bluuebunny/crossref_metadata_2025 huggingface dataset:
Triplet dipoles in the absorption spectra of dense rare gas mixtures. I. Short range interactions
Authors: Guillot Bertrand , Mountain Raymond D. , Birnbaum George
Published: July 2002

A theory is proposed to evaluate the induced-dipole moment occuring when three dissimilar atoms are mutually interacting. Based on the one-effective electron model, the theory predicts that, at short and intermediate distances, the triatomic dipole moment originates from three different processes, namely, overlap, quadrupole-induced, and dipole-induced. These three-body dipoles have then been implemented into a molecular dynamics simulation in order to generate the collision-induced absorption spectra by rare-gas mixtures. For Ar–Kr liquid mixtures, the irreducible three-body contributions to the spectral density are found so important that they exceed two-body contributions. This is due to a profound cancellation between two-body dipoles which does not occur between irreducible three-body dipoles. However, the comparison with experimental data is poor because of a shortcoming of the model calculation which does not take into account long-range dispersion interactions. On the contrary, a quantitative agreement is obtained for the fundamental vibration band of compressed H2 –He mixtures, indicating that exchange overlap, three-body dipoles play the leading role for such light systems.
Model Hamiltonians for atomic and molecular systems
Authors: Carlson J. , Moskowitz Jules W. , Schmidt K. E.
Published: July 2002

A model Hamiltonian, designed to allow larger systems to be treated with the Green’s function Monte Carlo method, is introduced for atomic and molecular systems. The model reduces the statistical variance associated with Green’s function Monte Carlo calculations by reducing potential energy fluctuations in the core regions. By performing calculations of Li, LiH, and Li2 we show that this method can be used to obtain energy differences with much less computer time than required for the complete interaction. Increases in efficiency for larger systems will be even greater.
Level-to-level vibrationally inelastic rate constants for Li2* –X (X=He, Ne, Ar, Xe) collisions
Authors: Magill Peter D. , Scott Thomas P. , Smith Neil , Pritchard David E.
Published: July 2002

We present 1088 level resolved rate constants for the vibrotationally inelastic collisions Li*2(vi=9, ji)+X →Li*2(vf, jf)+X with Li2 in the A 1∑+u state, ji =8, 22, and 42 and X=He, Ne, Ar, and Xe at T∼600 °C. At high ji, the results display a quasiresonant enhancement of vibrational transfer and a striking lack of dependence on target gas (X). At ji=42, the jf distributions for Δv≡vf−vi≠0 are quite narrow and peak at jf=ji+Δj with Δj=−4Δv. Internal energy conservation would require Δj=−6Δv; hence our use of the prefix ‘‘quasi’’ above. At high ji, the vibrationally inelastic cross sections are large: e.g., 4.5 Å2 into the single final level Δv=−1, jf =46 and over 80 Å2 total for Xe. Absolute errors as small as 6% have been routinely attained by using an analysis procedure which averages data from several fluorescence bands after correcting for variation of Franck–Condon factors with j.
The ionization threshold of N,N,N′,N′-tetramethyl-p-phenylenediamine in dense fluid ethane; effects of fluid density and temperature
Authors: Faidas H. , Christophorou L. G. , Datskos P. G. , McCorkle D. L.
Published: July 2002

The ionization threshold IF of N,N,N′,N′-tetramethyl-p-phenylenediamine in ethane, has been measured in the density (ρ) range 0.15–13.30 M/l and over the temperature (T) range 295–413 K, using a multiphoton ionization conductivity technique. The IF was found to be a function of both ρ and T in the ranges studied. At a fixed T (=373 K), IF was found first to decrease with increasing ρ and then to level off at densities of ∼10 M/l. For ρ≥11 M/l and T=295 K, the IF was found to increase with increasing density. At constant density (ρ=5.90 M/l ) IF decreased with increasing T between 323 and 413 K. These results are analyzed and discussed in relation to the effect of ρ and T on the electron conduction band energy V0 and the medium polarization energy P+. The changes in the IF with ρ are attributed to the dependence of V0 and P+ on ρ, while the dependence of IF on T is attributed to the effects of T on V0 rather than on P+.
Analytical second derivatives of two electron integrals over s and p Cartesian Gaussians
Authors: Schlegel H. Bernhard
Published: July 2002

An efficient algorithm for computing second derivatives of two electron integrals is outlined and has been coded for s- and p-type Cartesian Gaussians. This method is an extension of a previous algorithm for two electron integral first derivatives [H. B. Schlegel, J. Chem. Phys. 77, 3676 (1982)] and is suitable for scalar machines with modest memories. Timing data indicates that the present algorithm is 3–6 times faster than a scalar version of the Rys polynomial method. Furthermore, the computational time for the two electron integral second derivatives is only 2–3 times longer than for the integral first derivatives by the same approach. The code has been incorporated in the recently released version of gaussian 88.
Time dependence of donor–acceptor electron transfer and back transfer in solid solution
Authors: Lin Y. , Dorfman R. C. , Fayer M. D.
Published: July 2002

Electron transfer from an optically excited donor to randomly distributed acceptors followed by electron back transfer is treated theoretically for donors and acceptors in a rigid solution. The forward electron transfer process is described in terms of the excited state population probabilityPex(t) of the donor molecules, while the electron back transfer from the radical anion to the radical cation is characterized by Pct(t), the donor cation state population probability. Exact expressions for the ensemble averages 〈Pex(t)〉 and 〈Pct(t)〉 are derived. Numerical calulations are presented for the cation probabilities, the average cation–anion separation distance 〈R(t)〉, and the average cation existence time 〈τ(R)〉, using parameters which characterize the forward and back transfer distance dependent rates. Relationships among 〈Pex(t)〉, 〈Pct(t)〉 and the intermolecular interaction parameters provide detailed insights into the distance and time dependence of the flow of electron probability in an ensemble of donors and acceptors. The theoretical expressions can be used to calculate experimental observables. In particular, picosecond transient grating experiments are analyzed, and it is shown that by combining grating experiments (or other ground state recovery experiments) with fluorescence experiments it is possible to obtain the intermolecular interaction parameters for both forward and back transfer and a detailed description of the dynamics. The calculations presented here for rigid solutions are the precursor to the inclusion of diffusive motion of donors and acceptors to describe the dynamics of coupled electron transfer and back transfer in liquid solutions.
Effect of molecular orientation on electron transfer
Authors: Harland Peter W. , Carman Howard S. , Phillips Leon F. , Brooks Philip R.
Published: July 2002

K+ ions have been detected from the collision of a beam of fast (5–30 eV) K atoms with a beam of molecules (CH3I and CF3I) which were state selected and oriented prior to the collision. Collisional ionization is favored for both molecules when the fast K is incident at the I end of the molecule, even though the electrical polarity of the I ‘‘end’’ is different for the two molecules. For both molecules, the threshold for impact at the I end is ≊0.7 eV less than the threshold for impact at the other end. The effect of molecular orientation is most pronounced near threshold (≊5 eV) and almost disappears at higher (30 eV) energies.
Analysis of dipole–dipole coupling in isotopic mixtures of N2 on Ni(110)
Authors: Brubaker Mary E. , Trenary Michael
Published: July 2002

We have made a detailed comparison of experimental infrared spectra of isotopic mixtures of 14N2/15N2 on Ni(110) with spectra calculated according to the dipole–dipole coupling theory of Persson and Ryberg [Phys. Rev. B 24, 6954 (1981)]. The values of the electronic and vibrational polarizabilities used to calculate the spectra are αe =4.4 Å3 and αv =0.26 Å3 . The frequency shift and the intensities of the calculated spectra agree with the corresponding experimental quantities. The full widths at half maximum (FWHM) of the calculated data are narrower than the experimental spectra. The results show that a 36 cm−1 vibrational coupling shift can be accounted for entirely by dipole–dipole coupling. The negative chemical shift of 42 cm−1 is attributed to increased backdonation into the 2π* orbital due to band formation at higher coverages.
Structure of a polar fluid near a wall. Exact asymptotic behavior of the profile, relation with the electrostriction phenomena and the Kerr effect
Authors: Badiali J. P.
Published: July 2002

We study the asymptotic behavior of the position-orientation profile ρ(1) for a dipolar hard sphere fluid in contact with a neutral hard wall. First, by a virial expansion we show that ρ(1) is not totally determined by the classical image potential, i.e., by a dielectric continuum model. The exact expression of ρ(1) far from the wall is derived by using a renormalized cluster expansion. As predicted by the dielectric continuum model, ρ(1) exhibits an orientational structure and a long tail which decreases as the inverse cubic power of the distance to the wall. We first examine the density profile ρ0(1) which corresponds to the isotropic part of ρ(1). We show that ρ0(1) contains a part reminiscent of the image potential but also some contributions which depends on the pair correlation function and the triplet direct correlation function in bulk phase. When this last function is neglected, ρ0(1) can be considered as the result of a force balance in the interface. In the general result, the triplet direct correlation allows us to obtain a very compact expression for ρ0(1). It is shown that the asymptotic behavior of ρ0(1) reveals the same microscopic properties as the classical electrostriction phenomena which is observed in bulk phase and in presence of an external electric field. Thus, ρ0(1) can be considered as the result of a natural electrostriction induced in the interfacial region by the image potential. The term of lowest symmetry in the orientational structure describes the alignment of a molecule relative to the normal to the wall. Far from the wall, the orientational profile is proportional to the dipolar contribution of the Kerr constant. Thus, the asymptotic behavior of the profile and the Kerr effect are determined by the same function characterizing the alignment of dipoles. This alignment is observed via an external field in the case of the usual Kerr effect and it is naturally induced by the image potential in the interfacial region. The triplet direct correlation function gives rise also to some additional orientational structures which are ignored in the dielectric continuum model. The exact results derived in this paper include some bridge diagrams and consequently they are beyond the wall–particle hypernetted chain

approximation.
The stability of binary charged colloidal crystals
Authors: Shih Wan Y. , Shih Wei-Heng , Aksay Ilhan A.
Published: July 2002

The stability of a binary disordered substitutional colloidal crystal has been examined against the formation of a colloidal liquid. Phase diagrams are constructed by comparing the free energy of the liquid phase to that of the solid phases. The calculations show that the freezing density has a maximum as a function of the number fraction. The maximum in the freezing density becomes more pronounced when the particle diameter ratio differs more from unity. If the particles have very different diameters, the binary colloidal crystals are unstable against the colloidal fluid. The freezing density rises almost vertically when the number fraction differs from 0 or 1 by less than 6%. The pronounced stability of the liquid phase for particles with more different sizes may explain the formation of colloidal glasses as opposed to the formation of disordered substitutional colloidal crystals with particles of more similar diameters. The Hume–Rothery rule for a binary metallic alloy also applies to a binary charged colloid if the diameter ratio is taken from the diameters of the effective hard spheres which are determined by the interparticle interactions but not those of the bare particles.
Molecular solvent model for an electrical double layer: Reference hypernetted chain results for potassium chloride solutions
Authors: Torrie G. M. , Kusalik P. G. , Patey G. N.
Published: July 2002

The reference hypernetted-chain theory is solved for the structure of the double layer at the surface of large spherical macroions in a wholly molecular model of aqueous KCl. Detailed results are reported for the solvent and ionic structure throughout the double layer for surface charges up to 0.175 C m−2 and salt concentrations of 0.1 to 1.0 M. Concentration effects on the short-range structural features of the interface are discussed and related to the behavior of the same model at infinite dilution. There is a very rapid neutralization of the surface charge by a Coulombic adsorption of counterions into a narrow region near contact beyond which the system responds as though to a much lower effective surface charge. The relationship of these results to the properties of continuum solvent models and the implications for the electrostatic potential and differential capacitance of the double layer are discussed.
Analytical approach to molecular liquids. II. Solvation of ions in molecular fluids
Authors: Zhou Yaoqi , Stell George
Published: July 2002

Analytical equations are obtained for the pair correlation functions in the mean spherical approximation (MSA) for two charged hard-sphere ions in a model molecular solvent. Model solvents of dipolar dumbbells (which may be partially dissociative) or charged hard-spheres fused in a tetrahedral configuration are treated in an extended MSA. The resulting expressions for the Born solvation free energy of an ion are very similar to that obtained earlier by Chan et al. for a charged hard-sphere ion in a dipolar hard-sphere solvent. The solvent averaged ion–ion potentials are obtained in the hypernetted chain approximation. Comparisons among results of various solvents are also made. In order of increasing oscillation amplitudes of the solvent averaged ion–ion potential for given molecular density and dipole moment we have continuum ≪dumbbell≤tetrahedral<dipolar hard sphere.
Unstable periodic orbits, recurrences, and diffuse vibrational structures in the photodissociation of water near 128 nm
Authors: Weide Klaus , Kühl Klaus , Schinke Reinhard
Published: July 2002

The photodissociation of H2O in the second absorption band (X̃→B̃) is investigated in a completely time-dependent approach. The Schrödinger equation is solved by a time-dependent close-coupling method expanding the two-dimensional wave packet in terms of free rotor states. The vibrational degree of freedom of the OH fragment is fixed and only motion on the B̃-state potential-energy surface is considered. The calculated absorption spectrum exhibits a long progression of diffuse structures, ΔE∼0.1 eV, in very good agreement with the experimental spectrum. The structure is readily explained in terms of a recurrence of the autocorrelation function after about 40 fs. The recurrence, in turn, is attributed to special indirect trajectories which on the average perform one oscillation within the deep potential well before they dissociate into products H+OH. These trajections are ‘‘guided’’ by so-called unstable periodic orbits which persist to energies high above the H+OH(2 Σ) threshold. The existence of unstable periodic orbits leading to a recurrence of the autocorrelation function gives, for the first time, a consistent explanation of the diffuse structure in the absorption spectrum of H2 O in the second band.
Spikes in the sticking probability of light gas particles on solid surfaces
Authors: Buch V.
Published: July 2002

This study focuses on van Hove-type spikes in the sticking probability of light particles on crystalline surfaces. The spikes result from singularities in the density of surface phonons subject to the constraints of the energy and the momentum conservation. The sharp features in sticking probability as a function of incoming angle and energy are derived and demonstrated for a model H2/Cu(100) system.
Block Lanczos approach combined with matrix continued fraction for the S-matrix Kohn variational principle in quantum scattering
Authors: Yang Weitao , Miller William H.
Published: July 2002

An iterative method is proposed for calculating the S matrix in the Kohn variational approach. Instead of solving the system of linear equations directly, the method consists of a block Lanczos algorithm extended to complex symmetric matrices and a matrix continued fraction procedure. Applications to inelastic and reactive scattering calculations indicate a reasonable rate of convergence.
On the interaction between thermal electrons and polar solvents
Authors: Jay-Gerin J.-P. , Ferradini C.
Published: July 2002

An attempt to establish a correlation between the optical absorption and the mobility of thermal electrons, in solvents of various polarities, is presented. Implications of this correlation as to the peculiarities of the electron–solvent interactions are briefly discussed.
Electronic correlations in polyethylene
Authors: König Günter , Stollhoff Gernot
Published: July 2002

Results of a correlation calculation for the ground state of polyethylene, as obtained within the local ansatz, are presented. The starting point is a SCF ground state calculated within a double-zeta basis. For the correlation part, the basis is extended by including a set of polarization functions on each atom. The correlation computations are inexpensive and fast converging. On correcting for finite basis set deficiencies, an estimate of the binding energy is obtained which is very close to experiment. Pair correlation functions are also discussed. These compare well to those obtained previously for small hydrocarbon molecules and diamond, where similar bond patterns occur. The computation scheme itself contains a new feature, which allows for an improved use of the translational symmetry.
Low energy charge exchange cross sections between sodium and H+, H+2, H+3, N+2, H2O+, and O+2
Authors: Loch R. , Stengler R. , Werth G.
Published: July 2002

We confined ions of different molecules in a Penning quadrupole ion trap and observed the exponential loss of the stored ion cloud as the result of charge exchange collisions with a Na atomic beam passing through the trap. From the ion loss rates and the Na density we determined the charge exchange cross sections between Na and the ions noted in the title at a mean ion energy of 2 eV. The results are in reasonable agreement with theoretical expectations and other experiments.
A coupled anharmonic oscillator model for optical nonlinearities of conjugated organic structures
Authors: Prasad Paras N. , Perrin Eric , Samoc Marek
Published: July 2002

We present a simple model of coupled locally anharmonic oscillators which can be used to describe the optical nonlinearities in conjugated organic monomeric, oligoneric, and polymeric structures. We show that the method can very readily be used to explain the dependence of the band gap, the polarizability α, and the second hyperpolarizability γ on the number of repeat units of conjugated chain compounds by adjusting two parameters: the local anharmonicity term and the oscillator coupling constant. To illustrate the usefulness of this model, we have calculated the dependence of the band gap, the polarizability α, and the second hyperpolarizability γ, as the function of the number of repeat units for the oligomers of thiophene and benzene. The results predicted by the coupled anharmonic oscillator model are in good agreement with those of the experimental studies on thiophene and benzene oligomers recently reported by our group. In addition, the predicted power dependences of orientationally averaged 〈α〉 and 〈γ〉 on the number of repeat units are compared with those predicted by a free electron model, PPP method, sum-over-states method, and ab initio calculations.
Towards organic polymers with small intrinsic band gaps. II. Investigation of the electronic structure of poly(pyrenylene vinylenes)
Authors: Toussaint J. M. , Wudl F. , Brédas J. L.
Published: July 2002

Geometry optimizations and band structure calculations are reported on two polymeric vinylene derivatives of pyrene: poly(1,6-pyrenylene vinylene) and poly(2,7-pyrenylene vinylene). The results are indicative of a dramatic sensitivity of the electronic structure on the mode of connection of the two vinylene units to the pyrene rings, in agreement with the chemist’s intuition. An analysis of the atomic π orbital contributions to the one-electron wave functions shows that the HOMO–LUMO characteristics of both compounds are markedly different. The 1-6 mode of connection leads to an electronic structure similar to that of polyparaphenylene with a bandgap of 2.9 eV. The 2-7 mode of connection results in a significantly lower bandgap, around 1.7 eV, due to the stabilization of quinoid resonance structures.
Relaxation behavior in a bistable chemical system. Plateau behavior
Authors: Ganapathisubramanian N. , Reckley John S. , Showalter Kenneth
Published: July 2002

The relaxation behavior of the bistable iodate–arsenous acid reaction near a hysteresis limit has been investigated. Outside the bistability region, relaxations to the unique stable steady state may exhibit a plateau in concentration as a function of time, corresponding to a local minimum in rate of reaction. Very close to the saddle-node bifurcation defining the bistability limit, the relaxing system remains in the plateau for long periods of time. Experimental measurements of plateau behavior are compared to behavior predicted by a one-variable model of the iodate–arsenous acid system.
The Schwinger and Newton variational principles for the log-derivative matrix
Authors: Ramachandran B. , Wyatt Robert E.
Published: July 2002

We present the Schwinger and Newton variational principles for the log-derivative matrix. These methods have one significant advantage over their K, or T matrix analogs: the Green’s functions that satisfy the log-derivative boundary conditions can be made independent of the scattering energy, which means that all matrix elements between basis functions become energy independent, and hence need be evaluated only once. The convergence characteristics of these functionals are compared with those of the K matrix Schwinger and Newton functionals, for potential scattering problems. The amplitude density version of the Newton variational principle is then generalized to the multichannel case, and used to compute transition probabilities for a popular inelastic scattering problem at several energies. These results are compared to those obtained from the application of a discrete representation of the Kohn variational principle for the log-derivative matrix to the same problem.
Rotational dynamics in simple dense liquids
Authors: Evans Glenn T.
Published: July 2002

Recollision kinetic theory (RcKT) is applied to the calculation of the rotational friction coefficient of a hard nonspherical convex body in a dense fluid of hard spheres. The ratio of the rotational friction, fR , to its Enskog value, fER , is determined to be fR/fER≂1+1.06ρ* exp(5ρ*2), where ρ* is the packing fraction of the hard sphere solvent. RcKT accounts for the effect of caginglike correlated recollisions on molecular rotation. The above equation brings kinetic theory predictions of rotational friction and orientational correlation times into closer accord with experimental and molecular dynamics derived values.
Anisotropic rotational diffusion of entangled rodlike polymers under elongational flow
Authors: Teraoka Iwao , Hayakawa Reinosuke
Published: July 2002

Quantitative treatment of rotational diffusion of entangled rodlike polymer molecules in semidilute solution is extended to include anisotropy in the orientational distribution of rods. Noting that anisotropy has a similar effect to changing the polymer concentration of the size and form of a tube confining a rodlike molecule, we apply the mean-field argument to the hindered diffusion of entangled rods. The rotational diffusion constant, which depends not only on the orientation but also on the diffusion direction, is derived as a functional of the orientational distribution function. As an example of external force which causes anisotropy, we study stationary elongational flow and obtain the rotational diffusion constant self-consistently as a function of the strain rate. It is found the tube is not a round but an oblate cylinder.
Cooper minima and rotationally resolved resonance enhanced multiphoton ionization spectroscopy
Authors: Rudolph H. , McKoy V.
Published: July 2002

We demonstrate that a Cooper minimum, close to threshold, in photoionization via an excited molecular Rydberg state can have a dramatic influence on the ionic rotational branching ratios. It is also shown that this behavior can be exploited to produce ions selectively in a specific rotational level. To illustrate this effect we present the results of ab initio calculations for (2+1′) resonance enhanced multiphoton ionization via the O11 (23.5) branch of the H 2Σ+(3d,4s) state of NO, where a Cooper minimum is found in l=3 of the kσ and kπ continua at photoelectron kinetic energies of 2.6 eV and 2.9 eV, respectively.
Stirring rate effect in the closed, batch Belousov–Zhabotinsky system with oxalic acid
Authors: Ševčík Peter , Adamčíková Ľubica
Published: July 2002

Oxalic acid can produce oscillations in the Belousov–Zhabotinsky-type reaction in a closed batch system without a mechanism for bromine removal. The stirring rate has a significant effect on the reaction.
Light scattering power of randomly cut random coils with application to the determination of depolymerization rates
Authors: Reed Christopher E. , Reed Wayne F.
Published: July 2002

The scattering of light by a dilute monodisperse population of random-coil molecules is approximately described by the well-known function P(θ)=(2/u2)(e−u+u−1), where u=R2gq2, Rg is the radius of gyration of the random coils, and q=(4πn/λ)sin(θ/2) is the magnitude of the scattering vector. We show that if the molecules in this population undergo random scission, then P(θ) is still given by the above formula where, however, u is now equal to R2goq2+r, where r is the average number of scissions per molecule in the originally monodisperse population and Rgo is the original radius of gyration. It is suggested that this could be useful for determining depolymerization rate constants, and for investigating whether various forms of scission are actually random. Results from initial experiments which apply the theory to the acid hydrolysis of hyaluronic acid are presented. The rate constants found are in reasonable agreement with values in the literature.
Symmetry breaking in a chemical pinwheel
Authors: Kreisberg Nathan , McCormick W. D. , Swinney Harry L.
Published: July 2002

A transition that breaks the m-fold rotational symmetry of a chemical pattern of traveling waves is reported. The chemical waves travel around a thin annular gel, the inner and outer edges of which are in contact with continuous flow chemical reservoirs. Observations of the dependence of the rotating wave pattern as a function of temperature reveal several distinct states, including a wavelength-doubled state in which successive waves alternate in shape.
Ultrasonic and hypersonic properties of molten KNO3–Ca(NO3)2 mixture
Authors: Cheng Lap-Tak , Yan Yong-Xin , Nelson Keith A.
Published: July 2002

A picosecond time-domain light scattering technique is used to study the viscoelastic KNO3–Ca(NO3)2 60:40 glass-forming liquid mixture. By using scattering angles between 1.92° and 85.67°, acoustic frequencies from 50 MHz to 4 GHz are sampled. Together with existing ultrasonic and Brillouin scattering data, a temperature-dependent distribution of relaxation times is found to be well fit with a Cole–Cole distribution whose width changes from several decades in the glassy state to nearly single relaxation time in the high-temperature liquid state. The characteristic relaxation time is found to obey the Vogel–Tammann–Fulcher law with T0=338 K.
The microwave spectrum of the LiO radical
Authors: Yamada Chikashi , Fujitake Masaharu , Hirota Eizi
Published: July 2002

The rotational spectra of the 7LiO radical in the v=0 and v=1 vibrational states were observed in the millimeter and submillimeter wave region. The LiO radical was generated by the reaction of lithium vapor with N2O directly in a high temperature absorption cell. The rotational and centrifugal distortion constants, Λ-type doubling constants, spin–orbit interaction constant, and hyperfine interaction constants were derived through analysis of the rotational spectra observed in the present study combined with the radio frequency spectra of Λ-type transitions reported by S. M. Freund et al. [J. Chem. Phys. 56, 1467 (1972)]. The LiO radical was confirmed to have a 2Πi ground electronic state with an exceptionally large Λ-type doubling constant, as in the case of NaO. A pure precession hypothesis applied to the observed p Λ-type doubling constant gives the Σ–Π energy difference to be 2565 cm−1.
Inertial effects in butane stochastic dynamics
Authors: Pastor Richard W. , Karplus Martin
Published: July 2002

The range of validity of diffusive algorithms is studied by comparing the isomerization rates and rotational reorientation times from a series of Langevin dynamics trajectories of butane with the results of a Brownian dynamics (diffusive) trajectory, as well as with analytic approximations. It is found that inertial effects may be ignored for rotation at most liquid densities and contribute approximately 10% to the isomerization rate at water viscosities. For neat butane, or other short-chain alkane solvents at room temperature, the effect of inertial terms on isomerization rates is significant.
Direct inelastic scattering of N2 from Ag(111). IV. Scattering from high temperature surface
Authors: Kummel Andrew C. , Sitz Greg O. , Zare Richard N. , Tully John C.
Published: July 2002

We have measured the rotational state distribution and the angular momentum alignment and orientation of N2 scattered from Ag(111) at 540 K. Using resonance enhanced multiphoton ionization (REMPI), we are able to probe the scattered flux as a function of the exit angle θexit. For a modestly glancing incident beam (θi =30°) and incident translational energy, Ei =0.3 eV, the angular momentum alignment (tumbling vs helicoptering) at both quasispecular detection (θexit=35°) and superspecular detection (θexit=50°) is only weakly dependent upon the surface temperature. However, the angular momentum orientation (clockwise vs counterclockwise rotation) is strongly affected by the surface temperature. Raising the surface temperature from Ts =90 K to Ts =540 K causes the orientation to decrease substantially. Stochastic trajectory calculations were carried out in conjunction with the experiments. They reveal that at low temperature there is an averaging over two important initial conditions: the two-dimensional impact parameter and the molecular orientation geometry. At high temperature there is also an averaging over the instantaneous positions and momenta of the surface atoms. Hence, a given two-dimensional impact parameter and molecular orientation geometry results in a greater range of final J states, angular momentum polarizations, and velocities (exit angles) at high temperature than at low temperature. The resulting ‘‘smearing’’ accounts for the changes in rotational state distribution and polarization as a function of exit angle observed at high temperature. The major effect of averaging over the positions of the surface atoms (thermal roughening) upon the orientation of the scattered N2 is to increase the exit angle averaging rather than to increase the in-plane forces.
Multidimensional harmonic oscillator matrix elements
Authors: Fernández Francisco M. , Tipping R. H.
Published: July 2002

Multidimensional harmonic oscillator matrix elements of arbitrary products of exponentials for operators containing only linear and quadratic terms in the creation and annihilation operators are obtained. A simple and straightforward method is presented that leads to recurrence relations and generating functions for such matrix elements. Results are shown to be general enough to cover the cases of Franck–Condon overlaps, transition probabilities for time-dependent harmonic oscillators, and correlation functions for electronic spectra.
Temperature effect on nascent rotational state distribution of product MgH in reaction of Mg(3s3p1P1)+H2→MgH+H
Authors: Lin King-Chuen , Huang Cheng-Tsung
Published: July 2002

A pump-and-probe technique is utilized to yield a population distribution over the rotational quantum states of the nascent product MgH in the reaction of Mg(1P1) and H2. The resulting normalized profile of the MgH bimodal distribution at 693 K coincides with that at 733 K, as well as with the results obtained at 380 K by Breckenridge and co-workers. This temperature dependence demonstrates that the bimodality actually results from the insertive reaction alone. This conclusion is consistent with the isotopic effect.
Diode laser probing of the high-frequency vibrational modes of baths of CO2, N2O, and CO excited by relaxation of highly excited NO2
Authors: Chou J. Z. , Hewitt S. A. , Hershberger J. F. , Brady B. B. , Spector G. B. , Chia L. , Flynn G. W.
Published: July 2002

Quenching of highly excited vibrational states of NO2 in baths of CO2, N2O, and CO has been investigated. Dilute NO2 mixtures were excited by a pulse from an excimer pumped dye laser operating at 495 nm. The ν3 antisymmetric stretching modes of CO2 (2349 cm−1) and N2O (2223 cm−1) and the v=1 level of CO (2143 cm−1) were probed with continuous wave IR diode lasers. The amount of energy transferred from excited NO2 to the ν3 modes of both CO2 and N2O was found to be 3%±1% of the original excitation energy. On the other hand, a smaller amount of energy (0.9%±0.3%) was deposited into the CO vibrational mode for NO2 contained in a dilute CO bath. These results support a picture of very inefficient transfer from high energy states of donor molecules to the high-frequency vibrational modes of small bath molecules. Differences in the efficiency of energy reception by the high-frequency modes of these three molecules scale roughly as the infrared transition moments for the bath states indicating the importance of resonance effects in the energy transfer process, but a combination of short- and long-range force mechanisms is probably necessary to account for the observed differences.
Calculations of 1 eV–3 keV oxygen scattering from Ag(111) using a
b i
n
i
t
i
o pair potentials
Authors: van den Hoek P. J. , Kleyn A. W.
Published: July 2002

Classical trajectory calculations, using an ab initio Ag–O potential, are presented of O2 scattering from Ag(111) in a broad energy range. These calculations can explain the results of various O2/Ag(111) scattering experiments performed at different energies. For O2 scattering around 1 eV, the experimentally observed angular broadening in the scattered signal can be explained as an onset of surface rainbows. The degree of dissociation observed in scattering around 200 eV (incoming angle with respect to surface normal (θi=70°) and around 2 keV θi=85°) can be explained by a collision-induced dissociation. Since O2/Ag(111) is a very corrugated system, we also studied the effect of the parallel velocity on the scattering. With constant normal velocity, the ‘‘largest corrugation’’ (most energy loss and dissociation) occurs for θi between 30° and 60°. For grazing incidence (θi≊85°), the scattering shows the characteristics of scattering from a flat surface: energy losses only decrease the normal component of the incident velocity and energy losses and fractions of dissociated molecules are independent of the parallel component. However, the molecule gets more rotationally excited than in the case of scattering from a flat surface with the same normal incident velocity. Finally, we investigated the relative amounts of vibrational and rotational excitation leading to dissociation. For a flat surface, the dissociation turns out to be mostly due to vibrational excitation, but for a corrugated surface, also rotational excitation largely contributes to dissociation.
Spin-dependent force model of molecular liquids. III. Orbital and spin angular momenta degrees of freedom
Authors: Juanós i Timoneda J. , Haymet A. D. J.
Published: July 2002

We present a formalism for molecular liquids in which the interatomic force depends upon both the internuclear separation and on internal, quantum degrees of freedom. The internal degrees of freedom are spin and orbital angular momenta, and they are eventually approximated to yield a purely classical function for the interatomic interaction. We show that the interpolative character of the classical energy function is derived from a resolution of a model Hamiltonian. The formal derivation of the potential energy function is illustrated with an explicit calculation for some nonpolar and polar systems, and with discussion for ionic systems. Our derivation emphasizes an alternative to the ever more popular three-body potential energy functions for statistical mechanics studies of liquids. We outline the path to be followed in computer simulations and analytical studies of liquids with our pairwise-additive, spin-dependent potential energy functions.
Brownian dynamics study of the end-to-end distribution function of two-dimensional linear chains in different regimes
Authors: Bishop Marvin , Clarke Julian H. R.
Published: July 2002

Brownian dynamics computer simulations are used to investigate the shape of the mean-squared end–to–end distance distribution function for two-dimensional linear chains in three regimes: (1) excluded volume, (2) θ, (3) collapsed. It is found that Mazur’s function fits regime (1), that the Gaussian function fits regime (3), and that the changeover in behavior between these two functional forms appears to occur at temperatures well above the expected θ point.
Triplet electron paramagnetic resonance detection of porphyrinoids in fluid liquid crystals at elevated temperatures
Authors: Regev Ayelet , Levanon Haim , Murai Toshiaki , Sessler Jonathan L.
Published: July 2002

The photoexcited triplet state of two porphyrinoids, H2TTP and a Cd complex of Texaphyrin (Cd-TxP), oriented in liquid crystals were examined by time-resolved electron paramagnetic resonance (EPR) spectroscopy as a function of temperature. The low probability of molecular tumbling in the liquid crystalline phase allows the detection of triplets by fast EPR, at temperatures (300 K) which are above the hosts’ melting points, and where the fluid properties of the matrices are maintained. Line shape and kinetic analyses of the triplet spectra provide data on spin relaxation rates, interchromophore interactions, and on the dynamic behavior of the liquid crystalline hosts, driven by the external magnetic field.
High resolution vacuum ultraviolet fluorescence excitation spectrum and predissociation of Ã 1A″ HCN
Authors: Jonas David M. , Zhao Xinsheng , Yamanouchi Kaoru , Green Peter G. , Adamson George W. , Field R. W.
Published: July 2002

The Ã–X̃ fluorescence excitation spectrum of HCN has been recorded using 4-wave mixing in Sr vapor as a tunable vacuum ultraviolet source. The excitation spectra are notable for the high signal to noise ratio, resolution (FWHM ∼0.3 cm−1) of nearly all previously blended lines, and accuracy of wavelength calibration (±0.03 cm−1). Measured lifetimes for the first three bending levels of the Ã state [τ(000)1=13±7 ns, τ(010)1=130±24 ns, and τ(020)1<2 ns] qualitatively confirm the results of Hsu, Smith, and Wallace [Chem. Phys. Lett. 111, 219 (1984)]. Stern–Volmer fluorescence quenching studies reported here yield a best fit quenching constant for the Ã(000)1 rotational levels of Kq =73/μs⋅Torr, which is similar to the quenching constant of the Ã(010)1 level (Kq =24.3±0.4/μs⋅Torr).
Pure rotational spectrum of the mercury–argon van der Waals complex
Authors: Ohshima Yasuhiro , Iida Minoru , Endo Yasuki
Published: July 2002

The pure rotational spectrum of the mercury–argon complex has been observed by pulsed-nozzle Fourier-transform microwave spectroscopy. Rotational constants, determined for six major isotopic species of mercury, fix the mercury–argon distance of 4.053 Å. The eQq constant for 201Hg⋅Ar is estimated to be less than 0.5 MHz and indicates small perturbation on the electric charge distribution of mercury when the complex is formed.
A new accurate (time-independent) method for treating three-dimensional reactive collisions: The application of optical potentials and projection operators
Authors: Neuhauser Daniel , Baer Michael
Published: July 2002

This work describes a new (time-independent) approach to the study of atom–diatom reactive collisions in three dimensions. The method is based on the idea of converting a reactive multiarrangement problem into an inelastic single-arrangement problem. This conversion is done by applying optical potentials which are located at all exits of the reagents arrangement. The reactive transition probabilities are calculated applying flux formulas. The method is reminiscent of a previous time-dependent method successfully applied for both collinear and three-dimensional reactive collisions.
Resonant two photon ionization of V atoms: V+ electronic state distributions and J-resolved photoelectron angular distributions
Authors: Sanders Lary , Hanton Scott D. , Weisshaar James C.
Published: July 2002

We use one color, resonant two photon absorption to ionize a skimmed atomic beam of V formed by laser vaporization of a target rod in the throat of a pulsed nozzle. The resulting time-of-flight photoelectron spectra resolve the 3d4(5D), 3d34s(5F), and 3d34s(3F) electronic terms and the spin–orbit levels J=2, 3, and 4 within 3d34s(3F) as well. For nine different ionizing wavelengths, we present state-resolved photoelectron angular distributions, which yield anisotropy coefficients β2,i and β4,i and angle-integrated electronic branching fractions fi for each resolved feature i. Generally speaking, the photoionization cross sections are dominated by direct processes of the type 3d3(4F)4s4p(1P0)→3d34s(35F)+ks,kd whenever the leading term in the intermediate state wave function and the total available energy permit. All of the corresponding term-resolved angular distributions are qualitatively similar, with positive β2 and negligible β4. When the leading term of the intermediate state is of the form 3d3(2X)4s4p(3P0), where X is the orbital angular momentum of the 3d3 core, the V+ branching fractions are not easily understood. In these cases of indirect ionization, term-resolved angular distributions are typically nearly isotropic. We find no clear evidence of nonzero β4 parameters. The J-specific angular distributions vary substantially for J=2, 3, and 4 within the 3d34s(3F) term. The branching fractions at each ionizing wavelength permit us to extract state-specific chemical reaction cross sections of V+ in the accompanying paper.
A molecular beam-optical Stark study of the A 2Π–X 2Σ band system of YO
Authors: Steimle Timothy C. , Shirley Jeffrey E.
Published: July 2002

The permanent electric dipole moments of yttrium monoxide YO in its X 2Σ+, A 2Π3/2, and A 2Π1/2 states have been experimentally determined from an analysis of the Stark shifts in the optical spectrum of a molecular beam sample. The determined values are 4.45(7), 3.68(2), and 3.22(8) D, respectively. The results are discussed in terms of bonding models for YO and are compared to recent theoretical predictions.
Low-temperature approximations for Feynman path integrals and their applications in quantum equilibrium and dynamical problems
Authors: Mak C. H. , Andersen Hans C.
Published: July 2002

A new approximation for discretized equilibrium Feynman path integrals for both a single degree of freedom and for many coupled degrees of freedom is presented. The approximation is suitable for harmonic and anharmonic oscillators with discrete bound states, but not for free particles and scattering problems. It is capable of accounting for zero-point motion in an approximate way. When compared to the standard high-temperature approximation, the new approximation leads to more accurate results, especially in the limit of zero temperatures. When generalized to the computation of thermally symmetrized time correlation functions, this new approximation does not suffer from the usual phase cancellation problems and can be extended to time t≫βℏ/2 in some cases. The utility of this new approximation in equilibrium and dynamical problems is illustrated with several numerical examples.
End-point distributions for dense-phase chains in two dimensions
Authors: Tuthill G. F.
Published: July 2002

We have studied the probability distribution of end–end distances for noncrossing, noncyclic chain polymers at high concentrations in two dimensions, using Monte Carlo simulations and a lattice model. The scaling behavior of the distribution in the dense regime is found to be consistent with the existence of a dense critical phase for this system, and we show that when the fugacity for chain ends increases (the number of chains rises) the distribution crosses over toward that expected for ordinary self-avoiding walks (SAWs). We study excluded volume effects and estimate the exponent γ in the dense regime by examining the fraction of chains whose endpoints are on adjacent sites.
Optical spectra of molecular crystals: The effect of exciton band structure on spectra and linewidths
Authors: Port H. , Silbey R.
Published: July 2002

We discuss the spectrum (linewidths and positions) of the first triplet state of the molecular crystals anthracene, napthalene, and phenazine. These crystals have similar exciton bandwidths, but varying amounts of anisotropy in the intermolecular interactions. We analyze the linewidths at zero temperature using isotopic scattering as the broadening mechanism, and the temperature dependence using one and two phonon scattering processes as the broadening mechanism.
Presence of half-bound states in equilibrium: A separable potential estimate
Authors: Krishnan Mangala S. , Snider R. F.
Published: July 2002

For a gas in which there is a significant amount of dissociation, the presence of both bound and free states is well recognized. However, because of the interaction with other molecules in the gas,the pair density matrix can include terms which are off-diagonal in arrangement channel. Here an estimate in made of the magnitude and structure of these ‘‘half-bound’’ states at equilibrium. A model separable potential and the impulse approximation are used to make this estimate.
Influence of vibrational frequency mismatch on phase-space bottlenecks to intramolecular energy redistribution and molecular fragmentation
Authors: Tersigni Samuel H. , Gaspard Pierre , Rice Stuart A.
Published: July 2002

We report studies of classical models of unimolecular fragmentation of van der Waals complexes using the methodologies of Hamiltonian mappings and flows. The effect of frequency mismatch between harmonic molecular vibrations of a host molecule and the van der Waals bond-stretching motion is shown to dominate vibrational energy redistribution and fragmentation. Our results show that a crossover exists when the frequency mismatch is increased from a stochastic regime of resonances between the nonlinear oscillators to a regular regime where the system behaves like an integrable system. Our model systems include both collinear and T-shaped oscillator displacement configurations, as well as a many-oscillator system that included both classes of motions. Oscillator parameter ranges were used that approximate vibrations in C6H6⋅He and C6H6⋅Ar van der Waals complexes.