bluuebunny/crossref_metadata_2025 huggingface dataset:
Exact time-dependent quantum mechanical dissociation dynamics of I2He: Comparison of exact time-dependent quantum calculation with the quantum time-dependent self-consistent field (TDSCF) approximation
Authors: Bisseling R. H. , Kosloff R. , Gerber R. B. , Ratner M. A. , Gibson L. , Cerjan C.
Published: July 2002

The vibrational predissociation dynamics of a collinear model of the I2(v)He cluster is studied by numerically exact time-dependent quantum mechanics, and by the time-dependent self-consistent field (TDSCF) approximation. The time evolution for the initial excitation levels v=5, 11, 22 is explored. Excellent agreement is found between the TDSCF and the exact evolution of the wave packet; in particular the approximation reproduces well the dephasing events in the dynamics, and the measurable predissociation lifetimes. The results are very encouraging as to the applicability of quantum TDSCF as a quantitative tool in the study of van der Waals predissociation dynamics.
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o studies of the vibrational spectra and structure of phosphorus halides. I. Selection of basis functions and the spectra of PCl3
Authors: Shell R. , Steele D. , Gates P. N.
Published: July 2002

In an endeavor to establish a suitable ab initio basis set for the description of the structure and vibrational spectra of phosphorus chlorides, the effect of adding center-of-bond (cb) s and p functions to 3-21G* basis sets was explored. To ensure generality of 3-21G*cb results were compared with various simpler bases for a range of molecules containing second row elements. While geometries and vibrational frequency patterns are well reproduced using the 3-21G* basis, dipole moments are less adequately reproduced. By addition of s and p orbitals at the bond centers there is a marked improvement in the polar properties with the other properties being little affected. Use of only s orbitals in this context is, if anything, detrimental. The results are applied to a study of the simplest of phosphorus chlorides, PCl3. Among the extant literature force fields, field A of Cazzoli is identified as being near correct. As with this field, all the ab initio fields show that the experimentally estimated first order Coriolis constant of ζ4 is too negative by ∼0.2.
Rotational relaxation in the lowest triplet states of 2,4,6-triisopropylbenzophenones in solutions at room temperature studied by picosecond laser spectroscopy
Authors: Nakayama Toshihiro , Hamanoue Kumao , Funasaki Yasutaka , Teranishi Hiroshi
Published: July 2002

The triplet–triplet absorption spectra due to the lowest triplet states of meta- and para-substituted dicarbonyls of 2,4,6-triisopropylbenzophenone in solutions at room temperature shifted progressively to the blue with time on the subnanosecond time scale. The shift times were affected by changing the substituent X and their position of substitution as well as the viscosity of solvents. These results were interpreted in terms of the conformational change in the lowest triplet states.
Laser determinations of ‘‘hot band’’ quantum yields: Br*(2P1/2) formation in the continuum absorption of Br2 at 510–550 nm
Authors: Smedley John E. , Haugen Harold K. , Leone Stephen R.
Published: July 2002

Absolute quantum yields of Br* in the photodissociation of thermally excited vibrational levels of Br2 are measured by transient gain vs absorption probing of the Br*(2P1/2)–Br(2P3/2) transition at 2.7μm with an F-center laser. An etalon-narrowed pulsed dye laser with a linewidth of 0.04 cm−1 is used to excite continuum regions between the bound Br2(B–X) transitions of isotopic Br2(∼81% 81Br2) at selected wavelengths between 510 and 550 nm. The threshold wavelength for production of Br*+Br from v″=0, J″=0 is 510.7 nm. At 297 K the Br* yields vary from 85% at 510 nm to 12% at 550 nm. Absorption coefficient data measured with the laser together with the Br* quantum yields at both 297 and 373 K enable a direct determination of the B-state continuum absorption. The data are used to determine the individual contributions of the B 3Π(0+u), 1Π(1u) and A 3Π(1u) states to the continuum absorption in this spectral region. Good agreement is obtained between the measured B-state absorption coefficients and those calculated from the potentials of Le Roy et al., which were obtained by a fit to the total absorption data of Passchier et al. Deviations from Le Roy’s fit at very long wavelengths suggest an adjustment of the inner turning point of the B-state potential near the Br*+Br dissociation limit. The A-state continuum band has a peak absorption coefficient of ∼12ℓ mol−1 cm−1 at around 530 nm.
Effect of Ag and Cu overlayers on the surface-enhanced Raman scattering at Cu and Ag cold films
Authors: Gao Y. , López-Ríos T.
Published: July 2002

Surface-enhanced Raman scattering (SERS) of pyridine on coldly prepared Ag films was studied as a function of coverage of coadsorbed Cu atoms. Moreover, for cold Cu films, the progressive appearance of SERS with increasing thicknesses of Ag deposits was investigated with a 2.41 eV excitation light. We have found, with Auger measurements, a continuous growth of Ag on coldly deposited Cu films. Abnormal optical absorption related to SERS was also investigated.
Anisotropic friction in polymeric models. I. The elastic dumbbell and reptating chain
Authors: Baxandall L. G.
Published: July 2002

Expressions for the stress tensor of simple mechanical (bead–spring, bead–rod) models of polymers with anisotropic friction are derived using the phase space kinetic theory of Curtiss, Bird, and Hassager. The correspondence between such expressions and those postulated by Doi and Edwards for concentrated solutions of polymers suggests topological entanglement may be effectively modeled by anisotropic friction. In this first paper we consider the elastic dumbbell with anisotropic friction. A calculation of the zero shear rate rheological material constants η, ψ1,0, and ψ2,0, for a Hookean dumbbell is presented. The second normal stress coefficient ψ2,0 assumes a nonzero value with anisotropic friction. The stress tensor for a chain of such elastic dumbbells is shown to be that postulated by Doi and Edwards for topologically entangled, or reptating, flexible polymers.
Form and intrinsic optical activity in light scattering by chiral particles
Authors: Singham Shermila Brito
Published: July 2002

The effect of intrinsic optical activity on the scattering matrix elements of a particle with macroscopic chirality (i.e., form chirality) is examined. An extension of the coupled dipole method is used in which the optical activity of the medium is included in the dipole moments of the subunits which are arranged to describe the chiral structure. The magnitude of the difference in refractive indices of the medium for left and right circularly polarized light chosen for the calculations is realistic for biological systems. The scattering matrix elements which are sensitive to form chirality are found to be significantly changed by intrinsic optical activity of this magnitude and the effect is more important when the chiral parameters become small compared with the wavelength of light. The intrinsic optical activity has a significant effect on the circular intensity differential scattering (CIDS) of a chiral structure at forward scattering angles. The form and intrinsic contributions to CIDS are found to be additive.
Picosecond time-resolved and frequency domain coherent Raman scattering study of conjugated polymeric films: A soluble polydiacetylene, poly-4-BCMU
Authors: Swiatkiewicz Jacek , Mi Xin , Chopra Pratibha , Prasad Paras N.
Published: July 2002

We report the first study of both time-resolved and frequency domain coherent Raman scattering in a conjugated polymer, specifically a soluble polydiacetylene called poly-4-BCMU. Both CARS and CSRS spectra were recorded at room temperature and at 4 K in the region of –C=C– stretch on a ∼12 μm thick film of poly-4-BCMU in the red amorphous form. The relevant CSRS spectra of the blue crystalline form are also reported. Frequency domain study reveals the vibrational resonance frequency to be independent of the polymer molecular weight in the red form, but different for the blue and red form. The line shapes are asymmetric, but do not seem to fit the predictions of a simple model involving dominant two-photon resonance contributions. The observed vibrational dephasing in the time-resolved study is very fast at both room temperature and 4 K, being within the time-resolution available. From the combined analysis of the frequency domain and time-resolved studies at both room temperature and 4 K, an inhomogeneous mechanism of dephasing is inferred. The inhomogeneous dephasing arises from simultaneous coherent excitation of a distribution of vibrational frequencies originating from a distribution of polymer conjugation length.
Toward a Monte Carlo theory of quantum dynamics
Authors: Doll J. D. , Coalson R. D. , Freeman D. L.
Published: July 2002

We consider in the present paper an extension of numerical path integral methods for use in computing finite temperature time correlation functions. We demonstrate that coordinate rotation techniques extend appreciably the time domain over which Monte Carlo methods are of use in the construction of such correlation functions.
Monte Carlo path integration in real time via complex coordinates
Authors: Chang Johnny , Miller William H.
Published: July 2002

A method is described for Monte Carlo path integration that is valid for real time propagation. More specifically, it is shown how matrix elements of the complex-time propagator e−βcH, βc=β/2+it/ℏ, can be evaluated by straightforward Monte Carlo for values t≫βℏ/2. The key feature is that one distorts the path of the integration variables so that the kinetic energy part of the integrand is real. This in turn means that the paths are complex valued, but it is shown that, at least for barrier-type potentials, this causes no difficulties.
Observation of Rydberg transitions from the inner valence shell of ethane
Authors: Dillon Michael A. , Tanaka Hiroshi , Spence David
Published: July 2002

The electron impact spectrum of ethane has been examined in a region that includes ionization out of the inner valence shell. One diffuse structure and a progression of ten vibrational bands have been found in a 4 eV range below and to some degree overlapping the 2A2u ion threshold. Evidence indicates that the observed transitions belong to the symmetry forbidden Rydberg series (2a2u)2→(2a2u, npσ or npπ).
Concentration dependence of polymer dimensions. I. Theoretical access based on empirical laws for the dimensions in chain pairs with I-tuples of intermolecular overlaps
Authors: Olaj O. F. , Zifferer G.
Published: July 2002

An analysis of the configuration of pairs of self-avoiding five-way cubic lattice chains has revealed that on average the relative (squared) chain dimensions XI associated with any I- tupel of intermolecular overlaps (double occupancies) within fair limits of accuracy may be treated as an arithmetic series, ΔXI=(I−1)ΔX2, or, alternatively, also as a geometric series, XI=XI−12, for not too large values of I. Based on these findings comparatively simple expressions for the coefficient characterizing the first order concentration dependence kX of the squared end-to-end distance and the squared radius of gyration have been developed built up from X2 and the excluded volume functions h(z) and ψ(z) for the arithmetic series case, or h(z) and h(X2z), for the geometric series case, respectively. kX values calculated according to these expressions coincide well with those evaluated directly for lattice chains and exhibit at least fair agreement with those evaluated experimentally by SANS of SAXS techniques for real polymers.
Morphology of the ground state of a model five-atom system
Authors: Cheng-Guang Bao , Lim T. K.
Published: July 2002

An expansion in harmonic-oscillator basis sets and a convenient generalized Talmi–Moshinsky transformation are used to extract the wave function of the ground state in a quantum-mechanical five-boson cluster interacting pairwise through central soft-core potentials. The wave function, in turn, is manipulated through specially defined density functions to reveal the structures and spatial configurations favored in the system. The bipyramidal shape wins slightly over the square pyramid. In addition, the first ‘‘pictures’’ of constituent-particle movement within this microsystem are obtained.
Kinetic theory of dense fluids subject to an electric field and the Onsager–Fuoss theory of ionic conductivity
Authors: Eu Byung Chan
Published: July 2002

The generalized diffusion equation is obtained for pair distribution functions from the kinetic theory of dense fluids presented in the preceding paper. The derivation shows that the kinetic theory contains the Onsager–Fuoss theory of ionic conductivity under the set of assumptions and approximations made for the derivation. It also provides various molecular formulas which may be in principle calculated by statistical mechanical means, and ways to generalize the theory. It is shown that the drift velocity obtained can be identified with the one in the Onsager theory. A nonlinear mobility formula is also presented which reduces to the linear theory formula holding near equilibrium.
Calculation of the thermophysical properties of ground state sodium atoms
Authors: Holland Paul M. , Biolsi Louis
Published: July 2002

Transport properties of dilute monatomic gases depend on the two body interaction potentials between the atoms. When two ground state sodium atoms interact, they can follow either of two potential energy curves corresponding to the Na2 molecule in the X 1Σ+g or the 3Σ+u state. Transport collision integrals and second virial coefficients of monatomic sodium have been calculated by accurately representing quantum mechanical potential energy curves with the Hulburt–Hirschfelder potential. The generally good agreement of calculated viscosities and second virial coefficients with the available experimental viscosities and with previously calculated virial coefficients provides further evidence that this approach can be used for accurate estimates of thermophysical properties under conditions where experimental data are sparse or unavailable.
Lattice models of polymer solutions: Monomers occupying several lattice sites
Authors: Nemirovsky A. M. , Bawendi M. G. , Freed Karl F.
Published: July 2002

An exact field theory is presented to describe a system of self-avoiding lattice polymer chains with arbitrary regularly branched architecture. Equivalently, the chains can be viewed as linear and as composed of structural units (monomers) having a chosen shape and size and therefore each occupying more than one lattice site. The mean field approximation coincides with Flory’s theory, and it does not distinguish among chain geometries. However, we develop a systematic expansion for corrections to mean field approximation in powers of z−1 where z is the lattice coordination number. The entropy per site, the pressure and the chain insertion probability are computed for various chain architectures to O(z−2). At equal lattice site coverages per chain and total polymer volume fraction, the more compact the polymer chain geometry the higher is the insertion probability.
Accurate partial resonance widths for collinear reactive collisions
Authors: Schwenke David W. , Truhlar Donald G.
Published: July 2002

We present three methods for obtaining resonance energies and total and partial widths from scattering matrix elements as a function of energy in the vicinity of an isolated resonance. The methods are applied to 14 resonances in collinear reactive scattering systems, and they yield stable and reasonably consistent results. In 12 of the cases the partial widths sum to significantly less (23%–76%) than the total width even though the eigenphase sum fits the generalized Breit–Wigner form very well. The quantal partial widths are used to compute branching ratios for comparison to approximate results.
Many-body crystal field calculations. I. Methods of computation and perturbation expansion
Authors: Ng Betty , Newman D. J.
Published: July 2002

The practical implementation of biorthogonal perturbation techniques to calculate one- and two-electron crystal field parameters is described. The systems and basis sets used in the calculations are described in detail, and several crucial aspects of the numerical procedures are discussed. A complete listing of second order perturbation diagrams with associated denominator corrections is given.
A classical mechanical analysis of vibrational dephasing and rotational energy redistribution in CO–Ar
Authors: Farantos S. C. , Flytzanis N.
Published: July 2002

The role of rotational excitation of CO in the van der Waals (vdW) CO–Ar system in the vibrational dephasing and rotational energy redistribution has been studied with the classical trajectory method. At low angular momentum the motion is chaotic for a wide range of energies. As the rotational quantum number increases the rotational mode is decoupled from the stretching mode and the trajectories show regular patterns. The predominant feature of the dynamics is the impulsive collision of Ar with CO and this is identified as the main route to chaotic behavior in vdW species. An explanation of the numerical results is provided, using the standard mapping for the motion around an 1:n resonance between the diatomic and orbital angular momentum.
Classical energy transfer in forced oscillator models of inelastic scattering
Authors: Gazdy Bela , Sun Qiyan , Bowman Joel M.
Published: July 2002

A compact formalism is presented to solve the classical equations of motion for a general linearly driven parametric oscillator. It is shown that quantum transition probabilities obtained from an operator algebraic technique can be expressed in terms of classical energy transfers. The success and limitations of the DECENT method for vibration–translation energy transfer in molecular collisions is discussed and illustrated numerically in a simple model.
Molecular reorientations and local structure in nitrate aqueous solutions
Authors: Rosi B. , Fontana M. P.
Published: July 2002

We present an experimental study of anionic reorientational dynamics in Cu(NO3)2 and Ca(NO3)2 aqueous solutions. The anion reorientational autocorrelation time τor has been obtained by Fourier band shape analysis of the Raman 1050 cm−1 N–O stretching vibration line and by a multiple Lorentzian fitting of the depolarized Rayleigh wing spectrum. Its evolution as a function of solute concentration and of the nature of the counterion present (Cu2+, Ca2+) has been analyzed. Deviations from the Stokes–Einstein proportionality relation between Raman reorientational correlation times and the values of shear viscosity of the system have been used to obtain the microscopic structure contribution to the viscosity itself. From the depolarized Rayleigh wing spectra we have been able to investigate the contributions of both water and the nitrate ion to the reorientational relaxation in the solutions and to assess the contribution of collective effects.
Molecular dynamics of liquid water in a circularly polarized external field
Authors: Evans M. W. , Lie G. C. , Clementi E.
Published: July 2002

The dipole moment of the water molecule has been subjected to an external circularly polarized field of force and the effect on the molecular dynamics evaluated through novel laboratory and moving frame cross correlation functions (ccf ’s) using molecular dynamics computer simulation. The field symmetry is such as to make possible the existence in both frames off diagonal elements of the ccf ’s which disappear at field off equilibrium. The laboratory frame ccf ’s involve simultaneously the rotational and translational motion of the molecule and are not describable with contemporary theories of ‘‘rotational’’ or ‘‘translational’’ diffusion. The simulation shows laboratory frame birefringence in the orientational and rotational velocity autocorrelation functions (acf ’s) in the y (or z) and x axes for a field applied in the x axis. This birefringence is linked to the appearance of ccf elements induced by the circularly polarized field, and its experimental measurement would provide indirect information on the time dependence of these ccf ’s.
Defects in the electronic structure of poly (p-phenylene vinylene): Electronic spectra, electrochemical behavior, and molecular orbital calculations
Authors: Obrzut J. , Karasz F. E.
Published: July 2002

The electronic spectrum of thin films of poly(p-phenylene vinylene) (PPV) in an insulating neutral form exhibits three absorption bands with maxima at 6.12, 5.06, and 3.08 eV. The first two maxima are related to localized molecular states and the broad band at 3.08 eV is described in terms of a vibrationally coupled exciton model. Upon electrochemical doping of PPV, two additional absorption bands appear at 0.80 and 2.10 eV which are associated with a dication whose charge is spread over a few monomer units. Thin films of PPV can be electrochemically converted from the oxidized (conducting) form to the neutral (insulating) form repeatedly in LiAsF6/propylene carbonate solution without evidence of decomposition. The E0 measured using cyclic voltammetry is 3.85 V vs a Li/Li+ reference electrode; however, the observed optically active defects occur in a region of higher potential.
LEED structure analysis of the clean and (2×1)H covered Pd(110) surface
Authors: Skottke M. , Behm R. J. , Ertl G. , Penka V. , Moritz W.
Published: July 2002

From an analysis of the low-energy electron diffraction (LEED) intensities we have determined the oscillatory distortion of the topmost interlayer spacings of the clean and the (2×1)H covered Pd(110) surface as well as the exact adsorption geometry in this latter structure, which is formed at T<180 K at a coverage of θH≤1.0. An R-factor analysis was used for quantitative comparison with the experimental data. The oscillatory distortion of the clean surface— d12=−5.1±1.5%, d23=+2.9±1.5%, d34 at its bulk value of 1.37 Å (with R factors RP=0.22 , RZJ=0.14 )— is found to be reduced by the H adlayer to d12=−2.2±1.5%, d23=+2.9±1.5%, and d34=bulk value. The H atoms are adsorbed on quasi-threefold sites with equal distances of 2.0±0.1 Å to the two nearest Pd neighbors in the topmost and the closest Pd atom in the second layer, leading to an effective radius of the H atom of rH=0.6±0.1 Å. The long-bridge adsorption site, (octahedral) subsurface sites, or a hydrogen induced reconstruction via a lateral displacement of topmost Pd atoms by more than ±0.1 Å can clearly be ruled out. These structural data, which are in good agreement with those of ordered Had structures on Ni(110) and Fe(110), characterize the (2×1)H structure on Pd(110) as being a typical adsorbate structure. There is no indication of either direct occupation of distinct subsurface or of near surface adsorption sites in this structure, nor does it open up channels for surface penetration, e.g., by a strong distortion of the topmost Pd substrate layers. In contrast to Ni(110) and Rh(110) the island growth of the (2×1)H structure on Pd(110) indicates predominantly attractive, indirect adatom–adatom interaction which, however, are of distinctly different nature than those causing island formation in the Pd–hydride phase.
Time-resolved fluorescence and hole-burning line shapes of solvated molecules: Longitudinal dielectric relaxation and vibrational dynamics
Authors: Loring Roger F. , Yan Yi Jing , Mukamel Shaul
Published: July 2002

We develop a microscopic theory of time- and frequency-resolved fluorescence and hole-burning measurements of polar, polyatomic molecules in a polar solvent. The line shapes are expressed in terms of gas phase spectroscopic parameters of the solute, vibrational relaxation rates, laser pulse shapes, and the dynamics of a solvation coordinate. These dynamics are then related to the frequency and wave vector dependent dielectric function of the solvent. Both fluorescence and hole-burning line shapes are predicted to show significant line narrowing at short times, and to undergo broadening and a red shift as the solvent relaxes. We propose hole burning as an alternative to fluorescence measurements in probing solvation dynamics. The time scale of the solvent induced line shift and line broadening is found to be independent of the shape of the solute, in contrast with previous work. The effects of vibrational relaxation are distinguished from those of solvent relaxation.
Intermolecular vibrational coupling associated with the 2ν2 features of the Raman spectrum of strontium nitrate
Authors: Jayasooriya U. A. , Kettle S. F. A. , Lugwisha E.
Published: July 2002

A comparative study of the Raman spectra of chemical and physically mixed crystals of Sr(14NO3)2 and Sr(15NO3)2 shows that the doubled features observed in the 1630 cm−1 (2ν2) region result from intermolecular vibrational coupling and not from multiple site occupancy.
Cooperative ordering of ammonium ions in NH4ReO4: A pseudospin model
Authors: Taylor D. R.
Published: July 2002

The coupling between ammonium ion orientation and the symmetric strain 2ezz–exx–eyy in scheelite NH4ReO4 is expressed in terms of an Ising pseudospin model with a longitudinal field. The model implies continuous ordering in one orientation with no phase transition in agreement with heat capacity and x-ray data.
Matrix elements for rotating Morse oscillators using the Elsum and Gordon approximations
Authors: Piñeiro A. Lopez , Moreno B.
Published: July 2002

In this paper, formulas and recursion relations for the matrix elements of the operators {1−exp[−a(r−re)]}n, exp[−a(r−re)]n, and (r−re){exp[−a(r−re)]}n are derived for a rotating Morse oscillator within the Elsum and Gordon approximation. These equations can be used to calculate the diagonal (v=v′, J=J′) and off-diagonal (v≠v′, J≠J′) matrix elements.
Measurement and modeling of nitrogen resonance line profiles from an electrodeless discharge lamp
Authors: Wood David R. , Skinner Gordon B. , Lifshitz Assa
Published: July 2002

Experimental profiles of the 1200 Å resonance triplet of atomic nitrogen were measured for a variety of operating conditions of an end-on electrodeless lamp, and corresponding absorption curves were calculated. Each source profile was determined by fitting parameters to an empirical two-layer model, then convoluting with the instrumental function for comparison with experimental data. Each three-component profile was fitted with three adjustable parameters: an absorption parameter for each of the two layers and a third absorption parameter to adjust for radiation trapping. Curves of absorption as a function of atom concentration, calculated from these profiles, are very similar to the shock tube calibrations of Thielen and Roth in which a source of similar design has been used.
Classification of the tunneling-rotational energy levels of ammonia dimer in the molecular symmetry group
Authors: Nelson D. D. , Klemperer W.
Published: July 2002

The symmetry properties of NH3 dimer are analyzed in the molecular symmetry group and the physical assumptions underlying the choice of this group are detailed. Several low lying tunneling states are predicted. Two of these states transform as four-dimensional (G) representations of G36, the molecular symmetry group. These two states can have the same microwave selection rules as those found in the two experimentally observed states of NH3 dimer. That is, the microwave transitions can follow pure rotational selection rules (no tunneling splittings) even though interchange tunneling is included as a feasible motion in the molecular symmetry group. The observation of these selection rules is interpreted as internal rotation stabilizing the system against interchange tunneling. Hence, in these particular states the interchange tunneling is quenched. The group theory predicts this picture of ammonia dimer to be appropriate if certain internal rotation interactions are large compared to the interchange tunneling matrix element. These conclusions are derived through correlation diagrams which are used to provide physical insight into the nature of the two states of G symmetry and approximate electric dipole selection rules for microwave transitions arising from these states. The selection rules for the infrared transitions of this dimer which correlate to the ν2 fundamental of NH3 are also given.
Experimental ESR method for investigating slow-orientational diffusion of normal nitroxides with 14N(I=1)
Authors: Lee Sook , Ames D. P.
Published: July 2002

The general theoretical-experimental method recently formulated for investigating dynamic properties of slow-orientational anisotropic ESR hyperfine centers under orientational diffusion is applied to normal nitroxide radicals with14N(I=1). This application permits the determination of slow-orientational correlation times, motional inhomogeneous linewidths, and temperature-dependent hyperfine coupling constants from the first- and second-derivative peak positions of nitroxide ESR hyperfine structures in polycrystalline-amorphous condensed matter.
Multiphoton ionization study of intra- and intermolecular effects on the photodissociation of methyl iodide
Authors: Sapers Steven P. , Vaida Veronica , Naaman Ron
Published: July 2002

A time-of-flight mass spectrometer (TOFMS) was used to obtain multiphoton ionization (MPI) mass and wavelength spectra of methyl iodide. The sample was studied in a molecular jet environment which allowed preparation of isolated molecules and/or clusters of different size. Excitation was performed to access the dissociative valence A state and the predissociative Rydberg C state. Valence state excitation produced no TOF ion signal for (CH3 I)n, n=1, but produced I+2 for n=2 and CH3 I+ for large n. The presence of CH3 I+ is an indication of cluster ‘‘caging’’ of the chromophore and consequential stabilization of the valence state by intermolecular interactions. REMPI spectra of the Rydberg C state origin and C–I stretch produced results that reflect the longer lifetime of this intermediate state. Spectral line shapes and intensities, and mass fragmentation patterns are discussed in light of the photodissociation dynamics of the intermediate Rydberg state and the effect of intermolecular interactions on this process.
Computations simulating experimental observations of complex bursting patterns in the Belousov–Zhabotinsky system
Authors: Bar-Eli Kedma , Noyes Richard M.
Published: July 2002

A model based on five independent composition variables has been tested by simulating the complex bursting patterns observed experimentally for a Belousov–Zhabotinsky system in a flow reactor. If the system is in a region of constraint space where all stationary states are locally unstable, and if malonic acid concentration in the feed is decreased at constant flow of bromide ion, simple limit-cycle oscillations persist until the system reaches the Hopf bifurcation which generates a stable stationary state with low concentration of bromide ion. If bromide ion concentration in the feed is increased at constant flow of malonic acid, more and more small-amplitude oscillations are interspersed between trains of fewer and fewer large-amplitude excursions until a Hopf bifurcation leads to a stable stationary state with a high concentration of bromide ion. Patterns may be quite complex but usually repeat regularly. If a firing number is defined as ΣS/(ΣS+ΣL), where the summations represent the numbers of small- and large-amplitude oscillations in a complete pattern, then this firing number increases monotonically with increasing bromide ion in the feed. Narrow regions of chaotic mixing of patterns are also observed. The computations agree spectacularly with the careful experimental observations of Maselko and Swinney. Two separate curves are generated by the Hopf bifurcations which limit local stabilities of stationary states containing low and high concentrations of bromide ion, and the intersection of these curves provides a rationale for the ‘‘cross-shaped phase diagrams’’ which define regions of bistability and of oscillations in constraint spaces for such systems.
Quantum correlations in a model for multiple-photon absorption
Authors: Brunet Jean-Philippe , Leforestier Claude , Wyatt Robert E.
Published: July 2002

A multimode model for molecular multiple-photon absorption was developed recently. Neglecting quantum correlations, Heisenberg equations of motion exhibit chaotic solutions and the energy absorbed grows approximately linearly in time. Corresponding fully quantum calculations are reported on a generic three-mode system. Quantum resonances are located for which the total absorption remains bounded.
The effect of the exchange mode dynamics on vibrational phase relaxation at surfaces
Authors: Green-Johnson Willeta
Published: July 2002

The problem of abstracting information from vibrational line shapes at metal surfaces has received much theoretical and experimental attention within the last decade. Currently the pathways to vibrational phase relaxation at surfaces thought to contribute the most are (a) coupling to a lower frequency mode in energetic contact with the phonons of the substrate bath—also called the ‘‘exchange mechanism’’, and (b) lateral dipole–dipole interaction between admolecules. The present work advances the understanding of the former contribution, that of the exchange mechanism. It is a comparative study of the line shape due to a novel treatment of the exchange mode dynamics against that due to the usual assumption of the energy diffusion limit model of the exchange mode dynamics. It is found that by allowing for phase relaxation of the lower mode as well as energy relaxation, the line shapes differ in shift and width, and for certain range of coupling strengths (of the high frequency mode to the low frequency mode, of the lower mode to the bath, etc.), the differences can be experimentally discerned. The work also suggests a unified convention for reporting the coupling between the system modes (called δω throughout the body of the paper), and the damping parameter η.
Calculation of the Auger spectrum of liquid water
Authors: Liegener C.-M. , Chen R.
Published: July 2002

The Auger spectrum of liquid water has been calculated by a Green’s function method applied to a dimer in the effective field of point charges simulating the neighboring molecules. The positions of the molecules have been determined by a Monte Carlo calculation. The obtained theoretical spectrum has been compared to the theoretical and the experimental Auger spectra of gas-phase water and to the experimental Auger spectrum of ice.
Experimental and INDO/CI calculated gas phase C1s shake-up spectra of C6H6, C6H5OH, and C6H5CH2OH
Authors: Nordfors D. , Nilsson A. , Mårtensson N. , Svensson S. , Gelius U. , Lunell S.
Published: July 2002

High resolution x-ray excited (1487 eV photon energy) shake-up spectra of C6H6, C6H5OH, and C6H5CH2OH are presented. A large number of new structures are observed and an interpretation is made, for the first π–π* shake-up states, using INDO/CI calculations. The C1s shake-up spectra of C6H6 and C6H5CH2OH are similar but differ substantially from the C6H5OH C1s shake-up spectrum. This is explained as originating from the conjugation of the lone pair O2p orbitals in the latter case. It is also shown that the inelastic scattering excitations fall exactly under the shake-up spectrum. This observation has important consequences for the assignment of shake-up spectra from aromatic ring molecules in the solid phase.
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o calculations of the vibration–rotation spectrum of HCS−
Authors: Senekowitsch J. , Carter S. , Werner H.-J. , Rosmus P.
Published: July 2002

The three dimensional near equilibrium potential energy and dipole moment surfaces of the electronic ground state of HCS− have been calculated from correlated MCSCF-CI electronic wave functions. These data have been used in perturbation and variational calculations of the bound and electron detachment anharmonic vibration–rotation levels. The electron affinity EA0 is calculated to be 0.41 eV and the equilibrium geometry to be RCH=1.111 Å, RCS=1.687 Å, α=106°. The fundamental vibrational band origins and integrated absorption band intensities are predicted to be 2648 cm−1/1318 cm−2 atm−1 (CH stretch), 1140 cm−1/145 cm−2 atm−1 (bend), and 911 cm−1/50 cm−2 atm−1 (CS stretch) in HCS−. The components of the dipole moment functions are given analytically. The dipole moment in the vibrational ground state of HCS− has been calculated to be 2.122 D. Radiative transition probabilities among low lying vibrational levels have also been evaluated. It is found that the radiative lifetimes vary in a mode-specific way. The theoretical photoelectron spectrum of HCS− and DCS− is reported.
Diffusion limited reactions on highly anisotropic surfaces
Authors: Meakin Paul
Published: July 2002

Monte Carlo simulations have been used to investigate diffusion limited annihilation reactions of the type A+A→0 on anisotropic two-dimensional surfaces. Two models in which diffusion is restricted to channels parallel to one of the axes of a square lattice were used. In model I A species annihilate other A species in the same channel but cannot pass A species in adjacent channels. For this model the time dependence of the number of sites occupied by A can be described in terms of the scaling form N(t)=k1/3f f (k2/3ft) if A is fed at a constant rate (kf) to the surface which is empty at time t=0. In model II A species annihilate other A species in adjacent channels but do not react with A species in their own channel. For model II the scaling form given above also describes the time dependence of the number of occupied sites for simulations starting with an empty surface. However, if the initial surface coverage is high, then the time dependence of the number of occupied sites is given by N(t)=kαf f (kβft) where α≂0.3 and β≂1.0. In this case the scaling function f(x) has the form f(x)∼x−0.3 at short times and f(x)=constant at long times.
Laser spectroscopy of calcium and strontium monoazide free radicals
Authors: Brazier C. R. , Bernath P. F.
Published: July 2002

We have synthesized the gas-phase metal azides CaN3 and SrN3. These ionic metal monoazides were found to have linear geometries. The positions of the Ã2Π–X̃ 2Σ+ and B̃ 2Σ+–X̃ 2Σ+ electronic transitions were determined as well as several vibrational frequencies. The 0–0 band of the Ã 2Π–X̃ 2Σ+ system of SrN3 has been rotationally analyzed by laser excitation spectroscopy yielding a Sr–N bond length of 2.26 Å.
Reaction rate constants of HO2+O3 in the temperature range 233–400 K
Authors: Wang Xiuyan , Suto Masako , Lee L. C.
Published: July 2002

The reaction rate constants of HO2+O3 were measured as a function of temperature. HO2 was produced by the reaction sequence: Cl+CH3OH→CH2OH+HCl and CH2OH+O2→HO2+CH2O. HO2 was detected by the OH(A–X) fluorescence produced by photoexcitation of HO2 at 147 nm. The reaction rate constants are represented by k(T)=(1.8±0.6)×10−14 exp[−(680±148)/T] cm3/s in 253–400 K, and are nearly a constant of (1.3±0.3)×10−15 cm3/s in 233–253 K.
Inelastic electron tunneling spectra of 1- and 2-naphthoic acids
Authors: Ratinen H. , Kiviharju M.
Published: July 2002

Inelastic electron tunneling spectra of 1- and 2-naphthoic acids were measured at 4.2 K with the aluminum electrode biased negatively and positively with respect to the lead electrode. The naphthoic acid molecules were transferred onto the alumina surface by a vacuum deposition technique. The complete inelastic electron tunneling vibrational spectra are presented and the line positions are listed in the table. We have observed an enhanced noise level in the energy range above 1800 cm−1 in the tunneling spectra of some multiring aromatic dopants. The noise appears in a particularly strong form in the spectra of 1- and 2-naphthoic acids. One of the aims of the present experiments was also to study the character of this noise.
Fine and magnetic hyperfine structure in the A 2Π and X 2∑+ states of yttrium monoxide
Authors: Childs W. J. , Poulsen O. , Steimle T. C.
Published: July 2002

A molecular beam optical and rf–optical double resonance spectroscopic study of numerous vibrational components in the A2Π3/2–X2∑+ subband of gas phase YO has been performed. The fine and magnetic hyperfine parameters for v=0–4 of the X2∑+ state have been determined. The observed strong vibrational dependence of the spin–rotation parameter γ(X2∑+) is proposed to arise from a perturbation from two as yet unobserved low-lying 2Π states coming from a three open-shell configuration. The magnetic hyperfine parameters for the X2∑+ state can be interpreted in terms of a single unpaired electron model. An estimate for the A2Π excited state dipolar parameter has been established.
Theory for the chi parameter of polymer blends: Effect of attractive interactions
Authors: Curro John G. , Schweizer Kenneth S.
Published: July 2002

Using an integral equation theory for polymer blends, we have calculated the effective chi parameter for a binary blend of Gaussian polymers interacting with identical hard core repulsive and dissimiliar attractive interactions between chain segments. Our results predict a parabolic composition dependence for χ in qualitative accordance with recent neutron scattering experiments and Monte Carlo simulations. We also find that the attractive interactions induce structural correlations which tend to significantly reduce χ and thus stabilize the miscible mixture.
Excess electrons in polar molecular clusters
Authors: Barnett R. N. , Landman Uzi , Cleveland C. L. , Kestner N. R. , Jortner Joshua
Published: July 2002

Electron attachment and localization to water and ammonia clusters, (NH3)−n and (H2O)−n (n=16–256), is studied using path-integral molecular dynamics simulations, employing an electron–molecule pseudopotential which includes Coulomb, polarization, exclusion, and exchange contributions. Due to the nature of the interactions the ammonia clusters do not form stable well-bound surface states for any size, in contrast to the case of medium size water clusters. The onset of bound excess electron states in ammonia clusters occurs for size n≥32, via an internal localization mode, in agreement with experimental data.
Crossed beam study of Na*(2P3/2)+O2→Na++O2−. The dependence of the total cross section upon Na* alignment
Authors: Tung C. C. , Reck Gene P. , Rothe Erhard W.
Published: July 2002

The title reaction, and its SO2 analog, have been measured as a function of the laser-prepared Na* p-orbital alignment. Within the 5%–8% experimental error, the total cross section is independent of the alignment.
Near threshold photofragmentation dynamics of ICl–Ne A state van der Waals complexes
Authors: Drobits Janet C. , Lester Marsha I.
Published: July 2002

The nascent ICl product state distribution is examined following vibrational predissociation of ICl–Ne A(vA=23) complexes via an optical–optical double resonance technique. ICl fragments are produced in approximately equal proportion in the vA=22 and 21 levels of the A(3Π1) state. Low rotational levels of ICl A(vA=22) are primarily populated, JA=1–8, accessing only a small portion of allowed states. ICl A(vA=21) fragments are produced with up to 69 cm−1 of rotational energy in a broad distribution which extends to nearly the energetic limit. Homogeneous broadening of the ICl–Ne feature indicates a vibrational predissociation lifetime between 2.3 and 50 ps.
Finite size scaling in polymer systems
Authors: Halley J. W.
Published: July 2002

We discuss finite size scaling for common models of polymer systems, both in the dilute and in the concentrated cases. In the dilute case, we find that some extension of ideas of Redner and Reynolds shows that the transition to a collapsed phase should be regarded as thermodynamically first order. In the concentrated case, paying careful attention to the relationship of the number of polymers to the number of monomers per chain length shows that the effects of an analogous chain collapse should be observable in polymer melts confined to pores of submicron size. We discuss some possible experimental consequences.
The Leslie coefficients for a polymer nematic liquid crystal
Authors: Lee Sin-Doo
Published: July 2002

We have calculated the Leslie coefficients for a nematic system composed of long rod-like molecules by starting with molecular kinetic theory. We parameterize the friction coefficient of the hydrodynamic interaction to provide a better understanding of the dynamic behavior of a nematic liquid crystal. The results obtained for effective nematodynamic viscosities differ qualitatively from others in which the hydrodynamic viscous stress is neglected. The Gaussian distribution function is also utilized to illustrate the main features of the asymptotic behavior of the nematodynamic viscosities as the molecular length increases. We have presented conclusive results which agree with predictions made by the infinite chain model as the hydrodynamic stress becomes important. In particular, it is demonstrated that the elongational flow viscosity diverges in the long chain limit.
The onion method for multiple perturbation theory
Authors: Cross R. J.
Published: July 2002

We develop a method of successive approximations for molecular scattering theory. This consists of a recipe for removing from the Schrödinger equation, one by one, the wave functions of a set of approximate solutions. The radial wave function is expressed as a linear combination of the well-behaved and singular solutions of the first approximation, and a set of coupled differential equations is obtained for the coefficients of the approximate solutions. A similar set of coefficients is obtained for the next approximation, and the exact coefficients are expressed in terms of the approximate coefficients to yield a set of second-level coefficients. The process can be continued like pealing off the layers of an onion. At each stage the coupled differential equations for the coefficients is equivalent to the Schrödinger equation. Finally, one can either ignore the remaining coefficients or approximate the coupled equations by a simple perturbation theory.
Electron localization in water clusters. I. Electron–water pseudopotential
Authors: Barnett R. N. , Landman Uzi , Cleveland C. L. , Jortner Joshua
Published: July 2002

A local pseudopotential for the interaction of an electron with a water molecule in the electronic ground state is developed. The potential contains Coulomb, adiabatic polarization, exclusion, and exchange contributions. The potential is suitable for a description of excess electron states in water clusers, and for studies of electron solvation in water. Quantum path integral molecular dynamics simulations of electron localization in water clusters employing this potential yield results in agreement with available experimental data and all-electron quantum chemical calculations.
Determination of rotational constants using nuclear quadrupole hyperfine structure: The microwave spectrum and dipole moment of vinyl iodide, obtained using a computer-controlled spectrometer
Authors: Cramb David T. , Gerry M. C. L. , Lewis-Bevan W.
Published: July 2002

The microwave spectra of vinyl iodide, CH2=CHI, in its ground and first excited vibrational states, have been measured in the frequency range 20–108 GHz. The spectrometer used was a 100 kHz Stark modulated instrument, interfaced to a Digital Micro-PDP 11/23+ computer for control and for data acquisition and storage; this interfacing is described. The spectrum contains strong a-type transitions and very weak b-type transitions; all contain 127I quadrupole hyperfine structure, with several large perturbations. A procedure specially devised for analysis of such spectra, which takes advantage of the perturbations, was applied to produce accurate values of constants that might otherwise be unobtainable, and to permit assignment of the b-type transitions. Both components of the dipole moment have been measured. The centrifugal distortion constants and inertial defects have been compared with those calculated from a published harmonic force field, modified for the out-of-plane vibrations. A partial structure has been obtained.