bluuebunny/crossref_metadata_2025 huggingface dataset:
Interfacial behavior of phase separated asymmetric compressible binary polymer blends
Authors: Lifschitz Masha , Freed Karl F.
Published: July 2002

Our theory for the interfacial properties of compressible binary polymer systems [J. Chem. Phys. 98, 8994 (1993)] is generalized to treat phase separated asymmetric binary blends. The theory is illustrated by using a Sanchez–Lacombe approximation for the homogeneous system free energy and a Cahn–Hilliard–de Gennes expression for the square gradient contribution. These two ingredients enable the same theory to describe both the compositions of the coexisting phases and the interfacial properties. The coupled equations for the two concentration profiles are numerically unstable, and our perturbation–variational method for symmetric blends is extended to treat blends with asymmetry due to differing polymerization indices and/or interaction parameters. The zeroth order approximation, which uses a hyperbolic tangent interpolation between the generally different densities in the coexisting phases, is found to be extremely accurate in predicting the interfacial tensions and widths. Illustrations of the theory are provided for systems with upper and with lower critical solution temperatures, as well as with hour-glass miscibility gap type phase diagrams. The higher order perturbation–variation contributions mainly affect the total density variation through the interface, exhibiting several different patterns of density redistributions to lower the interfacial free energy. Computations are also provided for the pressure dependence of the interfacial properties.
Intrinsic non-RRK behavior: Classical trajectory, statistical theory, and diffusional theory studies of a unimolecular reaction
Authors: Shalashilin Dmitrii V. , Thompson Donald L.
Published: July 2002

The nonstatistical behavior of a unimolecular reaction at energies well in excess of the threshold is examined. This behavior is sometimes referred to as ‘‘intrinsically non-Rice–Ramsperger–Kassel–Marcus’’ (RRKM). It is well known that microcanonical unimolecular rates computed by using classical mechanics can deviate from the predictions of statistical theories, particularly at high energies. The simplest manifestation of this behavior is that rate constants as a function of energy cannot be represented by simple expressions such as the RRK equation, k(E)=ν(1−E*/E)s−1, with a single set of parameter values over a wide energy range; more specifically, fits of the classical RRK expression to trajectory results frequently yield values for the effective number of degrees of freedom s that are significantly smaller than the ‘‘theoretical’’ values 3N−6. In the present study, rates were calculated for the unimolecular dissociation of dimethylnitramine, (CH3)2NNO2, by simple N–N bond rupture over wide energy ranges by using classical trajectories and Monte Carlo transition-state theory. The formalism of a diffusional theory of chemical reactions is used to develop a model that relates classical reaction rates to intramolecular vibrational energy redistribution (IVR). This model is based on the assumption that the molecular modes can be separated into reaction coordinate and energy reservoir modes. It is shown how this model can be used to extrapolate high-energy, nonstatistical classical trajectory rates to the low-energy, statistical region.
Small angle scattering from porous mass fractal solids
Authors: McMahon Phillip , Snook Ian
Published: July 2002

The small angle scattering of radiation or particles from many porous solids may be described in terms of fractal geometry. To date most examples have been surface fractals but a few studies have suggested that some porous solids may be mass fractals. We give a derivation of the scattering law from such a solid and present an experimental example of such scattering behavior.
The structure of a Langmuir monolayer of methyl eicosanoate as determined by x-ray diffraction and Brewster angle microscopy
Authors: Foster W. J. , Shih M. C. , Pershan P. S.
Published: July 2002

Relaxed Langmuir monolayers of methyl eicosanoate were studied using Brewster angle microscopy (BAM) and grazing incidence x-ray diffraction (GIXD). The structure of the various phases in this system was determined and compared to previous isotherm and microscopy measurements. At low pressure and low temperature, a crystalline phase with tilt toward nearest neighbor, I(L2″), is observed. At a temperature of ∼14 °C and low pressure there is a transition to a phase with tilt toward next-nearest neighbor, F(L2′). Finally, as the temperature continues to be raised at low pressure, there is a transition to a phase where the two F(L2′) peaks have the same values of Qxy, here called the τ phase, with a Rotator IV-like structure. At high pressure and low temperature, an untilted, orthorhombic phase, U′(CS), is observed. As the temperature is increased, a second untilted, orthorhombic phase, U(S), is observed, beginning at ∼12 °C. Above 21 °C and at high pressure, a Rotator-II-like phase is seen, with an undistorted, untilted structure. Finally, results from BAM measurements are used to monitor the texture of the film in the different phases.
The resonance fluorescence polarization of free rotors: Methyl iodide in methane and carbon dioxide
Authors: Ziegler L. D. , Fan R.
Published: July 2002

The polarization of the resonance fluorescence of symmetric top rigid rotors is described by a third-order density matrix treatment of resonance emission and a sum-over-all-rovibronic states scattering-tensor invariant framework. Within this theoretical approach the resonance fluorescence depolarization is a function of the excited electronic state population and rovibronic coherence decay rates, as well as the electronic absorption/emission line shapes. This description of the depolarization of resonance fluorescence is contrasted with that of resonance Raman in terms of angular momentum selection rules and dependence on material relaxation parameters. In contrast to resonance Raman emission in solution, the accompanying resonance fluorescence polarization is found to be most sensitive to the resonant excited state lifetime when this population decay time is of the order or less than rotational periods. These effects are demonstrated for excitation resonant with the B-state origin of CH3I vapor in high pressures of CH4 and CO2. The solute–solvent interaction responsible for the pure dephasing of the resonant optical coherence does not appear to cause orientational redistribution of the excited chromophore, at least on the time scale of the CH3I B-state origin lifetime. The influence of excited electronic B-state rovibrational pure-dephasing effects on the resonance fluorescence polarization measurements are discussed.
Nonadiabatic dynamics and electronic energy relaxation of Cl(2P) atoms in solid Ar
Authors: Krylov A. I. , Gerber R. B. , Coalson R. D.
Published: July 2002

The dynamics of Cl(2P) atoms in a solid Ar matrix is studied, with emphasis on electronic energy relaxation of excited states, and on p-orbital reorientation effects. The method used follows Tully’s approach for nonadiabatic molecular dynamics simulations, which treats the electronic degrees of freedom quantum-mechanically, and the atomic motions classically, allowing for ‘‘hopping’’ of the atoms between different potential energy surfaces. We introduce an extended version of this method, to handle ‘‘Berry Phase’’ effects due to the doubly degenerate Kramers pairs of states present in this system. The role of both electrostatic and of spin–orbit interactions between different electronic states is incorporated in the treatment. The simulations yield a time scale of 13 ps for the energy relaxation of the highest excited electronic state of Cl(2P). A time scale of similar magnitude is found for the depolarization of this state. However, the time scale for orbital reorientation at thermal conditions is only 0.7 ps. This is attributed to the fact that at thermal conditions, only the two lowest electronic states are populated. The physical mechanisms of these basic radiationless decay processes are discussed on the basis of the simulations.
Time-dependent morphology changes in thin silver films on mica: A scaling analysis of atomic force microscopy results
Authors: Semin David J. , Lo Alan , Roark Shane E. , Skodje Rex T. , Rowlen Kathy L.
Published: July 2002

The postdeposition evolution of the morphology of a thin Ag film on a mica substrate was studied using a combination of experimental and theoretical techniques. Atomic force microscopy (AFM) was used to follow the surface morphology as a function of time at temperatures in the range 30–56 °C. The AFM images were numerically processed to obtain the distribution function of island sizes, defined as island height (h), as a function of time, f(h,t). The Ag films were observed to coarsen, i.e., small Ag islands disappeared while larger Ag islands increased in size. The island height distribution function was of a scaling form, f(h,t)∼f[h/h̄(t)], where h̄(t), the mean island height, increased monotonically as a power law h̄(t)∼tβh up until a crossover time t×. The experimental results for this low temperature annealing process are most consistent with a mechanism whereby the film coarsens through an island–island coalescence process. From the temperature dependence of the annealing kinetics, it was found that the coarsening process is thermally activated and has an activation energy of 13±2 kcal/mol. It was observed that the coarsening process terminates past the crossover time yielding a stable asymptotic distribution of islands which was independent of temperature (in the range 30–100 °C). Thus, it is suggested that a Ag film can be stabilized at room temperature by subjecting the film to a low temperature annealing process.
A microscopic frictional theory for reactions in condensed phases: Influence of nonlinear couplings
Authors: Nagaoka Masataka , Yoshida Naoto , Yamabe Tokio
Published: July 2002

On the assumption of external bath equilibrium, a set of simultaneous linear generalized Langevin equations (GLE) for a microscopic Hamiltonian is derived, whose potential function includes cubic (i.e., nonlinear) coupling terms, which are linear in internal coordinates but quadratic in external bath coordinates. Furthermore, on the linear GLE treatment, a closed expression of time-dependent friction coefficient and a rate constant in the Grote–Hynes theory (GHT) are derived microscopically, reflecting the reactant and solvent structures. By comparing the rate constant of GHT with that of the multidimensional transition-state theory (TST) for the whole solution system, we conclude that these rate expressions are different from each other and the deviation is due to the dynamic effect via the nonlinear coupling among the reaction, internal, and external normal coordinates. Moreover, the friction coefficient depends on temperature and the deviation becomes larger with temperature increasing. By the second-order perturbation theory, we have estimated the deviation which is approximately equal to a transmission coefficient κ, for a real cluster reaction system: the formic acid–water–water system. We have obtained κ of 0.92, which is smaller than unity. A mode analysis shows that two hindered translational motions of the solvent with low frequencies prevent the reaction from proceeding. Besides, we have investigated the isotope effect of a medium water molecule and found that the dynamic isotope effect for the reaction is quite large, i.e., κ for heavy water is much smaller than that for light water. Not the change of the reactive frequency on the free energy surface but that of the frictional effect in the deuterium substitution mainly contributes to the isotope effect. Further, the temperature dependence of κ for the reaction has been estimated and it is found that κ becomes smaller with temperature increasing and the change of the frictional effect in temperature contributes to the temperature dependence of κ more largely than that of the reactive frequency on the free energy surface.
Multiphoton absorption by metal–metal long distance charge-transfer complexes in polar solvents
Authors: Dakhnovskii Yuri , Lubchenko Vassiliy , Coalson Rob D.
Published: July 2002

A theory of multiphoton absorption of mixed valence compounds in polar solvents is developed. When these systems interact with a strong laser field, there exist various absorption ‘‘channels’’ which correspond to one, two, three,..., absorbed photon quanta. The probability of each channel to be switched on or off is dependent on the laser intensity, which results in strong intensity dependence of the absorption spectrum. Bands can be eliminated from the spectrum simply by changing the laser intensity. The physical picture of channels is justified by a derivation based on a kinetic master equation for the flow of electronic population. Calculations are carried out for the case where the polar environment can be modeled by a single high frequency (‘‘quantum’’) vibrational mode plus a classical bath represented by a collection of low frequency harmonic oscillators. The spectrum is found to be only weakly sensitive to the quantum vibrational mode. Suggestions for experimental verification are made.
Isomers of SO2: Infrared absorption of SOO in solid argon
Authors: Chen Li-Shiun , Lee Cheng-I , Lee Yuan-Pern
Published: July 2002

Sulfur dioxide (OSO) isolated in solid argon at 13 K was irradiated with light at 193 nm from an ArF excimer laser. Weak absorption lines at 1006.1, 1004.7, and 739.9 cm−1 observed after photolysis are assigned to sulfur superoxide (SOO); the doublet near ∼1005 cm−1 is due to matrix site splitting. The assignments are based on results from 18O-isotopic experiments. Calculations using the B-P86 and the B3-LYP density-functional methods were made for three isomers of OSO; energies, structures, vibrational wave numbers, and infrared intensities were predicted for each species. Although observed line positions are nearly identical to those predicted with theory for cyclic-SO2, experimental relative IR intensities and 18O-isotopic shifts agree with those predicted for SOO. The mechanism of formation of SOO in a matrix cage is discussed.
The kinetics of the bimolecular A+B→0 reaction in condensed matter: Effects of non-equilibrium charge screening
Authors: Kuzovkov V. N. , Kotomin E. A. , von Niessen W.
Published: July 2002

The kinetics of the bimolecular A+B→0 reaction between charged reactants is studied in two dimensions, i.e., on a surface. The theory is based on the Kirkwood superposition approximation for three-particle densities and the self-consistent treatment of the electrostatic interactions defined by the non-uniform spatial distribution of similar and dissimilar reactants. Special attention is paid to pattern formation and many-particle effects arising from reaction-induced formation of loose domains containing similar reactants only. It is shown that the critical exponent α characterizing the algebraic concentration decay law, n(t)∝t−α, differs strongly between symmetric (DA=DB) and asymmetric (DA=0) reactant mobilities. This effect is abnormal from the point of view of standard chemical kinetics. It arises directly from the specific spatial distribution in the system as in ‘‘raisins A in a dough B.’’ At long reaction times the asymptotics of the interaction potentials is of non-equilibrium type at large relative distances. The accumulation kinetics in the presence of a permanent source is studied. Results of the microscopic formalism are compared with a previous mesoscopic theory.
The determination of the infrared radiative lifetimes of a vibrationally excited neutral molecule using stimulated-emission-pumping, molecular-beam time-of-flight
Authors: Drabbels Marcel , Wodtke A. M.
Published: July 2002

The authors present a new experimental method for measurement of collision-free infrared radiative lifetimes for single quantum states of a vibrationally excited sample. This method provides a more direct route to the infrared Einstein A coefficients than has been previously possible. Results for NO(X 2Π, v=21 and v=7) are presented. Comparison to results of ab initio calculations shows excellent agreement. A controversy regarding the relative intensities of first overtone and fundamental emission intensities in NO is laid to rest. The most complete least squares analysis of existing data was carried out to derive the electric dipole moment function (EDMF) to an accuracy of ±0.02 D between 0.9 and 1.7 Å.
Effect of chemical reaction on diffusion of diluted gas: Simulations by means of two Monte Carlo methods
Authors: Nowakowski B. , Lemarchand A.
Published: July 2002

Two Monte Carlo methods are used to simulate a dynamics of molecules of a foreign gas A, highly diluted in a carrier gas C. Diffusion in the presence of bimolecular reaction A+C→products is studied. Nonequilibrium corrections to the reaction rate constant and diffusion coefficient are calculated for a wide range of molecular mass ratio mA/mC and activation energies of the reaction. Some differences between simulation results of the two methods are observed. The deviation of the velocity distribution from the Maxwellian form is quantified by means of the fourth order cumulants. The nonequilibrium effects are most significant in the Lorentz range, that is for mA≪mC. The simulation results prove that the theoretical predictions based on a perturbative solution of the Boltzmann equation are valid for not too small values of mA/mC, but are not correct in the Lorentz range.
Time–frequency theory of pump-probe absorption spectroscopy
Authors: Yan Yi Jing , Zhang Weimin , Che Jianwei
Published: July 2002

A variation of density matrix formulation based on the nature of field–matter interference in a mixed time–frequency domain is developed to study molecular pump-probe absorption spectra in condensed phases. Considered are both the integrated probe transmitant signals and the frequency-dispersed transient absorption coefficients for molecular systems with either two or three electronic surfaces involved. The present formulation is exact and applicable to any field with arbitrary timescale and shape, and it is valid when the pulses are overlapped as well as separated. The dual temporal-spectral effect of both the excitation and detection fields can be clearly elucidated via a transformation that preserves causality. The resulting field–matter interference picture of transient absorption in the correlated time-frequency domain is conceptually natural and physically transparent. The molecular dynamics, and the field–matter temporal/spectral coherence and interference phenomena can all be clearly demonstrated.
Microphase separation of crosslinked polymer blends in solution
Authors: Benhamou M. , Derouiche A. , Bettachy A.
Published: July 2002

We consider here a polymer blend made of two crosslinked polymers A and B of different chemical nature, in the presence of a common solvent assumed to be good for both species. To study the solvent effects on the microphase separation of the mixture, we first apply the so-called blob model which is a direct consequence of the renormalization theory. In this model and at large scales compared with the screening length ξ of the semidilute regime, polymer chains A and B are viewed as sequences having blobs of size ξ as subunits. Within this model, we calculate the structure factor enabling us to derive all microphase critical properties. The main result is that, the swelling effects simply lead to a renormalization of these properties, and that the renormalization factors are powers of the monomer concentration. Quantitatively, such a renormalization is in agreement with the fact that, in presence of a good solvent, the mixture is more compatible. The blobs picture is a mean-field approximation, to go beyond this approach which neglects the strong fluctuations of the density near the critical point, we reconsider the problem using the renormalization-group techniques. We show that these strong fluctuations lead to a microphase critical behavior characterized by nonclassical exponents, which are found to be identical to those of the Ising-type system.
A percolation theory of ionic clustering in electrolytes
Authors: Given James A. , Stell George
Published: July 2002

We use the methods of continuum percolation theory to develop a consistent, essentially analytic theory for the properties of the restricted primitive model (RPM) of electrolytes. Contributions to the thermodynamic properties of this system are divided into two types; those from pairs of ions in the same cluster, and those from pairs in different clusters (we call these IN and OUT contributions, respectively, for brevity). We give exact expressions for the IN contributions as weighted integrals over the ionic pair connectedness functions. We give an exact analytic solution for these functions in the generalized mean-spherical approximation. The OUT contributions are calculated by replacing the system of ionic clusters by a system of charged hard spheres having the same statistics, and using the analytic results available for the latter system. Because the method requires no input from simulations, it can be readily adapted to treat many different electrolyte systems. Our method closely models simulation data for the thermodynamic quantities of the RPM. An earlier note [J. Chem. Phys. 96, 9233 (1992)] sketched our theory and compared our results to electrolyte data. Here we present in detail the analytic basis for our method. In future papers we expect to present detailed numerical results.
Accessing highly vibrationally excited states of water using double resonance vibrationally mediated photodissociation
Authors: Barnes Rhett James , Gross Adam F. , Sinha Amitabha
Published: July 2002

Vibrational overtone-overtone double resonance combined with vibrationally mediated photodissociation is used to access highly excited vibrational states of water in the region of 22 000–25 000 cm−1 with good sensitivity. The double resonance aspect of the vibrational excitation process allows us the ability to access vibrational states of different symmetry than possible through direct absorption. In addition, the technique provides the means to examine state resolved photodissociation dynamics of this prototypical molecule from strongly mixed vibrational states as well as study collisional relaxation in this system.
Local free energy calculations in simulations
Authors: Vlot Margot J. , van der Eerden Jan P.
Published: July 2002

In this paper we apply two independent methods to determine chemical potentials locally, the overlapping distribution method of Shing and Gubbins and thermodynamic integration from an Einstein crystal, to the same Monte Carlo simulation. The system is a Lennard–Jones crystal with a surface near the melting point. We demonstrate that the overlapping distribution method results in reliable free energies in the surface region, whereas thermodynamic integration is preferable for the bulk part of the system. In this way we succeeded to check, for the first time, chemical equilibrium between surface and bulk. Such a consistency check is essential whenever one uses Monte Carlo or molecular dynamics simulations to study equilibrium properties of crystal surfaces, since relaxation times easily exceed acceptable simulation times.
Electronic dephasing and electron–phonon coupling of aluminum phthalocyanine tetrasulphonate in hyperquenched and annealed glassy films of ethanol and methanol over a broad temperature range
Authors: Reinot T. , Hayes J. M. , Small G. J.
Published: July 2002

The electronic dephasing (spectral dynamics) and electron–phonon coupling of aluminum phthalocyanine tetrasulphonate (APT) in glassy films of ethanol and methanol were investigated by nonphotochemical hole burning over a broad temperature range, ∼5–100 K. Films formed by hyperquenching (∼106 K s−1) at 4.7 K were studied as well as films that were subsequently annealed at temperatures up to ∼170 K. Results are compared against those for APT in glassy water [Kim et al., J. Phys. Chem. 99, 7300 (1995); Reinot et al., J. Chem. Phys. 104, 793 (1996)]. As in the case of water, the linear coupling is weak with a Huang–Rhys factor S∼0.4 but the mean phonon frequencies for ethanol and methanol of 26 and 17 cm−1 are considerably lower than the 38 cm−1 value for water. These modes are assigned as pseudolocalized with significant amplitude (libration) localized on APT. Below about 8 K, the electronic dephasing/spectral diffusion is dominated by coupling to the tunneling intrinsic two-level systems of the glass. At higher temperatures the electronic dephasing is dominated by the exchange coupling mechanism, which derives from diagonal quadratic electron–phonon coupling. Here, for both ethanol and water, a pseudolocalized mode(s) at ∼50 cm−1 is operative. This frequency corresponds to a peak in the spectral density of the liquids which for water is due to the transverse acoustic mode. The results show that the modes responsible for linear and quadratic coupling are distinctly different. Implications of this for optical coherence loss in liquids are considered. Novel results from annealing experiments are reported and discussed in terms of the complex phase diagrams of ethanol and methanol. Formation of the glass from the supercooled liquid just above the melting point of a crystalline phase leads to a marked reduction (∼10×) in the homogeneous width of the zero-phonon hole at 4.7 K. This is interpreted in terms of a reduction in the density of intrinsic two-level systems due to reduced structural disorder of the glass formed from the supercooled liquid. As in the case of water, the highly efficient hole burning in glassy ethanol and methanol is observed to become highly inefficient upon formation of a crystalline phase as predicted by the Shu–Small mechanism for nonphotochemical hole burning. The close connection between this mechanism and Onsager’s inverse snowball effect for solvent dynamics around an instantaneously created point charge or dipole in a liquid is emphasized.
Influence of the head group size on the direction of tilt in Langmuir monolayers
Authors: Schmid F. , Lange H.
Published: July 2002

A model of rods with heads of variable size, which are confined to a planar surface, is used to study the influence of the head group size on tilted phases in Langmuir monolayers. Simple free energy considerations as well as exact zero temperature calculations indicate that molecules with small head groups tilt towards next nearest neighbors, and molecules with larger head groups towards nearest neighbors. This provides a possible explanation for recent experimental results, and for details of the generic phase diagram for fatty acid monolayers.
Square-well fluids confined in planar slits
Authors: Song Yong-Jun , Kim Soon-Chul
Published: July 2002

A simple perturbative approach, which was proposed by Choudhury and Ghosh, has been employed to predict the structural properties of confined square-well fluids and compared with the computer simulation. The calculated results show that the Choudhury–Ghosh approximation well describes the structural properties of confined square-well fluids compared with the computer simulation.
Accurate analytical representations of the core-electron densities of the elements 3 through 118
Authors: Cioslowski Jerzy , Piskorz Pawel , Rez Peter
Published: July 2002

The core-electron density in a molecule is defined as a sum of perfectly transferable, spherically symmetrical atomic contributions ρAC(Z,N,r). Analytical functions can be fitted to ρAC(Z,N,r) with a charge-conserving algorithm. The relativistic core-electron densities of the elements 3 through 118, obtained from numerical multiconfigurational Dirac–Fock calculations, are accurately represented by linear combinations of 50 s-type Gaussian primitives arranged in even-tempered basis sets. These representations are well suited for the augmentation of valence-electron densities produced by semiempirical methods and approaches involving effective core potentials. Calculations of the electronic properties of atoms in the TiCl4 and CdH2 molecules that employ such augmentation are presented.
Charge shifting in the ultrafast photoreactions of ClO− in water
Authors: Lim Manho , Gnanakaran S. , Hochstrasser Robin M.
Published: July 2002

The reaction dynamics of ClO− in water following femtosecond ultraviolet photolysis is investigated by measuring time-resolved absorption and anisotropy. Ab initio calculations show that light absorption induces charge shifting from the O− atom to the Cl atom. Molecular dynamics simulations predict that the charge shift is followed by the destruction of the solvent structure around the O atom and its formation around newly formed negative charge on the Cl atom. An ultrafast (∼60 fs) transient absorption change is observed and likely corresponds to the inertial part of the destruction of the solvent structure around the newly formed neutral O atom of the excited state OCl−. The early time anisotropy of −0.13±0.05 decays on the 230 fs time scale and is attributed to the dissociation along a new reaction path toward Cl+O− that is seen independently through the evaluation of the spectrum of Cl. The remaining anisotropy decays within 6 ps due to rotational diffusion of the ion. Probe wavelength dependence of the longer time dynamics (1.3–7 ps) is proposed to be the vibrational relaxation of the vibrationally hot ground state of ClO−, the generation of which can be described as electron transfer from the excited state OCl− to the ground state ClO−.
Temperature dependence of optical dephasing in an organic polymer glass (PMMA) from 300 K to 30 K
Authors: Nagasawa Yutaka , Passino Sean A. , Joo Taiha , Fleming Graham R.
Published: July 2002

The temperature dependence of the optical dephasing mechanism in an organic polymer glass, polymethylmethacrylate (PMMA), was studied from 300 K to 30 K using the dye IR144 as a probe. Transient grating and three pulse photon echo measurements were made, and the three pulse photon echo peak shift (3PEPS) was recorded as a function of temperature. The peak shift data reveal time constants of ∼6 fs and ∼60 fs, along with vibrational beats and a long-time constant value for the peak shift. The 6 fs component is attributed to intramolecular vibrations and the 60 fs component to librational degrees of freedom of the PMMA itself. This contribution appears slightly underdamped and the fitted spectral density matches well with the Raman spectrum of PMMA. The two ultrafast decays are insensitive to temperature. For temperatures above 80 K the long-time peak shift increases linearly as temperature decreases but at 80 K the shift levels off and decreases for temperatures between 80 and 30 K. Fit values for the inhomogeneous width (500 cm−1) and the reorganization energy (378 cm−1) describe the initial value of the peak shift, its decay, the absorption spectrum, and the three-pulse photon echo signal quite well at both high and low temperature. We were not very successful in describing the temperature dependence of the long-time peak shift, although the insensitivity of the dynamics to temperature could be qualitatively accounted for. At low temperature the imaginary portion of the line shape function, which is temperature independent, contributes significantly to the response, while at high temperature the dephasing is dominated by the real part of the line shape function. A more sophisticated model is required to quantitatively describe the data.
Kinematic mass model of activated bimolecular reactions: Molecular shape effects and zero-point energy corrections
Authors: Perdih Marko , Miklavc Adolf , Smith Ian W. M.
Published: July 2002

The recently proposed simple collision model of activated bimolecular reactions which takes into account the nonspherical shape of molecules and includes the effects of reagent rotation has been studied in considerable detail, in order to determine its range of applicability. By taking the limit of hard nonspherical molecules in which the formulation becomes rigorous it is possible to show that the model is likely to work well if the shape of the potential surface is prolate in the region of the reaction barrier, as long as a direct reaction mechanism is involved. The model cannot be expected to work for diatomic reactants if the shape of the potential surface is pronouncedly oblate because of the complicated reorientation effects which such a surface exerts on the trajectories and which do not appear amenable to simple modeling. The original model is improved by including the zero-point energy corrections on the assumption of vibrational adiabaticity en route to the barrier. With the improved model, cross-sections are calculated for the reactions: H+F2, O+H2, N+O2, O+HBr, O+HCl, O+DCl, and the results are compared with those of quasiclassical trajectory calculations and, in the last two cases, also with those of quantum mechanical calculations. The possible origins of the significant discrepancies which occur in some cases are also discussed.
Quantum-mechanical wave packet calculation of photoinduced surface reaction: O2/Pt(111)
Authors: Strömquist Johan , Gao Shiwu
Published: July 2002

We present a two-dimensional quantum-mechanical wave packet study of photoinduced reaction of O2 on Pt(111) within a mechanism of hot electron/hole excitation of a molecular resonance. Based on three constructed potential energy surfaces including the molecule-surface and intra-molecular coordinates, photoexcitation is simulated by nonadiabatic electronic transitions between the ground state (the chemisorbed O2−) and a negative ion resonance state (the O22− shape resonance) or a neutral O2 state. The wave packet dynamics exhibits a fast energy exchange between the two bonds, giving a comparable yield for desorption and dissociation. The calculated branching ratio, BR=0.5–1.0, between desorption and dissociation and the mean kinetic energy of the desorbed molecules 〈Ekin〉/2kB= 990 K are comparable with the reported experimental data measured in desorption by nanosecond laser pulses, while the vibrational temperature is first predicted by this calculation. In addition our results indicate the importance of a proper treatment of damping effect in a coherent wave packet after deexcitation.
A calculation of the entropy and free energy differences between the α and β phases of solid nitrogen
Authors: Kuchta Bogdan , Rohlender Krzysztof , Swanson David , Etters Richard D.
Published: July 2002

The transition between fcc and hcp solid phases of N2 is examined by calculating their Helmholtz free energies using the Monte Carlo method. This shows that the transition occurs at about 41 K, with an entropy change very close to the experimental value. No plastic phase intermediate between the α and β structures is indicated. However, within a few degrees of the transition, orientational exchange of the molecules from one body diagonal to another commence. Another precursor to the transition are considerably increased orientational fluctuations.
Vibrationally and rotationally resolved angular distributions for F+H2→HF(ν,j)+H reactive scattering
Authors: Dharmasena Gamini , Phillips Timothy R. , Shokhirev Kirill N. , Parker Gregory A. , Keil Mark
Published: July 2002

Angular distributions for individually resolved ν, j states from the F+H2→HF(ν,j)+H chemical reaction are measured for the first time. Vibrational and rotational resolution is achieved simultaneously by applying laser+bolometer detection techniques to crossed-beam reactive scattering. In addition to backward-scattering HF(ν=1, j=6) and HF(ν=2, j=5), we also observe HF(ν=1, j=6) products scattered into the forward hemisphere. The results are in qualitative agreement with fully three-dimensional exact quantum reactive scattering calculations [Castillo et al., J. Chem. Phys. 104, 6531 (1996)] which were conducted on an accurate potential-energy surface [Stark and Werner, J. Chem. Phys. 104, 6515 (1996)]. However, the forward-scattered HF(ν=1, j=6) observed in this experiment is not reproduced by quasi-classical calculations [Aoiz et al., Chem. Phys. Lett. 223, 215 (1994)] on the same potential-energy surface.
The hypermagnetizability of molecular oxygen
Authors: Jonsson Dan , Norman Patrick , Vahtras Olav , Ågren Hans , Rizzo Antonio
Published: July 2002

The hypermagnetizability and the hypermagnetizability anisotropy of the oxygen molecule are computed using cubic response theory applied to multi-configurational self-consistent field wave functions. The effects of basis set, electron correlation, frequency dispersion, zero point vibrational averaging and pure vibrational contributions are discussed. The result for the anisotropy (Δη=+2.65 a.u. at λ = 632.8 nm), even taking into account possible limitations in the treatment of electron correlation and in the incompleteness of the basis set, maintains a different sign and is more than two orders of magnitude smaller than the experimental values published in the literature. Possible reasons for this large discrepancy are discussed.
The millimeter wave spectrum of copper monoxide: CuO
Authors: Steimle T. , Namiki K. , Saito S.
Published: July 2002

The pure rotational spectrum of 65CuO in its ground X 2Πi state was recorded in the 198–384 GHz spectral region using a dc-sputtering absorption spectrometer. The determined 65Cu(I=3/2) hyperfine parameters are a=214.1(35), bF=−483.6(94), c=72.5(95), d=134.5(34), and eQq0=−16.9(54) MHz, where a three standard deviation error estimate is given in parentheses. A plausible molecular orbital description for the X 2Πi state based upon the determined parameters is given.
Spin–orbit branching in the photodissociation of HBr: Time-independent, time-dependent, and semiclassical calculations
Authors: Péoux Gérald , Monnerville Maurice , Duhoo Thierry , Pouilly Brigitte
Published: July 2002

The dynamics of the photofragmentation of HBr is treated within time-independent, time-dependent, and semiclassical methods. The calculated relative cross sections for formation of the two accessible fine-structure channels [Br(2P1/2) and Br(2P3/2)] agree well with the experimental results, both in magnitude and in dependence on photon excitation wavelength. For relatively small photon wavelength (λ=193 nm), vertical excitation in the Franck–Condon region populates preferentially the A 1Π state, and only three states (A 1Π, the Ω=1 components of the a 3Π and 1 3Σ+), coupled by the spin–orbit interaction, are invoved in the dissociation process. For larger photon wavelength (λ=243 nm), the product branching is governed by initial excitation in both the A 1Π state and the a 3Π(Ω=0) component. Comparison of the redistribution of the time-independent photofragment fluxes as a function of the H–Br separation with the temporal evolution of the populations within a time-dependent framework shows that the two methods, although based on a different point of view, provide equivalent mechanistic information on the dissociation process.
Nonequilibrium grand ensemble method for dense fluids
Authors: Eu Byung Chan
Published: July 2002

A nonequilibrium grand ensemble method is developed for dense simple fluids by using an irreversible kinetic equation whose collision term is required to satisfy a few conditions. We show that the kinetic equation satisfying such conditions is sufficient for constructing a formal theory of irreversible processes, which is consistent with the thermodynamic laws, and an nonequilibrium extension of the Gibbs equilibrium ensemble theory. As in the equilibrium ensemble theory, nonequilibrium macroscopic (thermodynamic) variables are given in terms of the nonequilibrium partition function. This formal theory supplies the evolution equations for macroscopic observables which are consistent with the thermodynamic laws and an integral part of the nonequilibrium ensemble method. They generalize the classical hydrodynamic equations of Navier, Stokes, and Fourier to nonlinear transport processes occurring far from equilibrium. The evolution equations also include the constitutive equations for the fluid which describe transport properties of the substance when the collision term in the kinetic equation is explicitly modeled by taking into account the details of molecular collision events in the system.
Reactions of Ni38 with N2, H2, and CO: Cluster structure and adsorbate binding sites
Authors: Parks E. K. , Nieman G. C. , Kerns K. P. , Riley S. J.
Published: July 2002

The gas-phase reactions of nitrogen, hydrogen, and carbon monoxide with Ni38 are studied as a function of reagent pressure at several reaction temperatures. Saturation coverage of the cluster is found at Ni38(N2)24, Ni38H36, and Ni38(CO)36. These saturation levels are consistent with the metal core of the ligated cluster having the structure of a truncated octahedron in each case. An alternate fcc structure derived from a 40-atom truncated tetrahedron is consistent with the nitrogen data, but not with the hydrogen or carbon monoxide results. In addition, the nitrogen uptake data indicate that the bare Ni38 cluster also has the structure of a truncated octahedron or possibly a deformed truncated octahedron. There is no indication that Ni38 has an icosahedral or polyicosahedral structure. The nature of the binding of the three reagents to the cluster is discussed. Evidence is presented that CO initially binds to atop sites, but following saturation of these sites a local rearrangement to bridge sites occurs that allows an increase in coverage to the observed saturation at Ni38(CO)36. At high reagent pressures all three reagents cause adsorbate-induced structural changes to isomers that bind more ligands and whose structures have yet to be determined.
Negative ion photoelectron spectroscopy of the ground state, dipole-bound dimeric anion, (HF)2−
Authors: Hendricks Jay H. , de Clercq Helen L. , Lyapustina Svetlana A. , Bowen Kit H.
Published: July 2002

We present the mass spectral and photoelectron spectroscopic results of our study of (HF)2−. Our main findings are as follows. The (HF)2− anion was observed experimentally for the first time, confirming the 20 year old prediction of Jordan and Wendoloski. The photoelectron spectrum of (HF)2− exhibits a distinctive spectral signature, which we have come to recognize as being characteristic of dipole bound anions. The vertical detachment energy (VDE) of (HF)2− has been determined to be 63±3 meV, and the adiabatic electron affinity (EAa) of (HF)2 was judged to be close to this value as well. Relatively weak spectral features, characteristic of intramolecular vibrations in the final (neutral dimer) state, were also observed. We have interpreted these results in terms of slight distortions of the dimer anion’s geometric structure which lead to an enhanced dipole moment. This interpretation is supported to a considerable extent by theoretical calculations reported in the companion paper by Gutowski and Skurski.
Photodissociation of IBr near 267 nm by center-stripe analysis of two-dimensional image
Authors: Kim Yong Shin , Jung Young-Jae , Jung Kyung-Hoon
Published: July 2002

The photodissociation dynamics of IBr has been investigated near 267 nm by a two-dimensional photofragment imaging technique. Speed and angular distributions for Br(2P3/2) and Br(2P1/2) fragments are obtained from the center-stripe analysis of their images. Using the speed distributions, three dissociation channels are proposed with the formations of I(2P3/2)+Br(2P3/2), I(2P3/2)+Br(2P1/2), and I(2P1/2)+Br(2P3/2). All three paths, judging from their anisotropy parameters, have shown the mixed characters of parallel and perpendicular transitions with different degrees. Relative quantum yields for the three channels are also determined and resolved into their anisotropic transition types. With reference to a recent work at 304 nm [Jung, Griffiths, and El-Sayed, J. Chem. Phys. 103, 6999 (1995)], we present the complete assignment of the photoexcitation of IBr leading to the three channels in the uv band region: the perpendicular transition to Π11(2341) state predominantly for I(2P3/2)+Br(2P1/2), the parallel transition to Π0+3(2341) mainly for I(2P1/2)+Br(2P3/2), and the mixed transitions of the parallel to Π0+3(2341) and/or to Σ0+−3(2422) and perpendicular to Π13(2341) states for I(2P3/2)+Br(2P3/2) channel, respectively. The nonadiabatic couplings and the order of vertical energy levels between excited states are discussed in terms of difference of the relative quantum yield at different excitation photon energies.
Lattice theory of a multicomponent mixture of monodisperse polymers of fixed architectures
Authors: Ryu Jong-Hoon , Gujrati P. D.
Published: July 2002

We present a lattice theory for a multicomponent mixture of p distinct polymeric species, each of a prescribed architecture but without any cycles and s monomeric species along with a solvent species, the latter playing the role of a reference species whose amount is controlled not by any activity but by the sum rule of fixed amount of material. The theory is an extension of our previous work on a binary mixture of polymers in bulk or a general mixture next to a surface. The model allows for nearest-neighbor interactions between unlike species. The chemical bondings are allowed to be between monomers (of the same species) that are nearest-neighbor. The resulting theory is obtained by solving the model on a Bethe lattice. The theory has a very simple structure and supersedes random mixing approximation to which it reduces in a special limit, the random mixing approximation limit, see text. We study the behavior of a ternary system numerically and compare it with that of a binary system. We also compare the predictions of our theory with simulations and find them to be consistent. However, our theoretical predictions are inconsistent with the conventional Flory–Huggins theory. Thus, our theory is superior to the Flory–Huggins theory.
Shear viscosity and dielectric constant of liquid acetonitrile
Authors: Mountain Raymond D.
Published: July 2002

Molecular dynamics has been used to evaluate the predictions for the coefficient of shear viscosity and for the dielectric constant for three-site models of acetonitrile as these properties are important when simulating processes in mixtures. The model of Edwards et al. [Mol. Phys. 51, 1141 (1984)] provides a value for the shear viscosity that is close to the experimental value and a value for the dielectric constant that is about 18% less than the experimental value. The model of Jorgensen and Briggs [Mol. Phys. 63, 547 (1988)] provides values that have larger deviations from the experimental values. The model of Edwards et al. is recommended as the three-site model of acetonitrile to use in simulations of mixtures.
One-electron versus multielectron effects in the near-threshold C 1s photoionization of acetylene
Authors: Kempgens B. , Kivimäki A. , Köppe H. M. , Neeb M. , Bradshaw A. M. , Feldhaus J.
Published: July 2002

The C 1s partial photoionization cross section and asymmetry parameter β of the C2H2 molecule have been determined with high-energy resolution between threshold and 360 eV. The C 1s shake-up satellite spectrum is richly structured; the cross section of several satellites increases strongly near threshold, which indicates that there is a conjugate contribution to their intensity. It is shown that the large enhancement in the total photoabsorption cross section between 305 and 320 eV, previously attributed to a σ* shape resonance, is largely due to this photon energy dependence of the shake-up transitions.