a wavelength range from 370–835 nm. We critically discuss the experimental method used and the deconvolution procedure applied to the resulting data to yield absolute absorption cross sections. The spectra contain much information on the cluster-size–dependent transition from molecular to bulk-like optical response and are interpreted in terms of various computational approaches ranging from classical electrostatic to ab initio large scale configuration interaction.
Accurate quantum mechanical calculation of thermally averaged reaction rate constants for polyatomic systems
Authors: Makri Nancy
Published: July 2002

A new approach to the calculation of quantum mechanical Boltzmann averaged reaction rate constants for polyatomic systems is described. The rate constant is obtained by integrating a set of coupled first order temperature-dependent differential equations, the number of which grows linearly with the size of the system. This is accomplished by (i) representing the complex time evolution operator in mixed position and operator form and (ii) introducing two-body correlations in the conventional time-dependent self-consistent field approximation, as suggested recently [Chem. Phys. Lett. 169, 541 (1990)]. The method is accurate and numerically stable; it is therefore expected to find considerable utility in the study of gas phase bimolecular reactions with the reaction path Hamiltonian formalism, as well as in the calculation of rate constants for reactive processes in condensed media.
Cylindrical manifolds and reactive island kinetic theory in the time domain
Authors: De Leon N.
Published: July 2002

In a series of recent publications we discussed the concept of cylindrical manifolds and their role in mediating the reaction dynamics of chemical reactions. The cylindrical manifolds were used to develop a chemical reaction rate theory we called reactive island (RI) theory. RI theory was cast in terms of the map dynamics between n Poincaré mapping planes—which were referred to as the ‘‘n-map.’’ Therefore ‘‘time’’ did not explicitly appear within n-map RI theory. In this paper we extend n-map RI theory to the time domain. The formal theory, cast as a master equation, is used to obtain the temporal RI model. Temporal RI theory is applied to a two degree-of-freedom system exhibiting dynamical chaos. The results of temporal RI theory are compared with classical Rice–Ramsberger–Kassel–Marcus (RRKM) theory and it is found that even under, presumably, ‘‘ideal’’ dynamical conditions, RRKM theory can be in serious error with numerical simulation. It is also seen that temporal RI theory accurately accounts for the rates at energies far above the barrier, where RRKM theory is not expected to be applicable. We also discuss some rigorous comparisons between the decay rates obtained from n-map and temporal RI theories.
Experimental distinction of electric and magnetic transition moments
Authors: Jonas David M. , Solina Stephani Ann B. , Field R. W. , Silbey R. J.
Published: July 2002

The rotational band structure of a magnetic dipole vibronic transition is exactly the same as that of an electric dipole vibronic transition. It is often assumed that molecular vibronic transitions are electric dipole, introducing an ambiguity about the change in symmetry if the transition is weak enough to be an allowed magnetic dipole transition and the symmetry of either vibronic state is unknown. In atomic spectroscopy, it is known that either the Zeeman effect or the polarization dependence of wide angle fluorescence interference can be used to distinguish among electric-dipole, magnetic-dipole, electric-quadrupole, enforced-dipole transitions, etc. This note points out that rotationally resolved double resonance using polarized light can be used to determine the type of an unknown multipole transition if the type (magnetic or electric) of the other double resonance transition is known. Stark and Zeeman effects are valuable special cases since the type of the Stark (or Zeeman) transition moment is known a priori to be electric (or magnetic) dipole. Published Stark spectra of formaldehyde-d2 experimentally prove Innes’ assignment of a magnetic dipole transition.
Theab initioeffective dipole operator of CH: Comparisons with semiempirical methods
Authors: Kanzler Alfred W. , Freed Karl F. , Sun Hosung
Published: July 2002

Matrix elements of the effective valence shell dipole operator μv for the CH molecule are computed in the nonorthogonal basis of atomic valence shell self-consistent-field (SCF) orbitals for carbon and hydrogen. Significant contributions emerge from two-center and two-body μv matrix elements. The former are generally neglected in semiempirical methods for calculating dipole matrix elements, while the existence of the latter is not acknowledged by these methods but should be averaged into semiempirical dipole matrix elements. The ab initio μv matrix elements contain significant correlation contributions, but these are entirely neglected in semiempirical schemes. This neglect of correlation contributions to the semiempirical dipole matrix elements stands in stark contrast to the ‘‘adjustment’’ for correlation of semiempirical Hamiltonian matrix elements. An important correlation contribution to the μv matrix elements is shown to arise from the usual molecular orbital polarization that is omitted in both semiempirical schemes and theoretical dipole matrix elements in the atomic SCF basis. Thus, the largest correlation contributions to μv matrix elements are found to emerge for those matrix elements containing the most polarizable orbitals that are directly involved in bond formation.
Fluid n-decane undergoing planar Couette flow
Authors: Padilla Paz , Toxvaerd So/ren
Published: July 2002

Molecular dynamic simulations of fluid n-decane undergoing planar Couette flow have been carried out. The n-decane chains were modeled using an anisotropic version of the united-atom model with fixed bond lengths. The results of our simulations verify the equivalence between the steady state time averages of the atomic and molecular stress tensors. In the range of shear rates which have been investigated, our results show that the values of the viscosity coefficient depend mainly on the shape of the intermolecular potential and they are insensitive to the shape of the torsion potential. Moreover, the viscosity coefficient as a function of the shear rate shows a region of shear thinning at low shear rates followed by a region of shear thickening. The study of the alignment induced in the fluid by the perturbing field shows that after a region of fast increasing of the degree of alignment with increasing shear rate, this reaches a maximum and slowly begins to decrease. This behavior can be related to the fact that at high shear rates the chains tend to adopt a more shrunken mean shape.
Mean field theory of the microphase separation in interpenetrating networks
Authors: Schulz Michael
Published: July 2002

A mean field approximation is formulated for the microphase separation transition of chemically quenched binary interpenetrating networks. For weakly cross-linked networks, microphase separation takes place, related to the competition that exists between elasticity and repulse of the two network components. As a result of the mean field approximation follows a characteristic size of the microphase, determined by the wave vector q*=5.45/〈R2〉. The ordered phase is characterized by a macrolattice with lamellar-, hexagonal-, or body-centered cubic lattice symmetry, which is dependent from the composition f and the effective interaction parameter χ.
Dissociation dynamics of CH3SH at 222, 248, and 193 nm: An analog for probing nonadiabaticity in the transition state region of bimolecular reactions
Authors: Jensen E. , Keller J. S. , Waschewsky G. C. G. , Stevens J. E. , Graham R. L. , Freed K. F. , Butler L. J.
Published: July 2002

These experiments use molecular photodissociation of CH3SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH3+SH→CH3S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments’ velocities and angular distributions determine the branching between the CH3+SH and the CH3S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S–H bond over the weaker C–S bond. Fission of the C–S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C–S bond fission and S–H bond fission is a factor of eight larger at 193 nm.

To probe the forces in the Franck–Condon region, we also measure the photoemission spectrum from dissociating CH3SH excited at 222 nm and compare it to the previous measurement at 193 nm. The 222 nm spectrum evidences emission into the S–H stretch and methyl stretch vibrations but not into C–S stretching modes, consistent with the dominance of S–H fission on the lower adiabat, while the 193 nm emission spectrum, reassigned here, has only a progression in the C–S stretch. The comparison of the spectra suggests a model in which stretching along the C–S coordinate on the bound upper state occurs as the amplitude couples nonadiabatically to the lower dissociative surface, allowing the molecule to access the region near the saddle point on the lower surface at extended C–S bond lengths. This results in better overlap with the C–S fission exit channel and thus an increased branching to C–S bond fission over that observed upon direct excitation to the lower dissociative surface at 222 nm. To further advance the experimental conclusions, we present collaborative calculations of the potential energy surfaces using the effective valence-shell Hamiltonian method developed by Freed and co-workers.
Drainage of thin liquid films between relatively smooth surfaces
Authors: Georges J. M. , Millot S. , Loubet J. L. , Tonck A.
Published: July 2002

Using a new surface force apparatus, static and dynamic measurements have been conducted to investigate the squeeze of a liquid from the contact between a relatively smooth sphere and a plane. Simple liquids and polymer melts (polyisoprene) have been studied, with different solid surfaces (mica, cobalt, gold, platinum, and steel). It has been found that an ‘‘immobile’’ layer of fluid is present on each solid surface which does not participate in the hydrodynamic flow of the liquid. The thickness of this film is relatively independent of the roughness of the solid, corresponds to a small number of molecular layers (1 to 5), and is influenced by the structure of the molecule. These fluids shows two different behaviors: for small molecules the thickness of the ‘‘immobile’’ layer is proportional to the viscosity; for larger molecules the thickness is proportional to the one-tenth power of the viscosity. During the squeeze process, when the solid surfaces are close, a ‘‘confined’’ layer occurs due to molecule entanglement. This phenomenon is described for a semirigid hydrocarbon.
Static dielectric properties of a flexible water model
Authors: Trokhymchuk A. D. , Holovko M. F. , Heinzinger K.
Published: July 2002

The linear response theory is applied to the interaction site model for the evaluation of static dielectric properties of a flexible water model. The atom–atom radial distribution functions derived from hybridization of the computer simulations and optimized cluster theory [A. D. Trokhymchuk, M. F. Holovko, E. Spohr, and K. Heinzinger, Mol. Phys. 77, 903 (1992)] are employed for the calculation of the polarization structure factor and the moments of the polarization correlation function. The color charge version of the dipolar hard sphere model [F. O. Raineri, H. Resat, and H. L. Friedman, J. Chem. Phys. 96, 3068 (1992)] proved to be helpful for the understanding of the effects of intramolecular charge distribution and flexibility on the dielectric properties of the interaction site model. The dielectric constant for the flexible BJH water model is calculated and the region of negative values of the static dielectric function is obtained. The possibility of a subdivision of this region into two parts at k about 1–2 Å−1 is pointed out.
The heat of formation of NCO
Authors: East Allan L. L. , Allen Wesley D.
Published: July 2002

The heat of formation of NCO has been determined rigorously by state-of-the-art ab initio electronic structure methods, including Mo/ller–Plesset perturbation theory from second through fifth order (MP2–MP5) and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. Five independent reactions were investigated to establish a consistent value for ΔHf,0○(NCO): (a) HNCO(X̃ 1A′)→H(2S)+NCO(2Π), (b) HNCO(X̃ 1A′)→H++NCO−, (c) N(4S)+CO→NCO(2Π), (d) HCN+O(3P)→H(2S)+NCO(2Π), and (e) NH(3Σ−)+CO→H(2S)+NCO(2Π). The one-particle basis sets employed in the study were comprised of as many as 377 contracted Gaussian functions and ranged in quality from [4s2p1d] to [14s9p6d4f] on the (C,N,O) atoms and from [2s1p] to [8s6p4d] on hydrogen. After the addition of bond additivity corrections evaluated from related reactions of precisely known thermochemistry, all five approaches were found to converge on the value ΔHf,0○(NCO)=31.4(5) kcal mol−1. Appurtenant refinements were obtained for the heat of formation of isocyanic acid, ΔHf,0○(HNCO)=−27.5(5) kcal mol−1, and hydrogen cyanide, ΔHf,0○(HCN)=31.9(5) kcal mol−1. The final proposals for ΔHf,0○(NCO) and ΔHf,0○(HNCO) resolve outstanding discrepancies with experiment and provide updates for thermochemical cycles of relevance to combustion chemistry.
Diffusion equation analysis of energy transfer
Authors: Green Nicholas J. B. , Robertson Struan H. , Pilling Michael J.
Published: July 2002

Collisional energy transfer in unimolecular systems is often described using a master equation. The lack of detailed knowledge of energy transfer events combined with the computationally intensive nature of this approach makes approximate methods attractive, especially for parametrizing experimental results. One such approximation is based on the diffusion equation. In this paper a number of diffusion equation approximations is examined. Troe and Nikitin have derived such approximations by truncation of the Kramers–Moyal expansion of the master equation. A new approximation, using a similar approach is presented here. The diffusion equation can also be obtained as the limit of a sequence of master equations as the collision energy transfer becomes infinitely small and frequent. The derivation of such a limiting diffusion equation is presented. A further set of approximations in which the energy transfer process is imbedded in a diffusion process is also examined. The accuracy of all approximations is assessed by comparison with full solutions of the corresponding master equations for energy transfer in azulene and ethane, and for the kinetics of methane dissociation.
Adiabatic population transfer with frequency-swept laser pulses
Authors: Melinger J. S. , Gandhi Suketu R. , Hariharan A. , Goswami D. , Warren W. S.
Published: July 2002

We present detailed experimental and theoretical results on population transfer with frequency-swept picosecond laser pulses. Here, we demonstrate that intense frequency-swept pulses, when applied in the adiabatic limit, lead to both more efficient and more selective excitation than do unmodulated laser pulses. The experimental work is performed on quasi-two-level systems (pentacene/p-terphenyl crystal and Na vapor), quasi-three-level systems (Na vapor), and on more complex multilevel systems (I2 vapor). We discuss the different characteristics of adiabatic population transfer in both few-level, and multilevel cases, and, in particular, present computer calculations to explore the effects of molecular rotations in multilevel adiabatic population transfer.
Lifetime of the CH3OH* intermediate in the reaction CH4+O(1D2)→CH3OH*→CH3+OH
Authors: van Zee Roger D. , Stephenson John C.
Published: July 2002

Subpicosecond lasers measured the appearance rate of OH X(v=0) following 267 nm photolysis of the CH4⋅O3 van der Waals complex. The rise of the OH A←OH X laser-induced fluorescence with respect to the photolysis/probe delay time, tD, was LIF(tD)=1−exp(−tD/τ) with τ approximately 3 ps, indicating that the reaction CH4+O(1D2)→CH3+OH involves a CH3OH* intermediate with that lifetime. No prompt OH(v=0) from a direct or fast reaction was observed.
Approaches to the electronic structure of incommensurately modulated insulating crystals
Authors: Mitchell John F. , Burdett Jeremy K.
Published: July 2002

A one-dimensional model of an incommensurately modulated insulator is investigated in perturbation theory using tight-binding theory with overlap. Results applicable in the sinusoidal regime are discussed in terms of the ‘‘structural resonance’’ of McConnell and Heine (Ref. ). The method of moments is used to explore ordering in one-dimensional chains, suggesting a novel electronic explanation of the driving force for the transition from disorder to order as a function of electron count (band filling).
Chemical and electronic properties of Pt in bimetallic surfaces: Photoemission and CO-chemisorption studies for Zn/Pt(111)
Authors: Rodriguez José A. , Kuhn Markus
Published: July 2002

On platinum zinc surfaces at 150–300 K, CO adsorbs only on Pt sites. CO molecules bonded to these sites exhibit a significantly lower adsorption energy (4–8 kcal/mol) and a larger O 1s binding energy (0.2–0.4 eV) than molecules bonded to clean Pt(111). To explain these changes in the Pt↔CO interaction, we have examined the electronic properties of Zn/Pt(111) surfaces using core and valence level photoemission, and ab initio self-consistent-field calculations. At 80 K, Zn atoms deposited on Pt(111) remain on top of the surface. Above 400 K, alloy formation occurs. In general, the formation of Pt–Zn bonds produces a large depletion in the density of Pt 5d states around the Fermi level, with a shift in the centroid of the Pt 5d band and 4f core levels toward higher binding energy. This is accompanied by an important redistribution of charge, in which Pt loses 5d electrons and gains (6s,6p) electrons. The electronic perturbations induced by Zn on Pt reduce its CO-chemisorption ability by weakening the strength of the Pt(5d)–CO(2π*) bonding interactions. The changes in the Pt properties observed after bonding this metal to Zn are as large as those found for Pt bonded to early transition metals, and much larger than those found when Pt is bonded to late-transition metals. In the bimetallic surfaces, the perturbations in the Pt properties increase when the fraction of empty states in the valence band of the metal bonded to Pt rises.
Thermal excitation of oxygen species as a trigger for the CO oxidation on Pt(111)
Authors: Yoshinobu Jun , Kawai Maki
Published: July 2002

Thermal excitation of adsorbed oxygen species is found to initiate the CO oxidation on Pt(111). We have prepared three different coadsorption systems to study the reactivity of different oxygen species; (1) CO on the O2 preadsorbed Pt(111) surface, (2) CO on the nearly perfect Pt(111) p(2×2)-O surface, and (3) CO on the disordered atomic oxygen-preadsorbed Pt(111) surface. Four CO2 desorption peaks (α-CO2 at 125 K, β3-CO2 at ∼225 K, β2-CO2 at ∼260 K, and β1-CO2 at 320 K) are observed. The desorption temperatures of CO2 strongly depend on the adsorbed states of oxygen species. We have shown that the α-CO2 state, β2,3-CO2 states, and β1-CO2 state are correlated with adsorbed O2, disordered oxygen atoms, and p(2×2) oxygen atoms, respectively. The difference in CO2 desorption temperature is related to thermal excitation of each oxygen species, which is derived from the structural information of coadsorbed states during thermal evolution by means of low-energy electron diffraction and infrared reflection absorption spectroscopy.
Ab initiostudy ofcis-butadiene valence and Rydberg states using the effective valence shell Hamiltonian method
Authors: Lee Sang Yeon , Freed Karl F.
Published: July 2002

Low-lying σ- and π-electron vertical excitation energies of s-cis-1,3-butadiene are calculated using the ab initio effective valence shell Hamiltonian (ℋv) method. The only experimentally known vertical excitation energy is that to the 1 1B2 state at 5.49 eV, while the ℋv computation in the π-valence space yields 5.62 eV. Calculated excitation energies to various valence and Rydberg states are in good agreement with theoretical multiconfigurational single reference state second-order perturbation theory calculations by Roos and co-workers and with values from other highly correlated computations. The ℋv calculations for cis-butadiene further investigate the dependence of the computations on the nature and the choice of molecular orbitals and provide the first comprehensive study of the convergence with respect to the enlargement of the valence space for π-electron systems. The present computations also represent the first ℋv treatment of the σ→π* and π→σ* excited states in conjugated π-electron systems, along with an analysis of the required degree of σ–π correlation within the valence (or reference) space. Vertical π-and σ-ionization potentials are also produced as a byproduct of the ℋv calculations for neutral cis-butadiene, providing the first predictions of these ionization energies. The computations conclusively reconfirm the high accuracy of the ℋv method.
Ab initiopotential energy surface and rovibrational energies of Ar⋅ ⋅ ⋅CO
Authors: Shin Seokmin , Shin Seung Koo , Tao Fu-Ming
Published: July 2002

The potential energy surface for the Ar...CO van der Waals complex is calculated by the supermolecular approach using fourth-order Mo/ller–Plesset perturbation theory (MP4) with a large basis set containing bond functions. The Hartree–Fock potentials are repulsive for all configurations considered. The second-order correlation energy accounts for most of the dispersion interactions. The MP4 potential energy surface is characterized by a global minimum of −96.3 cm−1 at Re=3.743 Å and θe=98° with the argon atom closer to the oxygen end. There are no local minima in the linear configurations. The linear configurations provide shallow barriers at both of the carbon and oxygen ends. The barrier height at the oxygen end is 13.6 cm−1 at R=4.04 Å, while that at the carbon end is 28.0 cm−1 at R=4.58 Å. The rovibrational energies of Ar...CO are calculated by the discrete variable representation method. The Ar...CO complex undergoes large amplitude hindered rotations in the ground state with a zero-point energy of 21.8 cm−1. The ground state lies 7.2 cm−1 below the carbon-end barrier. The bending excited state lies 3.1 cm−1 above the carbon-end barrier, making the Ar...CO complex a nearly-free internal rotor. The calculated bending excitation frequency of 10.268 cm−1 for vCO=0 is in good agreement with the experimental value of 11.914 cm−1 for vCO=1. The A rotational constant of 2.638 cm−1 derived from the K-stack origins of the ground state is in reasonable agreement with the experimental result of 2.475 cm−1.
Coupled-cluster calculations of the excitation energies of ethylene, butadiene, and cyclopentadiene
Authors: Watts John D. , Gwaltney Steven R. , Bartlett Rodney J.
Published: July 2002

The equation-of-motion coupled-cluster (EOM-CC) method has been used to calculate the vertical excitation energies of ethylene, trans-butadiene, cis-butadiene, and cyclopentadiene. The approximations used were the EOM-CC singles-and-doubles (EOM-CCSD) method and EOM-CCSD with a noniterative treatment of triple excitations, EOM-CCSD(T), EOM-CCSD(T̃). The basis sets were atomic natural orbital sets augmented with diffuse functions. Comparisons have been made with a series of complete active space second-order perturbation theory (CASPT2) results, which used the same basis sets and geometries, and experiment. For the Rydberg states the EOM-CCSD results were in good agreement with experiment and CASPT2. The results for the valence states show more variation. For ethylene CASPT2 exaggerates the Rydberg character of the V state, while EOM-CCSD provides a better description. For the valence states of the other molecules the EOM-CCSD excitation energies tend to be too high, but considerable improvements are given by EOM-CCSD(T̃). For the 1 1Bu/1 1B2 states of butadiene and cyclopentadiene EOM-CCSD(T̃) agrees well with experiment and CASPT2. The agreement for the 1 1Ag→2 1Ag and 1 1A1→2 1A1 transitions for trans-butadiene and cis-butadiene, which are not known experimentally, is not so good, with the EOM-CC results significantly higher than the CASPT2 ones. However, the EOM-CCSD(T̃) and CASPT2 energies for the 1 1A1→2 1A1 transition in cyclopentadiene are in quite close agreement and both agree well with experiment. EOM-CCSD(T̃) shows an average error in vertical excitation energies for all states of all molecules of 0.12 eV, and improves those for valence states from an error of 0.26 eV for EOM-CCSD to 0.13 eV.
A continuum rod model of sequence-dependent DNA structure
Authors: Manning Robert S. , Maddocks John H. , Kahn Jason D.
Published: July 2002

Experimentally motivated parameters from a base-pair-level discrete DNA model are averaged to yield parameters for a continuum elastic rod with a curved unstressed shape reflecting the local DNA geometry. The continuum model permits computations with discretization lengths longer than the intrinsic discretization of the base-pair model, and, for this and other reasons, yields an efficient computational formulation. Obtaining continuum stiffnesses is straightforward, but obtaining a continuum unstressed shape is hindered by the ‘‘noisy’’ small-scale structure and rapid helix twist of the discrete unstressed shape. Filtering of the discrete data and an analytic transformation from the true normal-vector field to a natural (untwisted) frame allows a stable continuum fit. Equilibrium energies of closed rings predicted by the continuum model are found to match the energies of the underlying discrete model to within 0.5%. The model is applied to a set of 11 short DNA molecules (≊ 150 bp) and properly distinguishes their cyclization probabilities (J factors) when compared both to experimental cyclization rates and to Monte Carlo simulations. The continuum model does not include entropic contributions to the free energy. However, because of its rapid and accurate computation of internal energy, the continuum model should, when combined with further work on entropic effects, be a useful method for computing experimental DNA free energies.