bluuebunny/crossref_metadata_2025 huggingface dataset:
Configurational relaxation and diffusion of a flexible polymer in a dynamically disordered medium
Authors: Loring Roger F.
Published: July 2002

A stochastic model of the dynamics of a flexible linear polymer is developed and analyzed. The medium in which the polymer moves relaxes on a time scale whose value is an adjustable parameter, and may therefore be taken to be either long or short compared to the time scales of polymer motions. The polymer is represented by a freely jointed chain, which moves by a ‘‘kink–jump’’ algorithm. At any instant in time, a fraction of the chain’s beads are immobilized by obstacles that relax on an arbitrary time scale. The connection of this model to random walks with dynamical bond disorder is established, and calculations of the correlation function of the end-to-end vector and of the mean squared displacement of one bead are performed using the dynamical effective medium approximation.
Threshold photoionization in HCL
Authors: Frohlich H. , Guyon P. M. , Glass-Maujean M.
Published: July 2002

The threshold photoelectron spectrum of HCl shows two bands (2Π3/2 and 2Π1/2) of unequal intensity and width with partially resolved rotational structure, the 2Π1/2 band being about half the 2Π3/2 one. Autoionization of Rydberg states with a 2Π1/2 core into the 2Π3/2 continuum, added to direct ionization, explains the difference observed on the bands which would otherwise be of equal intensities. Direct ionization is observed to proceed according to the rotation spectator model while the autoionization process is due to the combined effects of spin-orbit coupling and L and S uncoupling [Hund’s case (e)] yielding high rotational angular momentum transfer ΔJ≤7/2.
Collisional excitation of CO by 2.3 eV H atoms
Authors: McBane George C. , Kable Scott H. , Houston Paul L. , Schatz George C.
Published: July 2002

Vibrational and rotational distributions of CO excited by collisions with 2.3 eV H atoms have been obtained by monitoring the products with vacuum ultraviolet (VUV) laser induced fluorescence. Translational-to-vibrational (T→V) transfer is dominated by the dynamics of collisions occurring in the two wells on the H+CO potential energy surface, one characterizing the HCO radical and the other characterizing COH. The measured vibrational distributions agree well with the results of trajectory calculations performed on the ab initio potential energy surface of Bowman, Bittman, and Harding (BBH). The measured rotational distributions show two significant differences from the calculated ones. First, for v=0 the experiments find more population in J<15 than predicted. This discrepancy may be due to errors in the repulsive part of the BBH surface that is outside the HCO and COH wells, but inside the van der Waals well. Second, for v=1, the experimental distribution is flat from J=0 to J=10, whereas the calculated one rises from near zero at J=0 to a peak at J=12. This discrepancy appears to be the result of an excessively high ab initio estimate (by a few tenths of an eV) of the barrier for H atom addition to CO to form COH.
Rotation–vibration energy levels by a Hamiltonian reexpansion technique
Authors: Chang Bernard H. , Secrest Don
Published: July 2002

It is shown that by reexpanding the interaction potential for a molecule as a product of functions of the internal coordinates, the Hamiltonian matrix may be rapidly and efficiently computed and the rotation–vibration eigenvalues of the molecule may be easily computed for a number of rotational states. The Kratzer oscillator functions are found to be a rapidly converging set for this purpose. The method was tested on the HCN molecule using a potential for which accurate earlier calculations are available for comparison.
Lattice dynamics of α-CO from an a
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o potential
Authors: Janssen W. B. J. M. , Michiels J. , van der Avoird A.
Published: July 2002

Harmonic lattice dynamics calculations have been performed for the antiferroelectric P213 ordered phase of α-carbon monoxide and for two related structures with a different head–tail ordering of the CO molecules. The potential used is an ab initio CO–CO potential with its anisotropy expanded in spherical harmonics, as well as a site–site model fitted to this ab initio potential. Good agreement with experiment is obtained for the structure, the cohesion energy, and the phonon frequencies without any adjustment of the potential. Head–tail reordering of the molecules is energetically almost neutral if it is accompanied by a translational displacement along the bond axes, which is symmetric with respect to the inversion centers of the average Pa3 structure. This displacement is induced in particular by the strong head–tail anisotropy in the short-range repulsion. Translation–rotation coupling is found to be important; it affects especially the frequency of the lowest optical mode.
Monte Carlo electrostatic persistence lengths compared with experiment and theory
Authors: Reed Christopher E. , Reed Wayne F.
Published: July 2002

An off-lattice rotational isomeric state model Monte Carlo algorithm for a polyelectrolyte with Debye–Hueckel screening and no hard core repulsion is used to generate short (up to 150 unit) chains at (1) different ionic strengths, (2) varying uniform charge densities, and (3) pH–pK0 governed ionization. The mean square radii of gyration are related to the apparent total persistence lengths of the polyelectrolytes via the wormlike chain model. Near the random coil limit the apparent electrostatic persistence length varies approximately as the inverse square root of the ionic strength and linearly with charge density. The persistence length behavior is very similar in cases (2) and (3). These approximate power laws agree well with those found experimentally for hyaluronate and variably ionized polyacrylic acid. The original electrostatic persistence length theory, which does not contain excluded volume effects, predicts power law exponents which are twice these. Corrections due to polyelectrolyte excluded volume theories do not consistently yield good fits to the data, but do give ‘‘pseudo-,’’ or weakly changing power laws similar to those obtained from the Monte Carlo and experimental data. The similarity between the Monte Carlo and experimental results may indicate that the Debye–Hueckel approximation is reasonable, and that the neglect of hard core repulsion (e.g., zero chain diameter) is insignificant compared to electrostatic effects. This lends strength to the interpretations that electrostatic excluded volume effects explain much of the apparent deviation from the electrostatic persistence length theory and that the approximation of a smeared out line charge works fairly well, even when the actual charges are irregularly spaced.
Estimation of homogeneous nucleation flux via a kinetic model
Authors: Wilcox C. F. , Bauer S. H.
Published: July 2002

The proposed kinetic model for condensation under homogeneous conditions, and the onset of unidirectional cluster growth in supersaturated gases, does not suffer from the conceptual flaws that characterize classical nucleation theory. When a full set of simultaneous rate equations is solved, a characteristic time emerges, τn, for each cluster size, at which the production rate, and its rate of conversion to the next size (n+1) are equal. Procedures for estimating the essential parameters are proposed; condensation fluxes Jsskin are evaluated. Since there are practical limits to the cluster size that can be incorporated in the set of simultaneous first-order differential equations, a code was developed for computing an approximate Jssth, based on estimates of a ‘‘constrained equilibrium’’ distribution, Nequ, and identification of its minimum, Neql.
Spectra of dangling OH groups at ice cluster surfaces and within pores of amorphous ice
Authors: Rowland Brad , Devlin J. Paul
Published: July 2002

The intensity of infrared absorption bands, observed for large gas-phase ice I clusters and attributed to OH (or OD) groups dangling at the cluster surfaces, has been used to estimate the average size of ice clusters formed under fairly typical static cell conditions. The assignment of bands at 3692 cm−1 (2425 cm−1) to dangling surface groups has been established as credible by comparison with absorption bands presumably produced by dangling groups at the surfaces of pores in microporous amorphous ice. Using fairly standard assumptions about absorptivities and molecular sizes, an estimated cluster size of 25 nm has been obtained for clusters formed at 280 Torr and 80 K in a gaseous mixture containing a He to H2O ratio of 200.
Two-photon spectroscopy of Rydberg states of jet-cooled C2H4 and C2D4
Authors: Williams Bradley A. , Cool Terrill A.
Published: July 2002

Spectroscopic studies of two-photon resonant vibronic bands of the (π,3s), (π,3p), and (π,nf ) (n=4–18) Rydberg states of jet-cooled C2H4 and C2D4 are reported. New vibronic bands are resolved by these low temperature (<10 K) observations and previous vibronic assignments of two-photon 3s and 3p transitions are confirmed. A new vibronic progression is observed in the 3s spectrum, tentatively assigned to two-photon allowed 1B2g←1Ag transitions, vibronically induced via excitation of the b1u ν6 CH2 antisymmetric scissors mode. Polarization ratio measurements and identification of the previously unreported 3pσ origin band show that the energies of the 3p levels are in good agreement with theoretical predictions. Higher-lying members of the s- and p-Rydberg series are not observed, suggestive of the onset of strong predissociation. These first observations of the two-photon allowed nf-Rydberg series yield ionization potential estimates of 84 799±5 cm−1 for C2H4 and 84 918±5 cm−1 for C2D4 that compare favorably with previous measurements.
Transformation of iridium(110) (1×1) into (1×2) and spatial distribution of reactive carbon dioxide desorption
Authors: Matsushima Tatsuo , Ohno Yuichi , Nagai Kiyoshi
Published: July 2002

The spatial distribution of the desorption flux of CO2 produced on Ir(110) (1×1) and (1×2) surfaces was studied by means of angle-resolved thermal desorption and low-energy electron diffraction. The distribution is collimated along the bulk surface normal on (1×1). It is sharp in the [001] direction and sharper in the [11̄0] direction. This distribution is consistent with the model that the reactive desorption occurs on a short bridge site. On (1×2) surfaces, two-directional desorption was observed, which was collimated along the axis at the polar angle of 26 deg in both [001] and [001̄] directions. The distribution in the [11̄0] direction is collimated along the bulk surface normal. The reactive desorption was suggested to take place on a threefold hollow site on the declining terrace. The spatial distribution changed from the (1×1) type to the (1×2) type during the transformation of the surface structure. This structure change was confirmed by low-energy electron diffraction.
Close-coupled wave packet calculations of the scattering of oriented molecules from surfaces
Authors: Lemoine Didier , Corey Gregory C.
Published: July 2002

We report time-dependent close-coupled wave packet calculations of transition probabilities for the direct rotationally inelastic scattering of oriented NO(X 2Π) molecules from a rigid, flat Pt surface. A comparison is made with the recent experiments of Kuipers, Tenner, Kleyn, and Stolte [Phys. Rev. Lett. 62, 2152 (1989); J. Vac. Sci. Technol. A 8, 2692 (1990)] on NO molecules which were state selected with an electrostatic hexapole lens and oriented in a Stark field. Compared with unoriented molecules, the mean rotational energy is increased (decreased) by approximately 15% when the O end (N end) of the molecule is directed towards the surface, in qualitative agreement with experiment. An analysis of the rotational excitation mechanism in this collision system shows that the steric effect is strongly influenced by the dependence of the attractive region of the gas–surface potential on molecular orientation.
Infrared photodissociation of size-selected methylamine clusters
Authors: Buck Udo , Gu Xijia , Krohne Reinhard , Lauenstein Christian , Linnartz Harold , Rudolph Andreas
Published: July 2002

Infrared photodissociation spectra of (CH3NH2)n clusters were measured from n=2 to n=6 near the absorption of the C–N stretching mode of the monomer at 1044 cm−1 using a line tunable cw CO2 laser. The clusters are size selected by scattering them from a helium beam. The dimer spectrum shows a double-peak structure with a red- (1038 cm−1) and a blue- (1048 cm−1) shifted peak which is attributed to the nonequivalent position of the C–N in the open dimer structure. The larger clusters exhibit only one peak between 1045.4 and 1046.0 cm−1 which is caused by the equivalent position of the C–N in the cyclic structures of the larger clusters. Calculations of the minimum-energy configurations confirm these results. The linewidth increases by more than a factor of 3 for cold and internally excited dimers.
The direct near ultraviolet absorption spectrum of the Ã 2A2←X̃ 2B1 transition of jet-cooled chlorine dioxide
Authors: Richard Erik C. , Vaida Veronica
Published: July 2002

The direct absorption spectrum of the Ã 2A2←X̃ 2B1 transition of jet-cooled OClO is reported in the 21 000–33 000 cm−1 region. As a result of the rotational cooling afforded by the jet, the upper state level structure has been measured and analyzed in the absence of much of the spectral congestion present in the static gas spectrum. This cooling, in addition to the sensitivity and resolution of the interferometric detection, results in accurate determinations of frequencies and relative absorption intensities. All of the vibrational modes are observed in the 2A2 spectrum with unusually large intensities in the even quanta of the asymmetric stretch. A complete reanalysis of the spectrum yields new 2ν3 and 4ν3 energies of 886.8 and 1855.8 cm−1, respectively. The present assignment is supported by relative intensity and anharmonicity comparisons. We have reconsidered the issue of the intense even ν3 activity in terms of a double-minimum potential in the Q3 coordinate adjusted for the new energies of the asymmetric stretch. The results of this fit agree with experimental relative intensity and isotope measurements. The potential involves a C2v barrier height of 1153 cm−1 with a 0+–0− inversion splitting of 127 cm−1. This value is consistent with the 001 energy determined indirectly from anharmonicity-induced Coriolis perturbations present in the low energy (v1,0,0) levels assumed to be caused by (v1−1,2,1) interactions.
Progress of basis optimization techniques in variational calculation of quantum reactive scattering
Authors: Zhang John Z. H.
Published: July 2002

This paper describes several efficient basis optimization methods that we have developed in the application of S-matrix Kohn variational method to quantum reactive scattering. Specifically, we employ a minimum-K body-fixed representation combined with the use of quasiadiabatic basis functions for the expansion of the full reactive scattering wave function. This new basis function approach significantly reduces the size of the ‘‘larger’’ matrix of the final linear algebraic equation in the calculation of reaction cross sections. The accuracy of the calculation can be easily controlled by systematically increasing or decreasing the values of two parameters Kmax and α, and convergence to the full basis set results can be reached. Numerical test calculations are carried out for the 3D H+H2 reaction for the total angular momentum J=10 and for the 3D F+H2 reaction for J=0, 1, and 2. These calculations demonstrate that our basis optimization approach is very efficient for computing reaction cross sections. Since variational scattering calculations are ultimately limited by the size of the basis set, our method is a stride forward in the applications of variational approach to quantum reactive scattering.
A simplified model of oligosilane ionization energies
Authors: Ortiz J. V.
Published: July 2002

A b initio electron propagator calculations with effective core potentials on Si and a double ζ basis produce accurate vertical ionization energies and Feynman–Dyson amplitudes (FDA’s) for oligosilanes. The calculations determine variations in ionization energies and ground state total energies with respect to bond angle and dihedral angle distortions and obtain excellent agreement with photoelectron spectra and with previous all-electron calculations with larger basis sets. Second order, diagonal self energies contain the bulk of the relaxation and correlation effects. FDA’s accompanying the ionization energies are built from Si–Si bond functions. Phase relationships between neighboring bond functions determine how bond angles affect the ionization energies of Si3H8. The variation of ionization energies with respect to dihedral angles in the Si backbones of Si4H10 and Si5H12 depends on phase relationships between first non-neighbor bond functions.
Quasistable extreme motion vibrational states of HFCO above its dissociation threshold
Authors: Choi Young S. , Moore C. Bradley
Published: July 2002

The anharmonic mixing of highly excited vibrational states of HFCO above its dissociation threshold is studied by stimulated emission pumping spectroscopy. At 0.05-cm−1 resolution, individual molecular eigenstates were resolved and state mixings observed by the distribution of oscillator strength to nearby dark states of the background. Most of the zero-order vibrational levels observed in the energy range between 13 000 and 23 000 cm−1 are assigned to long Franck–Condon-allowed progressions of extremely high overtones of the C–H out-of-plane bending mode (ν6) in combination with the C■O stretching mode (ν2). The extent of state mixing of highly excited vibrational states is strongly mode dependent. For vibrational states with almost the same total vibrational energy, states with the most quanta in ν6 show the least coupling with other states. More strikingly, as the total energy is increased by adding ν6 quanta, the state mixing becomes weaker. For the vibrational states observed above 18 000 cm−1, those that have all of the energy in ν6 or at most one quantum of excitation in ν2 are extremely stable against state mixing. These appear to be the regular states or quasiperiodic trajectories predicted in some theoretical studies of two-dimensional systems. Extreme motion of the C–H out-of-plane bending mode seems to localize and prevent coupling with other modes.
The effect of solvent configuration on matrix-isolation solute line shapes
Authors: Winn John S.
Published: July 2002

A theory of the vibrational spectral line shape for matrix-isolated species is presented which is based on the perturbing influence of the solvent packing on a given normal mode of the solute. The theory is shown to be remarkably accurate in predicting relative line shapes in detail and in predicting absolute shifts of lines from site to site. Application of the theory to experimental spectra of OCS in Ar and Xe matrices has allowed assignment of the major features of the spectra to solvent sites of two general classes: one that is cage or crystalline in nature and the other that is cluster or amorphous in nature.
Time-dependent Monte Carlo studies of surface diffusion
Authors: Bowler Ann M. , Hood Eric S.
Published: July 2002

Surface adsorbate diffusion is examined using a numerical algorithm which incorporates a kinetic treatment in conjunction with a time-dependent Monte Carlo formalism. Nearest- and next-nearest-neighbor adparticle interactions are included. The method is based on a probabilistic description of adparticle jump events; and the diffusion rate is determined by the energetics of adparticle interactions on the lattice. In addition, the rare event problem associated with other theoretical treatments of diffusion is overcome by our highly efficient algorithm. Consequently, we are able to observe events, including ordering and island formation, which occur on time scales which are longer by orders of magnitude than those for simple adsorbate diffusion. Our initial investigations indicate that a variety of diffusion mechanisms may be operative depending on the adparticle interactions in the system. With nearest-neighbor interactions, our systems achieve a random walk limit at long times. We have also observed ordering and island formation, as well as a change in diffusion mechanism, as next-nearest-neighbor attractive energies are increased.
Measurement of the intermolecular vibration–rotation tunneling spectrum of the ammonia dimer by tunable far infrared laser spectroscopy
Authors: Havenith M. , Cohen R. C. , Busarow K. L. , Gwo D-H. , Lee Y. T. , Saykally R. J.
Published: July 2002

Over 150 lines in six tunneling subbands of an intermolecular vibration located near 25 cm−1 have been measured with partial hyperfine resolution and assigned to (NH3)2. The transitions sample all three types of tunneling states (A, G, E) and are consistent with the following assumptions: (1) G36 is the appropriate molecular symmetry group; (2) the equilibrium structure contains a plane of symmetry; (3) interchange tunneling of inequivalent monomers occurs via a trans path; (4) the 2C3+I limit of hydrogen exchange tunneling is appropriate; (5) tunneling and rotational motions are separable. A qualitative vibration–rotation tunneling energy level diagram is presented. Strong perturbations are observed among the states of E symmetry. This work supports the conclusions of Nelson et al. [J. Chem. Phys. 87, 6365 (1987)].
Potential energy surfaces from highly excited spectra using the bootstrap fitting method: Two-dimensional surfaces for water and ozone
Authors: Standard Jean M. , Kellman Michael E.
Published: July 2002

The bootstrap fitting procedure is implemented to obtain molecular potential energy surfaces for the stretching degrees of freedom of ozone using experimental spectra of highly excited vibrational states probed in recent experiments. The method has three key features: first, the use of a flexible anharmonic potential with parameters optimized in a nonlinear least-squares fit; second, the implementation of the bootstrap algorithm for fitting vibrational levels even in regions where the classical dynamics are globally chaotic; and third, the employment of a basis-set reduction technique called algebraic resonance quantization for the computation of eigenvalues of the potential. The approach was tested by using the stretching eigenvalues from a realistic empirical potential for water as input. Excellent agreement between the bootstrap potential and the empirical surface was found using levels up to 25 000 cm−1 with as few as 4 parameters. The method was then applied to experimental stretching levels of ozone. The potential energy surface obtained by our technique for the stretching modes of ozone is believed to be the best surface currently available up to 6500 cm−1.
Differential cross sections for rotationally state-resolved inelastic scattering of HF by argon
Authors: Rawluk Leslie J. , Fan Y. B. , Apelblat Y. , Keil Mark
Published: July 2002

We present differential cross section (DCS) measurements for scattering of HF by Ar. These crossed-beam experiments employ rotational state sensitivity, allowing determination of the DCS as a function of the scattered HF rotational state. The initial HF rotational distribution is generated by nozzle expansion, without further state selection. Its composition is mostly J=0 and J=1, with small admixtures for J>1. The DCS for each final state J′ is measured using a stabilized cw HF chemical laser, in conjunction with a rotatable liquid He-cooled bolometer. Measurable signals are obtained for scattering into 0≤J′≤5, where J′=6 is the thermodynamic limit for our collision energy of 120 meV. The measured DCS’s show a strong forward peak, largely from elastic scattering. In addition, the DCS’s evolve from a broad shoulder in the θ≊25°–40° region for J′=0—through a flattening of the wide-angle scattering for J′=2 and J′=3—to an increase in the scattering beyond ∼40° for J′=4. The DCS for scattering into J′=5 also shows increased intensity at wide scattering angles, but its onset is delayed until ∼70°. These features are shown to be independent of the laboratory → center-of-mass kinematic transformation. The wide-angle scattering into J′=4 and J′=5 corresponds to transferring up to 40% and 60%, respectively, of the available kinetic energy into HF rotation. Since the center-of-mass scattering angles are up to ∼110°, we interpret the observed features for J′=4–5 in terms of rotational rainbow scattering from the hard core of the HF+Ar potential energy surface. The origin of the shoulder for J′=0 scattering is less clear, but it may arise from the strongly anisotropic nature of the HF+Ar van der Waals attraction.
Universal exponents for a nonionic micellar solution of tetraethylene glycol decylether in water
Authors: Hamano K. , Kuwahara N. , Mitsushima I. , Kubota K. , Kamura T.
Published: July 2002

The angular distribution of light intensity scattered from a nonionic micellar solution of tetraethylene glycol n-decylether (C10E4) in water has been examined close to its critical point of mixing. Including multiple-scattering corrections to the light-scattering data, we have found the exponent γ=1.25±0.02 for the osmotic susceptibility χT and the exponent ν=0.63±0.01 for the long-range correlation length ξ in the temperature range of 1.03×10−4≤ε≤4.25×10−3, where ε=(Tc−T)/Tc. The values of the critical exponents obtained in this work are in good agreement with the theoretical and experimental ones for three-dimensional Ising systems.
Photoionization mass spectrometric studies of the isomeric transient species CD2OH and CD3O
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

Both CD2OH and CD3O were prepared in situ by the reaction of F atoms with CD3OH, and studied by photoionization mass spectrometry. The adiabatic ionization potential (I.P.) of CD2OH was found to be 7.540±0.006 eV, in good agreement with the photoelectron spectroscopy (PES) value, 7.55±0.01 eV. However, the adiabatic I.P. of CD3O was determined to be 10.726±0.008 eV, in marked contrast to the PES value for CH3O, 7.37±0.03 eV, but in the expected range based on reported values of ΔH0f(CH3O+) and ΔH0f(CH3O). From selected values of ΔH0f0(CH2OH+) ≤172.0±0.7 kcal/mol and ΔH0f0(CH3O) =5.9±1.0 kcal/mol, we deduce ΔH0f0(CH2OH) ≤−2.1±0.7 kcal/mol and ΔH0f0(CH3O+) =253.1±1.0 kcal/mol.
Differential cross section as a function of the product internal state: The F+I2→IF(v,J)+I reactive collision
Authors: Girard B. , Billy N. , Gouédard G. , Vigué J.
Published: July 2002

We have measured the differential cross section of the F+I2→IF+I reactive collision as a function of the rovibrational state of IF. This measurement was made by an analysis of the Doppler profiles of the laser-induced fluorescence detection signals, as suggested by Kinsey [J. Chem. Phys. 66, 2560 (1977)]. This paper describes in detail the experiment and the way we analyze the data to deduce the differential cross section. The shape of the cross section appears to vary noticeably with the rotational energy of IF and to a lesser extent with its vibrational energy. Several partial explanations are possible, and the choice remains not clear as long as we have no precise information on the potential-energy surfaces of the I2F system.
Model dependence of quantum isotope effects in liquid water
Authors: Del Buono Gabriela S. , Rossky Peter J. , Schnitker Jürgen
Published: July 2002

Path-integral molecular-dynamics simulations have been carried out for liquid water at room temperature using three different potential functions: ST2, SPC, and TIP4P. Quantum isotope effects on the liquid structure are computed in order to examine the dependence of these structural changes on the model used, and a comparison is made to corresponding measurements. The SPC model is found to be in excellent agreement with experimental results; for this model the oxygen–oxygen pair distribution function shows a change in shape and slight shift to smaller distance of the second-neighbor peak when going from D2O to H2O. In contrast, the other two models both show a distinct outward shift of this peak. This difference between models can be attributed to subtle differences in the direct interaction energies of second-nearest-neighbor molecules.
Bulk properties from finite-cluster calculations. VI. A finite-size perturbation theory for the Hartree–Fock energy of linear oligomers
Authors: Cioslowski J. , Lepetit M. B.
Published: July 2002

A finite-size perturbation theory that expresses the total Hartree–Fock energy and the orbital energies of a finite nonmetallic linear oligomer composed of N identical unit cells as power series in ν=N−1 is derived. The coefficients for the first three terms of these series are evaluated explicitly through the first order of the Hartree–Fock formalism. The form of the uncoupled second-order contributions is discussed. The resulting expressions contain infinite sums involving the Wannier orbitals of the bulk polymer. The presence of nonanalytic terms in the expansion is also commented upon.
Quadrature-based, coarse-grained treatment of the coordinate representation free particle real-time evolution operator
Authors: Sharafeddin Omar A. , Kouri Donald J. , Nayar Naresh , Hoffman David K.
Published: July 2002

In this paper we report a quadrature evaluation of the coordinate representation, short-time free particle propagator, 〈R‖exp(−iH0τ)‖R′〉. The result is the elimination of most of the highly oscillatory behavior in this quantity yielding in its stead a much smoother function, strongly peaked at R=R′. We view this as a numerical coarse graining of the propagator which leads to the intuitively reasonable result that for short times τ or large mass, the particle should not have a significant amplitude for R points that are far from R′. This leads to an interesting, and potentially useful, banded structure for 〈R‖exp(−iH0τ)‖R′〉. Calculations have been carried out both for zero and nonzero orbital angular momenta, for which we also give the exact analytic results, and the same behavior is found. The quadrature-coarse graining procedure still appears to retain the important quantum effects as demonstrated by subsequent use of the coarse-grained free propagator to calculate the scattering of an electron by a simple central potential. Results are in quantitative agreement with those obtained by alternative, numerically exact methods. The coarse-grained free propagator is, of course, independent of the potential, and we expect that it can provide a very useful tool for computing real-time dynamics for a variety of systems.
Charge trapping by H2O condensed onto thin films of Kr and Xe
Authors: Bass A. D. , Sanche L.
Published: July 2002

The energy dependence for electron trapping by H2O molecules condensed onto thin films of Kr and Xe has been measured between 0.4 and 11.0 eV. From these measurements, absolute cross sections for electron trapping were deduced and found to reach magnitudes in excess of 10−16 cm2 for incident kinetic energies smaller than 1.0 eV. This trapping is associated with electron capture by clusters of water molecules, a process related possibly to the solvation of the electron following the radiolysis of water. Near 0 eV, the cross section is so large that appreciable charging occurs when electrons of low residual energy are created at the onset of exciton formation in the Kr and Xe substrates. From 6.0 to 9.0 eV, further charge trapping proceeds via dissociative attachment, but a contribution from electrons scattered from electronic states of H2O cannot be excluded.
A unified theory of the dynamics of linear chain macromolecules: From unentangled to entangled polymer fluids
Authors: Szleifer Igal , Loring Roger F.
Published: July 2002

We present an analytically tractable model of the dynamics of a flexible linear polymer. The polymer is represented by a freely jointed chain that undergoes two types of motion. The first motion is a slithering along its contour that mimics reptation. The second type of motion is a generalization of the Stockmayer ‘‘kink–jump’’ algorithm that includes the effects of a medium that relaxes with a finite time scale. A fixed fraction of the beads are immobilized by obstacles that relax with a rate whose magnitude may be smaller than, larger than, or comparable to the rate at which a bead executes jumps in the absence of obstacles. The calculation of observables such as the autocorrelation function of the end-to-end vector, viscoelastic moduli, and the diffusion coefficient is related to the solution of a set of random walk problems with dynamical disorder. Analytical results based on the dynamical effective medium approximation are presented. Our results recover the behavior of an isolated (Rouse) chain and of a reptating chain in an entangled melt as limiting cases, and describe the crossover regime between these limits.
Internal state distribution of OD produced from the O(3P)+ND2 reaction
Authors: Patel-Misra Dipti , Sauder Deborah G. , Dagdigian Paul J.
Published: July 2002

The internal state distribution of the OD product from the O(3P)+ND2 reaction was determined in a crossed beam experiment. Rotational state populations in the OD v=0 vibrational level, and an upper bound to the v=1 to v=0 ratio, were derived from A 2Σ+–X 2Π laser fluorescence spectra. The OD internal state distribution was found to be very cold, with only ∼8% of the available product energy appearing as OD excitation. While this reaction is believed to proceed through D2NO and DNOD intermediates, the observed rotational state distribution is much colder than those predicted by statistical theories. The implications of these experimental results for the reaction dynamics are discussed.
Dissociative single, double, and triple photoionization of OCS in the region hν=20–100 eV studied by mass spectrometry and the photoion–photoion coincidence method
Authors: Masuoka Toshio , Koyano Inosuke
Published: July 2002

The photoionization of OCS in the photon energy region of 20–100 eV has been studied using time-of-flight (TOF) mass spectrometry and the photoion–photoion coincidence (PIPICO) method together with linearly polarized synchrotron radiation. The TOF mass spectrometer facilitates relatively efficient extraction of energetic ions, which is essential for quantitative measurements. The double photoionization threshold is measured to be 31.0±0.5 eV. Photoion branching ratios are reported for singly and doubly charged ions which are measured parallel and perpendicular to the electric vector of the incident photon beam. It is found that the ion intensities in the TOF mass spectra are affected by anisotropic angular distributions of photoelectrons relevant to the production of the OCS+, S+, and CS+ ions. PIPICO branching ratios are also reported, dissociation of OCS3+ into OC++S2+ being observed in the PIPICO spectra with a threshold at 63±3 eV.
E
J ensemble momentum sampling
Authors: Dumont Randall S.
Published: July 2002

The ‘‘EJ’’ ensemble (i.e., ensemble with fixed total energy and angular momentum) momentum sampling algorithm recently proposed by Nyman et al. [J. Chem. Phys. 93, 6767 (1990)] is verified to give correctly distributed atomic momenta. The proof is based on explicit analytic determination of the atomic momentum distribution generated by the algorithm. The simpler case of microcanonical momentum sampling is also treated using similar methodology.
Simulations of solvation in a Brownian dipole lattice
Authors: Papazyan Arno , Maroncelli Mark
Published: July 2002

We have simulated the solvation of ions in a solvent consisting of point dipoles which undergo diffusive reorientation while translationally fixed to the sites of a cubic lattice. The simplicity of this model allows us to thoroughly explore how the energetics and dynamics of solvation depend on factors such as solute charge, solvent polarity, and number of solvent molecules. Some of the main features observed are as follows. The orientational response of first solvation shell dipoles saturates for moderate solute charges, resulting in a nonlinear dependence of the reaction potential on solute charge. This nonlinearity is to a good approximation independent of solvent polarity and can be rationalized on the basis of a simple phenomenological model. One effect of the nonlinear solvent response is to cause solvation free energy wells of the sort considered in electron transfer theories to be significantly anharmonic. Surprisingly, this deviation from harmonic behavior has little apparent impact on solvation barriers to charge transfer. The time dependence of the solvation response deviates substantially from exponential behavior in the more polar systems studied. Solvation times (1/e times of the solvation response) are directly related to the magnitudes of fluctuations in the solvation potential. The dynamics of solvation for times ≤t1/e can therefore be understood in terms of purely static correlations between solvent molecules. Dynamical interparticle correlations are only important in determining the longer time behavior of the solvation response. In contrast to the long-ranged character of the solvation energy, only 20–30 solvent molecules are required to produce solvation times characteristic of bulk solvent.
Multiple-band theory of dynamics for interacting adsorbates coupled to phonons. I. Variationally optimized Hamiltonian
Authors: Reilly Philip D. , Harris Robert A. , Whaley K. Birgitta
Published: July 2002

A two-band generalization of the Hubbard model is developed in order to examine the different roles of adsorbate interactions with one another and with phonons. Variationally optimized unitary transformations of the Hamiltonian are performed in order to obtain dressed tunneling amplitudes. These display strong dimensional dependence and demonstrate fundamental differences between the effects of coupling of adsorbate density and of adsorbate vibrational excitation to the lattice modes. The occurrence of on-site asymmetric self-trapping induced by even weak coupling of interband excitations to the lattice is demonstrated, and the effect of this on the thermally averaged, intersite tunneling is investigated. Implications for surface transport and for surface reconstructions of hydrogen/metal systems are made.
A Monte Carlo study of surface diffusion coefficients in the presence of adsorbate–adsorbate interactions. IV. Attractive nearest-neighbor and repulsive next-nearest-neighbor interactions
Authors: Uebing C. , Gomer R.
Published: July 2002

Tracer, jump, and chemical diffusion coefficients have been determined by Monte Carlo simulations on a square lattice for the case of nearest-neighbor attraction and numerically equal next-nearest-neighbor repulsion. For this case only second-order θ vs μ/kBT isotherms were found, indicating that the system is in a single phase at all coverages. 〈(δN)2〉/〈N〉 varies only very slightly with temperature at all coverages indicating balance between repulsive and attractive adsorbate–adsorbate interactions. Tracer and jump diffusion coefficients also show only very slight temperature dependence, relative to the noninteractive case. The same is true for the chemical diffusion coefficient determined from the Kubo–Green equation as the product of jump diffusion coefficient and 〈N〉/〈(δN)2〉, DKG. For JNN/kBT<1, there is reasonably good agreement between DKG and DF, the chemical diffusion coefficient determined by the fluctuation method. For JNN/kBT>1, DF decreases markedly with decreasing T. It is hypothesized that this behavior results from the freezing in of enough long-range order for this particular set of interactions to make the fluctuation results differ from the Kubo–Green ones. The latter are more microscopic, and it may be that the fluctuation results depend on probe size.
Collective excitations in liquid water at low frequency and large wave vector
Authors: Sastry Srikanth , Sciortino Francesco , Stanley H. Eugene
Published: July 2002

We study the collective modes of water using molecular dynamics with the ST2 model potential, by calculating the dynamic structure factor S(q,ω). We identify the previously reported ‘‘first’’ sound mode to be due to the wave vector independent oxygen bending mode.
Interpretation of excited state Hartree–Fock analytic derivative anomalies for NO2 and HCO2 using the molecular orbital Hessian
Authors: Burton Neil A. , Yamaguchi Yukio , Alberts Ian L. , Schaefer Henry F.
Published: July 2002

A simple procedure utilizing the molecular orbital (MO) Hessian, the second derivative of the Hartree–Fock (HF) energy with respect to MO coefficient perturbations, has been proposed in the interpretation of anomalous analytic HF energy derivatives. The nature of the anomalous analytic self-consistent field (SCF) harmonic vibrational frequencies and infrared (ir) intensities of the 2B2 and 2A2 excited states of NO2 and HCO2 have been studied in detail. By suitable partitioning of the HF second energy derivative expression, an association between the abnormal force constants, the dipole moment derivatives, and specific solutions to the SCF coupled perturbed HF equations is established. This connection to unstable wave functions can now be used with the MO Hessian to predict these anomalies at the HF level. In addition, appropriate small complete active space SCF (CASSCF) methods have been utilized to overcome the inherent shortcomings of the HF wave functions, and determine more realistic values for the force constants and ir intensities.
Structure and dynamical behavior of fluid n-alkanes
Authors: Padilla P. , Toxvaerd S.
Published: July 2002

The structure and the dynamical behavior of two n-alkanes, pentane and decane in fluid state, have been investigated. The study has been done by molecular dynamic simulation. Most of the results presented here are for an anisotropic united-atom model, however, some results for a united-atom model are also shown in order to test the influence of the details of the model on the dynamical behavior of the chains. Evidence that the n-alkane chains are packed randomly in the fluid is presented. Several time correlation functions and relaxation times have been calculated. Our results show that while the diffusion of n-decane along its longest axis is favored, n-pentane translates as a spherically symmetric molecule. The results obtained for different torsion potentials show that the details of the torsion potential function play a significant role for the self diffusion coefficient as well as for the reorientational relaxation of the chains. Some considerations about the mechanism of the reorientation of the chain have been done in the framework of the Debye theory and Rouse model.
Solvation of Na+ and Cl− at the water–platinum (100) interface
Authors: Rose Daniel A. , Benjamin Ilan
Published: July 2002

The structural, energetic, and dynamical aspects of the solvation of Na+ and Cl− at the water–plantinum (100) interface are investigated by molecular-dynamics computer simulation. Although the structure of interfacial water is significantly different from that of bulk water, the structure of the ion–water solvation complex at the interface closely resembles that in the bulk. The free energy of adsorption is calculated as a function of the distance from the metal. It is nonmonotonic and is qualitatively very different for Na+ and Cl−. The shape of the free-energy curve can be explained in terms of solvation structure and the local perturbation of the interfacial water structure. The reorientation dynamics of water near the ion show that the structure-breaking effect of Cl− at the interface is much more significant than in the bulk, but that Na+ slows down water reorientation both in the bulk and at the interface. Collective solvent dynamics, as measured by equilibrium fluctuations of solvent–ion electrostatic energy, are surprisingly similar in the bulk and at the interface, in marked contrast with what continuum models predict. The mobility of ions near the interface is much less than it is in the bulk, which is also in disagreement with models which do not take into account the existence of a strongly adsorbed water layer.
Vibrational predissociation in S1 indole van der Waals clusters
Authors: Outhouse E. Allison , Bickel Grant A. , Demmer David R. , Wallace Stephen C.
Published: July 2002

The excited state dynamics of the indole(Ar)1, indole(d1)(Ar)1, indole(Ar)2, and indole(CH4)1 van der Waals clusters have been investigated in a free jet expansion. Excited state vibrational frequencies were determined using multiphoton ionization and fluorescence excitation spectroscopy. Time resolved emission spectroscopic techniques were used to determine vibrational predissociation rates and product state distributions. All of the clusters were found to predissociate when excited with sufficient vibrational energy in the S1 state. The predissociation dynamics were found to be consistent with a serial model in which energy transfer from the indole skeletal modes to the van der Waals modes precedes the dissociation step. The density of van der Waals vibrational states was found to be the most important factor in determining the predissociation dynamics.
Theoretical study of redistribution of light in Ca–He collisions
Authors: Pouilly Brigitte
Published: July 2002

The quantum theory of atomic collisions in the presence of a weak radiation field is used to describe the process of redistribution of light in Ca–He collisions. This work is devoted to the simulation of the recent experiment of Lin and co-workers [J. Chem. Phys. 89, 4771 (1988)] in which, the absorption profile of the (4s2 1S→4s5p 1P) transition and the dependence on the detuning of the spin-changing (4s5p 1P→4s5p 3P) transition were determined. Close-coupled calculations are carried out, based on model interaction potentials for the ground and the excited states of the Ca–He system. The calculated absorption profile for the production of Ca atom in the (4s5p 1,3P) states is in qualitative agreement with the experimental results. The blue wing/red wing asymmetry in the relative transfer to the 3P state, is interpreted in terms of flux transfer among the molecular states, which occurs when the Ca atom is approached by the closed-shell partner.
Evolution of the orientational and internal-state distribution of molecules in an expanding jet
Authors: Hulsman H. , Korving J.
Published: July 2002

We report experiments on the internal state distribution and angular momentum alignment in an expanding jet of Na2 molecules seeded in Ar. The results show that even at a distance of one nozzle diameter from the nozzle exit, the distribution function is already far from equilibrium. It is found that the deviation from equilibrium can be expressed in a polynomial in the total internal energy of the molecules. This result is very analogous to the outcome of a first order Chapman–Enskog treatment for molecules with internal states. The transition to molecular flow marks the breakdown of the simple relationship mentioned above. This points to the fact that data taken far away from the nozzle are by themselves of limited value for a test of calculated distributions starting from nozzle conditions. Further we present measurements of alignment for two rovibrational states of Na2 molecules. Far downstream the nozzle the results are in agreement with previous experiments in molecular beams. In the vicinity of the nozzle reorienting collisions of excited-state molecules play an important role. It appears that these collisions strongly depend on the rotational state of the molecules: the cross sections are inversely proportional to J′2.