bluuebunny/crossref_metadata_2025 huggingface dataset:
Mobility of Cl− ions in Ne, Ar, and Kra)
Authors: Thackston M. G. , Eisele F. L. , Pope W. M. , Ellis H. W. , Gatland I. R. , McDaniel E. W.
Published: February 2003

Using the Georgia Tech drift tube spectrometer, measurements were made at 300°K of the mobilities of 35Cl− in Ne, Ar, and Kr as a function of electric field and gas number density. (AIP)
The electron impact spectra of some mono-olefinic hydrocarbonsa)
Authors: Johnsonb) Kenneth E. , Johnston David B. , Lipsky Sanford
Published: February 2003

Electron impact spectra of ethylene, propylene, isobutene, trans-butene, cis-butene, trimethylethylene, and tetramethylethylene have been obtained at scattering angles of 0° and 90° and at impact energies from ?20 to 150 eV. The spectra scan an energy-loss region from 2.5–15 eV. All of the observed Rydberg transitions of the methyl derivatives are correlated to corresponding Rydberg transitions of ethylene. The missing π→3p transitions of ethylene are tentatively located via this correlation. Evidence is also presented for assigning the N→3R′ system of ethylene (at 8.26 eV) to a π→3px transition. Possible assignments of some broad continua above ?8–9eV as σ→π* and σ→σ* transitions are considered. In agreement with other reported large-angle electron impact spectra, no evidence is obtained for transitions that could be assigned to triplet Rydberg states. However the π→π* triplet transitions are all clearly located with transition energies in good agreement with those obtained by a variety of other techniques.
Dynamics of three-body half collisions. II. Instantaneous three-body decomposition in the photodissociation of trifluoroacetyl iodidea)
Authors: Krogerb) Peter M. , Riley Stephen J.
Published: February 2003

Photofragment spectroscopy of trifluoroacetyl iodide (CF3COI) at 266 nm shows evidence of an instantaneous decomposition to I, CO, and CF3 fragments. The transition dipole moment for photon absorption lies in the C–C–O plane within 20° of the C–I bond axis, and decomposition of the photoexcited CF3COI molecule is rapid (∼10−13 sec). A two-step dissociation model developed to explain acetyl iodide photodissociation does not apply to trifluoroacetyl iodide, indicating fluorine substitution has a dramatic effect on photodissociation dynamics. Monte Carlo trajectory calculations using a very simple potential energy surface are surprisingly successful in predicting the observed dissociation behavior.
A new functional form for representing vibrational eigenenergies of diatomic molecules. Application to H2+ ground statea)
Authors: Hashemi-Attar Ali-Reza , Beckel Charles L. , Keepinb) William N. , Sonnleitner Stephanie A.
Published: February 2003

We propose representing the vibrational eigenenergies Ev in a form Ev=D−(vD−v)m [L/N] where v is the quantum number, D is the energy at the dissociation limit, and vD and m are state dependent parameters. The symbol [L/N] denotes a rational fraction with polynomials in (vD−v) of degree L and N in the numerator and denominator, respectively. This permits interpolation between near equilibrium, where power series expansions in v have validity, and near dissociation, where (vD−v)m forms have theoretical basis. We apply this Ev expression with a variety of [L/N] to the Born–Oppenheimer potential of the H2+ ground state. A [2/2] fit yields more accurate first energy differences ΔE (v+1/2) than a polynomial or any other L+N=4 fit. For a given number of variable parameters, the best fits have terms of degree less than L or N missing in either the numerator or denominator. Our best overall fit has vD=19.76, m=6.08, L=2, N=4, and rms error in calculated ΔE (v+1/2) of only 0.006 cm−1. Difficulties that might be encountered in the use of the above Ev expression are discussed in some detail.
Self-absorption of molecular fluorescence, the design of equipment for measurement of fluorescence decay, and the decay times of some laser dyesa)
Authors: Hammond P. R.
Published: February 2003

At moderate concentrations (10−3 to 10−2 M), self-absorption of the fluorescence of a dye is a major obstacle to the accurate measurement of its decay time and quantum yield. Equations are derived for this self-absorption correction for the experimental condition where the correction is minimized namely for a thin sheet of dye medium betwen parallel plates. Simple equipment is described for the determination of decay times at such concentrations. An order of magnitude estimation relates the largest exciting flux density that may be employed for a laser dye so that the measured decay time is not distorted by amplified fluorescence. Tunable, short-pulse excitation has been achieved by a nitrogen laser pumped oscillator–amplifier dye-laser configuration in which a saturable absorber is used both to shorten the oscillator pulses and to prevent feedback from the amplifier. Decay times are reported for systems containing rhodamine 6G, rhodamine B, and kiton red S for a range of concentrations between 10−7 M to 10−2 M. Self-quenching of fluorescence is minor and is not evident at concentrations below 10−3 M.
Electron impact study of the energy levels of t
r
a
n
s-1,3-butadiene
Authors: Doering J. P.
Published: February 2003

The electron impact energy loss spectrum of trans-1,3-butadiene has been investigated at incident energies from 7.5 to 34 eV and scattering angles from 0 to 70°. Energy resolutions of 45 to 75 meV were used. The energy loss spectrum at 33 eV and 0° was found to correspond almost exactly to the optical absorption spectrum measured by McDiarmid. The only minor difference was a weak band which appeared in the energy loss spectrum at 7.80 eV. An investigation of the relative intensities of the three diffuse vibronic bands of the intense N→V1 transition centered near 5.92 eV showed that the relative intensities of these bands were not constant below incident energies of 15 eV and that whereas the low energy 5.74 eV vibronic band was enhanced at 0° scattering angle, the high energy 6.08 eV band was enhanced at 70°. These results are consistent with the presence of two forbidden transitions in the 5.7 to 6.2 eV energy loss region: a symmetry forbidden transition near 5.80 eV and a singlet–triplet transition near 6.10 eV—in excellent agreement with some, but not all, of the recent theoretical calculations on 1,3-butadiene. Detailed studies of the higher energy region above 6.5 eV energy loss did not reveal the presence of any underlying valence transitions although the results are not conclusive because of the intense Rydberg structure in this region. The Rydberg transitions at 7.08 and 8.0 eV were found to have differential scattering cross sections which were very different from those of the nearby transitions.
Quasiclassical trajectory studies of the chlorine–hydrogen system. III. Branching ratio, energy partitioning, and angular distribution in the reaction of Cl atoms with HD
Authors: Persky Avigdor
Published: February 2003

Results of three-dimensional quasiclassical trajectory calculations for the reaction of Cl atoms with HD are reported. Calculations were performed for the initial vibrational state v=0, rotational states J=0–4, and collision energies E between threshold and 15.0 kcal/mole. Reaction probabilities, cross sections, thermal rate constants, product energy partitioning, and product angular distributions, for the two competing branches Cl+HD→HCl+D and Cl+DH →DCl+H, are presented and discussed. The kinetic isotope effects kCl+HD/kCl+DH, kCl+H2/(kCl+HD +kCl+DH), and kCl+H 2/kCl+D2 obtained from this study and from earlier trajectory calculations are compared with experimental data and with results of TST and classical 1D trajectory calculations, using the same potential energy surface.
High temperature–high pressure thermal conductivities of ethylene and propane
Authors: Aggarwal Mahesh C. , Springer George S.
Published: February 2003

Thermal conductivities λ of ethylene and propane were measured in the temperature and pressure ranges 400–750 K and 0.1–2.65 MPa (ethylene) and 400–725 K and 0.1 to 0.6 MPa (propane). The data were correlated by expressions of the form λ=λ0(T) ×λp(P), with λ0 being a second order polynomial in temperature and λp a third (ethylene) or a fourth (propane) order polynomial in pressure. The results obtained were compared with previous thermal conductivity measurements.
Images of gas molecules by electron holography. II. Experiment and comparison with theory
Authors: Bartell L. S. , Gignac W. J.
Published: February 2003

Molecular images photographed by a variant of holographic microscopy proposed in Paper I are presented. ’’Holograms’’ were taken with an electron beam of 0.06 Å wavelength recorded up to a numerical aperture of 0.2 to yield an Abbe resolution limit of 0.15 Å. Experimental requirements and procedures are described. Images, representing rotational averages over an ensemble, closely resemble images calculated according to the theoretical expressions derived in Paper I. The theoretical treatment took into account the degradative effects of the zeroth-order beam transmitted through the hologram, finite numerical aperture and wavelength, the ’’false’’ peaks arising from the cutoff of the holograms at their centers, and nonoptimum first-stage filter functions. Illustrative examples include AsF5, a case chosen to satisfy standard holographic requirements, and two cases, SF5Cl and CF3OOCF3, selected to produce more complicated images.
Photon correlation spectroscopy of particle distributions
Authors: Gulari Esin , Gulari Erdogan , Tsunashima Y. , Chu B.
Published: February 2003

A least-squares method for correlation function profile analysis using a histogram approximation is described. The method is completely general, especially for bimodal distributions, and compares favorably with the method of cumulants. The measured photoelectron time-correlation function yields a histogram of the linewidth distribution which can be related uniquely to the particle size. The analysis is tested using simulated data with unimodal and bimodal size distributions. In our verification of the method using aqueous suspensions of Dow latex spheres, we have shown that our method is not only consistent with the results from electron microscopy, but that it is more precise and truly measures the hydrodynamic size of particles suspended in fluids.
Optimum two-body scattering T matrix elements on and off the energy shell for various potentials
Authors: Rosenthal C. M. , Haracz R. D.
Published: February 2003

Variationally optimized T matrices and phase shifts are reported for three potential scattering problems: (1) elastic scattering from a square well with a P state resonance, (2) elastic scattering from a Morse potential parameterized to fit S state nucleon–nucleon scattering data, and (3) off shell diagonal and nondiagonal scattering from the Reid potential. This last problem, involving a potential with long range attraction and short range repulsion, was selected for study because solving the three body problem within the context of the Faddeev formalism requires for input off shell two body T matrices arising from such a potential.
Interpretation of high spin⇌low spin transition in iron (II) complexes. I. A phenomenological thermodynamic model
Authors: Gütlich P. , Köppen H. , Link R. , Steinhäuser H. G.
Published: February 2003

A phenomenological thermodynamic model suited for the interpretation of the high spin⇌low spin transition in solid transition metal complexes is described. The model is based on the idea of the spin transition taking place through a significant phonon-assisted coupling between the electronic state of the primary spin change center and n−1 neighboring complex molecules undergoing secondary spin transitions and thus forming a cooperative domain as suggested by Sorai and Seki. The model has been applied to interpret the spin transition in polycrystalline solid solutions of [FexZn1−x(2-pic)3]Cl2⋅C2H5OH(2-pic=2-picolylamine) with 1.0⩾x⩾0.0009. The spin transition temperature Tc may be calculated using the effective changes of enthalpy and entropy, respectively, obtained from 1nK vs 1/T plots. Phenomenological expressions for the transition temperature Tc (x) as well as for the effective enthalpy and entropy changes ΔHeff(x) and ΔSeff(x) have been adjusted to the measured data. The average domain size for the pure iron compound has been calculated to be n≈3.5 which indicates comparatively weak intermolecular coupling strength for the present system. The domain size n depends markedly on the iron concentration x and tends to one at very low iron concentration. The volume expansion of the domain size n approximated by n (x) =1+noxγ, decreases rather strongly with falling iron concentration (γ≈1.7). In the pure iron compond, the sum of the intramolecular enthalpy contributions changes by some 550 cal mol−1 and that of the intermolecular enthalpy contributions, responsible for the cooperative effect, by some 200 cal mol−1 on going from low spin to high spin.
Rate constants for the reaction of atomic chlorine with methanol and dimethyl ether from 200 to 500 K
Authors: Michaela) J. V. , Nava D. F. , Payne W. A. , Stiefb) L. J.
Published: February 2003

Absolute rate constants for the reaction of atomic chlorine with dimethyl ether and methanol, Cl+CH3OCH3→HCl+CH2OCH3 (1),Cl+CH3OH→HCl+CH2OH (2), have been measured over the temperature range 200–500 K, using the flash photolysis–resonance fluorescence technique. In both systems, the results were independent of substantial variations in reactant concentration, total pressure (Ar) and flash intensity (i.e., initial [Cl]). The rate constants were also shown to be invariant with temperature. The best representation for this temperature range was found to be k1= (1.76±0.15) ×10−10 cm3 molecule−1 s−1 and k2= (6.33±0.70) ×10−11 cm3 molecule−1 s−1, respectively, where the error is one standard deviation. These are the first determinations of the rate constants for Reactions (1) and (2). These reactions are theoretically discussed and compared to the related reactions of Cl with CH4, C2H6, and H2CO. Molecules of the type CH3OX (where X=H, OH, etc.) are likely to be formed in the stratosphere as products of the oxidation of methane. The molecules CH3OH and CH3OCH3 are considered as prototypes of molecules containing the methoxy group. The potential importance of the fast reactions Cl+CH3OX in stratospheric chemistry is discussed.
Infrared and Raman intensities of polyethylene and perdeuteropolyethylene: Factor group splittings
Authors: Abbate S. , Gussoni M. , Zerbi G.
Published: February 2003

In a previous work we have calculated the infrared and Raman intensities of polyethylene applying the electro-optical parameters obtained from simple molecules to a model of single and isolated chains. In this work the validity of such an approximation is discussed. From the comparison of the calculated and experimental intensity ratios of the components of factor group splittings it is concluded that a Hornig’s-type gas oriented model with no sizeable electrical interactions between molecular chains accounts for the observed infrared and Raman data.
Laser induced fluorescence studies of the reactions of barium (1S, 3D) and strontium (1S, 3P) with halogenated methanes
Authors: Solarz R. W. , Johnson S. A.
Published: February 2003

Laser excitation of barium and strontium atoms followed by laser induced fluorescence detection of chemical reaction products is used to study the dynamics of reactive collisions of these reagents with halogenated methanes. We report state-to-state distributions in reactions of ground state Ba and ground state Sr, (1S0), and laser excited Ba(3D2 and 3D1) and Sr(3P1) with CH3X, CF3X (X=F,Cl,Br), CCl3F, and CCl4. We demonstrate that in certain cases reaction can be initiated by electronic excitation of the atomic reagent. We also show that the vibrational energy distribution of the diatomic alkaline earth halide product is in each case determined principally by the nature of the reacting halogenated methane irrespective of the surface on which the reaction is initiated. Measurements of the relative reactivity of different spin orbit states of excited barium are also presented.
Electronically activated chemisorption of nitrogen on a copper (100) surface
Authors: Tibbetts Gary G.
Published: February 2003

The chemisorption on copper (100) of nitrogen gas activated in the gas phase by monoenergetic electrons from 0–100 eV energy was monitored. By comparing these results with gas phase measurements, it is seen that, whereas unexcited N2 does not chemisorb, both neutral and ionized atomic nitrogen chemisorb strongly on copper (100). Calculations imply that each atom of atomic nitrogen to strike the clean copper surface chemisorbs.
Studies of electronegativity equalization
Authors: Rayb) Naba K. , Samuelsc) Leonard , Parr Robert G.
Published: February 2003

With electronegativity defined from density functional theory (Ref. 1) calculations are carried out illustrating the electronegativity equalization that takes place as atoms of different electronegativities come together to form diatomic or polyatomic molecules. By generalizing a discussion by Pasternak (Ref. 8), the simple bond charge model for diatomic molecules is shown to fully encompass the electronegativity concept, and within that model predictions are made and tested of the electronegativity χ, force constant k and bond length R of a molecule AB, from the electronegativities χA and χB and covalent radii rA=1/2RAA and rB=1/2RBB of the atoms A and B. The electronegativity is given by the formula χR=χArA+χBrB. Also examined is a simple charge transfer model for electronegativity neutralization, in which the electronegativity of an atom in a molecule is assumed to be a quadratic function of the number of electrons on the atoms. Electronegativities of some polyatomic molecules are predicted.
Emission of NO2* formed in the photodissociation of N2O4
Authors: Inoue Gen , Nakataa) Yoshio , Usuia) Yoshiharu , Akimoto Hajime , Okuda Michio
Published: February 2003

Visible emission was observed when the equilibrium mixture of NO2 and N2O4 was irradiated with near-UV laser light. The excitation spectrum from 295 nm to 365 nm resembles the absorption profile of N2O4, and the intensity is proportional to the number density of N2O4. This emission was ascribed to NO2* formed in the photodissociation of N2O4; N2O4+hν→NO2*+NO2. The quantum yield of this process is of the order of unity, and the fluorescence yield of direct excitation of NO2 in this wavelength range is less than 10−4.
Dynamical approach to exciton migration in disordered molecular solidsa)
Authors: Blumenb) Alexander , Silbey Robert
Published: February 2003

We present a dynamical model for the rapid change in relative emission from traps and supertraps in molecular solids when the trap concentration is varied. Good agreement between experiment and the theoretical model is obtained in all cases. The physical picture of this model is similar to that of a percolation model and contrasts with the model based on a transition from localized to extended states.
The fine structure of the B 2Σ+ and a4Σ+ states of CN as determined by molecular anticrossing spectroscopy
Authors: Cook J. M. , Zegarski B. R. , Miller Terry A.
Published: February 2003

The anticrossing between the B 2Σ+, v=11, N=20 and the a4Σ+, N=20 states of CN has been investigated in detail. The width and shape of the anticrossing signal are shown to be dependent upon the fine structure constants of the two states. Computer fits to this line shape have determined the spin–rotation constants of the states, the spin–spin constant of the quartet, the zero field separation, and a firm upper limit to the strength of the perturbation between the states.
The estimation of total SCF energies from orbital energies
Authors: Heaton M. M.
Published: February 2003

SCF calculations were performed with the polyatom programs on Li2 and C26+.
Comment on ’’Brillouin scattering studies of orientationally disordered sodium cyanide and potassium cyanide single crystals’’
Authors: Satija S. K. , Wang C. H.
Published: February 2003

Recent Brillouin backscattering geometry studies of sodium cyanide are reported. The (100) longitudinal acoustic phonon frequency is measured and used, together with index of refraction and density data, to determine C11 and other elastic constants.
Infrared–infrared double resonance on a molecular beam of NH3
Authors: Matsushima Fusakazu , Morita Norio , Kano Satoru , Shimizu Tadao
Published: February 2003

A novel laser spectroscopic method with a molecular beam is successfully demonstrated. This technique is widely applicable for studies of collisional relaxation on a variety of molecular transitions, even when untunable infrared molecular lasers are used. The effect of collision-induced transitions between the vibration–rotation levels of NH3 are investigated by infrared–infrared double resonance. It is found that the rate of excitation energy transfer is closely related to the molecular energy level structure.
Chemi-ionization in collisions of metastable neon with xenon
Authors: Neynaber R. H. , Tang S. Y.
Published: February 2003

Absolute and relative cross sections were obtained for the associative-ionization (AI) reaction Ne*+Xe→NeXe++e and the Penning-ionization (PI) reaction Ne*+Xe→Ne+Xe++e. The studies were made by a merging-beams technique for a relative kinetic energy of the reactants from nominally 0.01 to 0.3 eV for AI and 0.01 to 10 eV for PI. The Ne* represents a composite of Ne(3s 3P2) and Ne(3s 3P0). Relative cross sections are compared with those measured by other investigators and with values obtained in this laboratory for the Ne*–Ar and Ne*–Kr systems. At thermal energy the absolute cross section and rate coefficient for total ionization are compared with values obtained by other techniques.
Relaxation of a n-octane fluid
Authors: Weber Thomas A.
Published: February 2003

This paper reports the results of a series of molecular dynamics calculations on a n-octane fluid at the normal density and several temperatures. The torsional interconversion between gauche and trans bonds has been investigated as a function of bond position and temperature. It is found that intermolecular interactions retard trans/gauche relaxation and appear to influence the trans/gauche ratio of the fluid.
Absorption of ultrasound in Cs–NH3 and Cs–ND3 solutions
Authors: Fenner D. B. , Bowen D. E.
Published: February 2003

Ultrasonic absorption has been measured in liquid cesium–ammonia and cesium–deutroammonia solutions over a range of solution concentrations passing through the metal–nonmetal transition and extending downward in temperature to within a few °K of the solid ammonia-solution phase boundary. In neither of these two types of solution have we found any evidence of a liquid–liquid phase separation as does occur in most every other kind of metal–ammonia solution. Further, the sound absorption we have measured indicates a uniformly low value of α/f2 for the Cs solutions within this region. The measured α/f2 was found to be independent of the sound frequency between 10 and 54 MHz. We have inferred the absence of concentration fluctuations and in turn the absence of any nearby phase separation and critical point in these Cs solutions. A comparison is also made with our earlier measurements of absorption in Na and Li solutions which showed distinctly different behavior. Clearly then the metal–nonmetal transition does not always lead to concentration fluctuations and anomalous sound absorption in metal–ammonia solutions.
Formation of SiH bonds when SiO is deposited on alumina
Authors: Eib N. K. , Gent A. N. , Henriksen P. N.
Published: February 2003

Thin films of silicon monoxide have been deposited on alumina surfaces and examined by inelastic electron tunneling spectroscopy. A strong absorption peak at 2210 cm−1 reveals the formation of SiH bonds in the presence of trace amounts of water. It is inferred that a reaction between SiO and –OH groups to yield SiH bonds is catalyzed by the alumina surface.
Quantum topology of molecular charge distributions. II. Molecular structure and its change
Authors: Bader Richard F. W. , Nguyen-Dang T. Tung , Tal Yoram
Published: February 2003

This paper illustrates how the concepts of atoms and bonds may be given definite expression in terms of the topological properties of the charge density, ρ (r), and how, as a consequence of these identifications, one is led to a definition of structure and to a phenomenological analysis of structural stability. This approach finds its natural expression in René Thom’s general analysis of structural stability as it applies to a system whose behavior is describable in terms of the gradient of some scalar field. Chemical observations are made in real space, and thus chemical behavior is determined by the morphology of a system’s charge distribution and its evolution with time. The analysis of the topological properties of ρ (r) via the associated gradient vector field ∇ρ (r), reduces to the identification of the critical points in ρ (r). Two types of critical points assume special roles in the analysis. A (3,−3) critical point, a maximum in ρ (r), is an attractor and is identified with the position of a nucleus in the molecular system under study. The basin of the attractor defines the atom associated with the corresponding nucleus. A (3,−1) critical point defines the interatomic surface separating two neighboring atoms, and the bond path linking their nuclei, the line along which the charge density is maximum with respect to lateral displacements. Hence, neighboring atoms are defined to be bonded to one another and the network of bond paths, for a given nuclear configuration, determines its molecular graph. Structure is defined as that set of molecular graphs which contain the same number of bond paths, linking the same nuclei. Thus a change in structure necessitates a change in the number and/or arrangement of bond paths. The making and/or breaking of chemical bonds associated with such a change is topologically a discontinuous process, and the associated change in structure is therefore, abrupt: a continuous change in the nuclear coordinates, the parameters which control the behavior of the system, can lead to a discontinuous change in molecule’s behavior. A point in control space defining the nuclear configuration for which such discontinuous behavior is observed, is called a catastrophe point. The set of catastrophe points thus partitions nuclear configuration space into regions of different structure. The breaking or making of bonds is a catastrophe of the bifurcation type, resulting from the formation of a singularity in ρ (r), whereas the switching of a bond from one nucleus to another is a catastrophe of the conflict type. It is shown that the analytical description of the formation of a three-membered ring structure from all possible neighboring structures (as illustrated for H+3 and H2O) is provided by the unfolding of a particular type of catastrophe, the elliptic umbilic.
An accurate intermolecular potential for helium
Authors: Aziz R. A. , Nain V. P. S. , Carley J. S. , Taylor W. L. , McConville G. T.
Published: February 2003

A simple realistic and precise empirical intermolecular potential is proposed for helium. It possesses nearly the correct Hartree–Fock repulsion as well as the correct long range behavior. It was fitted to recent accurate intermediate temperature second virial coefficients and thermal conductivity data as well as high temperature viscosity values. It is able to predict second virial coefficients over an extended temperature range from 1.5 to 1475 K. Above 100 K it reproduces substantially all of the transport properties to within experimental error in a manner superior to all other potentials in existence. Below 100 K where the transport data are less reliable, it produces a good representation of the isotopic differences in the viscosity. It also predicts differential cross sections reasonably well. In spite of a few remaining discrepancies, when all the different macroscopic properties are considered, the potential produces the best representation of the helium interaction available at this time.
Dissociation dynamics of collinear triatomic systems by the R-matrix method
Authors: Numrich Robert W. , Kay Kenneth G.
Published: February 2003

A straightforward computational technique is developed for the quantum mechanical study of unimolecular decay. It is applied to collinear triatomic systems in which the central atom interacts with one terminal atom through a harmonic oscillator potential and with the other terminal atom through a Morse oscillator potential. Stationary state wavefunctions for these systems are generated over an energy grid by applying the Wigner R-matrix method with Buttle correction. Projections of the stationary wavefunctions onto nonstationary wavefunctions describing metastable states of the triatomic molecule are computed from the R-matrix basis set expansion of these functions. Time dependent state-to-state transition probabilities and final product distributions are then calculated from the projections by Fourier transform and quadrature techniques. The observed time evolution is analyzed in terms of contributions from bound states, resonance states, and branch cuts. Rapid nonexponential decay observed for a variety of initial states is attributed to branch cut contributions to the time evolution.
A planar–nonplanar conformational transition in conjugated polymer solutions
Authors: Patel G. N. , Chance R. R. , Witt J. D.
Published: February 2003

Solutions of certain urethane substituted polydiacetylenes undergo dramatic, reversible color changes on addition of a nonsolvent. In some cases, the lowest energy optical transition of the polymer shifts by more than 5000 cm−1 (21 300 cm−1 to 15 900 cm−1 when the nonsolvent is added). The color changes are shown to be due to a planar–nonplanar conformational transition of the polymer backbone. The planar, fully-conjugated conformation is stabilized by intramolecular hydrogen bonding between urethane functionalities on adjacent substituent groups.
A proposed formula for the energy of an atom in a molecule
Authors: Politzer Peter , Daiker Kenneth C. , Trefonas Peter
Published: February 2003

A formula for calculating the energies of atoms in molecules is proposed. It expresses these energies in terms of the total electrostatic potentials at the various nuclei. For each atom in a group of 52 molecules, the calculated change in its energy in going from the free state to the molecule is presented. In general, the more electronegative of the atoms in any given molecule are found to have lower energies than in the free states, while the less electronegative ones have higher energies than as free atoms. These atomic energy changes are discussed in terms of charge transfer between the atoms and the degrees of polarization of the atoms.
Rare gas–graphite interaction potentials
Authors: Rybolt Thomas R. , Pierotti Robert A.
Published: February 2003

The interaction of neon, argon, krypton, and xenon with the graphitized carbon black P33 (2700) is examined using the virial coefficient treatment of physical adsorption. Adsorption isotherm data in the low coverage, Henry’s law region are used to carry out this analysis. A modified Buckingham potential is selected to model the gas–solid interaction potential. A best fitting technique is used to make an unambiguous selection of the interaction potential parameters. A cubic power law decay is found to be the proper choice for the attractive portion of the potential, while the appropriate screening parameters for the exponential, repulsive portion of the potential are determined to be 0.0181, 0.0196, 0.0198, and 0.0215 (nm) for Ne, Ar, Kr, and Xe, respectively. The adatom–adsorbent internuclear separations are 0.307, 0.333, 0.336, and 0.366 (nm) and the gas–solid interaction energies are 384, 1113, 1467, and 1928 (°K) for Ne, Ar, Kr, and Xe, respectively. A surface area of 10.8±0.3 m2g−1 is obtained for the graphitized carbon black.
Collisional ionization of electronically excited lithium
Authors: Mariella R. P.
Published: February 2003

Total cross sections are reported for the ionization of Li in the 42Sy2 state in collisions with F2, Cl2, and NO2.(AIP)
Comment on the structure and stability of (CO2)−2
Authors: Rossi Angelo R. , Jordan Kenneth D.
Published: February 2003

Spin restricted and spin unrestricted Hartree–Fock calculations are presented.
Thermal energy charge transfer from He+ to O2 : Kinetic energy, nature, and reactivity of the O+ product ions
Authors: Mauclaire G. , Derai R. , Fenistein S. , Marx R. , Johnsen R.
Published: February 2003

From kinetic energy measurements in an ICR cell, the charge-transfer reaction between thermal He+ and O2 is shown to produce 64% of ground state O+ ions with 1.8 eV kinetic energy, and 36% of electronically excited O+* with 0.3 eV kinetic energy. These O+* react with O2 with a rate constant of 6.5×10−10 cm3s−1. The nature of this excited state, 2D° or 2P°, is discussed. A resonant, long-distance electron jump explains all the experimental data.
Molecular properties of N2 and CO2 as functions of nuclear geometry: Polarizabilities, quadrupole moments, and dipole moments
Authors: Morrison Michael A. , Hay P. Jeffrey
Published: February 2003

A b initio static polarizabilities, quadrupole moments, and electronic energies of ground state N2 and CO2 have been calculated self-consistently as functions of nuclear geometry in the Hartree–Fock approximation using the finite-field method. In addition, permanent dipole moments for the infrared-active vibrational modes of CO2 have been determined. For N2, the effects of electron correlation on these molecular properties have been studied by performing generalized valence bond (perfect pairing) calculations with the same basis sets as were used in the Hartree–Fock calculations. These basis sets are constructed from energy-optimized contracted sets of nucleus-centered Gaussian-orbitals to provide a flexible representation of the entire molecular charge cloud, including the highly polarizable outer regions. Results of these calculations are compared with experimental and other theoretical values where possible.
Trajectory analysis of a kinetic theory for isomerization dynamics in condensed phases
Authors: Montgomery John A. , Chandler David , Berne Bruce J.
Published: February 2003

We use an exact microscopic formalism to study the implications of a stochastic model of isomerization dynamics in liquids. In the model, a reaction coordinate moves in a multistable potential and is coupled to a thermal bath via random collisions which occur with a specified average collision frequency. The nonlinear dynamics for this system is solved numerically. It is found that the usual linear rate law for isomerization is valid for any nonzero collision frequency if the activation barrier to reaction is sufficiently high. The reasons for this behavior are discussed at length. With appropriate parameter choices, we can draw conclusions concerning the trans–gauche isomerization of n-butane in liquids. Transition state theory is found to overestimate the rate constant by at least a factor of 2 to 3 at any collision frequency. The collisional contribution to the volume of activation is calculated. At 1 atm, the result is an order of magnitude larger in size than the transition state theory activation volume. Furthermore, this collisional contribution has a strong pressure dependence that should be observable experimentally.
High temperature expansion for the Gaussian core model
Authors: Stillinger Frank H.
Published: February 2003

Exact series coefficients through eighth order are reported for the high temperature expansion of the Gaussian core model free energy. Although this expanison has a vanishing radius of convergence it can be summed by a Borel integral transform. It is demonstrated that this Borel transform has an intimate connection to ’’crystallites’’ in the stable fluid phase which causes it to display a replica of the distribution function for large crystallites. This last observation permits implementation of a transform subtractive procedure which generates metastable extensions of fluid properties into the supercooled regime below the thermodynamic freezing temperature.
Low-energy electron–molecule scattering: Comparison of coupled channel treatments of e−N2 scattering at 13.6 eV using various approximations to the static and exchange potentials and an approximate polarization potential
Authors: Rumble John R. , Truhlar Donald G.
Published: February 2003

We consider vibrationally and electronically elastic electron scattering by N2 at 13.6 eV impact energy. We use four combinations (S, SP, SE, and SEP) of the static (S), exchange (E), and polarization (P) potentials, four target molecular orbital wave functions (INDO/1s, INDO/1s with double zeta core, and Cade–Sales–Wahl and Ermler ab initio), and two local exchange approximations (semiclassical and Hara free-electron gas). We also consider the effect of using an accurate wave function for the spherical average and lowest order anisotropy and an INDO/1s wave function for higher order anisotropies, as well as other combinations. The effects of approximations to the various terms in the interaction potential and the target wave function are illustrated by calculations of the partial integral cross sections.
Laser-collision induced chemical reactions: Collinear exchange reaction model on two electronic surfaces
Authors: Light John C. , Altenberger-Siczek Aldona
Published: February 2003

A collinear model of bimolecular exchange reactions in the presence of an intense laser field coupling two electronic potential energy surfaces is developed. Close coupling calculations for two model systems were carried out as a function of the initial energy, E, the laser coupling, μ E0, and the laser frequency, h/ω. It is shown that the reaction probability can be greatly altered for some systems by intense off-resonant laser radiation. A suggestion is made for laser frequency up-conversion using exothermic chemical reactions.
Vibrational relaxation of DF (v=1–4) in D2, H2, N2, HF, and CO2
Authors: Bott J. F.
Published: February 2003

The deactivation of the upper vibrational levels of DF by H2, D2, N2, HF, and CO2 has been studied with the technique of laser-induced fluorescence. The upper vibrational levels were produced by sequential photon absorption in which DF (v=0) was pumped first to DF (v=1) and subsequently to DF (v=2), DF (v=3), and DF (v=4) by photons from a pulsed multiline DF chemical laser. The deactivation rates (V−V+V−R,T) for all the collision partners except D2 scaled with vibrational level as vn with n=1.9 to 2.0 for v=1 to 4. Similar studies have indicated the somewhat larger value of n=2.7±0.2 for HF (v) deactivation by diatomic molecules.
Quantum theory of line shapes in coherent Raman spectroscopy of gases and liquids
Authors: Bjarnason Jón Örn , Hudson Bruce S. , Andersen Hans C.
Published: February 2003

The fully quantum mechanical theory of coherent Raman scattering in gases and liquids is developed. In this theory, the electromagnetic radiation is described as a quantized field, and the quantum mechanical transition amplitude for the scattering process is calculated. This is formally different from the usual semiclassical theory in which the radiation field is characterized as a classical electromagnetic wave and nonlinear susceptibilities are used to describe the interaction of radiation and matter. The Doppler broadened line shape for the ideal gas is calculated and found to be non-Gaussian with a width that is 1.2 times larger than the Doppler width for spontaneous Raman scattering in the forward direction. This differs from a previously published result. The rotational–vibrational band shape is shown to be related to a correlation function for rotational–vibrational motion. This correlation function is the same as that describing spontaneous Raman scattering, but the relationship of the correlation function to the spectrum is different. In the absence of correlations between rotation and vibration the spectrum can be related to a Laplace transform (with an imaginary argument) of a rotational correlation function. The line shape is calculated for two special cases, free rotational motion and Brownian rotational motion.
Chemisorption geometry of hydrogen on Ni(111): Order and disorder
Authors: Christmann K. , Behm R. J. , Ertl G. , Van Hove M. A. , Weinberg W. H.
Published: February 2003

The location of a half monolayer of ordered hydrogen adatoms on Ni(111) has been analyzed by Low-Energy Electron Diffraction (LEED), Thermal Desorption Spectroscopy (TDS), and Work Function (Δφ) measurements. It is found that the hydrogen atoms are arranged in an overlayer of graphitic structure with a (2×2) unit cell with respect to the substrate unit cell. In the ordered regions, the hydrogen adatoms occupy both types of three fold hollow sites without a detectable difference in the Ni–H bond lengths between the two sites. The Ni–H bond length is found to be 1.84±0.06 Å, corresponding to an overlayer-substrate spacing of 1.15±0.1 Å. The relation between this structure and its observed order–disorder phase diagram as a function of temperature and hydrogen coverage is discussed. The disorder is discussed in detail, and a novel ’’atomic band structure’’ interpretation is given.
Aspects of the percolation process for hydrogen-bond networks in water
Authors: Geiger A. , Stillinger F. H. , Rahman A.
Published: February 2003

Sets of configurations selected from three molecular dynamics simulations for liquid water have been analyzed for the distribution of hydrogen-bond clusters. Two simulations correspond to water at 1 g cm−3, while the third corresponds to highly compressed water at 1.346 g cm−3. An energy criterion was adopted for existence of a hydrogen-bond between two molecules. As the cutoff value for bonding increases (becomes more permissive), a bond percolation threshold is encountered at which initially disconnected clusters suddenly produce a large space-filling random network. At least for the model studied, any chemically reasonable definition of ’’hydrogen-bond’’ leads to this globally connected structure though a few disconnected fragments inhabit its interior. Although some polygonal closures can exist, the critical percolation threshold is apparently well predicted by Flory’s theory of the gel point for dendritic polymerization.
Vibrations and force constants of the hexagonal lithium iodate crystal
Authors: Crettez J-M. , Misset J-P. , Coquet E.
Published: February 2003

Force constants of the α-LiIO3 crystal are calculated based on the thirteen k=0 infrared and Raman active vibrations. Setting up a generalized valence force field for the IO3 group and both valence and central forces for the external interactions has allowed us to obtain good agreement between experimental and calculated frequencies. The introduction of translational and librational coordinates related to the normal coordinate analysis leads to a precise assignment of the frequencies and shows that the degenerate bending vibrations of the IO3 groups are strongly coupled with the translational vibrations of the lithium atoms.
Zero pressure lifetimes and fluorescence quenching of ICl(A 3Π1)
Authors: Harris Stephen J. , Natzle Wesley C. , Moore C. Bradley
Published: February 2003

Lifetimes of ICl(A 3Π1) are measured by laser-induced fluorescence. Zero pressure lifetimes for excitation between 589 and 669 nm range from 405±40 to 460±40 μsec. Quenching cross sections measured at 293°K near 603 nm vary between 110 Å2 for ICl itself and 0.7 Å2 for Ne. A radiative lifetime of about 650 μsec was estimated from the integrated absorption spectrum.
Study of hydracids trapped in monatomic matrices. II. Intermolecular potential calculations of the structure and spectral response of polymers
Authors: Maillard D. , Schriver A. , Perchard J. P. , Girardet C.
Published: February 2003

Intermolecular potential calculations are developed for hydracids trapped in rare gas matrices and the results concerning both geometries and spectral responses of low polymers are compared to the analysis of experimental data previously described. Geometrical structures, including matrix distortion, are calculated from a minimization of the total energy. Cyclic and planar conformations found for trimers and tetramers satisfy both criteria: the (molecular) packing tendency imposed by the isotropic Lennard-Jones potential and the highest order symmetry for this packing required by the electrostatic potential. In a second step, the corresponding librational and vibrational frequencies are calculated according to the usual techniques of separation of angular and vibrational variables. For librations, it is moreover possible to separate in-plane and out-of-plane motions, the first ones lying at higher frequency than the second; intensities of the corresponding bands are also calculated, taking into account the induction effects tied to the matrix, leading to a noticeable relative intensity change of the IR active in-plane/out-of-plane modes according to the nature of the rare gas. For vibration, the a priori calculations show that agreement with experimental data is obtained if we introduce with respect to their gas phase values an enhancement of multipolar moment derivatives with intermolecular forces. The enhancement parameter τCT is interpreted as an effect of charge transfer between interacting hydracids XH⋅⋅⋅XH and the best fit for dimers, trimers, and tetramers responses show that this parameter depends on the nature of both electron donor and acceptor molecules, the stronger effect being however observed for the acceptor one (2.5 for HCl and 6 for HBr).
Coherent trapping of a single excitation in a multimolecular system
Authors: Ruschin S. , Ben-Aryeh Y.
Published: February 2003

The theory of photon trapping is presented and discussed for collection of molecules. The main features of the phenomenon are given on the basis of symmetry considerations. General equations of motion are developed for a multimolecular system sharing a single excitation.
Analysis of neutron incoherent inelastic scattering spectra of normal and deuterated formic acid. I. Planar chain model
Authors: Johnson D. H. , Berney C. V. , Yip S. , Chen S.-H.
Published: February 2003

Lattice-dynamic analysis of solid formic acid based on a planar single hydrogen-bonded chain has been carried out in order to interpret the observed neutron spectra of HCOOH, HCOOD and DCOOH. We use the direct relationship between the spectral intensities and the vibrational amplitudes of hydrogen atoms to extract information concerning the dynamic modes involving the hydrogen atom motions. Force constants similar to those used by previous investigators were refined (by a least-squares adjustment routine) to try to achieve agreement with observed zone center vibrational frequencies and with peak positions and intensities in the neutron spectra. Molecular force constants for torsion and out-of-plane deformation were varied to achieve agreement between calculated and observed peaks in the 900–1100 cm−1 region. Two calculated phonon branches in the 100–200 cm−1 region were shown to be particularly sensitive to the hydrogen-bond stretching force constant. The calculation fits the HCOOH spectrum well, but a discrepancy of 30–40 cm−1 for an out-of-plane mode in HCOOD and DCOOH remains. A two-phonon calculation has been carried out which accounts well for observed features around 515 cm−1 in HCOOH and HCOOD.
Structure and dynamics of associated molecular systems. I. Computer simulation of liquid hydrogen fluoride
Authors: Klein Michael L. , McDonald Ian R.
Published: February 2003

Recently proposed models for the interaction between two hydrogen fluoride molecules have been used in molecular dynamics simulations of the liquid. Results on the structure and dynamics are presented and comparison is made with experimental data on self-diffusion and with neutron inelastic scattering measurements. The simulated liquid is found to be characterized by a high degree of chain polymerization, but no evidence has been found of the formation of rings.