bluuebunny/crossref_metadata_2025 huggingface dataset:
The nonuniform Percus–Yevick equation for the density profile of associating hard spheres
Authors: Henderson D. , Sokol/owski S. , Trokhymchuk A.
Published: July 2002

The adsorption of associating hard spheres on a hard wall by using the nonuniform Percus–Yevick equation for the local density and the pair correlation functions is studied. A comparison of the density profiles with computer simulation data indicates that this approximation predicts the fluid structure significantly better than the singlet Percus–Yevick and hypernetted chain approximations.
Solvent–solute interactions and the Raman CH stretching spectrum of cyclohexane-d11: I. Solvent-dependence
Authors: Remar Glenn J. , MacPhail Richard A.
Published: July 2002

We examine the gas-to-liquid frequency shifts and isotropic Raman linewidths for the axial and equatorial CH stretching vibrations of cyclohexane-d11 in 13 different solvents. The ‘‘perturbed hard-fluid model’’ of Ben-Amotz and Herschbach provides a consistent description of the observed shifts in terms of average repulsive and attractive solute–solvent interactions. The attractive part of the shift is dominated by dispersive interactions, with a small contribution from inductive forces. This analysis also reproduces the observed ratio of equatorial to axial attractive shift components if certain details of the cyclohexane-d11 geometry are accounted for when the repulsive part of the shift is calculated. The ratio of equatorial to axial peak linewidths is nearly the same for all solvents and is consistent with inhomogeneous broadening by attractive force fluctuations. A version of the Schweizer–Chandler theory of attractive force broadening, modified to incorporate interactions between the solute vibrational dipole and solvent dipoles, successfully describes the dependence of the observed linewidths on solvent dipole moment. However, the remaining portion of the linewidth does not correlate with solvent parameters characterizing dispersive force fluctuations in the manner predicted by the theory.
The hexagonal phase of wormlike micelles
Authors: Schoot Paul van der
Published: July 2002

A mean-field description is presented for the nematic and hexagonal ordering of intrinsically polydisperse semiflexible wormlike micelles interacting via a hard core volume exclusion potential. The predicted phase diagram is determined by two parameters, controlling the intrinsic rigidity of the micelles and their size in the isotropic phase before a liquid crystalline phase emerges. Increasing the micellar flexibility destabilizes the nematic phase, which disappears in favor of the hexagonal phase when the ratio of the persistence length and the diameter of the micelles is reduced to less than a certain minimum value. The minimum rigidity required to stabilize a nematic depends on whether the micelles in the isotropic phase can be viewed as rodlike or not. For micelles on average longer than roughly a persistence length, we find a nematic phase provided the persistence length is greater than about thirteen times the micelle diameter. For increasingly shorter rods, this value goes up; the nematic phase is ‘‘squeezed’’ out and we find a direct transition from the isotropic to the hexagonal phase over a larger flexibility range.
Lattice dynamics and hyperfine interactions of C60Fe(CO)4
Authors: Herber R. H. , Bauminger E. , Felner I.
Published: July 2002

Detailed 57Fe Mossbauer experiments over the range 85<T<210 K have been carried out on the tetra-carbonyl iron derivative of C60(I) as well as on several related organometallic solids. The observed isomer shift and quadrupole splitting parameters are consistent with a bonding interaction between the metal center and the ligand which involves a single carbon–carbon double bond of the buckminsterfullerene. The temperature dependence of the recoil-free fraction, compared to analogous data for maleic anhydride Fe(CO)4(II), suggests that the two previously observed low frequency lattice modes in C60 persist in I. Contrary to expectations, there is no significant temperature-dependent vibrational anisotropy (Gol’danskii–Karyagin effect) evident in the above temperature range.
193 nm laser photofragmentation time-of-flight mass spectrometric study of HSCH2CH2SH
Authors: Zhao H.-Q. , Cheung Y.-S. , Liao C.-X. , Ng C. Y. , Li Wai-Kee , Chiu See-Wing
Published: July 2002

The kinetic energy release spectra for SH resulting from the 193 nm laser photofragmentation of HSCH2CH2SH have been measured. On the basis of the observed maximum kinetic energy for the formation of HS+CH2CH2SH, a value of 74±2 kcal/mol is derived for the bond dissociation energy of HS–CH2CH2SH at 0 K [D0(HS–CH2CH2SH)]. Angular distribution measurements for SH yield an anisotropic parameter β=−0.4±0.1 for the HS+CH2CH2SH channel, indicating that the C–S bond fission is fast with respect to molecular rotation. The energetics for the formation of HS+CH2CH2SH from HSCH2CH2SH have been investigated using the Gaussian-2 (G2) and G2(MP3) ab initio quantum chemical procedures. The G2/G2(MP3) calculations give a prediction of 72.5 kcal/mol for D0(HS–CH2CH2SH), in excellent agreement with the experimental value. Ab initio first-order configuration interaction calculations have also been made to examine the possible excited state of HSCH2CH2SH involved in the photodissociation process and to rationalize the observed angular distribution for the HS+CH2CH2SH channel.
Ground and excited state adiabatic 2A″ and 2A′ potential energy surfaces of the (Kr–O2)+ cluster ion
Authors: Ramiro-Diaz B. , Wahnon P. , Sidis V.
Published: July 2002

The seven lowest adiabatic potential energy surfaces (PES) of the (Kr–O2)+ cluster ion in each of the 2A″ and 2A′ symmetries are calculated. The computational method involves configuration interaction calculations in a basis of a thousand projected valence-bond state functions. It resorts to diagonal corrections of the Hamiltonian matrix prior to configuration interaction and makes use of an l-dependent pseudopotential for Kr. The results are characterized by the shallowness of the 1 2A″ potential well and the absence of wells in the other PES investigated. The 1 2A″ equilibrium characteristics differ significantly from those proposed in other work. Notable effects on all the PES are observed when the O–O bond is stretched beyond 2.5a0. No 1 2A′−2 2A′ (nor 1 2A″−2 2A″) pseudocrossings are found that could explain, on the basis of mere electronic structure arguments, the available thermal energy charge transfer data. A 2 2A″−1 2A′ crossing actually exists but the related Coriolis coupling mechanism cannot be invoked for low energy <1 eV encounters. This draws attention to alternative more subtle dynamic charge transfer mechanisms possibly involving quartet-doublet spin–orbit transitions and/or (near) degeneracies of rovibronic, instead of mere electronic, energy levels. Evidence is shown for pseudocrossing mechanisms of population sharing between the 2,3A″ and 2A′ states. Similar evidence is shown for an O2(X 3Σ−g)→O2(a 1Δg) excitation involving two electron rearrangement.
Vibrational energy relaxation of HOD in liquid D2O
Authors: Rey Rossend , Hynes James T.
Published: July 2002

Molecular Dynamics simulation is used to study the vibrational relaxation of the first excited state of the O–H stretch for HOD dissolved in D2O. The technique applied is based on a Landau–Teller type formula, in which the solvent contribution is computed classically, while the quantum nature of the solute enters through the transition moments of the molecular normal modes. The experimental result for the relaxation time (≊8 ps) is accounted for, and the pathway to the ground state is determined. The relaxation proceeds through a sequence of intramolecular transitions initially facilitated by the solute internal anharmonicities. In particular, the anharmonicity allows an initial and rate-determining transfer to the first overtone of the HOD bend; a corresponding harmonic force field calculation in which this step is precluded yields a relaxation time that is three orders of magnitude larger. The excess energy is removed by the bath modes, which include rotations and translations of all molecules, including the solute. Relaxation by Coriolis coupling plays a minor but non-negligible role, while the centrifugal coupling contribution to the relaxation is negligible.
Radiative cooling of fullerenes
Authors: Hansen K. , Campbell E. E. B.
Published: July 2002

We have measured the metastable fragmentation of fullerene ions in molecular beams. The rates are significantly smaller than the evaporative ensemble prediction, consistent with an alternative cooling mechanism through emission of electromagnetic radiation. Modeling the competition between radiative and evaporative cooling yields information about the evaporative activation energy and emissivity of the carbon clusters.
Production of I*(2P1/2) in the ultraviolet photodissociation of α -branched alkyl iodides
Authors: Uma S. , Das Puspendu Kumar
Published: July 2002

Photodissociation dynamics of a series of α-branched alkyl iodides at excitation wavelengths of 222, 266, and ∼305 nm has been investigated by measuring the quantum yield (φ*) of I*(2P1/2) production. I* is found to be the major product at 222 nm and 266 nm from methyl and ethyl iodides but not from the higher α-branched homologs. On the contrary, I(2P3/2) is the major product at ∼305 nm for all the iodides. Assuming that I* originates from the 3Q0 state over the entire A-band, production of both I and I* in methyl and ethyl iodides at 222 and 266 nm is explained by invoking the curve-crossing mechanism in the upper state. The crossing probability (P) between the 3Q0 and 1Q1 surfaces for these two molecules has been estimated. At ∼305 nm, simultaneous excitation to the 3Q0 and 3Q1 states remains a distinct possibility. For higher branched (i.e., i-propyl and t-butyl) alkyl iodides, the mechanism for I* production is qualitatively different from that of unbranched iodides. Coupling of α-carbon bending vibrational modes with the C–I bond excitation as well as the actual time spent in the excited state surfaces in i-propyl and t-butyl iodides seem to be the reasons for altering the dynamics of dissociation drastically in comparison with that of methyl iodide.
On the role of electromagnetic field gradients in surface Raman scattering by molecules adsorbed on single crystal metal surfaces
Authors: Perry Scott S. , Hatch Steven R. , Campion Alan
Published: July 2002

Bands normally forbidden in the vibrational Raman spectra of molecules appear with some intensity when they are adsorbed on solid surfaces. The origin of this intensity can, in principle, be accounted for by two mechanisms. Since the symmetry of the adsorbed molecule is lower than that of the free molecule, relaxed selection rules result. Alternatively, the large gradient in the normal component of the electromagnetic field near a conducting surface can induce scattering through the dipole–quadrupole polarizability. To assess the relative importance of these two mechanisms, the field gradient at the surface of a Ag(111) crystal was varied by using different incident laser frequencies and its effect on the relative intensity of a putative quadrupole mode monitored. The intensity of the mode remained constant over the frequency region in which the field gradient varied. It was concluded that the field gradient mechanism is not important, at least for molecules physically adsorbed on flat surfaces.
Energy transfer processes in organized media. I. A crystal model for cubic sites
Authors: Vásquez Sergio O.
Published: July 2002

A general study of energy transfer processes in organized condensed matter is presented. Considering the nature of these media, i.e., the periodic properties of the lattice, a theoretical microscopic model is worked out for the general case. It takes full advantage of the translational periodicities in the crystalline phase, using the real discrete distribution of ions in the lattice. A general master equation is obtained and, for the sake of completeness, a particular case is analyzed for the dipole–dipole nonradiative mechanism for energy transfer processes in the elpasolite lattices where the Ln3+ impurities are in cubic sites.
Evolution of excitonic energy levels in ArN clusters: Confinement of bulk, surface, and deep valence shell excitons
Authors: Wörmer J. , Karnbach R. , Joppien M. , Möller T.
Published: July 2002

The evolution of excitonic energy levels (Wannier and Frenkel type) is investigated for ArN clusters in the range N=200–106 using fluorescence excitation spectroscopy. In the case of Wannier excitons, a pronounced blue shift of the absorption bands relative to the position in the infinite solid is observed. As a consequence of the lower dimensionality, the shift of the transition energy of surface excitons is considerably smaller than that of the bulk states of clusters. The evolution with size is discussed within several theoretical models for exciton confinement. In addition, model calculations are performed for bulk excitons which give good quantitative agreement with the experimental results. In the case of n=1 Frenkel or intermediate type excitons, there are blue and red shifts observed. The spectral shift of (3p→4s) and deep valence (3s→4p) excitations differs considerably. From the shift of the transition energies the exciton mass of the (3p→4s) exciton is derived.
Time-resolved infrared diode laser spectroscopy of the ν5 band of the cyanomethyl radical (H2CCN)
Authors: Sumiyoshi Yoshihiro , Tanaka Keiichi , Tanaka Takehiko
Published: July 2002

The infrared spectrum of the cyanomethyl radical (H2CCN) generated by the 193 nm excimer laser photolysis of chloroacetonitrile was observed by time-resolved diode laser spectroscopy. About 50 lines, involving those split into doublet due to the spin–rotation interaction, were assigned to rovibrational transitions in the ν5 (CH2-wagging) band of cyanomethyl. The molecular constants in the ν5 vibrational state were derived from the analysis of the observed wave numbers, resulting in the rotational constants, A=9.095 03(21) cm−1, (B+C)/2=0.335 363 4(41) cm−1, and (B−C)=0.011 503 6(71) cm−1, and the spin–rotation interaction constant εaa=−2.143(47)×10−2 cm−1, where the figures in parentheses are 2.5 standard deviations to be attached to the last digit, the constants in the ground state being fixed to the reported values from microwave spectroscopy. The band origin determined ν0=663.793 98(85) cm−1 is consistent with the value derived from the photo-detachment spectroscopy of the H2CCN− anion. Large changes in the rotational constant A and the centrifugal distortion constant ΔK on vibrational excitation from the ground state to the ν5 state are accounted for by the a-type Coriolis interaction of the ν5 vibrational state with the ν8 (CH2-rocking) and ν9 (in-plane ∠CCN-bending) vibrational states.
Relaxation after a temperature jump within the one-phase region of a polymer mixture
Authors: Merkle G. , Bauer B. J. , Han C. C.
Published: July 2002

Small-angle neutron scattering was used to observe the relaxation of the structure factor from a smaller to a larger fluctuation state after a temperature jump within the one-phase region of a miscible polymer blend. The blend studied is a 60/40 by weight mixture of a hydrogen bonded polymer blend which consists of hydroxy modified deuterated polystyrene and poly(butylmethacrylate). A peak clearly evolved during the transition from the initial state at Ti to the final state at Tf as predicted by linear as well as nonlinear theories. This relaxation process, which took more than 10 h, could be represented reasonably well by a simple linear approximation of the Cahn–Hilliard–Cook (CHC) theory without involving nonlinear calculations for the evolution of the dynamic structure factor. The measurement offers an alternative method for estimating the order parameter relaxation rate in the one-phase region. Also, this demonstrates experimentally the correct relaxation (to a higher fluctuation state) mechanism predicted by the linear theory of CHC type.
Dephasing of a solvated two-level system: A semiclassical approach for parallel computing
Authors: Spencer Clayton F. , Loring Roger F.
Published: July 2002

The statically broadened vibronic line shape of a molecular solute in a liquid solution may be computed from a knowledge of the equilibrium structure of the fluid. By contrast, calculation of the contribution of solvent nuclear motions to this lineshape requires the use of semiclassical mechanics. Liquid-state electronic spectra have previously been calculated with a semiclassical approach relating the line shape to fluctuations in the electronic energy gap as the fluid evolves classically on the ground-state potential surface. We propose an alternative formulation that incorporates dynamics on both the ground-state and excited-state surfaces. While more computationally intensive, this approach lends itself readily to parallel computation. Line shapes using both methods are computed for a Lennard-Jones solute in a Lennard–Jones solvent, for which the depth of the potential well characterizing solute–solvent interactions changes with electronic state.
The local energy approximation and the predictability of chain configurations in polymer melts
Authors: Neyertz Sylvie , Brown David , Clarke Julian H. R.
Published: July 2002

The configurations of a series of short polar model molecules for poly(vinyl chloride), PVC, and poly(ethylene oxide), PEO, have been determined from extensive molecular dynamics simulations of the corresponding bulk melts. Their conformational and configurational properties are compared to those sampled for the same models by a pivot Monte Carlo procedure based on the assumption that chain configurations in the melt depend only on highly localized near-neighbor intramolecular interactions. The comparison proves favorable for all neutral and polar chains, with the exception of the realistic model leading to the gauche effect in PEO. Discrepancies in the latter case are related to the failure of the single-chain local energy approximation to account for the specific competition between intramolecular ‘‘1...5’’ C–H...O and intermolecular C–H...O electrostatic interactions in the PEO bulk melts.
Approximate quantum scattering studies of the CN+H2 reaction
Authors: Takayanagi Toshiyuki , ter Horst Marc A. , Schatz George C.
Published: July 2002

Reduced dimensionality quantum scattering calculations have been carried out for the H2+CN→HCN+H reaction. A new potential energy surface, which has recently been developed on the basis of extensive ab initio molecular orbital calculations, has been employed. In order to study the effect of H2CN complex-formation on the hydrogen abstraction, three active degrees of freedom have been considered in the scattering calculations: the H-H internuclear distance, the H-GCN distance (where GCN is the center of mass of CN) and the angle between H-H and H-GCN. This reduces the problem to the usual atom–diatom scattering calculation for H2+A, where A represents a pseudoatom. A hyperspherical coordinate coupled-channel method has been used to solve the Schrödinger equation. The reaction probabilities calculated show that H2CN complex-formation mechanism is not important for the hydrogen abstraction channel in the energy range considered in the present calculations. On the other hand, complex-formation is important for inelastic processes such as H+HCN(ν,j)→H+HCN(ν′,j′), where ν and j are the C–H local vibrational and rotational quantum numbers of HCN. This is consistent with previous full-dimensional quasiclassical trajectory calculations. The reaction probabilities, final vibrational distributions, and thermal rate constants calculated with the present reduced dimensionality theory have been critically compared with those calculated using quasiclassical trajectories and with other approximate quantum scattering methods including the adiabatic-bend approximation and the rotating-bond approximation.
13C isotope effects for pentacene in p-terphenyl: High-resolution spectroscopy and single-spin detection
Authors: Brouwer A. C. J. , Köhler J. , Groenen E. J. J. , Schmidt J.
Published: July 2002

Weak satellites in S1←S0 excitation spectra of natural-abundance pentacene in p-terphenyl due to position-specific 13C isotope shifts were observed. An assignment of these satellites to the various carbon positions is derived. The extremely narrow inhomogeneous linewidth allowed selective excitation of pentacene molecules with 13C in specific positions. Spectra of the magnetic-resonance transition between triplet sublevels of such ensembles, and of individual pentacene molecules showed position-specific 13C hyperfine broadening.
Coexistence curve of polystyrene in methylcyclohexane. IX. Pressure dependence of tricritical point
Authors: Dobashi Toshiaki , Koshiba Tetsuya , Nakata Mitsuo
Published: July 2002

The effect of pressure on the tricritical point (TCP) of a ternary system, polystyrene (PS) I+PS II+methylcyclohexane, was studied by numerical analysis of the generalized Flory–Huggins equation for the Gibbs free energy. For the ternary system, the molecular weight of PS II M2t, total volume fraction of PS φst, volume fraction of PS II with respect to total volume of PS ξ2t, and temperature Tt at TCP were calculated for various values of the molecular weight of PS I M1 in the pressure range from 0 to 100 MPa. As M1 increases, the ratio rt=M2t/M1 and φst decrease, while Tt and ξ2t increase monotonically irrespective of p. The curves of rt and φst vs p have a maximum, while the curves of Tt and ξ2t vs p have a minimum, irrespective of M1. For small M1 these curves are nearly flat and roughly symmetrical with respect to the extrema. From these calculated results it is predicted that the tricritical solution at atmospheric pressure remains very near the tricritical state along the cloud-point curve in the temperature vs pressure diagram.
Structural studies of an organic liquid through the glass transition
Authors: Leheny Robert L. , Menon Narayanan , Nagel Sidney R. , Long Price David , Suzuya Kentaro , Thiyagarajan P.
Published: July 2002

We have performed neutron diffraction experiments on deuterated propylene glycol, an organic glass former, at temperatures ranging from where its dynamical response approaches that of normal liquids to below the point where relaxation times appear to diverge. Our studies extend over a very broad range of scattering wave vector (0.01<Q<30 Å−1). In contrast to models which predict clustering, we find no evidence in the liquid at any temperature for heterogeneities large on molecular scales. However, we do note subtle changes at shorter lengths. Using molecular dynamics simulations to model our results, we identify these changes with increasing density and increasing orientational order induced by hydrogen bonding in the liquid as it cools. Analysis of the orientational correlations between molecules reveals a strong dependence on their relative positions.
Statistical anharmonic unimolecular rate constants for the dissociation of fluxional molecules: Application to aluminum clusters
Authors: Peslherbe Gilles H. , Hase William L.
Published: July 2002

Anharmonic densities of state are determined for the Aln (n=5,6,12,13) clusters using a model analytic potential energy function. Relative anharmonic densities of state are calculated by the multiple histogram/Nosé dynamics method. Absolute densities for Al5 and Al6 are determined by Monte Carlo evaluation of the phase integral, while for Al12 and Al13 they are determined by adiabatic switching. The anharmonic densities of state are orders of magnitude larger than harmonic values based on the deepest potential energy minimum. At an energy equal to the cluster dissociation threshold, the anharmonic density is 56 and 4600 times larger than the harmonic density for Al6 and Al13, respectively. The anharmonic densities of state are used to determine anharmonic phase space theory rate constants for Al6→Al5+Al and Al13→Al12+Al dissociation. These rate constants are within a factor of 2 of the anharmonic microcanonical rate constants determined by using classical trajectories to calculate the initial decay rates for microcanonical ensembles of Al6 and Al13 clusters. The trajectories also show that the Al6 and Al13 dissociations have ergodic unimolecular dynamics. At the Aln→Aln−1+Al dissociation threshold, where only one Aln−1 conformation is energetically accessible and the harmonic model is accurate for the Aln−1 density of states, the anharmonic correction to the unimolecular rate constant is that for the Aln density of states. However, at higher energies anharmonicity for Aln−1 also becomes important and the anharmonic correction to the unimolecular rate constant becomes smaller. A modified Rice–Ramsperger–Kassel rate constant expression, with all degrees of freedom active and A and/or E0 made energy dependent, fits anharmonic microcanonical unimolecular rate constants for Al3, Al6, and Al13 dissociation. A simple Rice–Ramsperger–Kassel–Marcus model, used to analyze the experimental studies of aluminum cluster dissociation, gives accurate rate constants as a result of a fortuitous cancellation of errors.
Rubber elasticity: A contact-probability model with harmonic entanglement constraints
Authors: Allegra Giuseppe , Bontempelli Sergio , Raos Guido
Published: July 2002

This paper is devoted to the theoretical analysis of nonideality effects in stretched rubber samples. The joint effect of packing forces and topological constraints is accounted for by adopting harmonic potentials between all pairs of network atoms, in addition to fixing a suitable set of junctions at the macroscopic sample surface according to the James–Guth theory. The potential minima are set at the average, affinely deformed interatomic distances. The force constant of each interaction is proportional to the probability of interatomic contact in the undeformed state and is inversely proportional to the square strain ratio along any space direction, thus accounting for the variation of the entanglement concentration with sample stretching. The proportionality factor of the pair potential is an adjustable parameter of the theory. A periodic coarse-grained model is used and the sample free energy is evaluated through normal-mode self-consistent analysis. Both the Mooney effect and the observed radius of gyration of the chain strands projected along different directions are properly accounted for. The results are similar to those of the Ronca–Allegra theory, which is based on direct application of constraints to the junction fluctuations. However, the present approach also embodies features of the theories which adopt the tube model. Finally, the variation of the Mooney constant C2 with sample swelling is accounted for in a semiquantitative way.
Vibration–rotational spectroscopy of XH2 and XH3 type molecules near the local mode limit
Authors: Wang Xiao-gang , Zhu Qing-shi
Published: July 2002

Present work studies the effective rotational Hamiltonians and their vibration–rotational parameters for XH2 and XH3 type molecules near the local mode limit by including the diagonal matrix elements of coordinate operators when the bond anharmonicity is significant. An improved ‘‘α relation’’ is given for the local mode limit by taking the anharmonic bond oscillator wave function as the basis function. Then the rotational tunneling approach is extended to model the effect of nonzero interbond coupling for XH2 and XH3 type molecules, which provides a dynamical view of the rovibrational structure of the local mode states.
Coriolis-dependent Stark effect of the 2ν3 band of methane observed by saturated absorption spectroscopy
Authors: Sasada Hiroyuki , Suzumura Kotaro , Ishibashi Chikako
Published: July 2002

We studied the vibration-induced dipole moment of methane by observing Stark-modulation spectra of the 2ν3 band centered at 1.66 μm. The spectral purity of an external cavity semiconductor laser and radiation density in a Fabry–Perot cavity absorption cell are sufficiently high to record sub-Doppler saturated absorption lines with a spectral resolution of 0.8 MHz. First-order Stark shifts of the E-type symmetry components in the P(2), Q(2), Q(4), Q(5), R(2), R(4), and R(5) transitions were measured, and the vibration-induced dipole moments for the v3=2 state were determined. Their values clearly demonstrate the Coriolis-induced dipole term, yielding [5.79(5)+0.440(18)×(2L−3)] mD for the JR level, where L is defined to be −(R+1), 0, and R for the R=J−1, J, and J+1 levels, R and J are rotational and total angular momentum quantum numbers, and numbers in parentheses are 90% confidence intervals in unit of the last digit.
Variational transition state theory for electron transfer reactions in solution
Authors: Benjamin Ilan , Pollak Eli
Published: July 2002

Variational transition state theory is used to compute the rate of nonadiabatic electron transfer for a model of two sets of shifted harmonic oscillators. The calculations provide new insight on the suitability of the energy gap as a reaction coordinate. The relationship to the standard generalized Langevin equation model of electron transfer is established, and provides a framework for the application of variational transition state theory in a realistic simulation of electron transfer in a microscopic (nonlinear) bath.
Adiabatic and nonadiabatic effects of nuclear dynamics in spectra of decaying states: Auger spectrum of HF
Authors: Pahl E. , Meyer H.-D. , Cederbaum L. S. , Minelli D. , Tarantelli F.
Published: July 2002

An all ab initio calculation of the Auger spectrum of HF is presented which includes the effects introduced by nuclear dynamics. The involved potential curves of the core-ionized decaying state and the dicationic final states are computed by CASSCF. On these curves, the wave-packet dynamics is performed in an exact manner. Special attention is paid to the transition to the 1Π(2σ−1,1π−1) final state where an avoided crossing between this state and a satellite state is found within the region of decay. By vibronic coupling, the satellite gains intensity which influences the shape of the spectrum. The experimental spectrum is very well reproduced.
Tracer diffusion of highly charged polymer colloids
Authors: Härtl W. , Zhang-Heider X.
Published: July 2002

Starting from the monomer 1-1-dihydroperfluorobutylacrylate we synthesized a new model system of highly charged colloids with a low refraction index. By preparing a binary mixture of these particles with a small number of highly scattering polystyrene particles we could measure the time dependent self-diffusion coefficient at different ionic strength. As we recorded at the same time also the local order of the system by static light scattering we could scrutinize different empirical and theoretical relationships concerning the dynamics of our liquidlike ordered system.
The argon–diacetylene complex: An example of distributed interactions and transferable potentials
Authors: Bemish R. J. , Miller R. E. , Yang X. , Scoles G.
Published: July 2002

The first spectroscopic study of the argon–diacetylene complex is reported here. The rotationally resolved near infrared spectrum has been analyzed in terms of a conventional asymmetric rotor Hamiltonian, yielding a set of ground and vibrationally excited state rotational constants consistent with a ‘‘T-shaped’’ geometry, similar to that of Ar–C2H2. Using distributed potential parameters determined previously for the Ar–C2H2 system we have developed an empirical potential for Ar–C4H2. A ground state calculation using this potential and the collocation method gives a vibrationally averaged structure in good agreement with experiment. A tentative assignment is also made for the band origins of the Ar2, Ar3–diacetylene complexes.
Femtosecond site-selective probing of energy relaxing excitons in poly(phenylenevinylene): Luminescence dynamics and lifetime spectra
Authors: Kersting R. , Mollay B. , Rusch M. , Wenisch J. , Leising G. , Kauffmann H. F.
Published: July 2002

Exciton relaxation in poly(phenylenevinylene), PPV, has been probed by femtosecond luminescence-up-conversion. We report on excitonic luminescence profiles that depend on the spectral position of the detection window (ε̂=2.7,2.6,2.5, and 2.4 eV, respectively). In an attempt to reveal the transient steps implied in fs relaxation, we present a quantitative forward reconvolution fit procedure that is based on a microscopic incoherent transport model, including diagonal disorder, dipolar intersite coupling, and a density-of-states (DOS) of molecular site excitations. Special emphasis has been placed (i) on the analysis of luminescence lifetime distributions 〈φ(τ;ε̂,τ0)〉 which directly map out the spectra of hopping modes of energy-cascading neutral excitations, and (ii) on the rigorous evaluation of (radiationless) transfer population from high-energy subensembles to low-energy tail states of the DOS. We quantitatively show that the absence of significant rise terms in the S0ν=0←S1ν=0 luminescence transition of low-lying tail states is due to the spectral superposition of the S0ν=1←S1ν=0 luminescence decay from states near the center of the DOS which, owing to the predominantly positive amplitudes of their ultrafast decay channels, destructively interfere with the negative amplitudes (luminescence rise terms) of migratively prepared bottom states. Finally, the asymptotic branch of the luminescence lifetime distribution 〈φ(τ;ε̂=2.4 eV,τ0)〉 has been reconstructed in an unbiased fashion by inverting the high-precision, picosecond, single-photon counting luminescence data at ε̂=2.4 eV with the help of a regularized exponential series methodology. The measurements combined by numerical computing are in accord with the molecular picture of exciton relaxation in PPV in which neutral, localized excitations undergo predominantly incoherent excitation energy transfer among sites of fluctuating self-energies related to segmental subunits with distributed π-bond conjugation lengths.
Chemical potentials of hard polyatomic solutes in hard sphere fluids
Authors: Stamatopoulou Argyroula , Ben-Amotz Dor
Published: July 2002

Monte Carlo measurements of the chemical potential of hard polyatomics of different conformations dissolved in hard sphere fluids are reported. These are compared with analytical expressions derived from the hard fluid model, performed as a function of solute size and solvent density. An excluded volume approximation, which is exact at low density, is found to compare favorably with simulation results for solutes of low asymmetry. This equates the chemical potential of an arbitrarily shaped hardbody solute with that of a sphere of the same excluded volume. For solutes of high asymmetry systematic deviations from this excluded volume approximation are found. New analytical relations between the optimal effective hard sphere diameter and solute asymmetry are suggested. These yield quantitatively accurate chemical potential predictions for hardbody solutes of arbitrary size and shape, up to liquid densities.
Short-time viscous and density relaxation in glycerol and ortho-terphenyl
Authors: Roland C. M. , Ngai K. L.
Published: July 2002

Coupling model analyses of short-time relaxation data of glycerol from quasielastic neutron scattering and ortho-terphenyl (OTP) by molecular dynamic simulation are shown to account for both the time dependence of the correlation function and its temperature dependence. The latter is expressible over a limited temperature range as an Arrhenius relation with an apparent activation energy for the fast α-relaxation process, with EA=6.2 and 3.9 kcal/mol for glycerol and OTP, respectively. The viscosities of these glass formers have also been measured up to temperatures sufficiently high that the mean viscosity relaxation time, 〈τη(T)〉, becomes short, approaching the crossover time, tc≈2 ps, of the coupling model. When this condition is realized, 〈τη(T)〉 becomes comparable to τD(T) in both magnitude and temperature dependence. Hence, the asymptotic high temperature activation energy of 〈τη(T)〉 approximately equals the apparent activation energy of τD. The experimental data, supporting this connection between macroscopic variables such as viscosity and microscopic dynamics from neutron scattering and molecular dynamics simulation, indicate the significance of the fast α-relaxation process in both fragile and strong liquids.
On the transition from nonadiabatic to adiabatic rate kernel: Schwinger’s stationary variational principle and Padé approximation
Authors: Cho Minhaeng , Silbey Robert J.
Published: July 2002

For a two state system coupled to each other by a nonzero matrix element Δ and to the bath arbitrarily, the generalized master equation is derived by applying the well-known projection operator techniques to the quantum Liouville equation. The time-dependent rate kernel is expressed by an infinite summation of the perturbative terms in Fourier–Laplace space. The Schwinger’s stationary variation principle in Hilbert space is extended to Liouville space and then applied to the resummation of the rate kernel. The Cini–Fubini-type trial state vector in Liouville space is used to calculate the variational parameters. It is found that the resulting stationary value for the rate kernel in Fourier–Laplace space is given by the [N,N−1]–Padé approximants, in the N-dimensional subspace constructed by the N perturbatively expanded Liouville space vectors. The (first-order) simplest approximation satisfying the variational principle turns out to be equal to the [1,0] Padé approximant instead of the second-order Fermi golden rule expression. Two well-known approximations, the noninteracting blip approximation (NIBA) and nonadiabatic approximation, are discussed in the context of the [1,0] Padé approximants, based on the variational principle. A higher-order approximation, [2,1] Padé approximant, is also briefly discussed.
Reaction dynamics of atomic hydrogen with the hydrogenated Si(001) (2×1) surface
Authors: Kratzer P.
Published: July 2002

The formation of molecular hydrogen from the reaction of an ideal Si(001) (2×1) monohydride surface with gas-phase atomic hydrogen is simulated using classical trajectory calculations on an empirical potential energy surface. Apart from a direct Eley-Rideal reaction, reactions mediated by a “hot precursor” and induced desorption of homonuclear molecules are observed. For a reaction between light hydrogen and deuterium, the reactive cross section is large, ∼25 Å2, and the reaction product is mainly HD, with a few percent admixture of homonuclear hydrogen molecules. About 0.8 eV of the exothermicity of the reaction end up as kinetic energy of the molecules, while 0.6 eV and 0.3 eV are found in vibration and rotation, respectively. Exchanging the isotope offered in the gas-phase and in the adsorbate has a small effect on the cross section and internal energy distribution. Details of the energy distribution could help to distinguish between the different reaction mechanisms. On the basis of the calculations, several dynamical features are predicted that could be tested experimentally.
Bimodality in the dynamic response of a supercooled liquid
Authors: Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

In order to understand the emergence and the influence of the “cage effect” in the dynamic response of a liquid, a detailed theoretical study of the frequency dependent friction of a Lennard-Jones liquid is carried out by using the mode coupling theory. The frequency dependent friction becomes increasingly bimodal in nature as the density of the liquid is increased beyond the liquid–solid coexistence line. The bimodality is most clearly manifested in the frequency dependence of the imaginary part of the friction. The mode coupling theory provides a nice physical interpretation of these results which are also in good agreement with the existing computer simulations.
Effect of dipole-ion interactions on the Ginzburg temperature of ionic fluids
Authors: Schröer W. , Weiss V. C.
Published: July 2002

For the restricted primitive model (RPM), we have extended our approach to calculate the Ginzburg temperature of model fluids presented earlier to the Fisher–Levin DI level, that is, to a theory that also accounts for ion pairing and the interactions between free ions and ion pairs. Neither the results for the Ginzburg temperature ΔTGi* nor for the amplitude of the correlation length ξ0* change appreciably: Still we find ΔTGi* for the RPM to be 2–3 orders of magnitude smaller than for simple neutral fluids (Sutherland or square-well fluid), which is at variance with the results of Fisher and Lee. This discrepancy also finds expression in different numerical factors in the formulae for the correlation length in the low-density limit.
Grand canonical Monte Carlo and modified singlet integral equations for the density profile of a Yukawa fluid near a planar wall
Authors: Olivares-Rivas Wilmer , Degrève Léo , Henderson Douglas , Quintana Jacqueline
Published: July 2002

Results for the density profile for Yukawa molecules near a hard wall and an exponential attractive wall are presented for Grand Canonical Monte Carlo (GCMC) simulations, for the singlet hypernetted chain (HNC) integral equation and for a modified version of the Lovett–Mou–Buff–Wertheim (LMBW-1) which uses the exact contact value theorem. The results of the standard singlet HNC are quite poor. If the LMBW equation is modified (but still using the bulk direct correlation function) the results at high temperature become reasonable. However, the results at low temperatures, close to the bulk coexistence curve, are only a partial improvement. The contact value of the density profile is now quite good but the profile has the incorrect oscillatory behavior predicted by the unmodified approximate equation. The GCMC simulation results show a clear effect of drying at low temperatures.
Vibrational autoionization in polyatomic molecules
Authors: Jungen Ch. , Pratt S. T.
Published: July 2002

A general framework for vibrational autoionization in polyatomic molecules with nondegenerate ion cores is presented in terms of a nondiagonal quantum defect matrix. The expressions for the vibrational autoionization rate depend on two vibrational-mode-dependent factors; (1) a vibrational “zero-point amplitude” that describes the displacement along a given normal coordinate and (2) a quantum defect derivative that describes the effect of the vibration on the electronic degrees of freedom. The latter is closely related to the dipole moment derivative and thus to the infrared intensity of that mode. This connection provides insight into the recently observed mode dependence of the vibrational autoionization rates for NO2 and other small molecules.
Polymer chain binding with a flat adsorbent in the case of selective adsorption of segments: Monte Carlo simulation
Authors: Zheligovskaya E. A. , Khalatur P. G. , Khokhlov A. R.
Published: July 2002

Adsorption of a single regular AB copolymer chain on a flat impenetrable surface is studied by Monte Carlo simulation. Type A segments, which are regularly distributed over the chain length, are attracted to the surface with the energy ε, whereas for type B segments the surface serves only as an impenetrable obstacle. The critical adsorption energy is found as a function of fraction of type A segments φ. The dependencies of the average numbers of adsorbed sections (i.e., trains), loops, and tails and the average numbers of segments in these sections on ε are obtained. These dependencies are qualitatively different for homogeneous (φ=1) and heterogeneous (φ<1) chains. At φ⩽0.25, the lengths of train and loop sections become independent of the parameters φ and ε and determined solely by geometric peculiarities of the chain model under consideration. The average number and length of tails are always independent of the chain length. In the strong adsorption regime, the average numbers of loops and trains are proportional to the chain length for both heterogeneous and homogeneous chains. The segment density distribution for the adsorbed chain near the surface is different for homogeneous and heterogeneous chains in both weak and strong adsorption regimes.
Ab initio study of the photodissociation of HCO in the first absorption band: Three-dimensional wave packet calculations including the X̃2A′−Ã2A″ Renner–Teller coupling
Authors: Loettgers Andrea , Untch Agathe , Keller Hans-Martin , Schinke Reinhard , Werner Hans-Joachim , Bauer Cornelia , Rosmus Pavel
Published: July 2002

We present a comprehensive investigation of the Renner–Teller induced dissociation of HCO(Ã) following excitation in the first absorption band (Ã2A″←X̃2A′). The dynamics calculations are performed in the time-dependent picture by propagating two coupled wave packets including all three internal degrees of freedom. The corresponding potential energy surfaces have been generated from new electronic structure calculations. The quantities of interest are (i) the vibronic excitation energies in the Ã2A″ state, (ii) the state- and K-dependent lifetimes of the rovibronic states in the excited manifold, and (iii) the final vibrational–rotational state distributions of the fragment CO molecule. In all cases very satisfactory agreement with recent experimental results obtained from photofragment yield spectroscopy is achieved.
Improved treatment of inertia and non-Markovian effects on short-time dynamics of diffusion-controlled reaction based on generalized diffusion equation
Authors: Ibuki Kazuyasu , Ueno Masakatsu
Published: July 2002

Starting from a generalized diffusion equation and the Collins–Kimball boundary condition, we investigated the inertia and the non-Markovian effects on the time-dependent rate constant of a diffusion-controlled reaction at short times. In the short-time limit, we obtained the rate constant analytically, and found that the rate constant was independent of the friction coefficient, and was always smaller than the result of the classical Smoluchowski–Collins–Kimball (SCK) theory in which both of the inertia and the non-Markovian effects were neglected. At finite times, we obtained the rate constant numerically, and found that the decay of the rate constant was slower than that of the SCK result. When the non-Markovian effect became larger, the decay became much slower. Our results were consistent with a relevant theory based on a generalized Fokker–Planck equation. The results were compared with computer simulations, and a good agreement was obtained for the case of the maximum reactivity.