bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculations of the lattice mode spectra of proton ordered ices: A test of the accuracy of water–water potentials
Authors: Nielson Glen , Rice Stuart A.
Published: February 2003

We report the results of calculations of the lattice mode spectra of ices II, VIII, and IX, designed to test the accuracy of several water–water potentials. The potentials which give the best (but not accurate) descriptions of the lattice structures, densities, and energies of these ices are found to give the proper overall description of the distribution of their lattice vibrations and the changes in frequency induced by isotopic substitution. Just as none of the available potentials gives accurate predictions of the lattice densities and energies, none gives a quantitative description of the lattice spectra of these ices. Nevertheless, there are some features of the calculations for the several water–water potentials which are robust and likely correct for the real systems. The most important of these predictions is that there is relatively little translation-libration coupling in the proton ordered ices.
A Monte Carlo study of concentration fluctuations in a 2D aqueous solution
Authors: Kataoka Yosuke
Published: February 2003

A Metropolis Monte Carlo study of a two-dimensional aqueous solution-like system is carried out by making use of the 2D model of water [J. Chem. Phys. 75, 5864 (1981)]. The water–solute interaction consists of a Lennard-Jones potential and an anisotropic part. This anisotropic interaction has been introduced to stabilize a clathrate hydrate type structure and prevent the tendency for phase separation. The total number of molecules in the systems is always 576 and various static properties are obtained over a wide range of temperature, density and concentration. The Kirkwood–Buff parameters are obtained by the integration of the radial distribution functions. From these parameters, the mean square concentration fluctuation is calculated as a function of temperature, density, and concentration. The mean square concentration fluctuation has a maximum at an intermediate temperature. The free energy is obtained by Widom’s method. The lower critical solution temperature is predicted to be near the point where the concentration fluctuation is maximum. The concentration dependence of the thermodynamic properties at low temperatures is qualitatively in agreement with that of alcohol–water mixtures. The structure of the solution depends strongly on temperature and concentration.
An optimized harmonic reference system for the evaluation of discretized path integrals
Authors: Friesner Richard A. , Levy Ronald M.
Published: February 2003

A harmonic reference system for Monte Carlo evaluation of discretized path integrals is developed. Various equilibrium averages are calculated for a one-dimensional quartic oscillator, and compared to converged basis set results; it is shown that use of the reference system substantially decreases the number of discretized points (relative to the free particle reference system) necessary to compute accurate quantum mechanical expectation values.
Conductivity transitions in fluorine doped triarylmethane dyes
Authors: Tennakone Kirthi
Published: February 2003

The temperature variation of the conductivity of fluorine doped triarylmethane dyes are studied. It is found that this material undergoes semiconductor–metal–semiconductor transition. The mechanism behind the transition is discussed.
A carbon Auger line shape study of nitroaromatic explosives
Authors: Rogers J. W. , Peebles H. C. , Rye R. R. , Houston J. E. , Binkley J. Stephen
Published: February 2003

The C(KVV) Auger line shapes of a series of nitroaromatic explosives have been compared to condensed benzene. Energy shifts of the pi levels in TNB (1,3,5-trinitrobenzene) and TNT (2,4,6-trinitrotoluene) and charge redistribution caused by the resonance electron withdrawing effect of the nitro groups lead to changes in the Auger line shape of transitions involving these levels compared to those of benzene. In MATB (1-amino-2,4,6-trinitrobenzene), DATB (1,3-diamino-2,4,6-trinitrobenzene), and TATB (1,3,5-triamino-2,4,6-trinitrobenzene), the amino groups form resonant structures with the aromatic ring and donate charge density into the ring pi levels. This more than compensates for the charge density withdrawn by the nitro groups. As a result, an amino-carbon pi level is populated which leads to dramatic changes in the energy and intensity of the leading edge of the C(KVV) line shape for these compounds. Hole-hole correlation in the final state causes only small changes in line shape. The relative explosive shock initiation sensitivity of TNB and TNT, where the intermolecular bonding is weak, correlates with the observed Auger line shape changes and reflects the relative ring stability of these aromatic molecules. However, the addition of amino groups in MATB, DATB, and TATB decreases the ring bonding, as reflected in the Auger spectra, in favor of intra- and intermolecular hydrogen bond formation and the initiation sensitivity no longer correlates with ring bonding strength.
Surface polarization induced by an adatom in a dielectric
Authors: Girard C. , Galatry L.
Published: February 2003

The electric polarization induced inside a solid and close to its surface by the presence of an adatom is studied in the absence of electron overlap. It varies as the inverse fourth power of the distance between the atom and the surface and depends on the linear polarizability of the atom and on the quadratic (electro-optical) hyperpolarizability for a unit volume of the solid. The result is shown to be consistent with the formulation of other, related van der Waals polarization effects presently available in the literature.
Analysis of photogeneration in a doped polymer system in terms of a kinetic model for electric-field-assisted dissociation of charge-transfer states
Authors: Goliber T. E. , Perlstein J. H.
Published: February 2003

The photogeneration process in a 40:60 triphenylamine/Lexan system doped with a boron diketonate acceptor has been analyzed in terms of a kinetic model for field-assisted dissociation of charge carriers. The best fit of theory to experimental data is obtained if one assumes a Gaussian distribution of charge-transfer radii. A short-range mobility in triphenylamine/Lexan of order 10−2 cm2/V s is predicted assuming a purely coulombic electron-pair interaction. For the system studied, the most probable charge-transfer radius is 0.54 nm with a quantum yield for charge-transfer states of 0.93. The present model also predicts temperature-dependent quantum efficiencies for the doped system more accurately than does Onsager’s 1938 theory of geminate recombination.
Nascent internal energy distributions of MgH(MgD) produced in the reaction of Mg(3s3p1P1) with H2(D2)
Authors: Breckenridge W. H. , Umemoto H.
Published: February 2003

Nascent rotational quantum-state distributions of MgH(v=0,1) and MgD(v=0) have been determined for the reactions Mg(1P1)+H2→MgH+H, Mg(1P1)+D2→MgD+D. The distributions are bimodal, with the major components (∼90%) peaking at very high rotational quantum numbers and the minor components at approximately N=10. The MgH(v=1)/MgH(v=0) ratio is 0.7±0.2, and there is decreasing population in the higher vibrational levels. The ‘‘high-N’’ distribution is discussed in terms of energy release from bent MgH2 configurations resulting from preferential ‘‘side-on,’’ insertive attack of H2 by Mg(1P1). This is shown to be consistent with ab initio calculations of the relevant MgH2 potential surfaces. The deconvoluted high-N distribution for MgD(v=0) is closer to phase-space-theory predictions than is that for MgH(v=0), and it is suggested that HMgH and DMgD intermediates are formed with lifetimes nearly long enough for internal randomization of vibrational energy to occur. The minor ‘‘low-N’’ component could well be due to inefficient ‘‘end-on’’ attack of the H–H bond by Mg(1P1), but because of the complexities of the potential surfaces other possible explanations are discussed. The inefficient disposal of energy into vibration indicates very ‘‘late’’ energy release. Finally, detailed comparisons are made between these results and analogous studies of the reaction O(1D2)+H2→OH+H, and several striking similarities are noted.
A simple procedure for incorporating off-energy-shell effects into energy sudden scaling relations
Authors: Eno Larry
Published: February 2003

In this paper, a new approach for incorporating off-energy-shell effects into energy sudden (ES) scaling relations is described. The framework for the new approach is provided by the first order Born approximation which allows for the relating in an approximate fashion of off-energy-shell and on-energy-shell T-matrix elements. The relationship between this approach and a former procedure is discussed. Application of the off-shell corrected ES relation is made to the vibrational-rotational scattering of He–H2.
Rotational energy surfaces and high-J eigenvalue structure of polyatomic molecules
Authors: Harter William G. , Patterson Chris W.
Published: February 2003

A rotational analog of the vibrational potential energy surface is introduced for describing the rotational fine structure of polyatomic molecules. Classical trajectories on rotational energy (RE) surfaces are related to quantum rotational eigenvalue structure. Interpretation of RE surfaces shows how very different types of molecules may undergo dynamical symmetry breaking and a corresponding clustering of rotational energy sublevels for high angular momentum (J>10). Cluster splitting and spacing are calculated using semiclassical quantization methods. Some consequences of dynamical symmetry breaking such as mixing of nuclear spin species are discussed qualitatively.
Representable local kinetic energy
Authors: Cohen Leon
Published: February 2003

An infinite set of expressions for the local kinetic energy are derived using proper joint distributions of position and momentum. The joint distributions yield the correct quantum marginal distributions and are positive. This assures that the local kinetic energy is positive.
Clarification of the electronic asymmetry in Π-state Λ doublets with some implications for molecular collisions
Authors: Alexander Millard H. , Dagdigian Paul J.
Published: February 2003

The asymmetry of the orbital part of the electronic wave functions and electronic charge distributions in 1Π, 2Π, and 3Π Λ doublets is carefully examined, to clear up considerable past confusion on this subject. The results are: (1) For 1Π and 3ΠΩ=1 states the electronic wave function in the e Λ-doublet levels is symmetric with respect to reflection in the plane of rotation of the molecule and, in the f levels, antisymmetric. (2) For 2Π and 3Π0,2 states, in the Hund’s case (a) limit the electronic distributions in both Λ-doublet levels are cylindrically symmetric. (3) As the case (b) limit is approached, the F1 e and F2 f wave functions of a 2Π state acquire an increasing degree of symmetric character with respect to reflection in the plane of rotation, while the F1 f and F2 e levels acquire antisymmetric character. In a 2Σ+–2Π radiative transition, the main branch P and R lines probe 2Π levels which are symmetric with respect to reflection in the plane of rotation while the main branch Q lines probe levels which are antisymmetric. The reflection symmetry of Π-state electronic wave functions is shown to be independent of the number of π electrons, while the cylindrical asymmetry of the electronic charge distribution does depend on the electron occupancy. This formal analysis is used to draw implications for product Λ-doublet populations in photodissociation and reactive processes leading to Π-state molecules, and for collisions of Π-state molecules with surfaces.
The vibrational dipole moment function of HCN
Authors: DeLeon Robert L. , Muenter J. S.
Published: February 2003

Dipole moment measurements for the 100 and 011 excited vibrational states of HCN are presented. These, and previously measured, dipole moments are combined with infrared intensity measurements to obtain dipole moment functions for HCN in both normal and internal coordinate systems. The dipole moment results are 〈 μ〉100=2.980 53(6), 〈 μ〉011=2.976 9(7), and μe =3.016 D. The first dipole derivatives with respect to normal coordinates, from IR intensities, are μ1=+0.0029, μ2=+0.178, and μ3=+0.1111 D. These results plus the vibrational dependence of the dipole moments give the following second derivatives: μ11=−0.003 38, μ22=−0.0251, μ33=+0.0012 D. Dipole moments of several states of DCN are calculated and are in excellent agreement with experimental results.
Magnetic properties of the Ã 1A″ state of HCF
Authors: Butcher R. J. , Saito Shuji , Hirota Eizi
Published: February 2003

Zeeman spectra of several transitions in the Ã 1A″–X̃ 1A′ (000)–(000) band of HCF have been studied by the method of laser induced fluorescence. Lamb dip spectra obtained by the technique of intermodulated fluorescence allowed very high resolutions to be obtained, while the use of a circularly polarized laser beam in the Doppler limited spectrum enabled the absolute sense of the Zeeman splittings to be determined. Magnetic properties of the A state are shown to arise mainly from electron orbital interaction with highly excited vibrational levels of the ground state and, in addition, there are several very localized singlet–triplet perturbations.
Copper halide clusters
Authors: Martin T. P. , Kakizaki A.
Published: February 2003

Mass spectra are presented of clusters formed by quenching copper halide vapor in He gas. One peak in the mass spectrum corresponding to Cu14X+13 is unusually strong. Total energy calculations using an ionic model indicate that this cluster is a cube with rock salt structure. We conclude that the copper halides which normally crystallize into the zinc blend structure prefer the NaCl structure in the early stages of growth. One-photon ionization of small clusters was achieved using synchrotron radiation. By studying the wavelength dependence of the ionization efficiency and of the fragmentation, conclusions can be drawn concerning the energies of the Cu-d and halogen p electrons. For the diatomic monomers the halogen p electrons are most easily ionized. With increasing cluster size the energy of the Cu-d electrons moves toward the vacuum level resulting in reduced fragmentation of the cluster by loss of a halogen atom.
Photoelectron spectrum and structure of B2O2
Authors: Ruščić B. M. , Curtiss L. A. , Berkowitz J.
Published: February 2003

The He i photoelectron spectrum of B2O2 is presented. A comparison of ab initio molecular orbital calculations and the observed spectrum provides the most conclusive evidence to date that the geometrical structure is D∞h O–B–B–O. Even though the experiment is conducted at 1200 °C, vibrational structure is evident in the first two bands. A complex Franck–Condon fitting is used to infer the geometrical changes occurring when the various ionic states are formed. The results are in fairly good agreement with ΔSCF calculations. The orbital ordering in B2O2, πg,πu,σg,σu differs from that in the isoelectronic molecule C2N2.
Spectroscopy of the transition state (theory). II. Absorption by H‡3 in H+H2→H‡3→H2+H
Authors: Mayne Howard R. , Poirier R. A. , Polanyi J. C.
Published: February 2003

Absorption spectra of transition state configurations in the reaction H+H2→H‡3 →H2+H have been computed. The density of H‡3 in configuration space was obtained from a classical trajectory study of collinear reaction on the Siegbahn, Liu, Truhlar, and Horowitz (SLTH) ab initio potential energy surface (PES). Vertical transitions were assumed to an upper PES H*3 modeled on limited ab initio data; four different model PES for H*3 were examined. The effects of varying reagent collision energy, varying reagent vibrational excitation, varying isotopic mass, and varying optical transition moment were explored. Transition state spectra were also computed for thermal distributions of H+H2, at 300 and 1000 K. The transition state spectra obtained constituted a wing extending as far as 40 000 cm−1 to the ‘‘red’’ of the Lyman-α transition. As illustrated here, the wing exhibited features that reflected the dynamics of reaction on the SLTH PES.
Geminate recombination of molecular iodine. The role of A and A′ states
Authors: Kelley David F. , Abul-Haj N. Alan , Jang Du-Jeon
Published: February 2003

Picosecond absorption spectroscopy has been used to study the geminate recombination of I2 in room temperature solution. In all solvents studied (CCl4, CH2Cl2, CHCl3, hydrocarbons, and ethylene glycol) a transient absorption band in the 575–800 nm region was observed, and assigned to transitions originating from A(3Π1) and/or A′(3Π2) states. No spectral shift of this band was observed throughout its lifetime, making an assignment to a vibrationally unrelaxed ground state unlikely. Branching ratios for ground state vs A and A′ state recombinations were found to depend upon solvent viscosity. These results are explained in terms of the internuclear separation obtained upon dissociation. The geminate recombination rates into A or A′ states were found to be >(30 ps)−1 and ∼(90–140 ps)−1 into the ground state. At present we can not determine whether surface hopping to the ground state or subsequent diffusion is the rate limiting step in direct ground state recombination. Intersystem crossing rates from the A, A′ states to the ground state were found to vary from ∼(100 ps)−1 in hydrocarbons to (2.7 ns)−1 in CCl4.
A spectroscopic study of the excited states of dimethylzinc, dimethylcadmium, and dimethylmercury
Authors: Chen C. J. , Osgood R. M.
Published: February 2003

The absorption cross sections of dimethylzinc, dimethylcadmium, and dimethylmercury are measured from 3000 to 1750 Å using a computerized spectrometer with 0.6 Å resolution. Structured continua with long progressions of vibrational peaks are observed for all three molecules. This vibrational structure, which is explained by a direct, asymmetric dissociation process resulting in a methyl radical and a metal–monomethyl molecule, is identified with vibrational modes in the fragments. A discussion of the electronic states of metal–alkyl molecules is given.
Molecular aspect of the even–odd effect in cyanobiphenyls (nCB): Theoretical studies of the molecular geometrical conformation and optical anisotropy. I. n≤6
Authors: Lalanne J. R. , Rayez J. C. , Duguay B. , Proutiere A. , Viani R.
Published: February 2003

We report some theoretical calculations related to the conformation of 4-n alkyl-4′-cyanobiphenyls (n≤6), performed by means of a quantum mechanical approach and classical electrostatic chemical bond method. The main results of this study are (i) the planarity of the cyanobiphenyl skeleton, and (ii) a strong intramolecular perturbation of the conformational order of the alkyl tail were trans form is largely favored. Such a perturbation is expected to vanish for n≥6.
Low temperature specific heat of β-AgI, β-Ag2HgI4, and pyridinium Ag5I6
Authors: Madison M. R. , LeDuc H. G. , Coleman L. B.
Published: February 2003

We have measured the low temperature (∼1.8–20 K) heat capacity of the low temperature phases of the silver fast ion conductors AgI, Ag2HgI4, and pyridinium Ag5I6. All three materials displayed non–Debye heat capacities, however, following analysis of the phonon dispersion, no extraordinary behavior or precursory signs of the fast ion transition are seen.
Composite ionberg–iceberg model of aqueous nonpolar solvation and water exchange reactions
Authors: Phillips J. C.
Published: February 2003

A new model for aqueous solvation and water exchange reactions of large apolar solutes is proposed which radically extends the iceberg model of Frank and Evans. In the iceberg model each apolar molecular is encased in a cage of bulky water (a microscopic iceberg) whose extent increases with the size of the solute molecule. For small solute molecules the iceberg may have an incipient clathrate cage structure. In the composite ionberg–iceberg model when the solvent molecule is too large to occupy a clathrate interstice, a cloud of dissociated water ions nd(H++OH−) forms a core shell or sheath between the iceberg casing and the solute molecule, with nd increasing as the misfit between the solute and the cage increases. The ionberg model explains the slope and range of the linear phase space compensated activation volume-activation entropy correlations in associative and dissociative chemical reactions with apolar intermediates which are observed for solutes ranging from transition metal complexes to enzymes.
Modulated piezoreflectivity and the photoelasticity of molecular crystals
Authors: Luty T. , Eckhardt C. J.
Published: February 2003

Analysis of the optical response of a molecular crystal undergoing an external, periodic modulation is presented in terms of classical dipole theory. The case of modulated strain is treated in particular and identification of such strain with acoustic phonons with q=0 permits use of lattice dynamical theory to elucidate the acoustic phonon–exciton interaction. It is shown that extrema in the piezomodulated reflectivity (PMR) locate transverse and longitudinal exciton–polariton frequencies under specific experimental conditions. Model calculations demonstrate the great difference in the PMR for strong and weakly coupled systems. Relation of the PMR intensity to the elasto-optic coefficients and the storage of mechanical energy by the crystal permits extension of the technique to the study of such phenomena as internal strains and phase transitions where the partition of the mechanical energy among the various crystal excitations is of interest.
The Ar–NO van der Waals complex studied by resonant multiphoton ionization spectroscopy involving photoion and photoelectron measurements
Authors: Sato Kenji , Achiba Yohji , Kimura Katsumi
Published: February 2003

Using a 5% mixture of NO in Ar in a supersonic free jet, in the present work we have carried out measurements of the total ion current in the 380–385 nm laser wavelength region. We have also measured photoelectron kinetic energy spectra at individual ion current peaks. In the ion–current spectrum we have observed a new vibrational progression which consists of four peaks in the wavelength region longer than the peak of the two-photon transition of the free NO molecule NO(X, v″=0)  →2hν NO(C,v′=0). It has been concluded that the new ion–current peaks are attributed to bound-to-bound transitions of the Ar–NO van der Waals complex from its ground state to the two-photon resonant state expressed by Ar–NO*(C 2Π, v′=0), in which the NO component is in the 3p Rydberg state. The whole resonant ionization process studied may be expressed by Ar–NO(X, v″=0) →2hνAr–NO*(C, v′=0)  →hν Ar–NO+(X, v+=0). Each ion–current peak separation is about 50 cm−1, which may correspond to the frequency of the Ar–NO intermolecular stretching vibration, showing a strong anharmonicity. The dissociation energy (D0) of the Ar–NO*(C 2Π) state has been found to be 0.055±0.001 eV. From the photoelectron spectra, we also conclude that the adiabatic ionization energy of Ar–NO is Ia =9.148±0.005 eV and the dissociation energy of the Ar–NO+(X 1Σ) ion is D0=0.129±0.005 eV.
Relation between the numerical values of parameters appearing in an empirical formula for V0,A(Z) with the theoretical value of a coefficient in the Z expansion of the total energy of an atom
Authors: Anno Tosinobu , Teruya Hirohide
Published: February 2003

A semiempirical formula for the total energy E of an isoelectronic series of atoms obtained from a semiempirical formula [J. Chem. Phys. 79, 6162 (1983)] for V0,A(Z), the electrostatic potential at the nucleus, is compared with the expression for E in the Z expansion theory. It is shown that the coefficients of the first-order (in Z) terms in the two formulas agree closely. This agreement may be regarded as a justification of the V0,A(Z) formula.
Reply to ‘‘4He n-mers and Bose–Einstein condensation in He I
I’’
Authors: March N. H.
Published: February 2003

A reply to the comments of Ghassib and Chester1 on bound and Efimov states of 4He dimers2 and Trimers is presented. It is contended that Bose−Einstein condensation may not be a condition for the appearance of superfluidity in HeII. (AIP).
A molecular dynamics study of liquid drops
Authors: Thompson S. M. , Gubbins K. E. , Walton J. P. R. B. , Chantry R. A. R. , Rowlinson J. S.
Published: February 2003

We report molecular dynamics studies of small liquid drops (41–2004 molecules) in which the atoms interact with a Lennard-Jones intermolecular potential cutoff at 2.5σ and shifted by the potential at cutoff. We calculate the density profiles ρ(r) and the normal and tangential components of the pressure tensor pN(r) and pT(r), using both the Irving–Kirkwood and Harasima definitions of p. From these functions we calculate the surface thickness, the equimolar radius Re and surface of tension Rs, the surface tension γs referred to Rs, the length δ that appears in Tolman’s equation for γs, the pressure change across the drop, and the densities and pressures of the liquid at the drop center and of the gas. The variation of these properties with both surface curvature and temperature is studied, and the results are used to discuss the validity of Laplace’s equation for the pressure change, Tolman’s equation for the effect of curvature on surface tension, and Kelvin’s equation for the vapor pressure. We also make a qualitative comparison with previous theoretical calculations for drops using density gradient and integral equation theory.
Energy disposal in photodissociation from magic angle measurements with a crossed high-energy ion beam and laser beam: Photodissociation dynamics of the (N2)+2 cluster in the 458–514 nm range
Authors: Jarrold Martin F. , Illies Andreas J. , Bowers Michael T.
Published: February 2003

The principle behind obtaining photoproduct kinetic energy distributions from magic angle measurements with a crossed high-energy ion beam and laser beam is discussed and the results of an investigation of the photodissociation of the (N2)+2 cluster in the 458–514 nm range are reported. The N+2 product kinetic energy distributions are smooth and peak at relatively large values of kinetic energy (∼0.5 eV). An average product rotational excitation of at least 0.15 eV must occur since the product vibrational states were not resolved. A plot of the average product relative kinetic energy against the available energy is linear and can be accounted for using a simple impulsive model plus vibrational excitation due to geometry changes. The product angular distributions have the form 1+βP2(cos θ) with β=1.15–1.35 (with a linearly polarized laser). The effects of reactant ion rotation on the value of β were explored using a crude model and found to be small. The results indicate that photodissociation of (N2)+2 occurs by a direct transition to a repulsive surface. The transition is predominantly (>90%) 2Σ←2Σ and there was no evidence for the formation of electronically excited products.
Coupled channel distorted wave calculations for the three-dimensional H+H2 reaction
Authors: Schatz George C. , Hubbard Lynn M. , Dardi Peter S. , Miller William H.
Published: February 2003

A method for determining cross sections and other dynamical information based on the use of accurate coupled channel nonreactive wave functions in an evaluation of the distorted wave reactive scattering amplitude is developed for three-dimensional atom–diatom reactive collisions and applied to the H+H2 reaction. The nonreactive wave functions are obtained by using a truncated expansion in asymptotic vibration/rotation states to generate coupled channel equations in terms of the full Hamiltonian. Explicit reduction of the distorted wave scattering matrix expression to a real valued three-dimensional integral is given and methods for simplifying the calculation through the use of parity decoupling, even/odd decoupling, and the coupled states approximation are introduced. The application to H+H2 considers the Porter–Karplus potential surface. Comparison of reaction probabilities, opacity functions, differential and integral cross sections with corresponding results from exact quantum calculations indicates that the coupled channel distorted wave method is an excellent approximation as long as the total reaction probability for each partial wave is less than 0.1. This upper limit occurs at 0.60 eV total energy for H+H2. Above that energy, convergence with basis set size is oscillatory after an initial plateau. The coupled states distorted wave method is also found to be an excellent approximation, with results which are indistinguishable in accuracy from coupled channel distorted wave results. Extension of these approaches to a nonperturbative evaluation of the scattering matrix elements is discussed.
Vibrational excitation via shape resonances in electron scattering from the NO dimer
Authors: Sanche L. , Michaud M.
Published: February 2003

Vibrational electron-energy loss spectra of polycrystalline thin films (∼50 Å) of dimerized nitric oxide [i.e., (NO)2] have been recorded near 20 K at incident energies of 2,9, and 14 eV. The 2 and 9 eV spectra consist mainly of three vibrational peaks which are the first three members of two unresolved progressions ascribed to in-phase (ν1) and out-of-phase (ν5) vibrations of NO molecules within the dimer. The ‘‘background’’ intensity and the broadening of the elastic and energy-loss peaks are believed to result from unresolved multiple losses including vibrations within (NO)2 and phonon modes. At incident energies in the range 11–15 eV, a progression of seven vibrational members belonging to the ν1 and ν5 modes has been observed. This enhancement in the overtones of the fundamental frequencies is interpreted to arise from the formation of transient (NO)−2 states. The electron excitation functions recorded between 0–19 eV reveal the presence of two broad ‘‘humps’’ at 11.6±0.2 and 14.2±0.2 eV and a strong maximum near 2 eV. The latter is present in all decay channels including the v=1, 2, and 3 vibrational states of the ν1 and ν5 modes, the ‘‘background’’ losses and the elastic peak; but, the broad ‘‘humps’’ are exclusively related to vibrational excitation of the ν1 and ν5 modes. These two resonances appear to be derived from a splitting, via ‘‘through space’’ orbital interaction, of unfilled degenerate NO orbitals. In the 2 eV loss spectra the preponderance of a strong infrared mode (ν5) and the absence of anharmonicity in the vibrational peaks suggests that the vibrational excitation cross sections may be dominated by dipole scattering. No clear-cut evidence of compound states derived from the low-lying 3Σ−, 1Δ, and 1Σ+ NO− states was found in the dimer.
Resonance Raman spectra of c
i
s (CH)x and (CD)x
Authors: Lichtmann L. S. , Imhoff E. A. , Sarhangi A. , Fitchen D. B.
Published: February 2003

Resonance Raman spectra are reported for as-grown films of the organic semiconductor polyacetylene containing up to 96% of the cis isomer. Spectra were measured for (CH)x and (CD)x at 77 K at 20 cw excitation wavelengths in the range 458 to 680 nm. Excitation profiles for the principal Raman-active modes [at 910, 1250, and 1540 cm−1 in (CH)x and at 976 and 1470 cm−1 in (CD)x] and their first overtones and combinations are presented. Changes in position and shape of these cis Raman bands with changing excitation wavelengths are also presented. Competing luminescence processes following photoexcitation are examined, as are contrasting resonant features associated with the small fraction of trans isomer in these samples.
Ammonia dihydrate: Preparation, x-ray powder diffraction pattern and infrared spectrum of NH3⋅2H2O at 100 K
Authors: Bertie John E. , Shehata Mohamed Refaat
Published: February 2003

The dihydrate of ammonia has been isolated for the first time. Its x-ray powder diffraction pattern can be indexed on a cubic unit cell a=7.118 Å with a 95% confidence limit of 0.015 Å at ∼105 K, and Z=4. The number of diffraction lines, neglect of the hydrogen atoms, and the assumption that the atoms are on sites of multiplicity 4 or 8 yield P213 or P4232 as possible space groups. The infrared spectra suggest O–H--O distances of ∼2.76 Å, short O–H--N distances of 2.74±0.02 Å, and N–H---O distances of 3.1 Å or longer. A structure which fits these bond lengths well has been found in P213, with the oxygen and nitrogen atoms all on the 4a sites, of site group C3, with x=0.141 and 0.7111 for the oxygen atoms and x=0.3635 for nitrogen. The infrared spectra also show ν3(NH3) as a triplet, which means that the correct structure, including the hydrogen atoms, must have a lower symmetry than cubic, probably the orthorhombic P212121. That the crystal cannot be cubic is also seen from consideration of the apparent site symmetries of the water molecule under the cubic structures.
Enigma of observed temperature dependence of rates of ter-molecular ion–molecule association reactions and energy randomization time
Authors: Bates D. R.
Published: February 2003

The observed weak dependence of the rate coefficient for O+2+2O2→O+4+O2 on the temperature T below 75 K is explained by the dissociative channel provided by an excited electronic state of O+4 while the unexpectedly rapid decrease with T from 100 to 400 K is tentatively attributed to the finiteness of the energy randomization rate. It is suggested that the failure of simple theory to reproduce the observed variation with T of the rate coefficient for N+2 +2N2→N+4 +N2 is due to an incorrect assumption regarding the structure of N+4 when in the activated state.
Photoabsorption cross sections of OH at 115–183 nm
Authors: Nee Jan B. , Lee L. C.
Published: February 2003

The absorption spectrum for OH was obtained in the 115–183 nm region. The OH radicals were produced by a pulse discharge of trace H2O in few Torr of Ar. Absorption cross sections were obtained by calibration with absorption of the OH (A 2Σ+←X 2Π) transition. The features in the absorption spectrum are correlated with the excited states 1 2Σ−, D 2Σ−, 1 2Δ, B 2Σ+ and possibly others calculated by van Dishoeck, Langhoff, and Dalgarno. The measured cross sections are comparable with the calculated values.
Thermodynamic consistency conditions for the reference-hypernetted chain equation with arbitrary molecular potential
Authors: Lado F.
Published: February 2003

The free energy functional that characterizes the reference-hypernetted chain equation for molecular fluids is variationally minimized, leading to rules for the choice of reference system parameters that guarantee a significant improvement in internal thermodynamic consistency. This work generalizes a successful procedure developed recently for simple fluids.
The Kirkwood superposition approximation for hard rods at high pressure
Authors: Siders Paul , Kozak John J.
Published: February 2003

We examine the influence of the Kirkwood superposition approximation in the Yvon–Born–Green equation for the pair distribution function of a system of hard rods. We construct a linearization of the difference–differential form of the Yvon–Born–Green equation with Kirkwood superposition. By examining the Laplace transform of the linear equation, we construct the pair distribution functions essentially exactly, and predict a threshold for transition from damped to undamped oscillatory pair distribution functions. We show explicitly the closure for which this linear equation is exactly the Yvon–Born–Green equation. We compare the results to those obtained with the exact closure, and to numerical results obtained from the full nonlinear equation with Kirkwood closure. From these studies, we are able to show that it is the long-range, non-nearest-neighbor coupling, present both in the Kirkwood and linearized closures, that is responsible for the transition to undamped oscillatory pair distribution functions in the dimension d=1. We speculate on the relevance of this result to the behavior of the Yvon–Born–Green equation in d=2,3.
Dipole moments and potential energies of the alkaline earth monohalides from an ionic model
Authors: Törring T. , Ernst W. E. , Kindt S.
Published: February 2003

It is shown that the Rittner model is inadequate for the calculation of dipole moments for the alkaline earth monohalides. A modified model is proposed which takes into account explicitly the large charge shifts in the metal ions arising from the polarization. The new model is shown to give results consistent with available experimental data.
Phase transitions in polypeptide liquid crystals: (T,φ) phase diagram including the reentrant isotropic phase
Authors: Subramanian R. , DuPré D. B.
Published: February 2003

The phase diagram of polybenzyl-L-glutamate in a denaturant solvent mixture of dichloroacetic acid/dichloroethane has been mapped as a function of temperature and volume fraction of polymer. This polymer solution exhibits a reentrant isotropic phase at low temperatures where a conformational change in the macromolecule occurs. The liquid crystal is bordered by a low and high temperature biphasic zone where isotropic and anisotropic components are in equilibrium. The lower temperature biphasic region is broadened over that at high temperatures by the coexistence of random coils and polymer helices. Optical rotatory measurements in dilute solution distinguish between two conformationally different isotropic phases of this polymer, one with orientationally disordered helices and the other consisting of random coils.
Is chemical mismatch important in polymer solutions?
Authors: Joanny J. F. , Leibler Ludwik , Ball Robin
Published: February 2003

We study linear polymer solutions containing different chemical species within the framework of the direct renormalization method introduced by des Cloizeaux. We study the osmotic pressure and polymer radii of dilute solutions containing two chemically different polymers A and B both in good and θ solvent when the interaction between A and B monomers is repulsive. The central result is that in a good solvent in the asymptotic limit of infinite mass, polymers cannot tell each other apart: a polymer A cannot distinguish between a polymer A and a polymer B. The importance of the chemical difference between A and B monomers shows up only in corrections to scaling which are extensively studied. The study is made first for solutions of A and B chains having the same end-to-end distance and then generalized to the polydisperse case where A and B chains have different end-to-end distances. This allows to predict the behavior of the mutual A–B virial coefficient for two chains in a good solvent, in a θ solvent or in a selective solvent (good for A, θ for B). We then study semidilute solutions and show how the experimentally observed segregation between A and B chains is governed by corrections to scaling in a good solvent. An explicit expression is proposed for the osmotic pressure in that case. This study is also extended to very dilute solutions containing block copolymers.
Universal amplitude ratios for the surface tension of polymer solutions
Authors: Eisenriegler Erich
Published: February 2003

Arguments are given that (semi-) dilute polymer solutions are promising candidates for probing multicritical surface behavior in a real system. Multicritical surface properties of the energy density in a semi-infinite n-component Ginzburg–Landau model are discussed and used to obtain results for the monomer density and surface tension near the threshold for polymer surface adsorption in a good solvent. Directly at threshold, the surface tension is found to decrease with increasing monomer concentration. For a quantitative comparison with experiments several universal amplitude ratios are calculated. These relate surface critical properties at the so-called ordinary, special (or surface bulk), and extraordinary transitions of the semi-infinite Ginzburg–Landau model.
On the effect of fluctuations on explosive reactions
Authors: Dacol D. K. , Rabitz Herschel
Published: February 2003

A simple model for studying the influence of fluctuations in explosive chemical reactions is discussed. The model uses stochastic processes of bounded magnitude to describe the fluctuations. The exact solution of the model allows for a quantitative discussion of the effects of the fluctuations on physical quantities such as the critical value of the initial concentration that characterizes the explosive regime and the explosion time. Both quantities are found to be smaller when fluctuations are taken into account than when they are neglected.
CH stretch excitation causes conformational interconversion in ground state methyl vinyl ether but not in methyl nitrite
Authors: Ruoff Rodney S. , Kulp Thomas J. , McDonald J. D.
Published: February 2003

Molecules exhibiting conformational isomerism have been studied by spectrally resolving the infrared laser induced fluorescence from CH stretch fundamental excitation of methyl vinyl ether and from CH stretch fundamental and N=O overtone excitation of methyl nitrite. By comparison of the resolved fluorescence spectra of methyl vinyl ether with its absorbance spectrum, we have observed the first case of state selected ground electronic state intramolecular conformational interconversion in an isolated molecule. The distribution of conformers is altered drastically by the deposition of energy. The cis/trans ratio of the laser excited molecules may also be sensitive to the laser excitation frequency. Excitation of the N=O overtones of both cis methyl nitrite and trans methyl nitrite, which are well separated in frequency, gives resolved fluorescence spectra which mimic the absorbance spectra for cis methyl nitrite and trans methyl nitrite, respectively, thereby placing a lower bound for the conformational interconversion energy barrier from trans to cis of 3325 cm−1. The experiments were performed by exciting either molecule cooled in a pulsed nozzle expansion with light from an infrared optical parametric oscillator. The fluorescence spectra were recorded with a cryogenic Michelson interferometer.
Exterior electron model for Penning ionization. Unsaturated hydrocarbons
Authors: Ohno Koichi , Matsumoto Setsuo , Harada Yoshiya
Published: February 2003

A simple theoretical model is described for Penning ionization (M+A*→M++A+e−) in which a metastable-state rare gas atom (A*) extracts an electron from a target molecule (M) and ejects another electron into a continuum state. One of the most important factors governing Penning ionization is electron distributions of molecular orbitals which are directly connected with the electronic transition probabilities. Another key factor is the repulsive molecular surface which divides the unreactive electron densities in the interior region from the reactive electron densities in the exterior region, because the metastable atom A* cannot penetrate into the interior region. In the present model, exterior electron densities (EED) for individual molecular orbitals are considered to be proportional to Penning ionization branching ratios. Good agreements between ab initio MO calculations of EED’s and experimental branching ratios of Penning ionization were obtained for various unsaturated hydrocarbon molecules.
A new theory for vibrational and rotational energy transfer in the collisions of atoms with symmetric top molecules
Authors: Clary D. C.
Published: February 2003

A new three-dimensional quantum-mechanical theory is described for calculating vibrational and rotational relaxation cross sections and rate constants for the collisions of atoms with prolate symmetric top or near-symmetric top molecules. The technique uses a wave function expansion in vibrational states coupled with azimuthal basis functions which describe the spinning of the top about its symmetry axis. The infinite-order-sudden method is used for the total rotational angular momentum of the molecule. The technique is applied to the computation of vibrational relaxation rate constants for the collisions of C2H4 with He atoms. Comparison is made with results calculated previously by using the sudden approximation for both the total and azimuthal rotational motion. Good agreement is obtained with vibrational relaxation rate constants measured in an infrared double resonance experiment.
Feynman path integral and the Hückel model
Authors: Marañon Julio , Grinberg Horacio
Published: February 2003

The propagator of the Hückel model is obtained through the Feynman path integral formulation and the corresponding secular equations are generated via the saddle point approximation of the Gaussian integrals appearing in the path integral. The present formalism appears as a practical tool for the evaluation of thermodynamic and magnetic properties of molecular systems and constitutes an alternative method to those given previously using the Green’s function and Dyson equation to get self-energies and reactivity indices.
A new representation of potential energy curves for diatomic molecules
Authors: Arteca Gustavo A. , Fernández Francisco M. , Castro Eduardo A.
Published: February 2003

We present a new method for constructing potential energy curves that allows one to incorporate available experimental information systematically. It consists of a variable transformation that properly takes into account the behavior of the vibrational potential when the nuclei are far apart. As a result, the Dunham power series are changed into a sequence with improved convergence properties. Potential energy curves obtained in this way are free from spurious maxima and unphysical singularities. The method predicts dissociation energies for both ionic and covalent molecules that are in much closer agreement with experiment than those obtained by other procedures that make use of the same input data. Results are given for 20 alkali halides and 3 covalent molecules.
Determination of the conformational barriers to internal rotation of 3-fluoropropene from far infrared and low frequency Raman spectra
Authors: Durig J. R. , Zhen Mengzhang , Little T. S.
Published: February 2003

The far infrared (200–40 cm−1) and low frequency Raman (1000–20 cm−1) spectra of gaseous 3-fluoropropene have been recorded. The fundamental asymmetric torsion for the conformer which has the fluorine atom cis to the double bond has been observed at 164.62 cm−1 with four excited states falling at lower frequencies, and the corresponding fundamental torsion of the gauche conformer was observed at 108.00 cm−1 with three excited states observed at lower frequencies. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V2=459±29, V3=830±9, V4=18±8, and V6=−37±5 cm−1, with a ΔH of 304±20 cm−1 (869 cal/mol). It has been determined that the cis conformer is the predominant form at ambient temperature in the gas phase and, from a temperature study of the Raman spectrum in this phase, the enthalpy difference between the cis and gauche conformers was determined to be 263±25 cm−1 (752 cal/mol). The calculated cis to gauche, gauche to gauche, and gauche to cis barriers are 1201 cm−1 (3.43 kcal/mol), 498 cm−1 (1.42 kcal/mol) and 869 cm−1 (2.48 kcal/mol), respectively. An alternative potential function was also calculated utilizing the successive gauche asymmetric torsional transitions proceeding to higher frequencies. A second series of asymmetric torsional transitions were observed for the cis conformer which are believed to be due to ‘‘hot bands’’ associated with the first excited state of the CCC bend. Additionally, each of the torsional transitions for the cis conformer exhibits multiple splitting which is believed to be due to excited states of the CCF bend. These results are compared to similar quantities in some related molecules.